IFATCA - The Controller - 1972 by IFATCA

IFATCA JOURNAL OF AIR· TRAFFIC C()NTROL

AnniversaryIssue

DE.CEMBER

1972

VOLUME

NO. 1-4

(Cossor CSD 2000 series synthetic radar display)

The quality of the information presented to an air traffic controller is determined by the excellence of the equipments between the display console and the aircraft in flight. Cossor syste111sprovide the fl nest possible presentation of alpha-numeric and radar data. System improvements in development in Cossor include a SSR reflection suppression transmitter, Cossor Precision Secondary Radar (CPSR) and the Selectively Addressable SSR system (ADSEL).

~" Cossor ~

'W~

The range of Cossor equipments include:SSR Transmitter Site Equipments. Decoders-Active and Passive. Plot Extractors. Data Processors. Displays- Raw, Mixed and Synthetic. Alpha-Numeric Terminals. Airborne Transponders. SSR Test Equipment. For further information contact:-

Cossor Electronics Limited, Sales Division-Aviation and Surface Electronics, The Pinnacles, Harlow, Essex, England. Telephone Harlow 26862. Telex 81228.'Cables: Cossor Harlow.

IFATCA

JOURNAL

AIR

TRAFFIC

CONTROL

THE CONTROLLER Frankfurt am Main, December 1972

Volume 11 • No. 1-4

Publisher: International Federation of Air Traffic Controllers' Associations, S. C. II; 6 Frankfurt am Main 60, Bornheimer Landwehr 570. Officers of IFATCA: J. D. Manin, President; J. D. Thomds, First Vice President; R. Meyer, Second Vice Presi• dent; H. Guddat, Honorary Secretary; J. Gubelmonn, Treasurer; W. H. Endlich, Treasurer; Executive Secretary, T. H. Harrison. Editor: Wolter H. Endlich, Brewersstraot 18, Simpelveld, Holland Telephone: 4442-1250

CONTENTS Editorial The Importance of the "Human Factor" for Collision Prevention in the Terminal Area

3 5

Prof. Dr. H. von Diringshofen Fatigue and the Controller . . . . . . . . . John G. Wilson

Human Engineering Problems of Air Traffic Control Tasks

V. D. Hopkin Management Factors in Reducing ATC Stress

John T. Doiley Publishing Company, Production and Advertising Sales Office: Verlag W. Kromer&Co., 6 Frankfurt am Main 60, Bornheimer Landwehr 57a, Phone 43 43 25, 49 21 69, Frankfurter Bonk, No. 3-03333-9. Rate Card Nr. 2.

Some Neglected Psychological Problems in Man Machine Systems . . . . .

V. D. Hopkin The Control Load and Sector Design

Printed by: W. Kramer & Co., 6 Frankfurt am Main 60, Bornheimer Landwehr 57a.

Price per copy of this issue: DM 16.-. Contributors

B. A. Arod Mathematical Models for the Prediction of Air Traffic Controller Workload

S. Rotcliffe

are expressing their personal points of view

and opinions, which must not necessarily coincide with those of the International Federation of Air Traffic Controllers' Associations (IFATCA). tFATCA does not assume

responsibility for statements made and opinions expressed, it does only accept re• sponsibility for publishing these contributions. Contributions ore welcome as are comments and criti• cism. No payment can be mode for manuscripts submitted

for publication in "The Controller". The Editor reserves the right to make any editorial changes in manuscripts, which he believes will improve the material without altering the intended meaning.

33 35

The Name of the Game . . The Schiphol ATC Simulator

R. N. Horrison Russian Story

. . . . . . . . . . . . . .

Walter H. Endlich ond Bernhard Ruthy Developments in Helicopter IFR Operations Tirey K. Vickers

Some Aspects of Terminal Control in the Seventies

Tirey K. Vickers Wind Shear Problems in Terminal Operations

. . . . . . .

.....

Tirey K. Vickers The Improvement of Wet Runway Operation

Tirey K. Vickers Written permission by the Editor is necessary printing any port of this Journal.

for re-

Living with Vortices . . . . . . . . . . . . We're learning more about Clear Air Turbulence

Advertisers in this Issue: AEG-TELEFUNKEN (2); Cessor Electronics Ltd. (Inside Cover); Ferranti DSD Ltd. (4); RACAL Thermionics (38); SELENIA S. p. A. (Inside Bock Cover, Back Cover); I. W. Sharp Associates (27); STANSAAB (52).

Picture Credit: Arad (21, 24, 25, 26, 27); Elsner (33, 34); Endlich (39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51); Ferranti (37); Orr (87, 88); Ratcliffe (30); Telefunken (98, 99, 100); U. S. Weather Bureau (83); Vickers (53, 54, 55, 56, 57, 58, 59, 61, 62, 65, ., 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 80, ' 85, 86, 94, 95).

Tirey K. Vickers Tirey K. Vickers The Role of the Touch Display in Air Traffic Control N. W. Orr and V. D. Hopkin Lessons learnt in nine Years SATCO . . . . . . . . . . . J. S. Smit Falconry in the Air Command of the Royal Navy Lt. Commander D. D. Fairweother

82 87 90

. . . . . . . . . .

Digital Radar Plot Extractors . . . . . . . . . . . . . . .

Developments in Collision Avoidance

Tirey K. Vickers Dr. Heinz Ebert

Cherubs

■ ■ ■

... have been around for ages. They·re the only things flying that can take off and land in a room. They reach their destinations without navaids, thake short cuts through airways and fly at any height they want ...

141.017

. .. well, as far as we know. We don't know if they're still flying in today's dense air traffic - if they are they haven't been seen on the screens of our ATC radars. But perhaps that's the fault of our radars we get the Angels but not the cherubs.

AEG-TELEFUNKEN Fachbereich Hochfrequenztechnik 79 Ulm • PB 830 W.Germany

Radar equipments by AEG-TELEFUNKEN

Editorial More than ten years have passed since the publication of the first issue of this journal. Ten years during which the International Federation of Air Traffic Controllers' Association has grown from an initially European venture to a truly world-wide organization. Within that time we have attempted to provide our readers with interesting mation on Air Traffic Control and related subjects.

and timely infor-

It appears that this endeavour has met with some success, judging from the frequent requests for back copies of the journal, or for the reprinting of articles. The latter requests have been so numerous, and the back copies in such small supply, that we have thought it appropriate to re-publish in this anniversary issue a cross-section of the "bestsellers" of the past decade. Such an issue, it was hoped, would, to some extent, also meet the demands for a full set of all CONTROLLER volumes published since 1961, which are often made to the publishing house. It is considered to be of particular value to those IFATCA Member Associations who joined the Federation only recently. Although some of the articles, for instance the study "Living with Vortices" by Tirey K. Vickers, have been published many years ago they are as valid and timely as they were at the time of their original publication. The tremendous growth of IFATCA and its increasing activity on a world-wide basis have brought about some problems, not only in respect of the efficient administration and management of the Federation, but also for the publication of its journal. A point has been reached beyond which the production of the journal on a mainly voluntary basis becomes extremely difficult. It may soon become necessary to employ the services of a paid Associate Edito,. To cover the additional expenses incurring therefrom, and also to compensate for the continuously rising tariffs in the printing trade a slight increase of subscription and advertising rates will probably be unavoidable in the future. After all, the CONTROLLER rates have never been changed since 1961, when the journal was first published. These organisational changes, however, take time and until all relevant questions are settled, the publication of the CONTROLLER may have to be suspended temporarily. Meanwhile we shall attempt to design a new cover page and also review editorial policy. Just two thoughts on the latter subject: IFATCA has always attached great importance to the human element in Air Traffic Control, and many interesting papers on that subject have been published in the journal (cf. pp. 5, 7, 9, 13, 16, 20, 28). To maintain and enhance our efforts in that field the good relations between IFATCA and the "Stress in Air Traffic Control Research Association" (SATCRA) have been further consolidated. We are presently investigating the possibilities of SATCRA's direct involvement on subject-related editorial activities. Also under consideration is the making available to SATCRA on a regular basis printing space for the systematic treatment of this important subject in the IFATCA journal. The IFATCA Corporation Members, through their valuable assistance, have essentially contributed to the contents and format of THE CONTROLLER. We shall further increase the friendly contacts with the avionics industry, and endeavour to make available to such organizations as EUROCAE the consolidated experience of Controllers from all over the world. Close contacts between System Designers and Air Traffic Controllers will undoubtedly result in industry being better informed about the Controllers' requirements and Controllers about the equipments which industry can offer. Finally, I would like to express sincere thanks to those who have kindly assisted me in launching the IFATCA Journal and keeping it in orbit during its first decade. Walter H. Endlich

It's the world of air traffic control. We've been in it since the 'fifties and have designed both operational systems and simulators for training and evaluation. Our programmers talk to controllers a lot and get to know their problems. They learn not only what information a controller wants but how he wants it presented. Take the situation today. More advanced equipment means that more and more information is coming in, and a controller needs help in sorting the wheat from the chaff. So we're automating air traffic

control ... processing the data so that the controller is freed from a flood of detail but can receive early warning of the development of conflict situations, and can then concentrate on resolving them while leaving the system to monitor all that's going smoothly. It's all helping to create the conditions that will be essential for the high speed, high volume jct travel of the seventies. Ferranti are at the heart of it. Ferranti Ltd., Digital Systems Division, Bracknell, Berkshire, England, R G 12 l RA

Canada - Ferranti-Packard Limited, Industry Street, Toronto 15, Ontario. U.S.A. - Ferranti Electric Inc., East Bethpage Road, Plainview, New York 11803.

air traffic control for the seventies.

FERRANTI 0S32/2

Im)

Prof. Dr. H. von Diringshofen

The Importance of the so-called "Human Factor" for the Reliability of Collision Prevention in the Terminal Area As long os there is no automatically operating and sufficiently reliable instrument system for preventing aircraft collisions, the avoidance of such accidents is dependent on the following human factors in flight safety: I. 0 n Boord, on the ability of the pilot when flying under visual flight rules to evade other aircraft in time. II. 0 n the ground, on the obility of air traffic control to provide for the necessary safety separation in the airspace under control. Neither of these is possible at the present time with the requisite reliability. In many aircraft, pilots have a very restricted field of vision. They cannot continuously search the airspace for other aircraft because they have a great deal more to do, particularly after take-off and when approaching an airfield for landing. Thus there occur frequent intervals in the observation of the airspace, in which, under certain circumstances, other aircraft may approach unnoticed within a critical distance on a collision course. In practice these intervals are often considerably longer than necessary. It is o problem of training in flight discipline, to keep them as short as possible. But even with the best intentions on the port of the pilot, these intervals cannot be reduced to less than 10 seconds in most cases. This con have critical consequences, particularly in the terminal areas of airports with their large numbers of aircraft in flight,even when the approach speeds ore low. In this connection, on article by A. Zeller in the periodical "Aerospace Medicine", from the viewpoint of "Mon when anticipating collisions in the air", is very informative. According to this article, 33 such collisions occured in the United States in 1958. Of these, 80% were within a radius of 30 miles of airports, mostly under visual weather conditions, at moderate speed. In half of these accidents the cause was: other aircraft not seen; in 25% faulty estimation of distance and in some 20%, too late or wrong evading action. It would thus be unrealistic to believe that pilots, when flying under visual flight rules, constantly watch the airspace so effectively, that collisions can be prevented with requisite reliability. As a result, satisfactory collision prevention is at the present time only possible by means of air traffic control from the ground. This is supported by the very small number of aircraft collisions under weather conditions that only make flying possible under instrumenf flight rules. Unfortunately present day technical means and statutory conditions ore invariably inadequate for identifying and determining the altitude of every aircraft in the controlled airspace and to pass the necessary information to its pilot through the medium of radio telephony. For this eoson the radar controllers con only inform the pilots with ·""',c, they ore in communication about non-identified air-

craft when their flight path might be a collision course. As such, warnings ore given much more frequently with the altitude data missing than should otherwise be necessary, and as the pilot notified can often not see the aircraft in question, because it is for off at a different altitude, these warnings cause anxiety and finally induce non-observance. In the Air Traff1c Control profession it is a natural tendency for the controller to deem himself largely responsible for protection against collisions in the airspace he controls. He therefore experiences the collision situation that he has not suff1ciently clarified, because of non-identification of aircraft on radar, as a considerable psychological burden, which often weighs more heavily than the strain of controlling densely crowded airspaces, in which controlled \FR-flights ore. operating. The extensive use of radio-telephony equipment in private and sports aircraft, and the introduction of secondary radar with automatic data transmission systems for identification, flying altitude and speed, could certainly improve the reliability of collision prevention in terminal areas and notably relieve the controller. Radar installations operating threedimensionolly and an automatic proximity-warning from the ground ore most desirable but are only to be expected in a distant future. There ore not many professions, in which the sense of responsibility for the lives of others is generally so highly developed, as in the Air Traffic Controller's profession. At the present time air traffic controllers work at the focus of air traffic, often up to the limit of the strain they ore capable of supporting, for reliable accomplishment, pressed for time as they are and under the pressure of great responsibility. They are aware of the present technical inadequacy of their air traffic control system which can still not be avoided and experience fairly often near-collisions which occasionally only have a happy outcome through favourable luck. They often hove to disentangle critical air traffic situations which demand the greatest concentrotion and rapid decisions. The life of more than l 00 people may depend on whether the air traffic controller is fully efficient or whether he is restricted in the accuracy and speed of his actions by fatigue or some other deterioration of his stole and his strength of vision. Under heavy stress on the air traffic-control staff, which is becoming ever greater and of longer duration as a result of the increase in density of air traffic at its focal points, human reserves of capacity for work ore so small, that every reduction in capability because of the inadequacy of the environmental conditions, such as unsuitable lighting, insufficient air conditioning, too high a noise level, uncomfortable seating and mD11yother things as well, may under certain circumstances have dangerous results, particularly if the staff position should make more than two hours of uninterrupted attention necessary under full strain. In this connection, air traffic controllers in the conventional control are particularly liable to disturbance, 5

because their task demands powers of memory and imagination which ore very easily affected unfavourably by fatigue. It is by no means infrequent for radar controllers lo note errors in the conventional control, which may result in the circumstances in a dangerous proximity of the aircraft. Aircraft cannot pull up short whilst in flight. They con only temporarily and in restricted ~umbers be confined to a holding pattern. This is the fundamental difference between the system of air traffic control and the control of rood, roil and sea traffic. Air traffic controllers form port of a very rapidly operating man/machine systeme in which human and technical factors of flight safety, both on the ground and in the air, comprise a functional unit. In this system the human factor in air traffic control, with regard lo preventing collisions in controlled airspace hos become just as important as the pilot. Because of the close functional link between pilot and controller, it appears to be advisable, with a view to improving flight safety, that the latter should often be given the opportunity of flying with the pilot in the cockpit of commercial aircraft, in ord\!r lo acquaint himself with the problems met with on board. Again, flying training for controllers, as it is carried out in Belgium, Fronce and Austria, with official facilities, appears lo be thoroughly logical. Pilots, loo, should in their own interests, take the trouble to get on insight into the problems of air traffic control and on appreciation of the limits of efficiency of this system. In the coming years, demands on air traffic control staff, and especially on controllers, will certainly increase considerably because during this period the growth of air traffic wil: presumably be substantially quicker than the relief afforded to the ATC staff by new technical methods and automation. This gives rise to the urgent requirement lo exhaust every possibility for technical improvements which con reduce the strain on the staff, even in the smallest details, and to spore no measures or expense in the process. Such onlhropotechnicol rationalisation, os well as physical and psychological operational hygiene, belong to the crucial port of the flight safety program. For this purpose, the engagement of on industrial doctor with good physiological and psychological ability and experience and with o particularly sympathetic understanding of the importance of air traffic control, is indispensable. Only a doctor of this type hos o sufficiently troined ability for recognising and correctly assessing a lock of operating hygiene and the capacity for making the most suitable suggestions for its elimination. However, he will need to colloborote closely with a suitable techn;cion. Without on industrial doctor, ATC is not only a curiosity, but also evidence that authorities concerned who do not wish to sanction the cost of such a doctor, foil to recognise the importance of the human factor in ATC with regard to flight safety.

of employment ond ore i_n o psycho-physical guarantees their full efficiency.

stole that

Such a requirement hos long been a matter of course for pilots of commercial and military aircraft. The medical supervision of the operating conditions in air traffic control and of air traffic controllers should be just os much a matter of course. Here special observation must be directed to the oppeorence of nervous symptoms and the indications of overfotigue, because these point lo o considerable reduced speed and occurocy of observation and action. In these coses prompt measures for relaxation con generally completely ond fairly quickly re-establish full efficiency ond ability to stand the strain. Nervous supervisors ore particularly unsuitable in air troffic control work, because of their harmful effect on the mental operoting atmosphere. Candidates with evident symptoms of nervous and psychological instability should not be put lo work in oir traffic control, even though as a result, candidates may be lost who might become suffi. ciently stable with increasing age. It would be very regrettable if the full importance of the human factor in oir traffic control, os regards the reliability of collisionprevention, would only then be generally realized, if demonstrated unambiguously by o series of collisions in the air with many deaths, attributable lo human fo;lure of air traffic controllers os o result of overstroin. In the coming years the probability of such demonstrations may occur much more often, if air traffic controllers find themselves increasingly pressed for time by the rising density of the traffic, that is, if the available time becomes less than is essential for the reliable accomplishment of their work. Then their capabilities will foll rapidly with the quotient: time available / time required, in the form of o quickly steepening curve. The time required by human beings for a quite simple, reliable action does not differ greatly individually and according to conditions. For correctly appreciating and mastering o complicated situation however, the differences in the time needed ore very great, os these ore governed by individual ability and the temporary physical and psychological frame of mind. It is for this reason that, when time is short, the reliability of collision prevention through air traffic control is so highly dependent on the professional ability and healthy frame of mind of the traffic controllers as well os on whether they are in o vigorous stole or exhausted at the time. Only automation con provide sufficient reserves of time in ATC as it is increasingly withdrawing the human factor from direct inclusion in the functional chain of this system. But there is stil I o long way to go to achieve this goal. Such complicated technical systems con, however, only operate with adequate reliability, if they are constantly supervised and maintained by human beings and if provision is mode for man to make the necessary compensation promptly by means of other technical methods in emergencies.

The foci that the number of collisions of commercial aircraft still remains very low, d<;>es not justify the assumption that this will be so in the future, unless steps ore token in all large ATC-centres to see that the air traffic controllers, employed there only after rigorous checks on their suitability and proficiency, ore given optimum terms

The physical and psycho-physical adaptability of man to oeronouticol technical science has its limits. The adaptability of technical science to man, however, is practically unlimited. It is therefore o much better prospect to adapt technical science to the shortcomings of mon, than to accustom man to the shortcomings of science.

First published in the Winter 1961/62 issue of THE CONTROLLER

FATIGUE and the CONTROLLER

"The controller, in applying the standards contained herein, should remember that he is completely responsible for the safety of humon life and of valuable aircraft and cargo. Human error is completely intolerable." How many ATC Manuals contain similar statements of ideal and human standards? To be read with an inner glow, and shown to the other agencies and users with pride ... Which is all very commendable, but despite such highflown ideals, which no professional controller deliberately compromises, human error does occur. There can hardly be a controller, engaged in IFR control, who has not same time or another seen an error committed. An error which, maybe, was caught in time and did not cause an incident or which did not cause an incident just because there wa: no other aircraft there, or which did, in fact, cause a "paper" incident, solved by a hurried transition to other separation standards - typically, radar saves the day. Very occasionally small percentage statistics work out and a genuine hazard to navigation is caused, leading to investigation, publicity, mudslinging, and a general lowering, for a time, of the mutual confidence so necessary between pilot and controller. Where and why do such errors creep in? Noticeably, they do not seem fo occur in the final phases of approach and landing, where all possible conflict has previously been resolved, nor do they occur in VFR circuit conditions, except where a pilot has failed to carry out his instructions - quite another consideration and outside the scope of this article. We are then left with the major problem areo, which is so susceptible to human error and where the implications ore so serious; that is the transfer of control data followed by the transfer of control, either between Centres, or sectors, or between units within a Centre's jurisdiction. Now, if we accept that no controller is normally and deliberately going to create a lack of separation, then we have to seek some factor which could affect his complete physiological state, and thus make him, in effect, o temporarily different person in his mental and motor reactions to the situation of the moment. When we set it out this way and then eliminate the more obvious external influences, such as personal illness, drug effects, etc., we find we are left with one culprit only - fatigue. This is, of course, an easy conclusion to orrive at, especially in view of its intangible nature. Possible this has something to do with the fact that very little published work exists dealing with fatigue in the ATC environment. Professor Dr. von Diringshofen's article on "The Human Factors in Collision Prevention" gives an excellent and authoritative viewpoint from the medical side of the fence whi!st ~rnold Fi~ld's add~ess to the Royal Aeronautical 1ety s symposium on Air Traffic Control, published in ril 1961,gives the viewpoint of the senior and wise con\'!r studying his colleagues in their environment.

by John G. Wilson

In the field of aviation medicine, however, for more study has been given to the effect of this insidious factor on aircrew, and it seems to be important that this material is not wasted, and is, in fact, analysed and related to our ATC environment. The general effects of fatigue ore unlikely to be different, since we are all human beings, but it is the functional results of the general effects that will be different for a controller and need to be considered known and understood by all of us so that we can opera!: most safely and efficiently, even whilst under its effects, and so that we can train ourselves and others to recognise its onset and stave off its effects. This is essential, for, with the general and growing shortage of personnel, the inescapable fact remains that the job must still be done, making it mandatory that we are always consciously looking for the symptoms and results of fatigue in ourselves and others and that our personal procedures and selfindoctrination are of the foil-safe variety insofar os the effects of fatigue are concerned. Many of the general conclusions upon which this article is based originally appeared in an article entitled "Fatigue", by Col. W.R. Turner, MD, USAF, published in "Interceptor" and subsequently in the RCAF "Flight Comment". So what is this fatigue that is so insidious - that so many managements try lo laugh out of existence? What ore its effects and results? When we ore fatigued, we experience drowsiness, lassitude, tiredness etc., however these factors are incidental and the important fact, which should always be present i~ every controller's mind, is that fatigue results in loss of efficiency and skill. The medical symptoms of circulatory instability, loss of weight, hypoglycemia, and disturbance of coordination are only overt symptoms. Because fatigue is on intangible, and we cannot measure or define it, we have difficulty in controlling it, but we can study the factors that produce it and their relationship to each other. First then the causes, which con be categorised into environmental and personal causes. In the environmental category we have: Long and continuous periods of duty. Operationally unsatisfactory equipment, which probably serviceable and technically adequate. Equipment irregularities "make do" situations.

or outages, with resultant

Traffic density. Actual position being filled. To these more operational factors, we must odd external physical factors such as: Uncomfortable Ambient humid.

seating.

temperature

conditions

too hot, cold or

High general noise level. Badly laid out position time and motion study.

from the point of view of

Now these fotigue-inducing circumstonces ore all external factors, but we ore human beings and ore subject to internal stresses as well. let us consider some of these internal or personal factors: Boredom Concentration Frustration Attention Uncertainty

Responsibility Anxiety Apprehension Panic Fear

Quite a list, is it n~t? Now let us odd some more: Hunger and thirst lock of experience Family illness Financial problems Known personal inadequacies Suspected teamwork inadequacies Unsatisfactory personnel management from front office So now we hove compiled this formidable list and loaded it onto the controller's bock, what is the result? An error is mode in something which is relatively simple and easy on face value, on error of the type which always seems so inexplicable and incomprehensible to subsequent investigation. Now we ore told that the Cambridge cockpit studies on fatigue, conducted on a Spitfire simulator, come up with several conclusions.These ore set out with the original wording modified to fit our ATC environment: l.

Motor responses suffer as fatigue develops. (Hove you ever caught yourself, for example, clearing on aircraft to a completely different destination from that on the strip, or passing an estimate to the wrong Centre, apparently without cause?)

2. Fatigue produces a willingness to accept lower standards of accuracy and performance. (The "couldn't core less" attitude.) 3. Fatigue tends to induce a shift from control based upon the situation, as it appears on the board, to rule-ofthumb control. (The "habit" of always giving that particular flight its initial descent at that particular fix.) 4. Fatigue induces a failure to check all the control tors before implementing a control decision. (The same situation as 3.)

fac-

So much for the facts - whot, then, ore some of the more obvious dangerous results or reactions of this fatigue effect? Failure to remain alert Carelessness Drowsiness Day-dreaming Lock of stability Loss of self-control Becoming aware of on error but being apathetic about correcting it Irritability Irrationality

One typical effect is some times referred to as ''hearing cross-eyed" or "transposition error". It seems to be caused by a breakdown in the communication path within the individual where, for exomple, one heors information, understands, reacts, and instigates a motor response to write or speak - somehow, what was heard does no I gel written or spoken as originally heard, as for example, the taking of an estimate or progress report. Possibly in the fatigued slate, the unconscious is able lo slip in a previous and unrelated memory out of the filing system, however this is purely uneducated speculation. Nevertheless the individual will be quite convinced that what he wrote was what he heard, because he hos no other memory of the train of events, but somebody else, monitoring the exchange, may well be horrified lo see a responsible and competanl colleague write down on altitude quite different to that being d ictaled by the adjacent Centre, or a time quite different from the aircraft's actual report. Of all fatigue symptoms, it is perhaps the most dangerous in the ATC environment, because of the vast amount of control data which is passed verbally between Centres, and will be for some years to come. The potentiality for incidents due to this sort of error is considerable. How, then, con we reduce the effects of fatigue? In one word training. The development in the individual, through his own, or formal, indoctrination, of routine safe habits, habits which may be somewhat restrictive, or phraseologies which may use extra words, in light traffic conditions, but which, in conditions of heavier traffic and fatigue inducing circumstances ore a safe rock upon which a controller may stand, whilst he assembles his deteriorating concentration and completes the solution to the problem. Develop the habit of always reading bock time and altitude on on estimate or progress, which gives someone else the opportunity of picking up your error before it gets serious. Develop the excellent American practice of using every communication concerning a flight as on opportunity of double-checking the altitude. Stick rigidly to routine and procedure and indoctrinate yourself to do this normally, and you will do it automatically when you ore fatigued. "Be flexible and expedite" is fine, if you ore in top form mentally, but if you let your normal operation become a non-routine flexible affair, then you ore overtaxing ·and relying heavily on your concentration, which is one of the first factors to deteriorate under the effects of fatigue. The more procedurolised and routine the operation, the less the concentration needed to run it, which results in more of the controller's mind being available for the non-standard problem, or in a bigger cushion against fatigue effects.

Once again a formidable list, however the one which is perhops most worthy of more detoiled consideration is the last - irrationality - doing something quite inexplicable with dangerous implications; in addition, and this seems to be characteristic of this fatigue symptom, the individual often cannot remember or explain the action.

Finally, the supervisory responsibility - and here it should be stated that although the controller himself has a duty to report when he feels he is suffering the effects of fatigue, the onus is upon the supervisor to be looking for it and to ring the changes before it gets dangerous. Detecting fatigue is, in itself, fatiguing, but it con be done and should be done, even though the results may only seem to be negative - incidents that do no I happen however awareness of and alertness to the symptoms itself aids in preventing the typical errors. Every level of management has a responsibility in eliminating the fatigue factor. When long hours of shift cannot be avoided, fatigue m us I be recognised as a potential hazard and sensible precautions token at the on-the-floor supervisory level.

First published in the January 1965 issue of THE CONTROLLER.

Human Engineering Problems of Air Traffic Control Tasks by V. D. Hopkin RAF Institute of Aviation Farnborough, Honts

Medicine

In the following paper Dr. Hopkins describes the contribution which the psychologist can make to the evolution and testing of air traffic control system, together with certain human factors problems which are associated with air traffic control tasks. Solutions to many of these problems can be devised only by working from first principles and

by conducting evaluation trials to confirm the acceptability of recommendations. This paper has been read to the 7th Conference of the Western European Association for Aviation Psychology, Eurocontrol, Brussels, Sept. 4-8, 1967.

Introduction

tern is already in being, and that better advice could have been given if the question had been asked before the system was built and brought into use. This leads to the psychologist contributing at earlier stages in the evolution of the system, at first in the testing and evaluation of proto• types and eventually at the earliest planning and design stages when the system is formulated. It is at this earliest stage that the psychologist may make his most valuable contribution by obviating the need for extensive and time consuming remedial measures after the system is in being and by ensuring that the system as built is intrinsically more efficient than it would otherwise have been. In parallel with this progress from specific ad hoe problems to contributing at the planning stages is the extension of the psychologist's contribution into related areas. For example, advice on the efficiency of a given display will probably mention ambient lighting, the design of symbols, the layout of work spaces, and so on. Attempts to evaluate the display lead to consideration of system evaluation, system measurement and simulation, communication, and features of the working environment. The tendency can develop, and must be resisted, for the psychologist to think of himself as a one man expert. This he is not. He may, however, fulfil! the role of catalyst in ensuring that the multitude of requirements of various other specialists are not ignored when the system is devised. The psychologist has a contribution to make, for example, on ambient lighting problems, but this is in addition to and not instead of those of the medical specialists and the lighting engineer. Effective work on system of this kind implies good relationship with people from many disciplines, some of whom may need to be convinced that the applied experimental psychologist can make a valid contribution. Normally this is not difficult to prove and his contribution is soon accepted, although the onus of proof is, rightly, on the psychologist.

At the present time, extensive planning and evaluation of future air traffic control systems are in progress. The impact of automation on air traffic control tasks and procedures is being felt in cu'rrent systems and in the formulation of systems for the future. Current air traffic control systems have been dealing with increasing traffic loads during the past few years and the traffic is expecled to increase at a rapid rate for many years to come. Future systems must therefore be able to handle higher densities of traffic than current ones. They must also handle more varied traffic. Most current aircraft types will retain their counterparts in future systems, but the advent of supersonic aircraft and of increasing numbers of small executive and private aircraft will extend the range of traffic types which future air traffic control systems must deal with effectively. The size and complexity of current and future air traffic control systems pose many human engineering problems, some of which are of a standard type encountered in most man/machine control systems while others seem peculiar to air traffic control tasks. Extensive human factors work on air traffic control problems is a relatively recent innovation, and in many instances the full contribution which human factors can make towards solving such problems has not yet been appreciated. The multiplicity of problems is, however, beyond the current capacity of human factors resources to tackle in detail, and it may therefore be some years before human engineering plays its full part in air traffic control.

The Role of the Psychologists When advice on human factors, and more particularly the advice of an applied experimental psychologist, is sought for the first time, it is usually about a small problem in a system in operation where for some reason the operators have signally failed to perform as expected. Practical recommendations can be made to produce a significant improvement in performance or the reasons for the poor performance can be made clear and the prerequisites for an improvement defined. In this way the psychologist shows that he has a useful contribution to make and this is commonly first established in relation to the solution of simple problems in current operational systems. Usually his ·advice makes the point that the solutions available are .some ways unsatisfactory and limited because the sys-

The Application of General Principles Many of the problems encountered in air traffic control tasks are standard ones associated with any man/ machine control system. Advice is provided, when the system is being formulated, on the feasibility of the envisaged tasks in terms of the work load they will involve and the facilities required for their performance. This is without prejudice to loter evaluation trials but ensures from this early stage that the work loads demanded are unlikely to

When problems are of a general type associated with all systems it is often possible to suggest, on the basis of previously published findings, what sort of solution is likely to be best. There are, however, other problems which seem specific to air traffic control tasks and can seldom be solved by consulting a book. Such problems may require either a fuller exploration of all the factors involved before an acceptable solution can be· formulated or evaluation trials, because no acceptable solution can be achieved without further empirical evidence.

become traditional so that the burden of proof lies with those who wish to change it. Situations where dim light is still essential are now rare. It may still be advisable, if the operator has a raw radar picture and an identification or vigilance task, to keep the ambient illumination low to prevent some signals falling below the visual threshold. However, many current displays do not present raw radar information but synthetic information, including digits, the brightness of which can be adjusted by the operator. In the past the assumption has also been made that if in the same working environment there are large wall displays for several operators and local radar and other displays for individual operators, and if their ambient lighting requirements are not the same, it is best to optimize for the radar display rather than for the remainder. This traditional approach has often been adopted even when the radar information forms a relatively minor part of the whole task. Now that the assumptions behind this traditional approach have been recognised as such, the reaction tends to be to consider the other extreme of starting .from the assumption that the ambient environment for air traffic control tasks should meet normal office lighting ·requirements as laid down by statute, ond that if radar displays cannot be used satisfactorily in such an environment some cowling or other local modification to the lighting should be provided. This assumption also may have serious disadvantages and produce problems of its awn. It is, for example, inadvisable to view o dark display against o m~ch brighter background or to have pools of bright light or dim light within o room. Associated with this move towards more pleasant and higher levels of lighting is a tendency to reject ambient lighting on a narrow bond. With amber radar display, it was customary to hove either white minus amber lighting or brood blue bond lighting as a means of providing a reasonable level of ambient illumination without degrading the radar display excessively. This was a temporary solution: recent advances in radar display design and presentation make it possible to consider the suitability of normal office lighting in rooms where air traffic control tasks ore performed. Ambient lighting fixtures con interact with the angles at which tubes are mounted and the positions occupied by general displays. Any glass/per~pex fronted surfaces can produce cross reflections and glare which may be difficult to get rid of. Normally this problem con be solved without experimental trials by computing angles of reflection and, if necessary, by building simple mock-ups or scale models. Trials conducted to evaluate the system as a whole or selected tasks within it con also verify that the recommended ambient illumination is acceptable and creates no unforeseen problems.

Ambient Lighting

Coding of Displayed Information

An example of a problem which, though including novel features may nevertheless be solved from first principles without recourse to experimentation, concerns the difficulties in designing satisfactory ambient lighting for air traffic control tasks. Adjacent facilities may have incompatible lighting requirements and a compromise must be devised which sacrifices neither system efficiency nor the operator's wellbeing. During the Second World War, displays presenting radar information had to be in dim ambient light. This

Although in air traffic control tasks it is often possible to determine from first principles what information should be displayed, it is much more difficult to formulate in detail the order in which information should appear on the displays. Much displayed information is subject to frequent changes or updating and very little of the displayed information is permanent. When an operator is concerned with an aircraft it is likely to remain under his control for only a few minutes and he is usually dealing with several aircraft concurrently. Information about aircraft under '

be so excessive os to be completely impracticol. At this stage mony features of the task ore considered to ensure thot they seem acceptable, but this is done according to basic principles ond does not yet include evaluation trials. For example, the checks ore mode, using drawings or models, to ensure that ony displays which on operator must use will be cleorly visible from his working position, and tolerances can be suggested for the size and other chorocteristics of disployed informotion. Similarly the seating, console design, keyboards, communication facilities and other feotures of the tosk ore considered in relation to environmental ond other factors such as lighting, ventilation, mointenonce requirements, work and rest schedules, the physical position of operotors ond so on. An attempt is mode to structure all the envisoged tasks so that they will be within the capabilities of the operators and will meet the conditions necessary for acceptoble performance. At a somewhat later stage in the evolution of the system, the psychologist may participote in formulating the information content of displays in terms of the correct level of detail, the optimum methods for its presentation, ond the preferred methods for coding and classifying information. Similarly with controls the psychologist may suggest the most appropriate type of control and its probable optimum characteristics such as sensitivity and control display relationship. Communication facilities will also be examined in relation to alternative methods for transmitting information. Care is taken to ensure that various features of the environment ore os near optimum as they can be, given that some may have potentially incompatible and conflicting requirements. At all stages the aim is not just to meet all human factors requirements but rather to do so within the restrictions imposed on the system by other limiting factors. If human factors requirements could be stoted and treated as relevant at an early stage in the evolution of the system it is more likely that the f-inal solution will meel all these requirements without jeopardising others.

Specific Air Traffic Control Problems

control may be disployd chronologically, or by assigning a display position to each aircraft in the system. In a chronological display, the order in which information about the aircraft in the system is displayed is determined by the order in which the aircraft hove entered the system or the order in which the operator must deal with them. To maintain this order the data about each aircraft must be moved to the next display position when on aircraft leaves the system or to accommodate the information about another aircraft entering the system. Thus information about a given aircraft hos no fixed position on the display. This is the method currently adopted with information on flight strips. Alternatively it is possible to allocate a fixed position on the display to on aircraft when it enters the system and to retain the information about that aircraft in the some position until it leaves the system. When a new aircraft enters the system it is allocated whatever vacant position is available on the display. When the display is viewed as o whole the aircraft in the system do not appear on the display in any logical order or sequence. More collation of in.formation about different aircraft is required, and the operator may hove to search for on aircraft if he does not remember where the informtion about it appears on the display. Some method must be available to indicate if a position on the display now refers to a new aircraft and not to the one previously in that position. Often neither the length of time during which aircraft ore in the system nor the procedures the operator must follow ore the some for all oircroft, so that gaps may occur within a list or display whichever method is adopted. Coding by fixed position is not usually a practical way of organising the display data, although it may be possible to code according to relative position. The gist of the problem is that some of the difficulties inherent in dealing with air traffic control data occur because certain standard mf'thods for coding and presentation connot be used. Selective Information Retrieval

When the content of a display hos been suggested, it is advisable to confirm by on evaluation trial that all the relevant factors hove been foreseen and that the formulated display is satisfactory in practice. Other developments to some extent influence this procedure. With the advent of outomotion and computer storage techniques the familiar large permanent displays of oil the information needed at any time for several tasks ore giving way to smaller local displays of the information required for a given task at a particular time, with a range of colldown facilities for retrieving from data stores relevant information whenever it is required. Although this in turn presents its own human engineering problems these ore the more standard ones ossocioted with the selective retrieval and display of stores data. Limitations on calling down information as it is required may also be set by the amount of computer storage available. One reason why large wall displays ore becoming outmoded is that the increasing density and complexity of modern air troffi.c has rendered traditional methods of display cumbersome and impractical. When many aircraft ore in the system and a great deal of information about each must be displayed, the sheer quantity of information is beyond the capacity of general displays and it is necessary to have recourse to a more selective system restricting the displayed information to what is essential for the task in hand. Although on air traffic control system must

have the capacity to handle mony aircraft concurrently, it must also function efficiently with low density traffic, for instance during the night. An influence on the design of tasks and displays is this requirement for the system to deal effectively with a Jorge range in traffic density and therefore in operotors' work load. One outcome is the design of tosks so that they can be split between operators when traffic is heavy or omolgomated when it is light. Workspaces must often therefor!l- allow work shoring or task omalgomotion and these requirements are reflected in the location of consoles and facilities ond in the design of communication systems. Whatever the task loading, all the necessary facilities must be to hand. Criteria for Evaluating Performance

Certain air troffic control tasks permit standard methods of evaluation. For example, it may be possible to compare the efficiency of different keyboards as input' devices by preparing suitable experimental material and playing it on tape to each operator. Valid comparisons con then be mode between operators and between input devices. For many tasks this simple and straightforward method of evaluation is inappropriate because of complex interacting effects within the system. The procedures adopted by a given controller may not be entirely within his discretion but depend on interactions with the pilot. There ore also substantial individual differences between controllers in the way in which they would resolve a given situation. Although it is possible in theory to remove such differences by imposing rigid instructions, in practice there may be no justifiable rationale for doing so since it may not be possible to specify with sufficient precision the optimum way of performing the task to justify rigid instructions, and extra and unfamiliar constraints would invalidate measures of the operator's performance. Operators may all be faced at the start of on experiment with the some situation to be resolved and measurements may be taken to ensure for example that they all resolve the situation without infringing any of the rules governing safety factors, separation standards between aircraft, delays and so on. If two operators both succeed in different ways in controlling their traffic safety then subsidiary criteria must be found for comparing their effectiveness, if such a comparison is desired. Given a situation where a controller has to bring a series of aircraft onto a given· airfield, it may be that under one controller most aircraft touch down without delay but one or two suffer long delays, whereas under another controller most aircraft hove a short delay but none hos o long delay. The best weight to assign to these factors in order to devise a scoring criterion is a matter for debate. It may be difficult to disentangle the effects of different methods and facilities for performing the task from the effects of the interactions between a given controller and pilot. Hence comparative evaluations of olte.rnotive methods or equipments must be done with core lest the findings become on ar_tefact of the personnel taking part, of th~ir training, of the methods of scoring, or of the arbitrary weighting assigned to a scoring criterion. Measures of task performance therefore tend ta be in generol terms without great emphasis on the detailed nature of errors, omissions, delays, or minor infringements or on the methods by which the performance of the task is achieved. For many factors, it is not clear what the scpring criteria should be. Even

among factors which are relevant and con be quantified the emphasis each should have may be a matter for debate. What weighting should be given for instance to the successful maintenance of all safety standards and separations, or to minor infringements of these to achieve major reductions in delays. On the whole, air traffic control always puts safety as the first requirement whereas in air defence operational efficiency may be of prime importance. In practice care must be taken to ensure, for example, that if any major delays occur to civil flights because of ATC safety requirements these delays are distributed evenly among the various airlines so that it is not possible to argue that the reputation for punctuality of a particular airline is being unfairly undermined by a particular air traffic control procedure. This is a further type of factor which is relevant to the performance of particular air traffic control tasks and must not be ignored. Many of the more rigid and tightly controlled procedures which could be laid down carry implications which would penalise aircraft flying on certain routes or from a particular starting point. Such penalties ore normally unacceptable and no solution which entails them would be entertained.

The Information Content of R/T Messages

Another problem which affects evaluation trials derives from the detailed nature of the R/T messages which ore the normal source of information for many traffic control tasks. These messages take the form of conversations between a ground controller and pilot or between ground controllers. Some of the information is redundant. Much appears so, but may not be as its substance may be recol led or used later, or may have on indirect influence on the content of other messages. There may be long silences occasionally lasting for some minutes but normally the operator will not know when a silence is forthcoming and must continue to listen for a new message which might begin at any moment with no signal that it is imminent. Thus a silent channel needs continuous monitoring and the silences cannot be utilised fully to perform other tasks unless it is possible to revert immediately at any time from the other task to the task in hand. Many messages may be irrelevant. Sometimes it is apparent at once from the aircraft's collsign because it is not on aircraft with which the operator is likely to be concerned. At other times it is not possible, without listening to the whole message, to know that it contains no new information requiring action, because for example it is repeating or confirming information already in the system. A detailed study of the factors in R/T relevant to errors and omissions made in on input task recently hos been concluded (Hopkin, Allnutt & Orr, 1967). Many of the factors thought to be relevant were found to hove no significant influence, and others were more important than hod been realised. The task of analysing and classifying the content of R/T is difficult because of the numerous factors involved and their complex interactions. Some method for controlling the task difficulty presented by samples of R/T is necessary in order to control the operator's task loading and to permit the normal experimental precaution of using samples of known complexity. In the post it has been almost impossible to obtain matched samples but although this cannot yet be achieved it is now becoming more feasible, and the complexity of

the problem hos been appraised. The ideal would be to know, from listening to a sample, how difficult it would be as an input task, because all the factors affecting its difficulty had been defined and weighted. This ideal is still a long way off. A further purpose in defining and weighting the factors in R/T samples which produce human factors problems is to allow convincing synthetic data to be made. This in turn would permit more rigorous control over the variables in the experimental material and would allow a more thorough investigation of input devices and tasks in air traffic control. Results obtained would then no longer be a function of the experimental material used. Until all the relevant factors hove been identified and brought ,under some sort of control, convincing synthetic data with known task loading cannot be made.

Conclusions Air traffic control tasks present a variety of difficult human engineering problems, many of which are not encountered elsewhere. The solution of these problems demands considerable effort which may not at first produce such clearcut findings as might be hoped for. One reason is that these tasks ore highly complex, particularly in terms of their information sources and the procedures leading to the input of data, and there are many variables involved with complex i nteroctions between them. Progress is now being made and this is an area in which human factors work con be expected to thrive and to produce results of major significance and importonce. Many human engineering problems ore first encountered in air traffic control tasks and when a successful solution has been devised it can have many applications elsewhere.

Reference HOPKIN, V. D., ALLNUTT, M. F. & ORR, N. W. (1967) Comparative Evaluation of the Johnson Touch Display: Some Factors affecting the Input of Air Traffic Control Data. Institute of Aviation Medicine Report No 382.

First published in the October 1967 issue of THE CONTROLLER.

Management Factors in Reducing ATCS Stress Presented to the international Symposium on Air Troffic Control, Morch 1969, Stockholm, Sweden

Basic Nature of the ATCS Job The air traffic control system is o man-machine system wh:ch places primary reliance on the human component, and the human operator is often the limiting factor in system output. The air traffic control system violates the first tenet of man-machine design - that is - "to design the system so the overage man with on overage amount of effort con carry out his port in the system." Unfortunately, there seems to be no feasible way to do this and so the system requires o controller who is highly selected and highly trained. The controller bears o heavy load on his memory and his ability to keep many things in his mind while arriving rapidly of well-reasoned solutions to complex problems under conditions of stress. Controlling air traffic is not necessarily o stressful activity, but in situations with o high traffic density the job of the air traffic control specialist becomes stressful because it requires pushing man's capability to its limit to maintain continuous peak performance. A disitinctive feature of air traffic control under heavy traffic is that the individual controller does not directly control his rate of work. The aircraft just keep coming. With high traffic density, the controllers are able to maintain a high performance level but, as they do, the stress

Human Factors affecting the rated Capacity of an Air Traffic Control System l. Skill and maturity of the ATC crew or its ability to control traffic under normal conditions.

A. Aptitude. The rated capacity or normal ability to handle traffic of an air traffic control system will be very much affected by the aptitude of the operator and his ability to move traffic swiftly but safely. This is not necessarily the same os the aptitude he expresses on tests, but is the aptitude he expresses on the job and in training by showing o high degree of ability to carry out the work of the air traffic controller. Individuals vary enormously in this sort of aptitude and some controllers can do o great deal more than others. The rated capacity of o given ATC installation will be greatly influenced by the aptitude of its crew members. B. Ex p e r i e n c e . Experience is a very powerful factor in this area and the ability of the air traffic controller to handle traffic increases markedly with experience for the first few years.

C. Age.

While the air traffic controller's ability to handle traffic increases with experience for a few years, eventuo lly it levels off and at some point in time will decrease to the point where he warrants retirement. However, this point in time is very indefinite and depends a great deal on the type of management that

by John T. Dailey, Ph. D. Special Assistant for Psychology Office of Aviation Medicine Federal Aviation Adminislrotion

level rises accordingly. Ultimately the stress level that they can solely tolerate can become the primary limiting factor on the workload that the system con handle with safety. The system must strike a proper balance between maximum performance and stress on the controller. This is accomplished by o complex set of rules for handling traffic under various conditions and by flow control and traffic restrictions when necessary to limit the overall volume of traffic handled by the system. The capacity of an air traffic control system to handle traffic is difficult to determine with precision. The characteristics of the hardware of the system set on upper boundary on the amount of traffic the system con handle but this level is rarely approached in practice for any sustained period of time. The capacity of the system is usually limited by the human factors and is affected by various management and staffing factors. One cannot, of course, really speak simply of the controller or think of stress os any simple type of force. Controllers in different situations vary greatly in the workload placed on them and any stress factors imposed by the job can be greatly magnified or minimized by management factors. Efficient management of an air traffic control system must be based on on understanding of the various human factors affecting the productivity of the controller an9 the stress level under which he operates.

has been exercised up to that point, and the amount of stress that hos occurred. Management factors could be manipulated to increase greatly the age at which the performance of an air traffic controller levels off and declines. 2. Morale. The general level of morale of the controller and his trust in management, both for present and in the future, is a powerful factor in influencing the rated capacity of the controller to do work with safety. If his morale is high, he will be able to 9perate at a higher production level than if his morale is low, with safety factors being equal. 3. Anxiety -

fear of mistakes.

A very important

stressful factor on the controller is fear of the consequences of mistakes that he is likely to make or fears that he might make. The ultimate of this, of course, is a collision with fatalities, but in many cases mistakes happen without o crash actually occurring. In many such coses, the consequences are formal reprimands or criticisms of the controller. How the management handles these con affect the rated capacity of the system and the stress on the air traffic controllers in it. The controller has o very special set of skills that ore reworded highly in air traffic control, but there is little market for them otherwise. Worry about premature termination of his career by loss of health or os a result of his mistakes can create a great deal of anxiety. This can become a major stress factor.

4. Fatigue. A. Each operator has his awn individual normal rated capacity for carrying out air traffic control work for a prolonged period without undue stress. He can operate at a higher capacity for short periods of time, but ofter fatigue begins;he has to rest or else endure increasing stress. If staffing is inadequate in o high density installation and adequate rest schedules are not possible, the stress on the air traffic controller is increased greatly and the capacity of the system ultimately is reduced. This will be reflected in o higher rote of flow control restrictions. Chronic fatigue con eventually reduce the rated capacity of the controller for carrying out work. In a sense, the air traffic control specialist is a "perceptual-motor or psychomotor athlete". These athletes differ enormously in their capacity for performance just a other types of athletes do. Athletes con maintain a much higher level of performance for a short period than they can for a longer period. If he maintains a high performance more often or longer than an optimal schedule, an athlete·s fatigue mounts rapidly, stress factors increase, and eventually his performance suffers seriously. Whenever extremely high performance is required in athletics in a cFucial activity - such as a relay race or pitching in a big league baseball came - the principle of the relay team is used to maximize the rote of performance that can be expected by o team of individuals engoged in such activity as running o long race or getting batters out in o crucial big league game. This principle is, of course, being used informally to some extent by many Towers and Centers, but there hos been no study of optimol schedules for this and the manning of the Stations or Centers hos not been adjusted to enable them to use this relay principle most effectively. Even with an overall shortage of controllers in o system, it might still be possible to allocate additional experienced personnel to o very few key High Density Centers to use the relay principle to increase the ability of the system to handle traffic with a minimum necessity for traffic restrictions. B. 0 p t i m a I o I I o c o t i o n o f o p e r o t o r' s t i m e. How on operator's time each week is allocoted could have o marked effect on the amount of stress on hi~ and the amount of work he is able to handle. Many studies of similar situations such os rodarwatch standers, submarines, space ships, long-range aircraft and the like, indicate that optimal work-rest schedules can do much to increase the amount of work that can be accomplished by a given crew in a given amount of time. It is important that we learn the best length of day and best schedules for o lternation of work and rest to maintain maximum productivity of the air traffic control specialist without undue stress and with most efficient use of personnel. In a bottleneck situation, such as a high density airport, it may sometimes be desirable to maximize the system's rated capacity to handle traffic. The increased capability could come from controllers working in relays with individuals being replaced at intervals for appropriate rest. This is equivalent to preventive maintenance on a system which is working at its maximum capacity and this is made possible by replacing its tubes or other components on a regular schedule before failure. The analogy here is that a man hos the capability to re14

cuperate during a rest period so that after proper rest he becomes essentially a "new tube". In the usual world of work, it is expected that the work schedule is such that man reports for work every day as essentially a "new tube" and operates at his normal rated capacity throughout the day. However, if the system is to be able to operate safely at a higher level, it is necessary to work individuals in relays. The question to be established here is the most effective way of using extra staff so rest periods during the day can make the man essentially a new tube after each short rest period. The exoct best schedule here is as yet unknown, but interviews with oir traffic controllers indicate that in most high density situations an adequate rest schedule would be o regularly scheduled lunch period and o short scheduled coffee break twice a day. In a few ultra-high density areas, it might be necessary to go beyond this and to limit the length of the workday for individuals on the line on extremely hot spots. It is important for the rest periods to be regularly scheduled and staffing would have to be adjusted to permit this.

5. Shift rotation pattern. One of the distinctive characteristics of the air traffic control system is that it is a 24-hour system where the man have to work around the clock at one time or another in shifts. In recent years it has been found that flying across time zones or changing shifts has a powerful effect on many of the biological or circadian rhythms of his biologicol functions. These functions rise to a maximum and then decrease to a minimum in a regular 24-hour pattern. Among the things that vary on this 24-hours schedule ore: A. Pulse B. Respiration C: Blood pressure D. Temperature E. Electrical skin resistance F. Excretion of: (1) Water (2) Potassium (3) Phosphorus (4) Sodium (5) Magnesium (6) Chloride (7) Numerous hormones (especially the stress hormones) G. Mental olertness and performance (including errors) H. Mood I. Motivation J. Hunger and appetite K. Quality of sleep L. Many other aspects of performance. When the individual changes to a new daily schedule by crossing time zones or changing work shifts these biological functions begin to adjust themselves to a new 24-hour rhythm with a new starting point. However, they do so at unequal rates and so, for several days, the biological rhythms are not in proper relation to each other ond this results in biologicol instability with too much of some functions and too little of others. Whenever the controller changes shifts, his biological rhythms are thrown out of equilibrium and take some time to re-establish themselves in a new stable 24-hour pattern after the shift has been made. When shifting is frequent, the individual will spend a considerable proportion of his time in a condition of biological instability. There is

evidence, such as in 20-hour Aight crews on long-range bombers, that young healthy individuals con grow accustomed to rapid changes of shifts and apparently function satisfactorily for some time. However, there is no information regarding the effects of many years of such rapid changing of shifts. There is much reason to believe this would be on unfovoroble health factor and no grounci; whatever for thinking it would be a fovoroble health factor. It is likely that changing shifts too often places additional stress on the already stressed air traffic controller. Over a long period of years this could be a really major factor in placing stress on the controller. Attention should be given to ways of reducing the rapidity of shift change of air troffir. controllers. One possibility is to give more choice in whether they shift or not. Some men prefer night work because they like to golf, fish, garden or perform other daytime activities and would prefer not to shift. To the extent this is done, the others might not hove to shift as often. It might be possible and would be highly desirable to hove the late shift staffed by semi-permanent volunteers and by men who on rare occasions stand several months duty on this shift. The remainder of the controllers could then rotate shifts without major changes in their sleeping patterns and ovoid disturbing their circadian rhythms. Very rapid shift rotation is often preferred by the controller because it helps squeeze out a few more extended periods off duty. Some extra staffing to ovoid this would be a good investment so that too rapid shift rotation might be discouraged. This should be considered o top priority at those few ultra-high traffic density installations where oil of the other stress-inducing factors ore at a maximum. Another way to ovoid these rapid shifts might be to hove shifts of controllers on duty on a work-rest basis for sixteen hours. The late shift requires very few people and need rotate very infrequently. There is much evidence that work as demanding as that of the controller could be carried out on a work-rest basis in 16-hour shifts with as much as 12 hours on the line and the other four hours being devoted to appropriate rest periods. Thus it would only require three days on duty to put in 36 hours on the line. Such a schedule would enable everyone to maintain a normal 24-hour routine. It should also lessen the need for stand-by time and minimize coll in. The saving in driving time of the controller could be appreciable. Other advantages of such a shift pattern would be to permit the controller to hove on active port in community affairs, attend college classes off duty, and hove regular programs of exercise. Research is needed on how to change shifts with minimum disruption of the 24-hour Circadian rhythms. We need to know not only how often- to change but also the best way to change. It would be possible to train the controller in methods of changing shifts with minimum disruption in biological stability.

4. The controller is subject to many stress-inducing factors and management practices can increase or decrease the stress to which he is exposed. 5. Stress on the controller can be reduced by increased staffing with proper allocation of rest periods during his shift on duty. 6. The present practice of rapid shift rotation is medically undesirable and should be modified. Reducing air traffic controller stress and increasing productivity (long-range recommendations)

l. It is recommended that action should be taken to develop a set of long-range objectives to work toward in order to minimize and equalize the stresses imposed on the controller. 2. The long-range goal should be to equalize stress on the air traffic control specialists in the various installations by such means as adjusting the work to the level of activity of the facility and by rotation from one facility to another. Staffing of the high density facilities would hove to. be mode adequate to facilitate free rotation. Of course, a sophisticated index of activity level would need to be used for this purpose which includes weighted combinations of several of the relevant factors which ore most important in establishing the true work level of the facility. Perhaps a reasonable base schedule for all air traffic controllers would be a 40-hour week with a scheduled lunch break and two short rest periods a day. This would give a net of about seven hours on line with 35 hours of active control per week. Where found necessary in high density installations, extra staffing should be provided to permit extra rest time so high productivity could be maintained at a constant and reasonable level of stress. During temporary periods of personnel shortages, the work week could be extended with adequate safety and no damage to the health of the controller, but this should be avoided as much as possible. 3. Acceptable ways af minimizing rapid shift rotation should be devised and adopted after proper trial and evaluation. 4. After taking the above steps to equalize stress and to reduce it to a reasonable level, the optimal length of career for a controller could be estimated and retirement provisions adjusted as needed. It is anticipated that management changes could do much to lengthen the productive career of on air traffic controller. The long-range goals for changes in on air traffic controller personnel system, after proper staffing and modification in coordination with employee groups might well then be adopted os a long-range set of goals to work toward minimizing the stress imposed on the controller and at the some time maximize the efficiency and productivity of the air traffic control system. Short-range recommendations

Summary 1. The air traffic controller is o psychomotor athlete, possessing on unusually high level of development perceptual motor and decision-making abilities. 2. Like other athletes he is subject to fatigue and cannot maintain his highest level of performance indefinitely. 3. Maximum levels of safe productivity of air traffic control teams can be achieved by working controllers in relays with optimal work-rest intervals.

l. Top priority action should be taken lo experiment with bolsterring one or more ultra-high traffic density installations with more experienced top-quality controllers in order to see to what extent this enables them to handle more nearly the volume of traffic possible within the limits established by the hardware of the system. 2. Air traffic controllers should be mode aware of the undesirability of rapid shift rotation in order that they might take advantage of any present opportunities to slow down the pace of rotation. First pvblished

in the April

1969 issue of THE CONTROLLER.

Some Neglected Psychological Problems Man Machine Systems By V. D. Hopkin• Poper to the 8th Conference of the Western European Association of Aviation Psychology - Zurich - September 2nd to 5th 1969.

Introduction

The Systems Approach

Mon machine systems, such os air traffic control and air defence systems, pose many problems which psychologis_ts con contribute towards solving. The psychologists who come in contact with such systems ore not usually representative of psychology os o whole, either in their special interests or in the methods which they employ. Most psychologists dealing directly with problems arising within man machine systems ore concerned either with the selection ond training of operators for those systems or with ensuring that trained operators con reach acceptable operational standards when performing their tasks. The main thesis of this paper is that becau~e most experimental psy· chologists working on systems problems hove thereby become specialists in one bronch of psychology, some practical problems in such systems ore attacked in on unduly rc,:ricted woy, and others ore neglected completely, either because the psychologist does not feel equipped to deol with them or because he foils to recognise thot they exist.

The systems approach depended originally on the claimed ability of psychologists to deal with the problems of human variability which occur in systems, but some of its shortcomings appear to be derived from its emphasis on system efficiency as the criterion for assessing the effects of any changes. Very recently more attention hos been paid to the operator in the system and to the extent to which the system con meet the needs of man at work, but there ore still many unanswered questions on the interactions between the operator's wellbeing and system efficiency, and on the longterm consequences of neglecting the operator's wellbeing. Examples of questions which arise in this context are the fol lowing. Con o man take pride in o task which he finds tedious? Is he able to use efficiently equipment which he dislikes? If o task, which con be done by an unintelligent man, is done by on intelligent one, will the latter act unintelligently? Some of these questions cannot be answered because the necessary evidence is not available, often because these questions hove never been asked. They ore examples of what I have termed neglected psychological problems in systems. Certain psychological problems become difficult to detect or to solve because they have been built into the system. It is still quite common for the decision on the grade of person who will perform a particular function to be taken before the task has been designed and before it is apparent what skills, responsibilities and training will be required in that task. It is also common to find that the desire for status symbols and for responsibility has become confused with the equipment needed for the job. A concept which arouses the suspicions of psychologists because it poses problems which are difficult to solve is flexibility, particularly with reference to the system design when it may in fact mean that the planners have not thought of all the possible procedures which could arise and of all the focilities needed to deal with them. One of the most simple reasons why certain problems are neglected may be traced ta the use of various labels and titles for operator's functions. These labels may imply that a function is being performed although it is not and cannot be because of the system design. The presence of someone called a supervisor is not in itself sufficient to ensure effective supervision. The presence of someone called a monitor does not thereby imply that monitoring is feasible. The presence of someone called an assistant does not imply that a task has been designed to be performed by a team with one member assisting another. These and other concepts may describe the intended rather than the actual functions of operators.

The Application of Psychology Whenever o psychologist applies his knowledge to solve problems in systems he seems to forget large ports of his own training. Mony psychologists working on these applied problems become quite proud of the fact, as it seems to them, that they hove a brood approach to practical problems and ore not hide-bound by blind adherence to a particular theoretical point of view. They think of these afflictions as being confined to their colleagues in universities, who may not come into direct contact with applied psychology. It is true that most applied psychologists ore willing to accept, as valid doto, not only behavioural measures but also physiological data, subjective comment, and probably tests of abilities and personality. However they also develop habits of thinking which imply that each problem is tackled in o restricted and formalised manner. Examples ore that display design is not related to visual imagery; systems ore not seen as organisations in the technical sense of the term; communication is not treated as a social process; case histories ore anathema; and if certain tasks are associated with poor attitudes attempts ore concentrated on modifying the tasks rather than on changing the attitudes. It is argued that some psychological problems within systems ore being ignored or neglected, and that the blame for this lies with the applied psychologist who is the only person with access to the system who may recognise and formulate these neglected problems and suggest how they might be tackled. Attempts such as that of Mackworth in his classic vigilence studies twenty years ago to relate applied findings to theoretical concepts hove become progressively more rare.

R. A. F. Institute of Aviation Medicine, Fornborough, Honts, England

Automation in Systems The main reasons for introducing automation into a system are firstly that it has become technically possible, secondly that it has become cost effective, and thirdly that it is thought that it will produce some benefit. Quite often, automation is introduced with the belief that it constitutes an improvement rather than with direct proof that it does so. It is rare to have such proof in the form of direct comparisons between the man and the machine performing the task. These comparisons are usually more feasible than might be expected, because when automation is introduced the task is not normally redesigned to be more appropriate for automation and it therefore remains in the same form as it was when done manually. Automation may also aim to reduce work load and increase the capacity of the system but, again, proof that it will achieve these aims is often not available. Much of the fundamental work which psychologists might have done in relation to automation has never been done. Little is known about attitudes towards automation either in terms of its expected effects on jobs or its expected effects on careers. A scale for measuring attitudes to automation, suitable for general use and with established norms, apparently does not exist and has never been developed, although the impact of automation on the jobs is of widespread concern. Limited evidence gained in relation to particular air traffic control jobs suggests that most operators expect automation to remove much of the drudgery and lead to a more even work load, and therefore although there are reservations many existing attitudes towards automotion seem to be favourable. In practice it would appear that some of the automated functions being suggested will not fulfil the expectations of operators but will reduce the opportunities for them to exercise their skills without reducing the routine parts of their tasks.

Operator's Preferences Detailed information is often obtained by experimental methods on the efficiency with which operators can use various pieces of equipment. Sometimes this is supplemented by questions on whether the operators like the equipment and on how they think it might be improved, but such questions on the operator's likes and dislikes often appear to be asked to provide an opportunity for the operator to express his views rather than to take the views into account. Insufficient work has been done on the factors which make an operator like or dislike a piece of equipment and on the effects which his likes and dislikes have on the efficiency with which he uses it. Whether extreme dislike of a piece of equipment by operators should ever be allowed to overrule performance measure if these are shown to favour the disliked equipment is a matter for dispute. By now, psychologists should have tackled these problems and be in a better position to advise on how important it is that the operator's likes and dislikes should be considered. There seems to be considerable anecdotal evidence that if operators seriously dislike a piece of equipment they may refuse to use it or may make it appear inefficient. The implication seems to be that if equipment is disliked it should be modified. It might be more profitable to discover what features of equipment engender dislike, Nhether this is an attitude which is commonly farmed on 0rt acquaintance with equipment or develops gradually,

or whether it might be better to study factor, which affect the development of attitudes and !,: ,11odify the attitudes rather than to concentrate on the equipment and modify the equipment.

Systems as Organisations An air traffic control or air defence system can be treated as an organisation, using the term in its technical sense as the rational co-ordination of the activities of a number of people for the achievement of some common explicit purpose. This means that the concepts and findings of organisational psychology may be expected to apply to these systems. Most psychologists in close contact with these systems however know little about organisational psychology and many view its subject matter and methods with some suspicion. A characteristic of organisations is that groups within them generate their own norms of acceptable behaviour and reasonable work load. The explanation for an air traffic controller·s ability to deal with high work loads may relate more to the standards expected of him by his colleagues and by tradition than to variables in his displays, controls, or communications. The methods of organisational psychology may show that any changes which reduce on air traffic controller·s dedication to his job, pride in it or commitment to it may have consequences reflected in his performance.

Display Evaluation Most research work done on systems components for ground control purposes has been devoted to displays, and particularly to the fine detail of display design. Not many papers have been published on the relative merits of vertical and horizontal displays, or large group displays versus individual displays, but work on the design of alpha numerics has become almost an industry in itself. Some psychological problems associated with displays have therefore not received the attention they deserve and in many instances problems hove been treated as purely related to displays when in fact they have much broader implications. For example, the choice of whether a vertical or horizontal display should be used depends not only upon display factors but on communications, since liaison problems with horizontal displays, where more than one person may sit at the same display, are quite different from those of vertical displays, which usually employ separate displays for each operator. Similarly, the choice of individual or group displays implies different uses of information and different requirements for liaison and consultation among operators. Many display problems have used a very limited range of measures in display evaluation. Display evaluation has concentrated on display variables rather than on task variables. One example concerns the tabulation of data about aircraft on cathode ray tube displays, where this may prove successful with small numbers of aircraft but imposes a considerable search task when the system is working near its peak loading and the operator must find information about a particular aircraft from a long list on his CRT display. Certain factors which may affect the value of displays appear to hove been ignored altogether. It is not clear whether these constitute psychological problems or not, since the basic work to discover if they qre relevant hos not

been done. One example concerns the relationship between displayed information and the air traffic controller's visual imagery. Systematic evidence is locking on the sort of visual images of traffic patterns which controllers usually hove, perhaps because this is a branch of psychology not particularly in fashion at the moment. In air defence, research on the layout of command and control displays hos neglected the mental processes and images associated with the operator's tactical thinking, so that the format of displayed command and control information may bear no relation to visual imagery. It is not known if it is important that it should do so.

Effects of Personality While it is not contended that personality is a feature of overriding importance in a system, it is suggested that its probable importance warrants for more attention than it hos received. Some basic research is required to establish how important personality is and whether in relotio;i to task performance in air defence and air traffic control systems it is best measured by factorial, projective, or other methods. It may be that certain tasks, such OS monitoring or watch keeping, which hove hitherto been treated as display problems could be treated as problems of selecting those with the most appropriate personality to perform long watch keeping tasks without becoming bored or highly inefficient. The possibility should be explored further of treating vigilence as a personality factor.

Time Estimation Display variables in vigilence tasks hove been manipulated in attempts to improve performance. This hos usually token the form of introducing false signals to increase the general signal frequency and also to impose greater regularity in signal occurrence. It is suggested that on important intervening variable here may be subjective impressions of time and that attempts to introduce false signals hove not token sufficient account of their effects on time estimation. The best signal rote may be that which makes time seem to pass most quickly.

Boredom and Use of Skills This factor of time estimation is port of the larger problem of the causes of boredom in performance of routine tasks. Extensive work on boredom within systems, particularly in industrial tasks, hos recently been done, but hos emphasised task changes as a means of alleviating boredom and hos not measured the effects of boredom on task performance, or tackled the problem by trying to select people who may be less likely to become bored. An operator may hove two tasks, one of which is tedious and occupies much of his time and the other is demanding and requires skill and quick thinking. As a result a highly skilled person is expected to spend much of his time doing a repetitive task, which makes him bored and often dissatisfied. More planning work is needed to assess how feasible it would be to split tasks of this kind, where it is unlikely that a single operator con be selected to do both equally effi18

ciently. Although there ore theories on factors affecting job satisfaction, these hove not, on the whole, been related to progressive automation in systems. Different possible· methods for alleviating boredom such as changing the task or selecting operators hove not been compared.

Case Histories Certain sources of information ore dismissed by psychologists working on problems in ground control systems because they ore held to be insufficiently scientific. One example is the case history. This tends to be considered as anecdotal evidence, which in a sense it is, but it may nevertheless provide useful information. If, for example, there is a case history of o flickering display producing symptoms of headache or distress, then the psychologist should be prepared to act on evidence of this kind rather than dismiss it. The symptoms moy be a reflection of the individual operator rather than a serious foult in the displays but it is the psychologist's task to determine, in conjunction with medical and other specialists, whether this is so and to decide whether he is foced with on isolated case or the symptoms ore likely to occur often enough for some selection procedure to become necessary or modifications to the equipment to be essential.

Communications Extensive psychological work hos been conducted on certain features of communications within air traffic control and air defence systems, while other communications problems hove been neglected almost completely. There hos been very detailed research on the use of intelligible words in routine communications and numerous experiments hove been conducted during the evolution of the ICAO alphabet used as o means of conveying unambiguous alphabet information. Some attention hos also been paid to the format and content of messages being transmitted and to the effect of the characteristics of transmission channels, such as their frequency, on the intelligibility of messages. Attempts hove been mode to apply information theory and to discover what is achieved by the high level of redundancy typical of air traffic control messages. Measures of communication work load ore somewhat arbitrary and their interrelations between communications and other tasks hove not been properly examined. The main emphasis hos been on communications between the pilot and the controller and the co-ordination problems between ground controllers hos been almost entirely neglected. There hos been a tendency to automate ports of the system for the sole purpose of reducing the communications workload. Attention hos not been paid to the full consequences of doing this. The psychologist could advise on what is gained by simply hearing another person's voice, particularly if it remains calm during on emergency, regardless of what information is in fact transmitted. The changes in communications occuring with automation may reduce or remove its functions as social processes. The controller must work more in isolation, with reduced contact with the pilot and with his colleagues. The implications of this kind of automation require more explanation than they hove received, both from the point of view of the system efficienc and from the point of view of the operator.

Dissociation Many current automation procedures appear to have the effect that each operator becomes progressively more dissociated. from whot he is actually doing. He moy come to think of his tosk. as dealing with symbols on o CRT display rather !han with aircraft which require control. Information is updated automatically for him instead of being obtained by speaking to another person. This and other factors inevitably tend to make operators in air defence and air traffic control systems more remote from their actual tosk so that a task which has same of the elements of a game is being substituted for the true task of controlling aircraft. It is too early to assume that this must be detrimental, either from the point of view of the operator or of the system. It is however unlikely to be irrelevant, ond it seems sufficiently important for the psychologist to attempt to measure whot the consequences are for the system and for the operator of this progressive dissociation of the operator from his real task.

Programmed Learning A further development in automation which may have social implications is the introduction of programmed learning whereby on operator learns to perform his task at o machine instead of being taught by an instructor. At present the general concensus of opinion seems to be that this method of teaching is efficient for many of the tasks performed in air traffic control or air defence at least in the short term, although there ore reservations an the long term retention of skills and procedures learned by automated teaching methods. One aspect of this development which has been relatively neglected is that hitherto teaching has been considered as a social process and this aspect of the training is reduced when teaching is automated. Compared with the extensive work that hos been done on other aspects of automated teaching this is o neglected problem.

be redesigned or done in a different way so that the operator may have to unlearn his previous habits and learn new ones. This process becomes progressively mare difficult with increasing age. Methods for overcoming this problems require more effort than they have so far received. A further neglected problem is that new tasks in automated systems should acknowledge whenever possible in their design, the existing skills and abilities far which many of the operators who will do the tasks have in the past been selected. The tasks will be performed mainly not by people specially selected for them but by those who have been selected to fulfil the same operational role in the post. If a task demanding a great deal of decision making evolves into a routine key pressing function, existing skills are no longer being used and dissatisfaction may result.

Effects of Equipment Faults During evaluation trials and with operational equipment, minor equipment faults may develop. If an operator is required to work with a piece of equipment which is faulty, it is important to know what information he requires before he begins to suspect that it is faulty, and how he treats o faulty piece of equipment compared with one which is working perfectly. Most operators find an imperfect piece of equipment frustrating, but it is not known what effects this frustration hos on their ability to perform their tasks or ta learn to perform them. This problem may become serious if, for example, short delays, detectable by the operator, occur in using a data input device whenever there is o high system loading. It would be predicted that such delays would disrupt the normal learning process, but it is not known haw wel I the operator can adopt to them, what sort of !')motional effects on the operators such delays hove, and what effects his changed emotional state in turn has on his performance. Certain problems of this type, difficult to measure, ore also receiving less attention than they deserve.

Conclusions Age and Experience Although administratively and sometimes politically unwelcome it is now widely acknowledged that for personnel doing most tasks in air traffic control and air defence performance is more closely associated with age than with experience. The practical result of this is that as operators get older the experience they gain is not sufficient to offset the decrement due to age, so that the result of the two factors combined tends to be o deterioration in performance with age. This becomes most pronounced when a technical advance in equipment requires o task to

This paper hos attempted to survey briefly what appear to be neglected psychological problems in man machine systems at the present time.The list is not exhaustive, though it may be found contentious and it is meant to provoke discussion. It may seem over ambitious to suggest that certain of these problems con be tackled at the present time. However it seems essential that they are formulated and in certain coses that steps be taken to establish whether they are important problems or not. It is contended that the applied psychologist is the best person to draw attention to the existence of these problems and to initiate work on them.

First published in the October

1969 issue of THE CONTROLLER.

The Control Load and Sector Design*

by Bar Atid Arad

Introduction

The structure of the Air Troffic Control Subsystem in the enroute environment is subdivided into well defined jurisdictional units for the exercise of control. These units, commonly known as "control sectors", subdivide the entire navigable airspace in the enroute environment. The magnitude, shape and orientation of these sectors vary considerably. The only planning criteria in existence today ore primarily directed toward manning and do not provide enough guidance for the proper and efficient design of the sector. At present there ore about 400 enroute control sectors in the continental U.S. Consequently, any improvement in sector design will yield appreciable benefits to the system, since more than 50 percent of the annual recurring system cost is directly proportional to the number of the operating sections. Moreover, total reduction in the number of sectors will reduce the total amount of sector associated equipment on a nationwide basis, save control Air-GroundAir frequencies, yield better frequency management and reduce cockpit load. The responsibility to provide a given level of service and the traffic activity in the airspace generates a requirement for a control effort. This required effort is consequently a basic measure of traffic activity or, conversely, the total traffic activity is a measure of the control effort required. This approach needs further clarification, the total control effort can be measured in two places: (1) at the control position, performed, or (2) in the airspace, mena.

by measuring

the actual work

the total traffic

pheno-

In case (1), the results do not necessarily indicate the relationship between the traffic, the airspace, the rules Reprinted from Journol of ATC by kind permission of the Editor.

and the effort of the control position. On the other hand, case (2) excludes all effort which does not directly affect the control of traffic. The second method is preferred, i. e., measurement of the traffic variables and definition of the effort required by the control position as proportional to the total traffic activity. This method was selected because the traffic and the airspace parameters ore, by nature, more tangible and measurable quantities. Any direct measures of the human effort both at the behavioral and the physiological levels could, at best, be used for cross validation of same basic assumptions. The control effort required has been defined as directly proportional ta the total traffic activity. The measurement of this effort must be: (1) sensitive to all the parameters rules of operation, and (2) consistent

throughout

of airspace, •

the navigable

traffic

and

airspace.

The effort required is not measured at the control position and, therefore, is independent of the human controller, the control equipment, or any combination of manmachine. These, however, ore of a great significance when the capacity of the sector is considered. It should be realized that for any given level of service and safety, the ratio between the total traffic activity and the internal capacity of the system will determine the "level of discomfort" to the user. In other words, when the total effort required by the control position exceeds the capacity, and the required level of safety and service ore maintained, the system will generate "discomfort" to the user (i. e., delays, change of original intent, etc.). On the other hand, by adjusting the capacity and the effort required, o given level of efficiency con be maintained. Moreover, any potential increase of capacity by implementing new and better equipment con be balanced

by delineating sector boundaries to fully utilize this latent capacity. In the following paragraphs we will attempt to describe how control loads con be measured and how a method of sector design con be developed.

Basic Concepts of Load

Safety is a binary concept; on operation is either so.fe or unsafe. The flow of air traffic is considered to be safe if the rules pertaining to minimum separation criteria are adhered to. The rules do not suggest that safety is o continuous concept but rather that it is a binary function where any instant in the present, and any other instant in the future ore considered safe or unsafe with respect to the relative position of the controlled aircraft. The concept "safe" and "unsafe" and the dividing line between the two is defined by the regulations; it is left to the con• trailer to decide whether or not the flow of traffic conforms to the regulations. When it does conform the flow of traffic is deemed safe, but where o situation develops, or may develop which infringes on the regulations, the flow of traffic is considered unsafe. Let us assume o situation where the flow of traffic is completely free of any intervention by control activity. It is very easy to show that, for many reasons, there is o natural tendency for this freely flowing traffic to converge. That is, in a free flow environment on aircraft will eventually pass from what we consider to be a safe situation to on unsafe one. In order to circumvent the natural tendency of uncontrolled traffic in a free flow traffic environment to develop on unsafe situation we provide a control system. The main function of this system is to provide a specified separation service; that is, to separate the aircraft in such a way that the whole flow of traffic will be safely maintained in accordance with the rules and regulations. In other words, we con define the c,/,ief function of the control system as the provision of continuous minimum separation between aircraft and the maintenance of o safe traffic flow. However, this activity imposes o load on the control system. This load does not, in any way, relate to the way we select to control traffic but only to the traffic activity and the natural tendency of aircraft to converge and violate our concepts of safety. We will coll this load imposed on the control position the AIRSPACE LOAD, because it is the kind of load which is created by the activity of the traffic within the airspace and is a reflection of what would hove happened to the traffic if no control activity hod token place. This Airspace Load will be designated as L,. But, in order to separate aircraft one from another and maintain o continuous safe flow of traffic, we have to do many other tasks at the control position. We hove to accept aircraft and to hand them off to other sectors, or to terminal areas. We hove to communicate with the aircraft, to write and update flight strips,_ organize the strips on the flight progress board, accept position reports, coordinate with adjacent sectors, etc. The load imposed on the control position by these activities does not depend on the natural tendency of the aircraft to converge. Because we hove elected to handle each and every controlled aircraft, this load will depend on the number of aircraft under control. Every aircraft passing through the system requires a certain amount of routine handling which is quite independent of aircraft interaction. The load

imposed by this routine handling will be called the ROUTINE LOAD, and will be designated as L,. Over and above this we should consider another load component which is imposed on the control position and hos nothing to do with the activity in the airspace or the number of aircraft that need handling. This load, which we will coll the BACKGROUND LOAD, and designated by L0 , is the load which is generated by the very fact that each controller hos to come to work and has to man his position whether there is traffic or not. The most important characteristic of this load is that it is entirely independent of both the traffic activity and the number of aircraft in the system. To sum it up, the total load imposed on the control position is mode up of three main components: L

= L + L, + L,

In Figure 1, o typical components. Where:

load curve is shown with its three

L is the total load, L0 1s the background load, L, 1s the routine load, and L, is the airspace load.

<l:

0 _J

--◊

NUMBER Figure l

OF Al RCRAFT

The three lood components

2. The Variables of Traffic and Control Before commencing the task of measuring the loads imposed on the control position we should clarify in our minds what ore the basic variables that govern the behovior of the traffic and the control functions. Furthermore, we should satisfy ourselves that these variables ore measurable and readily obtainable.

(1) The T roffic Variables: (o) The number of aircraft (N), the density and the distribution of the aircraft in altitude; (b) The speed of the traffic tion.

(V) and the speed distribu-

(2) The Rules - A given set of rules operotes on the traffic in order to ensure its safety. These rules, namely, the seporatian minima, require a quantitative expression (a) that quantifies the amount of protected airspace that envelopes aircraft in the controlled airspace. (3) The Airspace Variables - The flow of traffic is regulated not only by the rules of separation but also by the existence of on organized and highly regimented airspace. This airspace organization will be quantified by two basic variables: (a) The size .of the airspace under the jurisdiction of the control position (S), and (b) The flow organization (g). This last term (g) needs further clarification. Traffic assumes different shapes and forms. There is the random now of traffic and then there is a highly organized airway flow. The traffic tends to converge towards, and diverge away from terminal areas. Each form of flow organization will effect the control position differently and therefore we will need a quantified expression of the flow organization. (4) The Traffic Features - The traffic features characterize the traffic behovior in a given environment. They will give us additional information concerning the classification of the users and their immediate mission. For example, in a given environment, most of the aircraft could be air carrier types flying mostly straight and level, whereas in another environment only a small proportion of the aircraft may be air carriers and most of these aircraft may be transitioning to or from a terminal area. (5) Parameters - Finally, we need measures that relate the total traffic activity, the rules, the airspace and the traffic features to the effort which is required at the control position. These measures are: (a) The coefficient of routine load (K,) that quantifies the effect of the traffic features on the control position, and (b) The coefficient of the airspace load (K,) that quantifies the effort required to detect and resolve a conflict situation.

3. Work and Load The component of routine load (L,) is generated by every aircraft that traverses the sector. The control system is handling every aircraft irrespective of its. relationship lo other aircraft in the system. Thus, we could soy that the amount of routine work required is directly proportional to the number of aircraft -that traverse the sector. This relationship between the number of aircraft and the work to be done is similar in many respects to any other problem involving "work to be done". For example, let us consider the case where chairs hove to be moved from one room to another. There is a certain amount of work to be done and to move two choirs will require twice as much work as one chair. However, when a time element is introduced, we ore faced with a problem of a different nature. Going back to our simple example dealing with furniture moving, to move twenty choirs in one hour is a much easier task than to move the same number of choirs in five minutes. The total work accomplished is exactly the some in the two cases and yet moving twenty choirs in one hour is "a cup of tea" compared with accomplishing the some work in five minutes.

Thus the concept of "rate" has been introduced and ,n our particular case we will coll the rote of doing work "load". The routine load is that component of load which is directly proportional to the number of aircraft handled per unit of time. But, what if the chairs are not equal? What if some ore light and some heavy? Some easy to handle and some require special handling techniques? Or back to the airspace and the problem of routine load, what if the load imposed by each aircraft is somewhat different? In order to solve this problem, two things ore required: (l) A standard unit of measurement, and (2) a scale. Given these two prerequisites we con determine "how heavy is heavy" and "how difficult is difficult"? An establishment of a unit of measurement is nothing but a convention. We could, just to be difficult, determine by agreement that the standard unit of measurement will be an aircraft that lost one of its powerplants and is requesting priority to a lower altitude. However, this is not practical and since there is no particular reason to be difficult we will agree on o very simple, common, standard unit of measurement: - a "standard aircraft" in the IFR system will be o scheduled aircraft that has penetrated the sector ore □ of jurisdiction in a straight and level overflight when no interaction with other aircraft is considered. For all practical purposes we consider all air carrier aircraft as standard aircraft. We should realize that the uniformity of air carrier procedures and pilot capabilities is the prime characteristic that makes it a good standard measure. Therefore, other users of the airspace could be considered as good candidates for this standard category. However, for practical reasons we could not and will not examine each and every aircraft by itself but rather group them in accepted and well established categories. Thus, MATS aircraft_ should be considered as standard, whereas SAC aircraft, even on routine point to point flights, will be considered as "non standard". The work which is generated by one standard aircraft is called DEW (Dynamic Element of Work). That is, we decide that one DEW is equal lo the work generated by one standard aircraft over-flying the sector in a straight and level flight when no interaction with other aircraft is considered. The unit of load is called DEL (Dynamic Element of Load) and one DEL is equal to the role of doing work of one DEW in one hour.

1 DEL =

DEW hour

Now, we hove a unit of measurement and if we hod a scale we could "measure" every aircraft that traverses the sector in units of DEW. Add it up during one hour and find the routine load (L,) which is imposed on the sector.

4. The Coefficient of Routine Load Unfortunately the distinction between standard and non-standard aircraft is not sufficient to express the amount of handling which every aircraft will require. Aircraft differ not only in the classification of the user but also in their immediate mission. Some aircraft climb and descend, whereas others fly straight and level. Some ore being handed off vertically to or from upper layer sectors, whereas others ore handed off to adjacent sectors. Then again, some aircraft ore handed off to or from a terminal

area while others just overfly ii. Finally, there ore those thot try to get the best of everything: VFR oircroft that request odmittonce to the IFR system while in flight (popups). It is o foci thot the troffic feotures hove o repetitive tendency. We expect that if, in o given environment, 60 percent of the oircrolt ore air carriers on week days ond 30 percent on weekends, thot these features will repeat themselves from week to week. Then ogoin, o sector odjocent to o big terminol oreo is expected to hove more tronsitioning oircroft and certainly more oircrolt going to or coming from the terminal oreo, ond thot these feotures ore more or less constant ond repeat themselves. These repetitive troffic features ore, in fact, the characteristic feotures of the sector. In the high altitude environment, we will expect higher percentage of stondord oircroft ond in the vicinity of terminals it is only noturol thot o larger proportion of oircroft will climb or descend. Thus, if o "weight" con be assigned to any of the features, we will be oble to determine the coefficient of the routine load (K,) for every sector and this coefficient,expressed in DEW per aircraft will be the "characteristic number" of the sector. In Tobie l, the traffic features "specific weights" ore listed. These weights ore the results of extensive field surveys conducted in 13 ARTCCs.

Feature Standard oircroft Non-stondord oircroft Vertical hand-off Terminal aero hand-off Climbing and descending Pop-up

Po P, P, P, P, P, Table l

"Specific Weight" DEW

1.0 1.1 + .26 + .38 + .24 + 1.3

Traffic Features Weigh1s

Undoubtedly various other features could be listed and their "weights" measured experimentally. However, these odditionol refinements might hove o small ond 'insignificont effect on the determination of the routine load. Nevertheless, some traffic features should be examined more closely and in particular the specific weights associated with military traffic. The traffic lectures con be given in percentages (P). That is, P0 is the percentage of stondord aircraft and P, = (l 00 - P0 ) is the percentage of non-standard aircraft. In addition to this main classification P, is the percentage of aircraft handed off vertically; P, is the percentage of oircroft to or from terminal areas; P, is the percentage of aircraft that climb ond descend in the sector; and P, are "pop-ups" requesting impromptu admission to the IFR system. Now let us assume the following features: P0 = 50 per cent, P, = (100 - 50) = 50 percent, P, = 60 percent, P, = 20 percent, P, = 55 percent, and P, = 10 percent. This means that for every 100 aircraft that troverse the sector:

50 aircraft

will

generate

50 X 1.0 = 50 DEW;

50 aircraft

will

generate

50 X 1.1 = 55 DEW;

60 aircraft will generate = 15.6 DEW;

on additional

60 X

.26

20 oircroft will generate 7.6 DEW;

an additional

20 X

.38

55 aircraft will generate = 13.2 DEW; and

on additional

55 X

10 oircroft = 13

an additional

10 X 1.3

will generate DEW.

.24

Summing up oil the handling work we find that 100 aircraft generate 50 + 55 + 15.6 + 7.6 + 13.2 + 13 = 154.4 DEW or the overage aircraft will generate 1.54 DEW. Stated differently we soy that the value of the coefficient of routine load (K,) is: K, = 1.54 DEW per aircraft. This value hos been obtained by the traffic features whichare unique to o certain er:vironment. A different environment will generate on entirely different value of the routine load coefficient. Consider for example the following high altitude sector: Po = 95% (100 - 95) P, 20% P, 0 P, = 25% P, 0 Total:

or, K, =

111.7 100

95 X 1 5 X 1.1 20 X .26

25 X

95 DEW 5.5 DEW 5.2 DEW

.24

6.0 DEW

100 aircraft

= 111.7 DEW

= 1.12 DEW per aircraft.

The only difference oetween the two sectors is the traffic features and yet, on overoge aircraft in the first case generates .42 DEW more routine work than on overage aircraft in the second case. This difference in the amount of handling work required per aircraft is basically a function of the environment.

5. The Routine Load The routine work is on expression of work that hos to be accomplished in the routine handling of the troffic. This however does not indicote the load imposed on the control position. In order to determine the routine load, we have to introduce a time element that will express quantitatively the rate of doing work. Thus,

L, = K, Obviously,

the routine

lood will increase when:

(1) the number of oircroft (N) will increase; (2) the coefficient of routine load will increase; ond (3) the overage time that the aircraft ore in the sector (T) will decrease. For example, consider o sector adjacent too terminal area where the coefficient of routine load is 1.61 DEW per aircraft and the overage time under control is .4 hour. The routine load in this sector will be l.61 L, = -N = 4.25 .N DEL. .4 On the other hand, o high oltitude sector hoving a coefficient of routine load of 1.13 DEW per aircraft and an overage traverse time of .6 hour will yield 1.13 L, = -N = 1.88 N DEL.

In Figure 2 the two coses ore shown in o graphical form. It is evident that the sector adjacent to o terminal area is expected to impose much higher routine load per aircraft than o high altitude sector.

We should note that the determination of the rautinf: load (L,) has been achieved by using measurable quantities. The variables of the traffic features, number of aircraft and time are obtainable by measuring the traffic activity in the airspace. In fact, any one of these variables cou ld be readily obtained from tabulation and processing of the flight progress strips, these being the available record of the traffic activity. 0

...J

0 0

<l:

...J

..... :::, 0

(2) If the diameter of the balls remains constant and their number is unchanged but we increase their overage rolling speed, obviously the rote of collision will increase. Indeed, the rote of collision is directly proportional to the overage rolling speed. (3) Now, let us maintain the some overage rolling speed and the some diameter of the bolls but increase the number of bolls on the table. The rote of collision will increase but, this increase is not directly proportional to fhe number of balls but directly proportional to the square of the number of bolls. This undoubtedly requires some further clarification. It is evident that every collision involves a pair of bolls, thus, between two balls we could hove one collision. However, if three bolls, A, B, and C, ore present the following collisions ore possible: AB, AC, BC

dom movement, the average rote of collisions is directly proportional to the diameter of the bolls.

/..<,,/'

Furthermore, if four bolls, A, B, C, and D ore present we expect that the following collisions ore possible:

,'>

,·

❖

+-'

AB, AC, AD, BC, BD, CD

...J

Evidently on increase in the number of bolls increases considerably the number of all possible collisions. However, we ore not interested in the number of all possible collisions but rather in the overage value of the expected col-lision rote. This rate is directly proportional to the square of the number of billiard bolls on the table.

N-NUMBER OF AC Figure 2

The routine load in two typical sectors

6. The Airspace Load

We have defined the airspace load (L,) as that component of load which is generated by the requirement imposed on the control position to keep the aircraft separated in accordance with the accepted rules of separation. The question of whether one set of rules is safer or less safe than another set of rules is not our concern since we ore operating in accordance with on established set of rules specified quantitatively by the separation minima. Indeed our problem is reduced to a very basic question: How much violation of the separation minima is expected to occur if traffic should proceed uncontrolled? Since these violations ore defined as "conflicts" our problem is to determine the number of conflicts that ore expected to develop in the airspace? Consider· the following analogy: A given number of billiard bolls move at random on a billiard table. Every so often two or more bolls will collide. Moreover, if the some number of bolls should keep on moving at the some speed and maintain the some random movement, we could expect a certain overage number of collisions per unit of time. Now let us see how this rote of collision is affected by the following·variobles: (l) If the number of bolls and their speed remains unchanged but the diameter of the bolls is doubled we expect that more collisions will occur in a unit of time. In fact, it con be shown that, under conditions of ran24

(4) Finollywe should consider the case where all the above mentioned variables ore kept constant (diameter, speed, number of bolls) but we hove increased the size of the table. In other words the some number of bolls ore free to move in a larger area. The obvious answer is the right one - the rote of collision is inversely proportional to the size of the table. From this analogy we could learn quite a lot about the traffic behovior in the airspace: (l) the diameter of the billiard

boll is the separation

mi-

nima;

(2) the rolling speed of the billiard bolls is analogous to the traffic speed; (3) the number of bolls on the table is the number of aircraft under control; (4) the size of the billiard table is equivalent to the size of the sector; and (5) the average rote of collision of the billiard balls is analogous to the average number of expected conflicts between aircraft. The concept of "overage number of expected conflicts" needs some explanation. We will define "conflict" as a violation of the separation minima that would occur if no control action is token. That is, a conflict is something that is only expected to happen but (hopefully) is always prevented in time. In fact, once a conflict posses the expectation stage and no prevention measures ore token, the flow of traffic is considered "unsafe" and on "incident" is declared. The overage number of expected conflicts is a number that expresses the overage rote of possible conflicts that might hove occurred, under a given set of conditions, if no control action would occur.

We hove defined the airspace load os this component of the control effort which is required in order to prevent the development of on expected conflict into on incident. In our analogy of the billiord gome, imagine that we introduce o new element to the game. One of the players will olwoys attempt to generate collisions by either increasing the diometer of the bolls, their rolling speed, or the number on the table (he is blind-folded and not ollowed to aim his throws). The opponent is provided with o tool that enables him to forecast collisions ond prevent them. The load which will be imposed on the second player is, by our definitions, directly proportional to the overage rote of expected collisions. The billiard gome analogy assumes o random traffic but in octuol operational environment, various levels of organization ore possible. For example, airway flow, intersections of oirwoys, one directional airways, etc. Each of these flow organizations will affect the octuol number of aircraft that ore expected to participate in a conflict in o unit of time. Therefore, we hove to include in our expression of the expected number of conflicts, o number that quantifies the flow organization ond numerically relates the variables (o, V, N 2 ond S) to the actual numerical volue of the conflict rote (C). We coll this number "the flow organization factor". To sum it up, we hove a basic expression for the overage number of aircraft expected to conflict in one hour, for any given condition of:

(l) rules of separation -

o (nm/oc); (2) overage traffic speed - V (knots); . (3) number of aircraft under control - N (oc); (4) sector size - S nm'; and (5) flow organization - g (non dimensional number).

Area Av. Time Av. Speed Flow Organization Toblo 2

Sector A Sector B Sector C

Symbol

4000 nm' 8000 nm' 12000 nm' .4 hr. .9 hr. .6 hr. 220 knots 250 knots 350 knots

T V

9.5

T roffic features

Standard ac Nonstandord o/c Vertical H.O. Terminal H.O. Climb/descend Popups

12.0

ond variables

75% 25% 30% 80% 75% l 0°/o

10.5 In three typical

40% 60% 25°/o 0 30% 10%

sectors

95% 5% 15% 0 20% 0

Po P, P, P, P, P,

Traffic Features

Coeff. of L, l.8DEW/AC

l.37DEW/AC

Routine load

4.5

Airspace load

.081 N 2 DEL

N DEL 2.28 N DEL

.063 N 2 DEL

1.1DEW/AC K, 1.21 DEL

.078 N 2

ResuIts

Figure 3 illustrates the load imposed on the control position by the three sectors. We can see, for example, that 10 aircraft will impose: in Sector A ,n Sector B in Sector C

53 DEL 29.5 DEL, and only 20.0 DEL

The general expression for the expected number of conflicts is: 2a V N2 C = ---,-oc in conflict per hour

g s

t 10

The airspace load (L,) is the load which is expected to be imposed by the interaction of aircraft in the sector. The general expression of this load yields:

L, = 2 K, a V N

100 90

g s

Where K, is the coefficient of the conflict load and expressed in DEW per aircraft. The value of K, hos been obtained by field surveys involving about 300 controllers in 10 field facilities:

l conflict K,

= 2.8 DEW

0 <{

The expected number of conflicts (C) is expressed in ac/hr. and therefore the air space load L, is expressed by DEW/ hr. or DEL.

70 60

0 -'

40 30

7. The Control Load In summing up the expressions for L, and L, we get

=K ~ 1

2 K, a V N T + g S

In Tobie 2 we have listed some information three typical sectors: (A) A sector adjacent to o terminal area (B) Low altitude enroute sector (C) High altitude sector

·-

_, 0

1.4 DEW per AC in conflict

concerning -,L_c.._J--'--'--'1--'I

-1.I _.t___l__J_ I I I

10 NUMBER

I Ll_LJ

OF AIRCRAFT

figure 3 The total lood (L) in three typicol

sectors

The differences between the loads imposed on these three typical sectors is due to the differences in the traffic features, the traffic variables, the airspace and the flow organization. Figure 4 illustrates how a characteristic load curve and the variations of the traffic during a typical day gives us o complete picture of the amount of load imposed on the control position at any time. Furthermore, if we accumulate the load from the beginning of the watch to the end (8 a.m. - 4 p.m.) as shown in Figure 5, we will get the amount of total control work performed by each watch in units of DEW.

CUMULATIVE

LOAD DISTRIBUTION

IN A BUSY

.. , r"

DAY TIM(

Figure 4 A & B

by the interaction between aircraft (C), is completely independent of the way we select to control traffic. On the other hand, the constraints imposed on the system generate a requirement for a limited size sector and the routine load is a quantitative expression of the load imposed on the control position by the system limitations.

500

Following this line of reasoning we could define the effectiveness of our system as the ratio between the "objective" load imposed by the traffic activity and the total load (L): L2

Z50

200

The effectiveness increases with the average track length (s) for any given aircraft density. In other words, the best sector design will be achieved by maximizing the value of s. This criterion could be considered as necessary and sufficient for optimizing the design of o sector when random traffic is considered.

w 0

:.::

150

ir 0 ~

However, airway traffic requires additional considerations. Maximizing EL, by itself, is not sufficient and some other conditions have to be defined and applied. 100

The most efficient sector will be achieved by maximizing the following ratios: (a) S/s2 where s is the average track length (b) st£.s where ~s is the total airway length covered by the sector, and

(c)~

We will refer to ES = S/s2 as the area effectiveness, Es = s/"'£.sas the airway effectiveness, ·and to EL =8

j.-

10 11 12 I AM FIRST WATCH

·I·

9 10 II

PM SECOND WATCH

-l

Figure 5

8. Sector Design

There is a distinct difference between the routine load

L, and the airspace load L2 . The airspace load is generated by the total traffic activity and basically is o reflection of the desires and intents of the flying public to go from place to place. In fact, the total traffic activity as reflected 26

L2 L

as the

load effectiveness. The total effectiveness of the sector is: E = ES. E,. Er, where the values of E,, ES and EL are normalized to 1000/o between their minimum and maximum values. Maximizing the total effectiveness of the sector yields a very interesting result. The optimal sector (E Max) is achieved when each airway length is proportional to its average density. That is, if one airway has on average density of 6 aircraft per 100 nautical miles and another airway has an average density of 12 aircraft per 100 nautical miles. The length of the denser airway will be twice as long as the length of the scarcer one.

This method, however, does not define the size of the sector. Figure 6 shows the application of th~ principle· of proportional ports in a simple schematic airway structure. We con observe that any proportional increase in the air- . ways segments will, indeed, define a new sector size by a contour of load level. In order to determine the size of the sector we hove to determine the capacity of the system. Given a capacity level in units of load (DEL) the optimal size of the sector con be determined by maintaining a balance between the load imposed on the control position and its capacity. The importance of the principle of proportional port~ is that we con design the best sector for any given level of capacity. Thus, on improved environment and better machine aids at the control position will yield new and improved sectors if the traffic activity is properly measured, the control loads determined, the sectors ore designed in accordance with the principles of sector design and their size is matched to the system capacity.

Figure 6

The principle

of proportional

ports

First published in the July 1964 issue of THE CONTROLLER.

I. P. Sharp Associates

I ntersystems

Our congratulationsto the InternationalFederation of Air Traffic ControllersAssociations on the occasionof the 10th anniversary of this journal. We too have a considerableworldwideinterest in systemssoftwarefor Air TrafficControl.

Intersystems

I. P. Sharp Associates

Mathematical Models for the Prediction of Air Traffic Controller Workload Paper to the United Kingdom Symposium "Electronics for Civil Aviation"; 1969*

by S. Ratcliffe,B. Sc. Malvern,

Worcestershire Royol Rodor Estoblishment,

Summary Much effort and expenditure has gone into the automation of ATC, but we lack quantitative data about the controller workload which automation is meant to relieve. It is unreasonable to expect more than a rough measure of workload, however defined. A "mathematical model" takes the form of one or more algebraic expressions which purport to predict the workload. One way to construct such a model is to break down the controllers' task into a number of rudimentary components, to measure the time spent on each sub-task, to multiply these times by suitable weighting factors and odd to obtain the total load. Alternatively, a workload equation may be arrived at intuitively, and the coefficients adjusted to fit the observed facts. The paper is o critical review of models so for suggested, and of techniques for measuring controller workload. The author hos little confidence in any of the published results. When faced with a serious overload situotion, the controller normally preserves air safety and his own sanity by slowing down the traffic demand, by one means or another. In such o situation, tests on controller loading may be on insensitive method of measuring the traffic delays which ore of primary importance.

Introduction Given the scale of effort and expenditure that hos gone into the mechanisation of air traffic control, it is perhaps surprising that there is relatively little quantitative data about the nature of the nature of the workload which automation is meant to relieve. If there existed an adequate tool for the prediction of the various components of this controller workload, it might be a much easier matter to compare the economics of various possible control configurations which might be adopted to deal with a given task. It is o formidable task to get even on approximate solution to this problem. Controllers differ markedly in the amount of work they manufacture for themselves in o given situation; and in their ability to deal wit it. Interactions between different members of the control team may considerably confuse any simple arithmetical approach. "Workload" is not defined with any rigour and subjective estimates of work difficulty ore confused by the variability of controller ability. For the purposes of the present paper, the author is prepared to accept any definition of "workload" that is not in conflict' with common English usage and which lends itself to measurement. The scientific approach to the prediction problem is to set up o mathematical expression or expressions into which ore substituted the appropriate values of the parameters

Reprinted with kind permission of the author and the U.K. Ministry

of Technology

which ore deemed to characterise the problem, and which yield a predicted value for the ensuing workload. The algebra should be of the minimum complexity and the parameters of the minimum number necessary to give an adequate fit to the observed data. Published papers (1, 2, 3, 4) treat the number of aircraft under control as the main parameter and express the workload by an expression of the general form: -

L = o + bN + cN' .............................. (1) where a, b and c vary with the nature of the control task and the orgo nisotion. Equation 1 con undoubtedly be arrived at by arguing that it is convenient to use o power series expansion, that a and b are certainly non-zero, that o linear model is inadequate, and that if more than one additional term is added the experimental evidence (if any) will be inadequate to determine values for the coefficients. Techniques, other than pure intuition (2, 4), for the construction of workload models will be classified, in the present paper as "synthetic" or "analytic". A synthetic model is arrived at by assembling, by one method or another, o list of the various tasks which o controller must perform, e. g, "conflict search", breaking each task down into manageable components, and determining the amount of work involved in each component, and adding the results, with suitable weighting, ta determine the total work. Basically, this is the technique used in motion and time study (6). The "analytic" approach takes the actual ATC situation as a whole, and attempts to determine the contribution to the total workload due to each factor of interest by analysis of variance or other market research techniques. An alternative terminology would describe the "analytic" approach as "descriptive" - giving on account of the behaviour of the system; the "synthetic" approach being described as "prescriptive" - giving an account of how the system s ho u I d behove. The next two sections of this paper will consider these techniques in greater detail.

Synthesis If the controllers' task is to be studied as on assembly of sub-tasks, the first need is for a breakdown of the job into its components, and for the construction of a flow diagram showing the sequence of events required to deal with a known task. This is a coarser scale version of the problem facing a computer programmer who attempts to mechanise some aspect of ATC. It is not enough to categorise the various problems, it is necessary to know the strategy employed to solve them. Basically, the method is to interrogate one or more controllers. An extension of this technique, termed "instigated introspection" by some psychologists, requires the subject to solve a series of problems whilst simultaneously giving a running commentary on his mental processes. The difficulty here is that the commentary introduces unreality into the sit.uotion, the controller may describe, not what he usually does, but what

he thinks he does or even whot he thinks he should do. Further, there is the temptotion to spend more time in describing the easy processes and less on those, common in ATC, which ore extremely difficult to explain to on outsider. Leplot and Bisseret (5) used the technique of the previous paragraph, followed by a series of trials under laboratory conditions. Their study was confined to a particular controller task, the test for procedural conflicts. In the laboratory trials, the controller was faced with a flight progress board depicting a traffic situation at a fixed time. He was then faced with new aircraft wishing to join the system, or with requests for a level change. Measurements were mode of the time needed to solve each problem. The trial was repeated for on adequate sample of controllers and for a set of problems designed to stimulate each branch of the flow diagrams. If this technique is to provide on overall measure of controller workload, it is necessary to hove a measure of the relative frequency with which each sub-category of task will face the controller. It may be possible to measure these frequencies by analysis of the results of normal operation, but, except where there is already extensive mechanisation, the labour involved in data collection and analysis may be intolerable. In any event, this technique may not be applicable too hypothetical new organisation. An alternative is to use fast-time simulation (10) to-determine the magnitude of the various control tasks. By confining themselves to a procedural system, and by discussing the conflict detection task only, Leplot and Bisseret were able to ovoid the more subtle situations that arise when information is arriving by more than one channel at o time, e. g. by ear and eye, and where the controller may be uttering ritual words over the R/T ·and simultaneously be thinking of something different. The classical techniques of time and motion study hove been the subject of heavy criticism (7) even when they were applied to almost purely manual tasks. The application of this technique to a largely cerebral activity will need considerable justification.

Analysis The work by Bor-Atid Arod (1) appears to be one of the earliest attacks on o significant ATC system, and is almost certainly the most ambitious so for attempted. Arod chose to study the entire contemporary U.S. en-route ATC environment, with traffic loading and sector configuration as the main variables. The study breaks down into tliree components: (i) formulation of a workload model, (ii) experimental determination of coefficients, (iii) effect on workload of changes in sectorisotion. The present section will be concerned with (i) and section 4 with (ii). Arad states as on axiom that the control workload consists of three terms: (i) a "bockground" load, L0 independent of N, the number of aircraft under control, (ii) a "routine" load, L,, directly proportional to N, (iii) on airspace load, L,, proportional to N 2 . In practice, the L0 term is ignored. The term in L, Is assumed to be of the form: -

L,=K,r······································(2) N is the number of aircraft under control, T is the overage time that aircraft ore in the sector, and K, is the "coeffi-

cient of routine load" which varies with the nature of the task and the traffic mix. Traffi!: was, in fact, broken down into "standard" and "non-standard" aircraft, and additional allowances made for traffic in the following four categories: - Vertical handoff, - TMA handoff, Climbing and descending, - "Pop-up" (a/c demanding impromptu admission to the I FR system). A "specific weight", empirically derived, is then assigned to each class of traffic and the weighted mean of these weights is K,. The term in L2 is assumed by ref. 1 to be of the form: L, =

?.K,aVN2 gS

........

where a is a parameter (nm/oc).

·:..

. ....................

(3)

fixed by the rules of separation

V is the overage traffic

speed (kts) N is the number of aircraft under control (oc) S is the sector size (nm)' K, is the coefficient of airspace load. g is the "equivalent volumetric flow organisation tor" (see ref. 8 para. 1.6.11.2(9)).

fac-

Arod defines a unit of work as the load generated by "one standard aircraft overflying the Sector in straight and level flight when no interaction with other aircraft is considered". This is the "Dynamic Element of Work" or DEW. The unit of load is termed the "Dynamic Element of Load" or DEL. One DEL equals one DEW per hour. The units of L, and L,, ore therefore, DEL. The units of K, and K2 are DEW/o.c. There is a dimensional error in equation (3), since, as shown in ref. 9, Appendix 11,the right hand side of the equation does not have dimensions DEL. The reader should see ref. 9 for details of o tidied-up and dimensionally correct version of equation (3) which was used in the computer programme for the evaluation of the Arad model. It should be noted that the definition of a DEW is apparently local to the Sector under consideration. It is unfortunate that none of the published papers on the Arod model discuss the axioms on which it is based. These imply that the L, term represents work generated when aircraft enters or leaves a Sector, and that this work, presumably communications and data entry of one type or another, is directly proportional to the number of aircraft entering or leaving and independent of the time which the aircraft spends in the Sector. Similarly, the L, term is implicitly token to be proportional to the number of conflicts arising between oircraft in a given Sector. Since the difficulty with which o conflict can be resolved is itself a function of traffic density, i"t con easily be argued that higher order terms ore necessary in the workload equation. It is a relatively simple matter to measure the time spent on R/T or on inter-controller conversation, and although this does not measure workload, in DEW units or otherwise, the results throw on interesting light on the Arod axioms. It is necessary to spend only a little time listening to R/T to realise that the nature of the messages changes with looding. In o busy period, a burst of noise as the R/T key is flicked replaces the message "good day to you, sir" which might well be passed in a slack period. This phenomenon may well complicate the relationship between the routine workload and the number of aircraft.

50 PERCE NTAG£ LOADING

0 l_

__L

L___ _

__L

J....... _

_l..

.,__

_,__ __

..___--'----'---~--_,___-

2◄

NUM8£R:

32 OF AIRCRAFT

Figure 1

Total

speech versus traffic

level.

70 -

PERCENTAGE LOADING

I 20

0 L---.....L...---'-----'---_,___-~--~-~~-~--~-~--~-~~

28 NUMBER

A recent study (l l) at LATCC, West Drayton, has yielded, amongst other things, figures for the total speech load (expressed as a percentage of the study period) on controllers on Sectors 5 ond 11, as a function of the number of aircraft passing through the Sector during the half-hour period over which each loading was measured. To quote from ref. 11: - "Sector S's sphere of operations is a bidirectional airway bounded at its western extremity by a TMA and at the eastern by the convergence of two busy airways. The incumbent is frequently more heavily engaged in liaison than in actual controlling. On the one hand, he is engaged in almost continuous liaison with the Garston stack controller - on the other, he is liaising with the Sector 11 controller, initial descents on inbound aircraft, and climbs on outbound. The Sector 11 controller, in contrast to his neighbour is primarily engaged in active controlling, and since his area of operations is almost entirely over water, his duties in so for as inbound releases to, and clearances from airports, are concerned are limited with a consequent reduction in telephone workload". 30

OF AIRCRAFT

Figure 2 Curve fitting.

The present author has fitted to this data quadratic curves giving the least-squares best-fit prediction of speech loading as a function of the number of aircraft in the Sector (in a half-hour period). The results are shown in fig. l. The curve for Sector 5, in particular, suggests that there must be a major port of the total speech load which does not depend on the number of aircraft in the Sector. This conclusion is, at least partly, supported by the quotation from ref. 11 given in the paragraph above. The difficulties of curve-fitting are illustrated in fig. 2, based on the data from Sector 5. The best-fit curve is here shown together with the dots which mark the experimental results and elongated ''I's" which mark the ± 1 sigma limits about the mean loading for each traffic level. (Sigma was calculated on the assumption that the scatter about the mean for each level of loading constituted a sample from the same population). It will be seen that whilst the main features of the curve ore almost certainly correct, sampling errors may be playing a significant role. It would be interesting to have a larger sample of data (say,

l 0 times the present size, or about 600 half-hour periods) on which to work. Ref. 11 gave a breakdown of the speech load into various components: - R/T, intercom., GPO lines, and direct liaison. Unfortunately, the sampling errors for most of these components are even greater than those in fig. 2. because the samples are smaller, and the present writer has achieved no meaningful breakdown of the results. What does seem clear, at any rate, is that the Arad assumption that the workload curve passes near zero for zero traffic is seriously inaccurate for the Sector 5 results shown in fig. 1. Using methods which are discussed later in this paper, the F.A.A. mounted a study (9) to check the validity of the Arad model. This compared the accuracy with which three different models could predict the average order in which controllers would rank the difficulty of handling given numbers of aircraft in various sectors. The three models were: (i) the Arad model (termed the "M" model in ref. 9) (ii) a variant of (i) in which only the "routine load" component was used (the L, model) (iii) a model which assumed the load to be proportional to the "equivalent traffic count" i. e. the average number of aircraft under simultaneous control in the sector (the "E" model). Tests of the predictions of the three models against controller judgment showed that the differences of proximity were statistically significant, and that the order of closeness was:(i) the E model (ii) the M model (iii) the L, model. The difference between the E and L, models is particularly interesting. As was pointed out above, the L, term is calculated on the assumption that the routine load depends on the number of aircraft entering or leaving a Sector in a given period. The E model assumes that the load depends on the number of aircraft within a Sector simultaneously. It seems clear that the poor showing of the present Arod model could be improved by taking the E model as the routine load component instead of L, as presently defined.

Measurement of Workload It was pointed out at the start of this paper that "controller workload" has not been rigourously defined. If we ore prepared to adjust the definition of workload to suit the method of measurement, possible techniques include: (i) External observation of control activities (basicolly, the method used by Leplat and Bisseret). (ii) PhysiC!llogicol tests for strain in the controller. (iii) Simulator trials in which traffic levels ore pushed up to the point where the controller is saturated. (iv) Methods of detecting controller overload by measurement of his error rate, either in performing his normal task or in some ortificial base-load task involving, say, elementary arithmetic. (v) Controller judgment as assessed by questionnaire and interview (the method used by Arod and Jolitz). Method (i) has been discussed in seciion 2. Method (ii) does not seem. to hove been applied with any success. Method (iii) is perhaps capable of being used to form a quoli-

tative estimate of the relative capacity of two systems, but it is often impossible to define a precise point where the control system "breaks down". A controller who is approaching saturation will progressively adopt more and more tricks to reduce his workload and postpone his problems, possibly with a significant loss of expedition to the traffic concerned, until his mode of operation may eventually bear only a very rough relationship to that normally used. The technique has the added drawback that the process of building-up a saturation situation is necessarily a fairly slow one. Since it is clearly necessary in any measurement of workload to take a big enough sample of controllers and of traffic to keep sampling errors down to a reasonable level, the simulation process can become very expensive in time and effort. Method (iv) suffers to a somewhat reduced extent from the objections to method (iii), but has the added drawback that it is not easy to measure controller errors. At best, the process is laborious, and the results may be ambiguous. For example, if a busy controller hod decided that updating of a particular piece of data on his display was really irrelevant, and omitted to make the revision, would this constitute a mistake? If not, how con one be sure that this is not the explanation of an "error"? The base-load task avoids this difficulty, but there is now on additional unwanted variable, for controllers will differ considerably in the priority they accord to this task when the work-load begins to build up. The workers at the FAA (8, 9) apparently decided, in the face of the above difficulties, to adopt method (v). It can be argued that the consensus of controller opinion is based on a much larger sample of traffic situations than can be covered in any simulation, and, because the opinion is based on the real world, it ovoids the systematic errors that are always possible in simulation. It remains to obtain a large enough sample of controllers to reduce effects due to personal bias to a reasonable level, to devise a suitable technique for extracting a quantitative judgment'of control work-load, and to show that there is a meaningful consensus of opinion. in the evaluation of the Arad model, Jolitz (9) selected five air traffic control centres, and studied a total of 16 sectors, each of which hod been worked in common by at least two experienced controllers. The sectors were selected to have different functions, but each had "complete radar capability" and was bounded by other sectors that normally used radar handovers to the sector under study. Further, the selected sectors had average or above overage activity. Subject controilers, having experience on two sectors, A and B say, were faced with questions of the form: "How many aircraft under simultaneous control in sector A would you judge, on average, create the some load as N aircraft in sector B?" Twenty-four questions were generated by putting N = 6, 8, l O and 12 and by reversing A and B. These were arranged in pseudo-random order and put to each subject. After a lapse of about one week, the questions were rearranged and again administered to each subject. At the time of the first interview, the subjects were not told to expect the second. The FAA project team decided to eliminate from the data any set of answers which contained "reversals in the judgmental response". (The example quoted in ref. 9 is a subject who stated that the load due to 6 aircraft in Sector

A was equivalent to 4 aircraft in Sector B, but who, in reply to another question, stated that 8 aircraft in Sector A were equivo lent to l O in Sector B. No detail is given of the train of reasoning that led to this decision. It seems intuitively obvious that the workload in any Sector will increase monotonically, but it is for from obvious that the curves for two sectors will never cross. Consider, for example, the speech load curves of fig. l. A controller whose answer to questions comparing Sectors 5 and 11 reflected the situation depicted in fig. l. would hove hod his responses deleted as inconsistent. • Ref. 9 does not reveal the percentage of the replies that were censored out of the data collected, though it is possible to deduce from local irregularities in the sample size (in Tobie Ill, for example) that at least 50/o of the results were rejected for one reason or another.

4. Rosenshine, M.

5. Leplot, J. Bisseret, A.

6. Barnes, R. M. (Ed) "Motion and Time Study" Chapmon & Hall 1949. 7. Gillespie, J. J. "Dynamic Motion and Time Study" Paul Elek 1947. 8. Arod, Bor-Atid et. al.

"Notes on the Measurement of Control Load and Sector Design in the En-route Environment" FAA SRDS June 1964.

9. Jolitz, G. D.

"Evaluation of a Mathematical Model for use in Computing Control Load at ATC Facilities" FAA SRDS Report No. RD-65-69 June 1965.

Conclusion The objects of air traffic control ore stated to be "the safe and expeditious movement of air traffic". There are dangers in accepting without question the assumption that reduction of controller workload is a useful intermediate step in the search for more efficient ATC. When faced with a serious overload situation, the controller normally preserves air safety and his own sanity by slowing down the traffic demand, by one means or another. In such a situation, tests on the controller loading may be an insensitive method of measuring the traffic delays which are of primary importance. It is unreasonable to expect that "controller workload" can be defined or measured with the precision customary in the physical sciences, but attempts to dote at defining, predicting or measuring controller workload con be termed succesful only if judged by extremely relaxed criterfo.

"The Application of Automation to the Solution of Air Traffic Control Problems". FAA Third International Aviation R. & D Symposium - Automation in Air Traffic Control. November 1965. "Analyse des Processus du Traitement de L'information chez le Controleur de la Navigation Aerienne" Bulletin d'Etudes et Recherches Psychologiques XIV no. 1-2 pp. 51-67, 1965.

10. General Precision "Contract No. C/38/0/65 for the comSystems pletion of an Arithmetical (FastTime) Simulation Study". Final Report Vais. 1-111. January 1968.

Acknowledgement The author is indebted to Messrs. C. Dowling and W. Feison af the FAA who spent some time discussing the Arod model and its evaluation, to Mr. M. Rosenshine of Cornell Aeronautical Laboratory, and to ATCEU Hurn for permission to quote from their study of workload at LATCC West Drayton. Contributed by permission of the Director, R.R.E.Copyright Controller H.M.S.O.

References l. Arad, Bar-Atid, et al.

"Control Capacity and Optimal Sector Design" FAA SRDS Interim Project Report No. l 02-11R, December, 1963.

2. Ratcliffe, S.

"Congestion in Terminal Areas" J. Inst. Navgn. 17, 183 (1964)

3. Chandler, G. A.

"ATC Capacities at Sydney Kingsford Smith (Mascot) Airport and Controller Saturation Levels". J. Inst. Navgn. 18, 42 (1965)

First published in the January 1970 issue of THE CONTROllER.

The Name of the Game* In flying throughout these United States it hos slowly, but surely, downed upon me that pilots and tower operators hove established a very subtle and most times friendly bottle of wits. For lock of a better name we'll coll it Dog Eat Dog. Actually, it's a traditional game that hos gone on for years, but until now no method of keeping score hos been devised. I intend to lay down some general rules ond os you readers come up with your own ideas it won't be long before we will be oble to dream up o score cord and turn it in to the Ops Officer after every flight. The tower operators con send !heir's to the Federal Communications Commission in order to drow incentive poy for hozordous duty. Anyway, it's fun, and a stondord opening goes like this: "Goforth tower, this is Air Force Jet 60954, o transient T-33, twenty miles North, landing information, over." The tower comes bock: "Roger, 0954, londingrunwoyonethreeright, windsouthsoutheostottwe lveknotsgusti ngtotwenty, a lti metertwentyn i nen inetyfive, flytrofficpotternotfifteen hundredcoll in iii olth reeout." Of course the tower gives out this vital information in such o machine gun rapid burst of phrases that it completely defies understanding. The pilot is still trying to figure out whot in heck hos been soid, but like the stalwart trooper he is he comes bock with: "Roger, Goforth", ond starts fumbling through the letdown book to find o diogrom of the field. If he's lucky Goforth will only hove one runway ond then oll he hos to figure out is which way the wind is blowing. He might even be luckier and spot on aircraft landing or taking off, then he's got the direction hacked. The rest of the junk he can

foke. The tower knows thot the pilot hasn't understood one word of the landing instructions and hasn't the faintest idea of where he is, but he knows where the pilot is, so Reprinted with kind permission of the Editor, AEROSPACE SAFETY.

when the T-Bird should be on initial (provided, of course, the pilot hos flipped a coin, guessed right, or looked out ond hos the landing direction figured) the tower operator picks up his spy gloss, waits until the pilot is 3'/• miles out and beats it to him: "954, threeoninitiol, you'recleoredforarightbreak, callbosewithgeordown."

SCORE: Tower- 1; Pilot- 0 Then the tower closely watches the pitch into traffic ond as the nose gear locks he beats him to the punch again. For effect, this simple statement is given slowly ond distinctly, "54, recheck geor down, cleared to lond."

SCORE: Tower-2;

Pilot-0

The essence of this friendly gome lies in the foci thot no pilot wants to admit that he can't understand simple (even though rapid fire) landing instructions. After oll he's o pylut and it's really only a courtesy thot he coiled the tower in the first place. If he doesn't wont to ploy the gome he con untrop himself by saying: "Goforth tower. You were cut out. Please repeat landing instructions, starting with landing runway ond slow down a little bit, will yo?" This is clearly o foul, ond forfeits the gome. A guy like this would use o checklist, change underwear every doy ond attend flying safety meetings. To preserve the gome, therefore, woys must be devised to let the defense catch up with the offense. As is common in these offoirs, this means developing on offensive weapon for the defense. This must begin with the initial coll, he must never soy: "This is Air Force Jet 60954, o transient T-33, 20 miles north." He must soy only: "Goforth, this is 954, landing instructions." This makes the tower think he should recognize the aircraft ond you make small points if he comes bock with: "Aircraft coiling Goforth, say ogoin your complete identification, please." -The pilot says, "AF 60954". Then complete silence. You now hove the tower on the run. He's even forgotten why you called, much less know who you ore and where you ore. You force him to soy: "AF 60954, whot is present location, ore you o conventional or jet and whot do you wont?" 33

Like Tic Toe Toe, this is the key move - you must make him drag all this poop out of you - above all - don't volunteer anything. Then you soy: "Goforth, this is Jet 54, I'm over the 'big trees', landing instructions." Naturally the tower doesn't hove the faintest idea where the "big trees" ore but you hove mode him think he should know and he is flat shook by this time. He is so shaken that he gives you the landing info at a speed that even you con understand. Then he picks up his spy gloss to find out just where in the blazes you really ore.

The tower troop hos just poured himself a hot cup of coffee

► When you get ready to taxi out, don't tell or ask the

tower. Just go. When the tower sees you moving around he'll hove to ask you who you ore and where you're going. Score l point. ► Try to schedule your taxi to takeoff

SCORE: Pilot-1;

Tower-0

For extra points, you con reopen the game by saying: "Goforth, am I cleared into initial before these two other '33s out here?" You've got him on the ropes now and he's almost panicked 'cause he didn't know any other traffic was in the neighborhood (maybe there really isn't but why let him off easy?). He comes back bravely: "Roger, 54, cleared No. l. The two other aircraft northsouth eastwest of the field, pullup, break out and re-enter traffic."

SCORE: Pilot-2;

Tower-0

There ore other ways to make points and I'll list some briefly. This isn't complete 'cause by using ingenuity, a cunning and devious mind, you can come up with at least one every flight.

so you get to the No. 1 spot just as another plane is on a close final. Pretend you're going to take the active and listen to the immediate response from the tower. If you get away without a violation, score 3 points.

► Read bock a very complicated

and detailed departure without a single mistake. You've won the game for the whole day 'cause you've outhossled the ARTC sneakers and you've out shorthanded the tower.

Follow these rules carefully (plus those you make up) practice a few hours a day and in a few months you will find that you hove become a consistent winner.

GOOD LUCK! P. S. Please pass on any locally devised, underhanded, clever and/or cunning traps you think of. I'm always trying to odd on to my list. Besides some of the tower operators hove caught on to my tricks. Got skunked the last time out, 2 to 0. J LT

► Listen to the tower give landing

instructions to another aircraft, and when you make initial contact, you give the tower the landing instructions. Score 2 points, one for initiative, one for trapping the tower.

► Circle the field,

listen to the tower instructions, don't soy a word until you've pitched, then tell the tower where you are. This is sometimes dangerous but score l point anyway.

► Wait

until you're one mile out and request a straight in approach. This is very effective, particularly if you have reason to believe the tower troop has just poured himself a hot cup of coffee. Score 2 points, one for fiendishness, one for timing.

► When cleared for a touch and go landing, change your

mind at the last minute and ask for a full stop. If there's another bird on the active, take 2 points. 34

First published in the July 1962 issue of THE CONTROLLER.

The Schiphol ATC Simulator By R. N. Harrison•

In August this year installation work is due to begin on the Ferranti digital ATC simulator far the Netherlands Department of Civil Aviation at Schiphol Airport, Amsterdam. When the 1970 peak summ.er·traffic is over, the simulator will go into full operation far training, and later far evaluation studies in respect of new operational procedures. Plans far the first four yeors of operation cover the training of twenty five approach controllers and seventy five area controllers. The simulator hos o playing area of 240 nm by 240 nm to cover the Amsterdam FIR, it includes two primary and two secondary radars, and con handle up to thirty aircraft tracks simultaneously. Additional aircraft ore held on punched paper tape awaiting entry into the system ot the appropriate time. Provision is mode in the computer system far the automatic preparation of flight progress strips. The strips are printed on page printers with guillotine attachments similar to those already in use at Schiphol as port of the SATCO system. Separate printers ore provided far blue and yellow strips. The information for the flight progress strips is derived from the flight pion data fed to the computer. Preparation of the strips takes place before the start of the simu lotion exercise. During the exercise aircraft are controlled from four Aircraft Control Positions, each capable of handling up to fifteen aircraft. These ACPs, also known as blip drivers' consoles, are equipped with a keyboard, electronic data display, and simulated RT facilities. The EDD hos a reel· angular face tube approximately 30 cm by 23 cm capable of displaying alpha-numeric characters and symbols. The information displayed on it falls into six categories: Aircraft Doto, Computer Message, Amendment, Input Message, Answer Message, Exercise Time. The different categories of information each hove their own place on the display. Aircraft data is written in the upper left-hand quarter and computer messages in the upper right-hand quarter. The other sections ore arranged to run horizontally across the screen, one above the other in the lower half. Except for new information in the computer message section or on outstanding answer message, all data shown on the EDD relates to the aircraft selected by means of on aircraft key on the keyboard. This means that effectively each pilot is dealing with only one aircraft at a time, and does not hove to retain in his memory and information about the other aircraft allocated to him. He con rely on the computer to cue him when positional or other information is to be passed to the controller. Ferranti Limited, Bracknell, England.

The blip driver's keyboard accommodates groups of keys. These are:

four different

Aircraft identi1y, Alphabetical and numerical, Function, Executive. During the course of on exercise each aircraft identity key may be allocated to a succession of aircraft. To allow for easy change of identification, the association between the aircraft key and the callsign at any particular time is shown by means of a magnetic plaque located alongside the key. Initial association is done by the computer which lights up on aircraft identity key at the time on aircraft is due to enter the exercise. When the illuminated key is depressed details of the aircraft - including its collsign are shown in the aircraft data section of the EDD. The magnetic plaque bearing the aircraft callsign can then be positioned alongside the key. Procedure for association is similar when an aircraft is handed over to another ACP, and there is also provision for a blip driver to take control of on aircraft when its start time in the exercise hos not been specified. The simulator allows the blip driver a choice of almost seventy messages in respect of any one of the aircraft he is handling. Some of these are instructions, some ore requests for information, but wherever appropriate they equate to words spoken by the controller. The sequence followed by the blip driver is to depress the aircraft identity key followed by the function key and such alpha-numeric keys as are required. The message appears in the input message section of his EDD, and if he is satisfied with the content (i. e. he has mode no mistake) he does one of two things: a) In the case of an instruction he acknowledges (or reads bock) the message to the controller then presses the execute key, b) He presses the execute key immediately, then reads back the answer from the answer message section of the EDD. Checking of messages is not confined to the "pilot", and the validity check by the computer may cover as many as nine items in respect of a single message. Such validity checks ore additional to the sequence of events set in train by the initiation of an instruction. The processes are not dissim ilor, but instead of seeking reasons for not carrying it out the compu1er now concerns itself with assembling precise data on each aspect of 1he manoeuvre. The data may be in terms of the performance characteristics of the airrcoft type or the path to be followed for a standard instrument departure, and by its completeness it makes possible a degree of realism not previously attainable. More than one ACP con be associated with a single RT channel if the format of the exercise requires it, and in this case the computer automatically allocates aircraft between

blip drivers on the some frequency in order to keep their worklood in balance. Normally, however, each ACP hos its own frequency, and in on exercise with four sectors on oircrolt moy be handled by lour blip drivers in turn. Alternatively each blip driver position con be used for a separate exercise associated with o particular radar display. The only proviso in the case of Schiphol is thol, as there ore two area radar displays and lwo-opprooch rodor displays, the allocation of exercises mus! be such that lwo ore with on approach rodor and !wo with on area radar. Provision is mode for keeping !he video channels completely seporote for each exercise so that no controller se~s aircraft from another exercise. Furthermore, one or more exercises con be frozen independently without stopping or affecting the remainder. For both area and approach rodors the simulator provides co-located SSR facilities with passive SSR decoding at each controller position. Combinations of sloshes and bloomers ore used to indicate coincidence of mode, coincidence of mode and code, SPI, and occupation of o specified altitude layer. Different spacings of sloshes for area and approach rodors compensate for !he differences in operating range. CRDF facilities ore also available with simultaneous strobes from each of two DF stations on each of the Area video channels, and single strobes from o common DF station on each of the Approach video channels. It is thus possible, even when four exercises ore in progress simultaneously, to provide identification in the form of a fix as for as area radar is concerned, and on indication of bearing in respect of the approach rodor. Consideroble use of integroted circuits for the computer logic makes it possible to accommodate the complete simulator in o four-boy rock. (This does not, of course, include radar display backup.) Allocation of equipment to the four boys is as follows: Boy

Equipment

Boy 2

Central processor, computer interrupt core store.

power supplies.

Boy 3

Primary strobes.

Boy 4

Peripheral

and

secon:dory

control

echo

equipment,

generotion;

for ACPs; control

and drive

units for EDDs.

data from the manually-punched tapes, ond to query ony points which foll outside the bosic parameters loid down. An ILS glidepoth of 5c would be challenged because the outside limits laid down ore 1c to 4 . Following !he sotisfoctory production of the manuallypunched tapes, the computer produces o series of binary tapes to correspond to them. The correspondence is not exact because there is significant re-arrangement of information. The binary tapes ore physically very much shorter than the non-binary tapes, and for convenience they ore spliced to form o single entry, excluding wind ond flight pion information. One reason for keeping the flight pion tapes separate is the need to odd more aircraft during the course of on exercise as the earlier flights lerminote or leave the FIR. Each flight is fully progrommed from start to finish in terms of !rock and flight level, but it is worth noting ot this point that differentiation between controlled and pre-progrommed tracks is o distinction without o difference. A trock does not hove on existence until it is included in the exercise program, but once it is in the program it con be controlled quite freely without reference to the flightpath previously assigned to it. In practice a balance is struck whereby the degree of control is limited to such changes os the controller finds necessary. Provision is mode for a reversion to flight pion os soon as the need for o deviation from ii hos possed. While the preparation of on exercise requires o greol deal of work, much of this is of once-only nature. Either the some exercise con be re-used as it stands, or the environment portion used with different flight plans. The re-use of on exercise in no way affects the training value because each student controller hos to make his own decisions, and any of these decisions con affect the overall situation. When on exercise is to be re-run, it is necessary only to feed the binary tapes into the computer. Where o new exercise is being designed which hos similarities with on existing exercise it is possible lo use !he editing focilities ovoiloble with !he equipment to transfer such data as is required from on existing non-binary lope, slopping when required to insert the proposed changes. The sequence here is the some as !he original in that the first product is o nonbinory tape which is reod ond checked by the computer. The computer then produces a corresponding binary tape.

The computer itself is a Ferronti FM1600B, only seven inches hig~ and less than the widht of o boy wide. It uses 6-loyer printed circuit panels plugging into a 12-loyer printed circuit backboard. The computer memory consists of o 24 K core store built up in 4 K modules. No- disc or drum bocking store is required. The use of this sort of simulator does not require the establishment of a speciolist staff of programmers. Doto for on exercise is specified on pre-exercise forms. For the Schiphol simulator there ore twelve of these - eleven for fixed data and one for flight plans. The fixed-data forms cover such things as primary and secondary rod or performance, the number of reporting points together with the way they ore defined and used, ILS data, aircraft characteristics. Wind velocitie~ ore treated as fixed data but ore associated with flight plan information as for as computer entry is concerned. When the forms hove been completed they ore transcribed on to a punched tape using a !eletypewriter and then led into the computer. In the pre-exercise mode, the function of the computer program is to check and process

When !he simulator is delivered to Schiphol, prepared tapes for a number of exercises will go with it. Other lopes supplied will include test programs and diagnostic programs. Among the odvontoges of !he FM 16008 computer is the fact !hot the reol time supervisor program allows the periodic operation of short, foul! detection routines limeshored with the main progroms. If a foul! is detected, on organisational subroutine is introduced to indicate the existence of the fault to the people concerned.

Firsl published in the April 1970 issue of THE CONTROLLER.

The level of reliability envisaged is a high one based on on estimated meantime between foilures of 550 hours. This figure incidentally is similar to those achieved by severol comparable installations which hove been in service over a number of years. Two of the photographs included with this article ore of a scale model of the Schiphol simulator, and show in some detail the layout envisaged. Installation will be odiocent to the operational air traffic control room, and the second storey within the accommodation allotted provides a balcony from which it will be possible to watch the simulator in operation.

Keyboard for Schiphol aircraft control position. Alphonvmerics ore to a sfan• dord typewriter layout with oircroft identity keys above and function keys below.

Aircraft control position for Schiphol simulator showing blip drivers' display and keyboard.

Scale model of the Schiphol simulator showing radar operating positions. The bocks of the four bi ip drivers' positions ore in the upper port of the picture.

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Russian Story Impressions from a Visit to the Soviet Union Introduction

V. K. Mishinkin

(left), M. Cerf (centre), B. ROthy (right)

Following on invitotion by V. K. Mishinkin, then President of the Civil Aviation Workers Union (CAWU), an IFATCA delegation, consisting of President Maurice Cerf, Treasurer Bernhard Ri.ithy and Editor Wolter Endlich, visited the U.S.S.R. in September 1968 to study the Soviet ATS system. The report on thot mission wos first published in the April 1969 issue of THE CONTROLLER. Meanwhile V. K. Mishinkin retired os CAWU President; his successor, Mr. Zuev, recently wrote us about the development since 1968. " ... The Aeroflot route network hos been considerably extended and now connects some 65 countries in Europe, Asia, Africa and America. Many new airports and ground facilities have been huilt. Automatic takeoffs and landings are being made, ond new categories of aircraft, including the SST, ore introduced into operational service. By 1975 Aeroflot expects to carry 115 to 120 million passengers per year, widely using TU 154 and TU 144 aircraft. Considerable progress hos also been made on the social side. More than 800.000 m2 occomodation hove been built and made available to civil aviation workers. Net salaries went up by 22,7 %, and a further increase of 30 % is expected during the present Five Year Plan. An Institute of Aviation Medicine has been established which is now engaged in studying human factors and environmental conditions of civil aviation staff. Controllers, as well as other civil aviation workers, are entitled to free treatment at Aeroflot hospitals and sanatoria. The five-day week has been introduced. Staff having become redundant with the advent of automated systems were re-trained and are now being employed at other jobs. All projects are being carried out in close collaboration between Aeroflot and the CAWU."

CAWU

The Lomonossov University

The Russian ATC staff do not have an Air Traffic Controllers Association as such. They are all members of the Civil Aviation Workers Union (CAWU), jointly with pilots, navigators, stewardesses, meteorologists, maintenance personnel and all the other staff engaged in aviation.

Crowd queuing at the Red Square to visit the Lenin Mausoleum

The CAWU has about 450.000 members, a "Central Committee" and regional branches. The present Central Committee was elected at the 1967 CAWU Congress. It consists of 92 members, 17 of which are Aeroflot pilots. About one third of the C.C. members are women. The Central Committee elects a Board of 9 Officers (the "Presidium"), i.e. a President, a Secretary and the Heads of the following seven departments: -

Management; Salaries; Professional and technical matters, safety regulations,

etc.; -

Cultural affairs, education, sporting activities; International relations; Social Security; Living accomodation, housing projects.

The schedule of the tour President Mishinkin, fatherly host to the IFATCA delegation, had prepared a very comprehensive program. Despite of his many other commitments, he made it a point to personally accompany his visitors on their study tour whenever this was possible. In these endeavours he was very effectively assisted by the Secretary of the CAWU Presidium, D. I. Tiourine, by Eugenia S. Voitenka, charming

Secretary for International Relations, by Andre lvanovitch Plouzhnikov, and by the untiring, multilingual interpreter Valerie. The emphasis of the program was on a visit of the ATS facilities at Moscow Wnukovo airport, supplemented by trips to Leningrad and to Sochi on the Black Sea, where working sessions with local ATS staff were held. Quite frequently these working sessions started at breakfast, when the IFATCA delegates were joined by the airport commandant or the chief controller of the ATC facility. Over the evening meal shop talk still continued. Between the meetings no time was wasted either. Our Russian hosts hod worked out a clever itinerary which, on the way to the various ATS facilities and offices, brought us close to many monuments, places of historical events, museums, etc. Thus we obtained, more or less "en passant", a condensed lecture of Russian history and culture. Highlights of these peripheral activities were a visit to the Hermitage at Leningrade and a performance of ,,Sadko" in the Kremlin Theater. Further in the program were visits to social institutions. At Wnukovo, for instance, we saw a camp of the "Young Pioneers", which is supported by the civil aviation workers, at Leningrad we were invited to visit the airport Kindergarten and at the Black Sea, Lubov Kalachova, the Foreign Department Manager of the "Sochi regional council for the administration of the trade union health resorts" provided the I FATCA delegation with a thorough introduction to this Russian recreation area.

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Moscow Four airports are serving the Moscow area: Sheremetjevo, Wnukovo, Domodjedovo and Bilkovo. Sheremetjevo is mainly used by international flights and by flight to the Northern part of the USSR. Domodjedovo handles traffic to the Southeast and to the East. Bilkovo is the airport for General Aviation. Smaller type aircraft such as the AN2, AN24, lll4 operate from here. Wnukovo is the oldest of the Moscow airports. Domestic flights to the Southern holiday resorts, to the Moldavia region and to the Ukraine originate from here. Occasionally there is also some traffic to the Northern and Eastern parts of the Soviet Union, and finally, Wnukovo is the gateway for all government delegations. In the future Wnukovo will probably mainly be used by flights to the Southern holiday resorts. This category of traffic is steadily increasing. Traffic from Wnukovo to Sochi, for instance, has gone up about 25 % in 1967. At the time of the visit the daily movements averaged 400. Wnukovo is the home of Moscow Approach Control and Moscow Area Control.

The Approach Control Area hos a diametre of about 200 kms, with no upper limit. It is sub-divided into the Sectors North/West, South, South-East and East. The Wnukovo CTR extends from GND to 1200 mfrs. It is of irregular shape, the horizontal dimension varying between approximately 30 and 60 kms. The jurisdiction of the "ACC" extends to approximately 500-700 kms around Wnukovo, the entire area being subdivided into 9 sectors. Each Sector is accomodated in a separate room. This segregation would reduce the noise level in the operations room we were told. Sector to Sector coordination would, in any case, be effected by telephone, so that there wou:d not be an urgent reason for grouping all Sectors together in one big operations room. In most Sectors the work is shared between an "Upper" and a "Lower" Controller. The dividing line between their respective areas of responsibility is at 4,500 m. Remote R/T and radar stations provide for the coverage of the comparatively large area. Attempting to compare these units with a similar Western facility, one would probably think of a Forward Radar Station. They have direct 41

Reception at the Office of the Wnukovo Air• port Commandant

communication links with the "ACC" and, in addition to monitoring position reports ond reloying clearonces, they provide radar separation to selected traffic, if so requested by the ACC. The responsibility for the Sector, however, rests with the ACC Controller. Moscow Centre moves an impressive amount of traffic. About 1800 operations are controlled per day out of which some 1500 are arrivols and departures for the 4 Moscow airports, the rest are overflights. Unlike their Western colleagues the Russian Air Treffie Controllers utilise o distance-time graph as a traffic display. It consists of something like o small plotting toble of about 50 x 50 ems., across which a band of paper can be moved. The reporting points are listed on the horizontol axis of the display, the vertical axis carries the time, in 5-minute intervals. Each movement is represented by a diagonal line along which are written details of the flightplan. As time passes the controller moves the band of paper across the plotting table with a thumbwheel. We had some doubts as to the suitability of such a display system in a Western European environment, with the rather complicated route network and the high percentage of climbing and descending traffic. In the Soviet Union with its vast distances, however, the system seemed to work quite well. Its application is further focilitated by the fact that all domestic flights in the USSR are bound to operate in accordance with a very strict schedule. Precise flight plans are notified well in advance to the units concerned and a central coordinator assigns an appropriate slot to each flight. The pilot who misses his slot, for instance by not taking off at the prescribed departure time, might as well cancel his flight until a new slot is assigned to him. Naturally we were curious how international flights fitted into that system. Apparently these flights do not present any problem. First of all they only constitute a certain percentage of the overall traffic. Furthermore, the aviation administration can regulate the approximate arrival and departure times when issuing the diplomatic clearances. Should, however, an international flight ever clash with a domestic operation, our Russian colleagues do not hesitate to delay or even divert the domestic flighi'. Radar is heavily used both in Approach and in Area Control. It was interesting to notice that the entire radar equipment is available in duplicate, from the antenna to the scope.

ACC Sector with distance-time

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Distance-time graph, blow-up

Wnukovo Airport Commandant W. Tschernjakov (left) and IFATCA Pre· sident M. Cerf (right)

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Training The very timely subject of Controller training seems to be given due attention in the Soviet Union. The central institute for Controller training ist at Kiev, but there ore also local training establishments ot the larger airports. The Aeroflot training centre ot Wnukovo, for instance, provides extensive facilities for the indoctrination of pilots, navigators, radio and radar operators, stewardesses, maintenance personnel and Air Traffic Controllers. Each classroom is equipped for one particular subject. There ore individual laboratories for the various types of aircraft engines, ground and airborne radar installations, radio equipment, navaids, aerodynamics, meteorology, cabin crew, air traffic control, etc. We were quite impressed by the extensive use of highquality teaching aids, such as full size turbo engines, assembled and apart or cut into various cross-sections; a whole range of radar sets; a complete cockpit mounted to the outside of the building and connecting through a hole in the wall to one of the training rooms; and an obundance of large-scale, multicolour training posters, flip charts and diagrams.

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A big simulator room with, inter alia, a TU l 04 simulator is also accomodated in the building, which is already getting too small for all the training activities. This is not surprising, for the school has a considerable turnover. In addition to providing basic and advanced training, refresher courses ore frequently held ot the school. We were told that even the fully qualified pilots must have at least one month refresher training per year; an important subject of this training is the discussion and evaluation of recent aircraft accidents. The ACC training suite is an exact replica of one of the Moscow Sectors. Radar information is live, video-linked from the Wnukovo radar. Training in such an environment is probably very realistic. On the other hand it must be for more expensive than radar simulator training, and the 46

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operation of the target aircraft requires close coordination with the local ATC units. Until recently Controllers have been selected from former Aeroflot pilots, but to an increasing extent Controller Cadets are now recruited from high school graduates. The entry age is between 17 and 24 years. A thorough medical check is compulsory upon entry. The candidates are mainly selected on the basis of their academic qualifications. Aptitude tests are not used. The ab initio training lasts three years, the greater part of which are spent at the central ATC School mentioned above. This school must have an extremely good record; our Russian colleagues said there had been only one or two failures among the 300 students who hove been trained recently. After completion of the theoretical training, the future controllers are assigned ta field units for on-the-job training. OJT successfully completed, controllers are issued a licence which entitles them to exercise their task. All controllers are employed by Aeroflat. During their training they ore for a certain time assigned to a flight crew and must extensively fly throughout the region within which they are expected to work after their training.

Lecture room "Radionavigotion"

TU 104 Simulotor

TU 104 Simulotor;

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lecture room "Communications"

In order to maintain the knowledge thus gained on the flight deck, rated controllers ore required to make frequent route experience flights. Every two years each controller must go bock to the school for about three weeks refresher training.

Controller status and working conditions In the Soviet Union Air T roffic Controllers enjoy o considerable status. When we asked for o standard of comparison it was sugested to compare o Controller with o well-experienced electronics engineer holding o university degree. This recognition is reflected in the controller's salary, which was said to be about 30-40 0/o higher than that of o doctor or o high school teacher. The overage controller wages ore outlined below. High density facility

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In addition to their basic salary, Soviet controllers ore entitled to certain allowances. 400/oof the basic salary may be granted as on additional bonus if o controller hos not been involved in any accident or incident. A 13th month' salary may also be paid. At Moscow and Leningrad controllers obtain o l 00/o facility bonus. In the Eastern USSR on allowance of 50-1000/o of the basic salary is granted because of the difficult working conditions. Fringe benefits include free flights with Aeroflot and very reasonable rates at the CAWU sonotorio in the Southern holiday resorts. Although a controller's salary compares handsomely whit that earned in many other professions in the Soviet Union, applying Western standards he will still find it difficult to make ends meet. To obtain an impression of what o controller con buy for his salary, we were given the following approximate prices for food and other necessaries of life. kg butter kg bread kg meat kg potatoes dress coat car (Wolga)

3½ Rubels 14 to 16 Copecks 2 to 3½ Rubels 10 Copecks 50--100 Rubels 100-150 Rubels 5000--6000 Rubels (?)

The rates for gas, water, electricity and rent appear to be rather low. The rent is ea lcu lated on the basis of 13 Co pecks per square meter. Living accomodation is still a bottleneck. The "plan" foresees 9 m2 per person. Standard apartments for a family of three or four persons are now constructed in large numbers. The surface area af these apartments is 30 square metres and the rent approximately 420 Copecks. In the Moscow area controllers are usually working three shifts from 14.30--22.00, from 08.00--14.30 and from 22.00-08.00, followed by a rest period of 56 hours. During the shifts, breaks of 20 to 30 minutes are granted every l ½ hours. Controllers are entitled to at least 24 days annual (eave; in the Eastern and Northeastern regions this is increased to 30--36 days per year. Except for young staff who have just completed their ATC training, controllers may choose the facility at which they would like to work. Young controllers must serve three years at the unit to which they are assigned before they are permitted to transfer. The earliest retirement age for controllers is 55 years, provided they are working with radar. A controller can, of course, be compulsory retired at an earlier age for medical reasons. Staff not medically fit for air traffic control work may be assigned administrative tasks. A controller's pension depends upon the number of years in service. Maximum is 600/o of the last salary plus up to 100/ofor specific professional qualifications.

Aeroflot crew hostels

Lecture room • Air Traffic Control"

Air Traffic Controllers ore also entitled to use the recreation facilities established at all Russian airports for Aeroflot pilots.

At Wnukovo we were invited to visit one of these recreation centres. It is a mixture between hotel und sanatorium, managed by a woman doctor. The Wnukovo hostel has sleeping accomodation for 450 people, gymnastics hall, library and reading room, billard, hobby shop, etc. All Aeroflot crews oway from base live in these crew hostels and even the local flying pilots have to check in before they depart on a flight. There are usually 4 to 5 beds in one room to accomodate one flight crew; a special wing of the building is reserved for the stewardesses. This is a very healthy environment. No alcohol, no smoking, and before each flight all crew members have to take a thorough medical check. Crew members are not permitted to fly if they did not have at least 8 hours sleep within a reasonable period of time before the flight. Another interesting feature: duty rosters are prepared so as to allow pilots to alternate between destinations In the North and in the South of the Soviet Union.

Reception at the Leningrad station. ground

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Leningrad There is a saying that a visit to Russia is not complete without a ride on the "Krasnaja Strela". The journey from Moscow to Leningrad in this fast and comfortable train is, indeed, very pleasant. Frillies, thick upholstery and the humming of the Samovar create a cozy atmosphere. We boarded the "Red Arrow" at midnight and some seven and half hour later we arrived at Leningrad, on a beautiful, crisp and sunny morning. There is, alas, not enough space in the context of this article to describe the beauty of this city on the Neva, to which we were so expertly introduced by our kind hosts. Leningrad is the second busiest airport in the Soviet Union. The last count indicated 1.6 million passengers per year, transit passengers not included. 100 controllers in four shift are moving about 220-250 flights per day; the rush hours are between 08.00-10.00, 15.00-18.00 and 22.00-23.00, during which approach intervals are two to three minutes. Two IFR runways are available.

Leningrad, monument of Peter the Great

Leningrad connects to all big cities in the Soviet Union.

To Moscow alone there ore 15 flights per day. About half that number are daily shuttling Sachi.

between Leningrad and

The international quota of traffic is steadily increasing. Leningrad has regular services ta Amsterdam, London, East Berlin, Prague, Warshaw, Helsinki, Stockholm and Copenhagen. In addition to the normal passenger flights, a number of special flying activities originate from Leningrad, for instance geodetic surveys, fishing surveys and fish transport, agricultural flights and helicopter operations. The integration of these movements into the normal traffic does not cause any difficulties. Anyhow, air traffic control is not the bottleneck in Leningrad, but the limited capacity of the terminal building. A new building is now under construction. By 1975 the airport authorities expect 20 million passengers per year.

Leningrad, view from the modern "Sovietskoja"

hotel

Smolnii Poloce, the Headquarters of the Revolution

Leningrad Chief Controller

Briefing session at Leningrad airport

L. V. Rossochine explaining

the ATS system

Bernhard Riithy asked whether Leningrad airport has encountered any problems of snow removal. This seems, indeed to be the case. The system being used most effectively is to blow hot air on the runway with obsolete turbine engines which ore mounted on trucks. Experiments with chemical treatment of the runways look very promising. As to the many smaller aerodromes in the area, these ore not cleared from snow in the winter. The small aircraft serving them ore equipped with wheels and skis and can land almost anywhere.

Sochi

Sochi-Adler airport

5° at Leningrad, 30° C - CAVOK at Sochi, distance in between about 2100 kms. llushin 18 CCCP No. 75859 brought us safely from the Gulf of Finland to the Black Sea. Enroute we saw many aicraft. Indeed, Leningrad Moscow - Sochi seems to be the daily milkrun. Incidentally, one of the aircraft we saw, I guess it was an AN 12, was probably on a VMC restriction, or else there must have been something wrong with the distance-time graph. Sochi-Adler airport is located very close ta the seaside. A few miles inland commence the caucasian foothills. When we approached, the wind was from the sea. We had to come in from the land. That was quite an experience. One cannot say that the IL 18 is a small aircraft, but the skill with which the pilot manoeuvred her across the ridges almost made one think so. Thermical turbulence didn't make things easier for the crew. Naturally, one of the first questions we asked the commandant of Adler airport was related to the IFR approach procedures. In IMC approaches can only be made from the sea. Does this imply that the aircraft must land down wind? Fortunately, weather situations which would require this are relatively seldom. The problem will soon be solved anyway, because a new runway is being built. This will not only facilitate the approach in general but will also enable the big jets to land at Sochi-Adler airport. Sochi traffic is highly seasonal; at the time of our visit the peak was about 250 movements a day.

60 Controllers are providing approach and aerodrome control service. There is no ILS at Sochi, but the corner reflectors along the runway seem to indicate that PAR is available. Judging from the antenna installations on a small hill near the airport, the Sochi controllers must also hove a medium/ long range and a short range radar at their disposal. Sochi is the most popular Soviet balneologicol and climatological health resort. The city of Sochi, located about 35 kms NM of Adler airport, is the administrative center of a creation area called "Greater Sochi", which extends some 145 kms. along the coast of the Black Seo. The various holiday resorts along the coast are linked by train and by a dense helicopter service. The Foreign Department Manager of the "Sochi regional council for the administration of the trade union health resorts" is on attractive lady, Lubov Alexejevna Kalatchova. Lubov Kalatchova was a charming host and knowledgeable guide to the IFATCA delegation. During visits to the various centers of activity in the "Greater Sochi area", she gave us a comprehensive briefing on the management of

The sanatorium of the Metallurgists at Sochi

this huge project. Most of the unions have their own sanatorium at Sochi, and when the workers go down there on their annual holidays, it is not for "living it up" but in order to recharge the batteries. Hence one would probably look in vain for alcoholic beverages in the sanatorio. Instead, The "holiday package" includes comprehensive medical treatment, comprising climatological methods, seawater baths, physiotheraphy and regular visits to the Matsesta springs near Sochi, which contain a high level of free hydrogen sulphide, a variety of mineral salts, dissolved Methane and Nitrogene. The "Regional Council" is very interested in attracting foreign tourists at Sochi. There are already INTOURIST facilities and a number of big hotels, one of them with 2000 beds, are presently being built for the specific purpose of accomodoting international visitors. Some 120 kms. Southeost of Sochi, at Cap Pidzuno, a completely new complex with seven big hotels and various peripheral facilities has been built in a beautiful forest of pine trees right by the seaside. Cop Pidzuno is also open for froeign visitors. It occured to me that it was not so much "health orientated" as Sochi. Perhaps this is one of the reasons for meeting a far greater number of young people at Pidzuno than at Sochi. They obviously did not yet need the Sochi "Sanatorium Package".

A good-bye toast of the Sochi staff

Conclusion The visit of the USSR by the IFATCA delegation was undoubtedly most interesting, particularly in view of the fact that until now the Soviet Union hos been something like a "white spot" on our ATC mop. Not only did this study tour enable us to learn something about the ATS system in Russia, it was also a catalyst for establishing friendly contacts with Soviet controllers and to obtain a first hand impression of their working and living conditions. Moreover, apart of any professional aspects, the personal confrontation with this vast country, which constitutes one of the greatest powers of the world, is on experience no visitor of the Soviet Union will ever forget. First published in the April 1969 issue of THE CONTROLLER.

Holiday resort at Cap Pidzuna near Gagra, Georgia

•

us1ness

For many years we have been working in real time applications for complex Air Defence systems. Infonnation systems designed for handling large volumes of data, where the demand for immediate access and presentation of desired stored information is mandatory. That is how our Censor 900 real time information system was developed-A system including computers, processors, PPI' s, graphic and alphanumeric displays, radar extractors and data communication equipment. Our work within Air Defence gave us experience which was applicable also within civil Air Traffic Control, and in 1964 we installed our first ATC-system at Arlanda Airport in Stockholm. A complete automatic radar data handling and presentation system with automatic target tracking, processed flight plan data, SSR information. Since then nearly 10 years have passed. Today our solutions are accepted world-wide, and our ATC-systems and simulators are installed on various airports all over Europe. Scandinavia, Germany, Belgium, the Netherlands, France, Austria and 5-r-.11 n,~AAB Yugoslavia. ATC is our business. ,..,...,.,.~.

STANSAAB Elektronik AB • Veddestavagen 13 • S • 175 62 Jarfalla, Sweden ·Tel.: Stockholm 36 28 00 Telex: 17892 STASAB S · Cables: STANSAAB, STH.

Developments inHelicopter I FROperations by Tirey K. Vickers Fig. 1 VERTOL V-107 Helicopter, passing United Notions Building.

Some of the most interesting pioneering work in the field of flight operations today, is being accomplished by one of the shortest scheduled airlines in the world. The airline is New York Airways. Their pioneering is all aimed at the goal of becoming the first helicopter air carrier to obtain a supplementary ,FAA certificate for instrument flight operations. Lock of instrument flight capability hos been costly. In summer, NYA's 25-ppssenger twin-engine Vertol V-107 helicopters may shuttle 27,000 passengers a month, between ldlewild, Woll Street, and Newark. But winter is a different story. Lost winter, NYA hod to cancel 19% of its schedules because of weather. The economic effect of these cancellations was even worse, because of a fickle factor known as passenger loyalty. NYA hos found that after a prolonged interruption, passenger loads just don't jump bock to normal when the helicopters start flying again - sometimes it takes several days. Although schedule dependability is desirable for any airline, it's critically important to NYA, whose potential competitors (taxicabs) number in the thousands. Sa this year, NYA is doing something about the weather. However, IFR certification isn't something which con be achieved overnight. In this case, it involves the unprecedented task of certifying the pilots, the helicopters, and even the navigation system. The actual tests storied lost spring and ore expected to take about a year. The program is important to U.S. aviation, because it hos finally given the FAA a reason to develop IFR criteria for helicopter operations. (Up to now, helicopter operations hove been subject to a number of fixed-wing regulations.) The program is important to controllers, because one of its basic concepts, the use of precise pictorial navigation, could point the way someday to a major simplification of air traffic control procedures.

Technical Problems With commercial jet transports about to achieve allweather capability, why hove the slow, docile whirlybirds remained, for years, essentially VFR machines? For one thing, helicopter design hos hod to wait for the comparatively recent development of the compact,

high-power, low-weight, gas turbine, before anyone could market a commercial multi-engine helicopter with acceptable one-engine-out performance. But there hove been other problems. Almost every helicopter ever built hos been locking in aerodynamic stability. If its attitude is disturbed, the typical copter shows no particular tendency to return to straight and level flight - in fact, it may tend to diverge even more. As a result, constant corrective action on the controls is required. This makes it fatiguing to fly for long periods - and especially on instruments. The latter problem hos been prolonged by the technical log in developing special instrumentation for lowspeed flight. Up to now, helicopters have been forced to use flight instruments which were originally developed for fixed-wing aircraft. Helicopter pilots complain that these instruments ore not easy to interpret and follow, in hovering or low-speed operations. The IFR winter hazard of icing con have on especially serious effect on helicopters, in that on unequally-balanced build-up on the whirling rotor blades con lead suddently to a critical vibrotion problem. Another limitation hos been that the VHF and UHF navigation systems presently in use ore subject to lineof-sight cutoff problems, in some of the most important minimum-altitude areas desired for helicopter operations. These systems also lock flexibility, due to the siting problems which exist around built-up metropolitan areas. New York Airways now hos a tentative solution to each of these technical problems. The object of the IFR certifkotion program is to demonstrate conclusively to the FAA, that all these solution ore adequate.

Engine-Out Capability As shown in Figure l, the Vertol V-107 hos two rotors. They ore geared together, through a common drive shaft, to turn in opposite directions. The shaft is powered by two General Electric gas turbine engines, presently rated at 1250 horsepower each. The engines are connected to the rotor drive through individual overrunning (freewheeling) clutches. Should one engine suddenly fail, its clutch instantly goes into free-wheeling. The live engine con then assume the entire job of powering the. rotor blades, with-

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Helicopter.

out having to waste ony power in spinning the dead engine. Figure 2 shows how the power required for helicopter flight varies with forward speed. Hovering (flight 'at zero speed) requires high power. As forward speed increases, the power required to maintain altitude grows less and less; at or obove single-engine speed one engine con supply oil the power necessory to maintain oltitude. Beyond some optimum speed, any increase in forward speed requires on increase in power. Should one engine foil when the aircraft is below single-engine speed, the other engine cannot provide all the power necessary to maintain altitude. Therefore, the helicopter will settle, either until it contacts the ground, or until the pilot con nose it forward and accelerate it to single-engine speed, where the power available from the good engine is sufficient to stop the descent, further occelerote the aircroft, and stort climbing again. On the V-107, the optimum speed (where there is the greatest excess of power available, over power required) is slightly less than 65 knots. To maintain adequate safety margins, NYA plans to conduct oil IFR operations at o speed above single-engine speed, and preferably between 65 and 125 knots. After

tokeoff, pilots will accelerate to at least 65 knots before going on instruments; and will maintain ot least 65 knots when holding on instruments, or while making the instrument portion of the approach. Using this procedure, on engine failure need not result in o sudden loss of altitude. Keeping the IFR flight speed to ot least 65 knots also bypasses the low-speed instrument problem; at the.se speeds fhe flight instruments bebove more like they do in the fixed-wing aircraft for which they were designed. • Figure 3 shows another effect of maintaining the instrument letdown speed at 65 knots. To ovoid exceeding o descent rate of 500 feet per minute with unpressurized passengers, the descent angle in still air will be limited to about 4.3 degrees; which isn't much steeper than the usual glide slope for fixed-wing aircraft. After the pilot has the ground in sight, he con slow the oircroft and steepen the descent accordingly, if desired. To qualify for what the FAA calls Category A operation, the helicopter must be able to demonstrate o 150foot-per-minute climb with one engine out. It oppeors that the V-107 will be able to meet this requirement as long as its maximum gross weight of 19,000 pounds is adjusted for higher temperatures. Stability Augmentation

To reduce pilot workload, NYA's Vertol V-107's ore equipped with o stability augmentation system (SAS). Not exactly on automatic pilot, it is o pilot assistance device, which supplies o form of artificial stability, to compensate for the helicopter's lock of aerodynamic stability. The stability augmentation system works into the aircraft control system, somewhere between the pilot and the rotor hubs, as diagrammed in Figure 4. The SAS includes a gyro sensing unit which detects any deviation of the helicopter from o level flight attitude. When this occurs, the sensor activates o hydraulic servo system, which supplies o corrective action to the rotor controls, by changing the length of certain special push-rods known as extensible I inks. As shown in Figure 5, on extensible link looks like two tubular shock-absorbers, bolted end-to-end. Flexible hydraulic lines ore attached to each end of each section. When hydraulic pressure is supplied to the center lines, pistons within the tubular sections move outwards, extending the effective length of the member. When pressure is supplied to the outer lines, the pistons move inwards, reducing the effective length of the member. If the pilot do.es not move his control, any change in the length of the extensible link supplies o movement to the rotor controls. Each extensible link is designed as a double unit so that, if one unit should foil, the other unit doubles its travel to supply all the necessary movement. To insure that the pilot will always remain in control of the oircroft,

DESCENT ANGLE 65 KNOTS_,J (110 F.P.S.) Fig. 3

Limitation

of Descent Angle Due ta Forward Speed.

500 F.P.M ) ( 8. 3 3 F.P.S.)

any appreciable serious vibration

ROTOR CONTROLS

r---------SAS

---,

-----

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.--~--,

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Fig. 4

Block Diagram of Stobility

Augmentation

I I

PILOT

System (SAS).

i he SAS is designed so that its maximum corrective forces will never exceed 300/oof the control forces which can be exerted by the pilot. Thus, the SAS generates the little control corrections which gently mudge the aircraft back to an even keel whenever it is disturbed. Yet, because SAS works in series with the pilot's control movements, the pilot continues to fly the aircraft in a normal manner, and can override the automatic co;·rections if he desires, without conscious effort.

ROTOR

CONTROLS

in the ice load can set up a

The V-107 icing problem was investigated lost winter at the Royal Canadian Air Force installation near Ottawa. These tests indicated that the ice on the blades would tend to be self-shedding, down to on ambient temperature of -4 degrees Centrigrade. For colder conditions, NYA plans to protect the leading edges of the rotor blades with electrically-heated plastic covers. However, the large amount of electrical power required for these heaters may force NYA to install larger generators.

I ___

unbalance problem.

On operational factor in NYA's fovor is that their aircraft will seldom be more than five minutes away from the nearest landing pod; so pro!onged operotion in heavy icing conditions should seldom be necessary. Rooftop Operations Fifty-nine storeys above New York City's Grand Central Station is the heliport of the new PANAM Building, which is shown in Figures 6 and 7. New York Airways plans to fly in and out of ihis lofty perch, which should be a very convenient termi,--,I for midtown Manhattan.

f•nnrm 1•1~m~mno•

111

111 tnJrrrn1nn111nmn 111111 •••____ . --··••Ill Fig. 6

LEGEND

□ FLUID UNDER PRESSURE

Fig. 5 Operotion

of Extensible Link.

Icing The windsshields, pilot tubes, and engine inlets of the V-107 are equipped with anti-icing devicyes. However, the most serious potential icing problem of rotorwing aircraft is icing of the rotor blades. Because the crosssection of a rotor blade is much smaller than that of a typical airplane wing, the same thickness of ice buildup will hove a more critical effect on the lift and drag of the rotating airfoil. More quickly noticable, however, is that

Rooftop Heliport

atop new PANAM Building

in New York.

From the flight operations standpoint, the heliport has two interesting features. The curved horizontal edging around the perimeter of the landing area is actually on aerodynamic fairing, which has been carefully designed and wind-tunnel tested, to reduce turbulence and smooth the airflow across the roof, in gusty winds up to 50 knots. For night operations, the landing surface is illuminated by a large number of small bullet-shaped floodlights, around the perimeter railing. The lights ore carefully hooded, to avoid blinding a pilot; and ore spaced to provide a criss-cross textured pattern on the landing surface. The textured effect is designed to aid the pilot in judging his height above the surface. Starting in 1964, NYA plans to provide direct helicopter service to the New York World's Fair, using the rooftop heliport atop the Transportation Building, inside the fair grounds. Because of the spatial characteristics of rooftop heliports (small useoble areas surounded by practically nothing at all), landing and tokeoff techniques will be slightly different from those employed where plenty of room is available. As shown in Figures 8 and 9, the rooftop techniques are designed around certain combinations of speeds and altitudes which provide maximum safety margins in case of engine failure. 55

Fig. 7

Landing Surface and Perimeter lighting

Installation

of PANAM Rooftop Heliport.

HEIGHT LOSS

35 KNOTS

PILOT MAINTAINS AT LE AST 35 KNOTS DOWN TO LANDING DECISION POINT, 120- 150 FEET ABOVE LANDING PAD. THIS PROCEDURE PROVIDES AMPLE CLEARANCE FOR RECOVERY TO SINGLE-ENGINE SPEED IN CASE OF SUDDEN ENGINE FAILURE.

65 KNOTS

48 KNOTS SINGLE ENGINE SPEED

DECISION HEIGHT 120-150 FEET

l Fig. 8 Rooftop

Landing Procedure.

Fig. 9

Rooftop Tokcoff Procedure.

11® VERTICAL CLIMB AT

TO DECISION HEIGHT 115 FEET

II I I 11111I 56

L~~~~R E~S 1~i:t~~~i~

I I I

1000 FPM

PILOT CAN RETURN TO LANO AT A WITH ENGINE FAILURE AT OR AFTER REACHING 8, PILOT CAN ACCELERATE TO SINGLE-ENGINE SPEEO ANO BEGIN CLIMBOUT

SINGLE ENGINE SPEfO 48 KNOTS

35 FEET CLEARANCE ABOVE TAKE-OFF ELEVATION

I I:l~I I I I I 11111 I II :

Pilot Certification Most NYA pilots hove hod their fixed-wing instrument ratings for years. One of the primary tasks in the IFR certification program has been to draw up o course of training to enable these pilots to qualify for their helicopter instrument ratings. NYA, FAA, ond the Air Line Pilot's Association have all furnished ideas for this training program. The test is equivalent in scope, and required proficiency, to an FAA test for o fixed-wing rating in o twin-engine turboprop transport aircraft. The helicopter flight test includes single-engine operation as well as flight with SAS inoperative. As no suitable helicopter flight simulator is available, all training is being conducted in the actual aircraft. Qualification of NYA's 30 pilots is expected to require about 500 total flight hours. NYA hopes that these tests will result initially in authorized weather minimums of 300-foot ceiling and ½-mile visibility; with o possible reduction to l 00-foot ceiling and •/•-mile visibility at some distant date.

Navigation System Requirements: It isn't often that on airline hos to supply its own navigation system, but NYA hod to do it. Its special requirements for low-altitude navigation, around the concrete canyons of the New York area, could not be met by available U.S. systems. NYA needed a navigation system which would provide solid coverage, without reflections, clear down to the ground, in an area characterized by the largest collection of tall buildings in the world. They needed extremely accurate navigation, to find their way precisely to tiny pads like the PANAM and Woll Street Heliports. They needed on extremely flexible method of navigation, to follow pre-defined curved tracks which would avoid high obstacles and stay clear of the approach and departure lanes of fixed-wing aircraft. NYA needed a system which would provide closelyspaced parallel tracks so they could fly simultaneous schedules in opposite directions, at a single minimum-altitude flight level. They needed area coverage so they could modify their route structure and odd additional terminals as desired, without having to relocate ground facilities or add new ones. And, because piloting a helicopter requires both hands and both feet, NYA needed a navigation system which would show the pilot exactly where he was, at all times, without having to shuffle charts or twist knobs. So they got Decca. The ground stations of the New York Decca chain are operated by a subsidiary of New York Airways, and ore monitored continously by recorders installed in NYA's flight operations office at LaGuardia Airport. NYA's helicopters are all equipped with dual installations of the Decca Mark 8-A receivers and Flight Logs, as shown in Figure 10. During the course of the certification program, the reliability of the airborne equipment hos been increased tremendously by the adoption of a new solid-state digital computer (in place of the former analogue computer) to drive the Flight Log. Around New York VFR helicopter approaches and departures are often made at right angles to the fixed-wing runway in use. Keeping_ helicopters out of the stream of fixed-wing traffic is advantageous from the ATC ·stand-

Fig. 10 Dual DECCA Installation Helicopter.

in New York Airways

VERTOL V-107

point, as their relatively low forward speed could tie up on approach or departure path for a considerable period. With Decca, there is no particular reason why NYA's instrument flight paths should differ from those normally used in VFR weather. System accuracy on NYA's present routes will permit a pilot to navigate about as accurately on instruments, with a Flight Log, as he con in visual flight conditions using familiar neighborhood landmarks. Significance for ATC - Although most people don't associate navigation with today's critical ATC problems, the characteristics of the present navigation system hove a pronounced effect on the efficiency of the air traffic control system. Having just completed a study concerned with finding a suitable site for a new major airport in the New York metropolitan area, the writer is painfully aware of the restrictions which are imposed on 3-dimensionol traffic flow by the present singletrack airway system and its radial collision courses. To handle increasing traffic, radar controllers have had to compensate for these limitations by assuming more and more of the navigational responsibility themselves. However, the widespread employment of air route radar vectoring procedures has greatly increased the controller workloads of target tracking, communication, and coordination. Sometimes, as in the New York area on June 7, 1963, the total workload builds up to the point where severe flow restrictions must be imposed; the result is reduced capacity, and monumental delay statistics. Could a high-accuracy navigation system, with pictorial presentation for the pilot, reverse this current trend towards more controller workload per aircraft? Previous ATC research hos shown that the use of parallel dual-lane airways, in place of today's single-track diverging-converging routes, will eliminate much of the control workload and congestion presently encountered on busy climb and descent segments. Pictorial navigation, on parallel multi-track coded routes, promises a major reduction in controller workload, by providing more mutually non-interfering flight tracks, and then giving pilots the copability of following them accurately through congested areas, without controller assistance. Here perhaps is the most significant gain that any future system improvement could make, towards increasing the capacity of tomorrow's air route network. First published

in the January 1964 is,ue of THE CONTROLLER.

Some Aspects of Terminal Control in the Seventies By Tirey K. Vickers Paper present~d to the CATCA Convention Ottawa, 7th May 1969.

Introduction Two serious challenges will face the already overloaded ATC system within the next decode: One will be the introduction of new types of aircraft in large numbers; the other will be a relentless increase in traffic demand, up ta four times the present level by 1980. What can be done to meet these challenges? How will the resulting changes affect the terminal control job?

les International Airport. The Beech 99 and the DH Twin Otter are two of the most popular types of aircraft presently used for this purpose. It is to be expected that larger types of aircraft will ultimately be designed for commuter service; for economy, they will probably be powered by turboprops rather than jets because of their relatively low cruising altitudes and short stages lengths.

STOLs

Aircraft Types Old Aircraft The Twin Beech was introduced in 1935, the DC-3 in 1938; both types ore still around in large numbers. So, it is a safe bet that the terminal control system of the 1970's will hove t9 accommodate practically all the existing types of aircraft. However, there is an exciting array of new types on the horizon. How will they affect the ATC system?

Superjets The first of the superjets - the 747 - is already flying, and over 160 aircraft of this type alone ore on order today! By carrying up to three times the load of one of today's big jets, the superjets theoretically could simplify ATC by reducing the number of airline schedules. There is a limit, however, as to how far any airline can go in consolidating their schedules. Schedule frequency is still important as far as passenger convenience and time-saving are concerned - and these two factors are the main reasons why people buy airline tickets. So, don't expect a radical reduction in airline schedules when the superjets go into service. Any midair collision involving a superjet may trigger a public demand for higher separation standards for such aircraft, simply because of the much greater number of human lives at stake. We hope that this will never become necessary. The trailing vortices generated by the superjets, particularly on takeoff, will be of somewhat higher intensity than those generated by today's jet transports. More important, the greater diameter of the wake will increase the possibility of other aircraft, up to a fairly large size, becoming completely immersed in a single vortex and, thus, being rolled upside down. These reasons may force a requirement for greater longitudinal separation behind superjets, and for the complete segregation of small aircraft on runways which are considerably offset from those used by the superjets.

It can be expected that commuter aircraft designs will move more and more toward STOL capability in order to increase the potential utility of these aircraft. The key to high terminal capacity will lie ultimately in the use of multiple runways, and STOL runways should cost much less to build than conventional (CTOL) runways. The use of multiple STOL runways will enable these aircraft to save time and avoid the vortices of larger aircraft. STOL characteristics will also increase the utility and flexibility of operations into suburban airports and, perhaps ultimately, into downtown STOL-ports close to the population centers the aircraft are intended to serve. From the ATC standpoint, the performance characteristics of commuter-airline aircraft should pose no new types of control problems. In fact, the closer these aircraft get to true STOL characteristics, the easier they ore to control, provided they do not have to mix with fa~ t er types on the some approach path. STOL aircraft normally will hove on excess of engine power. This will give them a high rote of climb and get them clear of the paths of other aircraft immediately after takeoff. STOLs will be able to make instrument approaches with final approach paths as short as two miles coming down a six degree glide slope. Their relatively slow approach speed will permit the use of offset approaches, as shown in Figure l. It will also permit the use of short-radius turns to save on airspace and get the aircraft in and out of the terminal area· as quickly as possible.

Fig.

STOL approach paths offset for noise abatement purposes

Commuter Liners

VTOLs

At the other end of the airliner size range are the commuter liners, designed to connect the main airline hubs with smaller communities. Already, commuter. traffic accounts for 25 percent of the airline movements at Los Ange-

Further into the future will be the true VTOL aircraft, providing short-haul, city-center to city-center transportation, using their hovering capability to operate from rooftop Y-ports.

~l''i-.i~ .:d ! '••'

• : :.

·-----------··~.,~~

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Fig. 2

Converging multi -l~ne approach system

From the ATC standpoint, the ability to hover will give the VTOL one advantage over the STOL, as it will permit the simultaneous use of converging approach paths which stop short of the actual convergence point so that aircraft on different paths can still have lateral separation from each other. The converging approach concept may become applicable at V-ports and conventional airports as well, as it enables a multiple approach system to be concentrated into a minimum amount of airspace. The concept exploits the principle that a hovering capability eliminates the need for a missed approach path. About twelve years ago, we ran simulation tests of the concept shown in Figure 2. It proved to have an extremely high acceptance rate. Because vortex intensity is i n v e r s e I y proportional to the square of the airspeed, STOL and VTOL aircraft will generate quite powerful trailing vortices during low-speed flight. Overlying the whole future of VTOL development as well as citycenter STOL operations, is the aircraft noise problem. VTOLs and STOLs will depend on the use of relatively high power to maintain lift at low forward speeds. Unless significant advances can be made in the design of quiet engines, VTOL airliners may turn out to be economically impractical, and STOL aircraft may be restricted to socially acceptable sites a long way from city-centers.

SSTs At the opposite end of the speed scale will be the SST - another aircraft with a noise problem. It may have two noise problems; its gigantic engines may create a critical disturbance around airports and its shockwave at supersonic speeds will create a sonic boom swath about forty miles wide. Whether or not such aircraft will be allowed to fly supersonically over populated land areas will have a great deal to do with the number of SSTs which will be built. From the ATC standpoint, the SST will present a number of operational problems, most of which will fall outside the terminal areas. Within the terminal area will be the problem of its high fuel consumption while holding. This problem may force the ATC system into the use of a computerbased airport reservation system which will set up a definite time slot for each arrival, and then employ speed control in the last few hundred miles of the enroute area to absorb most of the delay enroute and, thus, avoid holding in the terminal area. The tremendous speed range of the SST can be exploited in such a system by controlling the time at which the aircroft makes its deceleration to subsonic speed. As shown in

for

VTOL

aircraft

Figure 3, a recent U.K. study indicates that it will be practical for the Concorde to absorb a known delay (to meet an assigned time slot) by advancing the "de-bang" (deceleration) time about one minute (20 miles) for each minute of delay,

T11,AC IW

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Fig.

Space-time graph showing procedure of advancing SST deceleration time in order ro absorb a known delay at destination.

The use of a landing reservation system of this type will require a master computer to keep track of the reservations; it will also require a much closer degree of coordination between the terminal arrival sector and the enroute sectors responsible for the control of the aircraft at the time the deceleration is mode. Most of this coordination con be accomplished by the computer. Because of the low span and high weight of delta-wing SSTs, the trailing vortices of such aircraft will be much

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Theoretical decay times of trailing vortices in calm air. Example:

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more intense than those of conventional aircraft, so the danger in on encounter will be proportionately larger. However, the more intense the vortices the foster they are propelled downward away from the flight path used by the generating aircraft, ond the faster the initial decoy rote. As shown in Figure 4, after about three minutes of dissipation time, SST vortices should be no stronger than those of other aircraft.

that great numbers of short-haul passengers will shift their personal habits and do their troveling between midnight ond 6:00 A.M. Instead, it is more probable that the ATC system will use this period to advantage for the bulk of doily freighter operations. Although better utilization probably will be mode of certain slack hours of the day, the real need is for greatly increasing the capacity -0f the system to handle traffic peaks. These peaks become most concentrated" in the terminal area.

Vortex Dissipation

The present methods of avoiding vortices may be summed up in six words: (l) Give them time; (2) Give them room! Before long, however, more active measures may be token to reduce the vortex hazard, at I e o s t over the runway its e If. The following measures ore now under study: l.

Determining m·ethods of introducing enough internal shear and turbulence by the generating aircraft (perhaps through retractable vortex generators on the aircraft) to hasten vortex break-up.

2. Using permanent arrays of ultrasonic or infrored detectors to worn controllers automatically whenever a vortex lies over the runway. 3. Using vortex fences of widely spaced bamboo poles, Christmas frees, or other frangible obstructions on the airport surface between parallel runways, to introduce shear and thus hasten the break-up of any drifting vortex before it reaches another runway. 4. Using surface-mounted air jets or vertical fans beside the runway to break up any vortices soon after generation.

Increased Demand Forecast

Air traffic demand is expected to double between 1968 and 1974, and to double ogoin between 1974 ond 1980. The ATC system is already badly overloaded in a number of key areas. What con be done to cope with the increased demond?

Spreading Schedules

A common suggestion is to make more use of the excess system capacity which still exists between midnight and 6:00 A.M. Reasons against this practice include the noise curfews which ore currently in effect at a number of key airports. More important is the reluctance of potential passengers to disturb their normal circadian cycle by traveling during the wee hours. The use of reduced fares as on incentive for passenger travel during the early morning period has been tried. However, night coaches have never been popular except on runs of 1000 miles or more. Most airline passengers travel on business; and the great majority of airline tickets ore for trips of less than 500 miles. For these passengers, most of the convenience and time saving would disappear if, for example, they hod to take o 02:00 departure for o 03:00 arrival ond then wait around till 09:00 to do any business at their destination. We do not expect 60

Flow Fundamentals

The flow of terminal area traffic is analogous in some respects to the flow of current in on electrical network. The outbound flow starts out in series (single file on o runway) and subsequently changes to parallel (different routes and altitudes). The inbound flow changes from parallel (different routes ond'altitudes) back into series (single file on the approach path and landing runway). Traffic density in any lone is analogous to current intensity; any restriction (such as interference with another lone) con reduce the rote of flow on that segment of the circuit. There ore just two ways of increasing traffic capacity; if you don't accomplish at least one of these, you can't augment the acceptance rote of the terminal area:

Increase the flow rote on any individual

(series) path.

2. Provide for the simultaneous operation (parallel) paths.

of additional

Airports

The flow of traffic in any given traffic lone is directly proportional to the overage ground speed of the vehicles and inversely proportional to the overage separation between them. As a result, for any given set of conditions, there is a fixed limit to the acceptance rote of a runway. You con raise this limit only by raising the overage approach speed (normally not desirable for safety reasons), or by reducing the average separation between aircraft. The present three-mile radar separation standard come about quite arbitrarily mony years ago - so arbitrarily, in foci, that it jumped 15 percent overnight when we went to nautical mile terminology (the-standard formerly was three statute miles). With today's aircraft and today's typical runway exits, the three-mile standard normally provides ample time to get the No. l arrival off the runway before the No. 2 arrival is committed to land. However, many of today's airports cannot utilize even the present minimum three-mile separation standard because of a lock of proper connecting taxiways. If your airport is in this category, the installation of adequate runway exits, runway entries, and taxiways could be the cheapest improvement you con make in upgrading traffic capacity. For night operations, these features need to be adequately marked and lighted. Improving the design of runway entry points is particularly important where the active runway must be shored by arrivals and departures. Too many runway en"tries still look like Figure 5. With this configuration, much time is lost in getting on aircraft into tokeoff position; to sandwich a tokeoff between two landings requires a very long interval between the landing aircraft.

A much more efficient proctice is to ploce the entry ot the end of the runwoy; ond to design it so !hot more thon one oircroft con hold cleor of the londing poth, but !hot ony selected oircroft con stort o rolling tokeoff independently of the others. Such on entry configurotion, os shown in Figure 6, increoses safety ond requires o minimum of runwoy occuponcy time by the deporlure. Thus, orrivols con sofely be spaced closer together with o consequent goin in acceptonce rote. With todoy's oircroft, ond present seporation stondords, it is difficult to exceed o total arrival ond departure rote of 42 IFR operations per hour on o single runway. If the demand is higher, o second runwoy is coiled for. When a seporote parallel or diverging runwoy is ovoilable for takeoffs, the landing runwoy can be used exclusively for landings. In this cose, the oirp6rt's total IFR arrival ond departure rote can be roised to around 64 operations per hour, with present aircroft ond present seporation standards. Whenever it is necessory for taxying oircraft to cross the runwoy in use, they create a runway occupancy problem which can interrupt the normal flow of tokeoffs and londings. This situation is chorocteristic of airports where two active runwoys ore locoted on the some side of the term inol building. The locotion ond geometry of the crossover has a large effect on its usobility. One improvement which can minimize the number of interruptions is to widen the crossover, os shown in Figure 7, so thot severol oircraft con cross together rother than one ot o time. If the IFR orrivol demond is greater thon 32 per hour, more than one instrument opprooch is needed. For simultoneous instrument opprooches, present criterio require thot parallel runwoys be spoced 5000 feet oport. However, this 5000 foot seporotion stondord originolly wos computed mothemoticolly for o sotisfoctory level of sofety which could be mointoined without controller intervention. Since it now appears that no duol approoch system would ever be commissioned without providing for adequate controller monitoring ond intervention, some reduction of the 5000 foot seporation standard probobly could be tolerated with sofety.

Fig.

Typical

Runway Entry

Fig.

High-speed

multi-access

runway

entry

I I

Approach Spacing

'~ ,

One factor which degrodes runwoy capacity in todoy's operations is the need to accommodate aircraft having widely different approach speeds. As shown in Figure 8, this tends to produce an excessively long approach interval whenever o slow aircraft follows o fast aircraft down the final opprooch path. In this case, the longer the common final approach path the longer the interval, ond the lower the landing occeptance rote· (although o shorp control team can sometimes use the gop to squeeze out another departure). One woy to reduce the size of the fast/slow interval is to have the pilot of the slow oircraft stoy ot o higher speed for as long os possible before final deceleration. This procedure is shown in Figure 9. All such adjustments need to be concluded early enough to give the pilot ample time to get the aircraft properly stabilized ot approach speed on the final approach poth before reaching the flare-out point. Another method of reducing- the effects of fost/slow opprooch intervols would take advantage of o wide difference in opproach speeds by simply turning the slow aircraft in close behind the fast one, on the knowledge thot the separation will increose all the woy to touchdown. This

11· u ,~ <: a: Fig.

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Crossover

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81.._------~----------------Fig. 8 Space-cime graph showing long approo.ch ln1crval which oormally occurs when a slow aircraft

follows a raster aircraft

down lhC approach pllh.

8.....____ Fig.

__.L

___________

___J

Reduction of approach interval which can be artamt."d by· keeping ~low a1rcrafl :n speed cornp,3t1bk wuh preccdinE: ,urcratc as long :is poss,bk.

procedure, shown in Figure 10, cannot be used in IFR conditions under today's rules. Probably the greatest objection to its use would be the risk of exposing the slow aircraft to the vortices so recently generated by the aircraft ahead. However, as the vortices are propelled downwards, this hazard could be minimized by making sure that the slow aircraft intercepted the common path no lower than the actual flight path which was used by the preceding aircraft. Looking ahead to the computer-based terminal approach system (a few of which may be operating in the late 1970's}, an alternate technique for reducing the effects of speed differentials would reshuffle the landing sequence many minutes ahead of time to minimize the number of last/slow pairs. The aircraft would be batched by speed class instead of being allowed to mix randomly in the usual first come, first served sequence. Such a procedure would require advance knowledge of all inbound traffic at least 30 minutes prior to arrival; it also would need the capability of generating and issuing individual speed control commands to individual aircraft up to 250 miles from their destination, in order to absorb most of the delay enroute. In order to attain more accurate spacing between aircraft feeding into the final approach path and, thus, maximize the acceptance rote for the separotion standards being used, the FAA has tried the use of approach spacing computers. Their results hove not been spectacular, however. Even though they showed a small gain in airport

acceptance rote, they required a very large and unnatural increase in controller workload. As a result, none are in oper_ation today. Simulation tests hove shown that comparable increases in traffic rates could be obtained by using simple spacing tables for the radar controller; however, such tables hove never been adopted by the FAA. It is expected, though, that provision of on approach spacing function will be o later addition to the terminal computer data processing program which will undoubtedly be in operation at a few major terminals before the end of the 1970's. Radar path stretching is presently the most frequently used procedure for spacing aircraft on approach. To operate additional runways and airports simultaneously will require additional approach and departure lanes through the terminal area. This, in turn, will tend to reduce the amount of airspace formerly available to any approach controller for path stretching purposes. Thus, to achieve high runway acceptance rates in the future, some ott-.er spocing procedure will hove to be used for the adjustment of approach intervals. Therefore, it is expected thot much more use will be mode of speed control in spacing arrivals at our busiest airports, within the next few years.

Departure Control

As airport traffic flow consists of departures as well as arrivals, the achievement of o high flow rate depends also on the ability to coordinate departures properly with arrivals. For example, o few seconds' delay in determining the position of on arrival can make the difference between a go and a no-go decision as for as a departure is concerned - and o few no-go decisions can greatly increase takeoff delays and airport congestion. One tool for improving arrival/departure coordination is a bright radar display for the tower cab to give the controller on instant check on the position of the nearest arrival before releasing the departure. The FAA is presently installing bright displays at a number of airports, with resulting improvements in capacity and safety. The same TV scan converter which feeds the bright display in the tower cab can also (with the help of microwave links} remote the radar display to nearby satellite airports, if desired. To handle departure traffic efficiently, any airport that requires a dual approach system will ol~o need at least one independent takeoff runway. Many major airports ultimately will be designed with multiple runways. One should remember, however, that additional runways, or even additional airports, will not necessarily increase total capacity unless they con be operated simultaneously. The use of an additional (but interfering) runway con actually reduce traffic capacity.

Noise Abatement

........____ Fig.

_,,_ _____________

10 Spucc·time graph showing Aircra(1 tl 1urning on final approach 2 NM hc:hind A1rcrar1 A In order w ob<ain 3 NM seporac1un a1 1ouchll,1wn.

___,

Aircraft noise is an important limiting factor in today's system. The resulting noise abatement procedures increase control workload and reduce airport capacity by forcing the use of inefficient combinations of runways as well as devious flight .tracks to ovoid noise-sensitive ·areas. The noise factor a lone could spell success or failure for the commercial use of STOL or VTOL aircraft. Efforts ta design quieter engines should be supported.

Steep Approaches

The use of steep approaches (glide slopes up to six degrees) is a possible way of reducing approach noise. NASA has been working on some radically new instrumentation which could improve steep approach operations by providing better guidance for the flareout before touchdown. If ultimately successful, the steep approach could not only reduce the noise problem but it could open up more low-altitude airspace for Y/STOL operations. It could also simplify ATC procedures and increase terminal area capacity at airports which ore spaced fairly closely longitudinally, by placing the glide slope of either airport just enough higher over the adjacent airport to permit completely independent IFR operation of each airport. This principle will become more important as the number of airports is increased.

Airspace Constraints

With conventional YOR/DME cockpit instrumentation, the pilot's navigational capability is limited essentially to flight along radial tracks to or from YOR/DME stations. As a result, our airway system has always been a spiderweb of single lanes. However, to resolve overtaking and headon traffic conflictions where the aircraft are using or crossing the same altitude level, multiple traffic lanes are needed. In such situations, where radar is available, the controller normally takes over the navigation of one or more of the conflicting aircraft by vectoring it off course, turning it on a parallel course, and then returning it to the initial course after separation hos been attained. In light traffic conditions this procedure con be handled satisfactorily. However, as traffic builds up and the number of traffic conflictions increase, this radar vectoring procedure becomes a formidable tgsk because of the extremely close attention and the large number of radio contacts it requires. Most of these contacts are time-critical because the pilot normally continues on his lost assigned vector heading until given a new one. The higher the workload per aircraft the lower the number of aircraft a controller (or a control sector) can handle simultaneously. This ultimately limits the handling capacity of the adjacent air route control sectors through which all the terminal area arrivals ond departures have to flow. In such a case, money spent for new runways and additional airports in this terminal area could be wasted if the air route sector workload is too high to accommodate the increased airport capacity. With increasing traffic, the trend in the post has been to divide and subdivide control sectors to keep the control load in each sector low enough for a sedor team to hond!e. Problems induced by this trend have included the need to increase the number of controllers, displays, and ATC communications channels accordingly. But, more critical to the operation of the system hos been the large increase in coordination workload caused by the larger number of people in the act and the additional transfers of control which hove to be made. Meanwhile, increased aircraft speeds have further compounded the coordination problems by shortening the time available for the controller's decision and communications functions. Thus, the continued reduction of sector size hos attacked the control workload problem from the wrong end. A

far more effective method is to reduce the total workload itself.

Area Navigatio.n

Control workload con be greatly reduced through the use of area navigation procedures. This is accomplished by small speciolpurpose airborne navigation computers with process the YOR/DME information in a form which enables the pilot to select and navigate not just radial courses to or from YOR facilities, but any selected track within coverage of the navigation facilities. When enough aircraft ore equipped with area coverage navigation devices, the controller con greatly reduce the potential number of aircraft conflictions by distributing his traffic on parallel traffic lanes which the pilots con then intercept and navigate themselves without radar vectoring assistance. • The controller's function will then revert from radar navigation to radar monitoring, a more simple procedure which requires for less radio communications, simpler decisions, and far less intense concentration on the individual aircraft. This, in turn, will alleviate another major limitation to ATC capacity today - the sheer amount of controller workload required to clear on aircraft through the system. A troublesome type of convergence problem occurs when a climbing or descending aircraft crosses another traffic lone. The basic difficulty is due to the uncertainty of predicting the altitude of the climbing or descending aircraft at the point of crossing. Unless radar con be used to monitor this situation closely, it usually is necessary for the controller to block a wide range of altitudes on the crossing route. An alternative to this waste of usable airspace is to interrupt the climb or descent and require that the oircraf maintain a constant specified altitude while crossing. This procedure con be highly restrictive when the aircraft hos to cross a series of routes - a characteristic problem around high-density terminal areas. The development of 3-D navigation, os recently demonstrated in the Eastern Air Lines STOL tests, appears to be on excellent solution to this problem. 3-0 navigation provides the capability for ATC to specify, and the pilot to follow, a prearranged "flight tube" in three dimensions with smooth transitions between successive specified woypoint/ altitude position. The use of suitably coded designators can simplify communications in the assignment of such routes by ATC. The ability of suitably equipped aircraft to navigate such assigned routes in three dimensions, without ATC assistance, would further reduce control workload.

Station-keeping (SK)

Possibly, as a spin-off from the current airline studies of collision avoidance systems or, perhaps, as an outgrowth of recent military development programs, it is expected that before the end of the l 970's some form of SK will be utilized in certain portions of the ATC system, to further reduce the control workload. ATC will assign to each aircraft a departure track which con be navigated with a high degree of accuracy Adjacent tracks will be as free of mutual interference as possible and traffic will be segregated by speed class, where necessary. Crossing tracks will be separated vertically at crossing points; head-on conflictions will be 63

eliminated by the use of laterally separated one-way tracks for opposite direction traffic flow. In this organized environment, aircraft should be able to maintain their own separation in the same s t re o m through the use of on airborne SK display. In climbing and cruising flight, the prior segregation of aircraft by speed classes in the various lanes will minimize the number of overtake situations or the need for speed adjustments. With SK, arriving aircraft may also be able to control their own declerotion so as to close up the spacing to the desired interval behind a designated ciirc·roft on the approach path.

and second-time-around effects. The resulting phenomena show up as false targets, false code readouts, target splitting, target cancellation, and poor resolution. Technical solutions are constantly being sought to alleviate some of these problems, most of which ore inherent in the SSR concept. Meanwhile, the number of transponderequipped aircraft is increasing al a rote of 1500 per month; currently, 22,000 U.S. general aviation aircraft are so equipped. Thus, we predict th·at terminal controllers will be living with the inherent problems of the SSR system throughout the 1970's. It is expected that ultimately this system will have to be replaced by a two-way data link based on selective interrogation.

ATC Displays

During the last ten years, terminal areas hove grown steadily in size as the jet holding areas hove been pushed higher and farther away from the airport. This trend has forced an increase in the range of terminal radars in order to cover the descent and approach paths. This, in turn, is forcing the replacement of the old ten-inch scopes with scopes sixteen or more inches in diameter, in order to provide an acceptable scale for the radar picture. Lorge scale scan-converted TV projected displays hove recently been tried out in the New York Common IFR Room. The general idea was to simplify coordination between controllers by producing o PPI display which was large enough for all to see simultaneously. Controllers agreed that the theoter-type projection is great for impressing visiting dignitaries. Unfortunately, expanding the radar picture also expands the imperfections and jitter of its TV roster, making it rather tiring to watch at close range. As traffic builds up, a common display, in carrying all the data for several controllers, tends to become extremely cluttered. Where SSR is in use, individual displays hove the advantage over common displays, of allowing each controller to select his own data of interest. Therefore, we do not look for o trend toward large-scale projected ATC displays within the next few years. Early ATC radars picked up precipitation areas which often blotted out large portions of the PPI display. The development of improved MTI and circular polarization circuitry was able to eliminate a large portion of this clutter from the primary radar picture. However, after a number of critical incidents in which aircraft were vectored inadvertently through extremely turbulent precipitation areas not shown on the PPI, it was determined that controllers definitely needed to know the location of such areas. An interesting compromise was the development of weather contouring, a radar circuit which draws on outline around precipitation areas at any selected threshold of echo intensity, but minimizes radar clutter by leaving the inside of the outlined areas blank, so that other radar data con still be seen. It is expected that this feature will be in wide implementation during the 1970's. During the post ten years, the implementation of SSR has provided an enormous improvement in traffic control through its ability to identify radar targets quickly, and so track such targets positively through all types of weather conditions. SSR is a good system - as long as air traffic density remains light and ground interrogators are spaced many miles apart. But, as the density of transponder-equipped traffic and the concentration of ground interrogators increase, more and more of the basic weaknesses of the system become apparent. These weaknesses include a susceptibility to garbling, reflections, shadowing, capture,

In this paper we hove tried to forecast tomorrow's terminal environment and to outline some of the possibilities for lifting the constraints which limit capacity of terminal areas today. Looking ahead ten years, we need to ask ourselves what will really be needed to handle four times the present traffic flow. Our personal opinion is that we must move toward o system in which controllers con monitor s t re o m s of traffic flow in preassigned independent lanes, instead of having to hand-carry each individual target through congested portions of the ATC system Tomorrow's terminal areas wil: be characterized by multi-runway, multi-airport operations. In turn, this will require high-accuracy area coverage navigation to obtain the closely-spaced parallel lanes necessary for simultaneous operation. For the concept to work, the navigation system must be good enough and the pilots proficient enough that controllers can afford to trust each pilot to follow his assigned track. Only then will controllers be able to monitor effectively a preplanned traffic flow and control by exception only, in o much more efficient ATC system than we hove today.

First published

Data Processing

In radar control, there ore three items of information - target position, identity, and altitude - that take precedence over all the rest. There is a need for the controller to be able to associate these three items quickly and positively for any aircraft under his control and to quickly transfer this data to the adjacent control sector. The provision of these functions has been the goal of vast development programs for automated radar displays. During the past decade, the FAA hos spent over $ 100,000,000 on flight data processing and the automation of ATC radar displays. Although all FAA Centers now have some form of automatic data processing, only two FAA facilities hove alphanumeric (togged-target) displays; these also happen to be the only two FAA facilities which con make use of Mode C altitude readouts from SSR transponders. A basic problem of automated displays hos been the addition of o new and rather unnatural kind of workload - the induced workload - caused by the need for the controller lo manipulate the controls of the computer/display. Until the omount of controller workload saved by the equipment is significantly greater than the induced workload, the automation equipment will hove very little effect on increasing system capacity.

Conclusion

in the October

1969 issue of THE CONTROLLER.

Wind Shear Problems in Terminal Operations

by Tirey K. Vickers

very lo_;,,,but a strong temperature inversion is present. Such conditions are most likely to occur under clear skies, during the late evening .and early morning hours. The temperature inversion (cool air at the surface, with warmer air aloft) results in a stable, stratified air mass, with relatively little tendency for mixing or turbulence. The result is a smooth, decoupled, laminar flow, in which the air aloft can be moving at a speed considerably greater, and sometimes in a completely different direction, than the air at the surface.

Introduction Long suspected as a contributing factor in a number of runway undershoot and overshoot accidents, the subject of wind shear is receiving increasing attention these days. The concern this time is centered mainly on the possible increase in wind shear hazards which may result from the proposed reduction of landing minima to !CAO Category 2 limits, at certain major airports.

Definition

The stronger the inversion, the greater the wind shear can be, before the smooth, streamlined, laminar flow breaks into turbulence and mixing begins. In extreme cases, the shear con reach 10 knots per 100 feet of height, before appreciable mixing takes place.Once mixing starts, however, the wind shear tends to disappear, as the turbulence speeds up the airflow at the lower levels.-The temperature inversion also tends to disappear as the air from the various temperature layers gets churned up together.

Wind shear - or more specifically, vertical wind shear - is the change or difference of the horizontal wind, with height. It is sometimes expressed as a vector change. It may also be resolved into its longitudinal and lateral components relative to a specific flight track or runway. Each component of the shear is expressed in terms of knots per 100 feet of height.

The tall-tower data shows also that a considerable wind shear can develop when the surface wind is over 17 knots and an overcast is present. When an air mass is moving across country, the airflow near the surface is slowed by skin friction and pressure drag. As a rnsult, the greatest amount of shear takes place at the lower levels, with progressively less shear in the levels above. Typical example: if the average shear from the surface up to 300 feet is 5 knots per 100 feet, the average shear from the surface up to 150 feet may be 8 knots per 100 feet.

Characteristics Although wind shear may occur at any altitude level, the zone of concern these days is that portion of the atmosphere which is most critical to the landing operation the layer from the ground up to a height of about 300 feet. During the past few years, considerable knowledge hos been gained about the characteristics of wind shear in this zone. This hos been done by the simultaneous recording of wind, temperature, and humidity data at a number of different heights, on toll towers at Upton, Long Island, Washington, D. C., Dallas, Texas, Philadelphia, Pennsylvania, and Lopik, Netherlands. In general, good agreement hos been found between the data collected at the different sites.

Effects on Aircraft Longitudinal (Headwind or Tailwind) Component. During steady flight, lift equals weight and drag equals thrust. If the aircraft encounters a sudden change in the longi-

The tall-tower data shows that strong wind shears often occur under conditions when the surface wind is

L L INCF?£ASIN0 HEADWIND

OR IN STE:ADYFLIGHT, LlfT

= W€!C,HT,

L D£CREASIN6 HEADWIND OR INCREASINC, TAILWINO DECREASES AIRSPEED, WI-IICH DECREASES LIFT AND ORM,.

DECREASIIVGTAILWINO INCREASES AIRSPEED, WI-I/CH INC/?f!M"ES 1.IFT AND DRA<;,

DRAG= THRUST. A

o--------

0 UNBALANCED FORCES PROl)()CE 1/CCELERAT/Olv ·:4··:

UNBIILANCED FORCES PRODVCC ACCE'LERAT/ON "A": AIRCRAFT TENDS TO RISE AND AIRSPUO n1ve>s ro t>fC~eliSE" GRAOUALI '( UNTIL FORCES ARE A~t.lN IN EQI/JLl8f?/l.ll\1.

w Figure l

AIRCRAFT TENC>STO SIIVI( AND AIRSPEoD TENDS TO INCREASE" GRAOUAlL '( UNTIL FORC.S ARF: AGAIN IN fQVILJBRIVM.

Effects of airspeed changes caused by wind shear

I"'

Figure 2

Increasing headwind component

tudinal wind component, the inertia of the aircraft prevents it from reverting instantly to a new groundspeed. Instead, the immediate effect on the aircraft is a sudden change in airspeed. This changes the lift and drag forces, as shown in Fig. 2. The airplane then accelerates in the direction of the stronger forces, until equilibrium is achieved with lift again equal to weight and drag again equal to thrust, at the new groundspeed. Some typical examples are described in the following paragraphs. Figure 2 shows the effect of an increasing headwind component on the final approach path. This type of wind shear can occur at the base of a low overcast. The immediate effect is an increased airspeed. The resulti,ng increase in lift tends to raise the aircraft above the intended path, thus increasing the possibility of on overshoot. If the runway is short, the pilot's only method of completing the landing is to duck below the normal glide slope and force the aircraft on to the runway. The some effect shown in Figure 2 can occur if the aircraft encounters a decreasing tailwind component on the final approach. path. This condition con occur during a temperature inversion, when the surface wind is almost calm. Here again the immediate effect of the shear is on increased airsf!)eed, with the increased lift raising the aircraft above the intended path, increasing the possibility of on overshoot. This particular condition has been responsible for many go-arounds (missed VFR approaches) on calm evenings, at airports with short runways. Figure 3 shows the effect of a decreasing headwind component on the final approach path. The immediate

WIND PRO/:ILI:'

effect is a decrease in airspeed. With the resulting decrease in lift, the aircraft tends to sink below the intended path. If the wind profile is as shown in Figure 3, the aircraft is sinking into on area where the airspeed is even lower, thus further accentuating the problem. Unless adequate corrective action is taken to increase the airspeed and reduce the sink rote, the result can be a hard landing, an undershoot, or even (in extreme coses) a complete stall. The pilot who anticipates such a shear condition con reduce the hazards listed above, by using an approach speed somewhat higher than normal, to allow on adequate margin for the airspeed losses which will occur. Figure 4 illustrates the case of an underpowered or heavily loaded aircraft climbing into an area with a decreasing headwind or on increasing tailwind. The immediate effect is a decreasing airspeed, which reduces the lift and causes the aircraft to sink below its initial climb path. The problem now is to accelerate the aircraft again to a safe or optimum climb speed. However, if no reserve engine power is available, the pilot's only solution is to make a further sacrifice in climb gradient, levelling off to pick up speed and then climbing out at a somewhat higher airspeed and lower gradient than was used initolly. The main hazard in this situation is the possibility that the total loss in climb gradient may exceed the safety margin initially allowed for clearing obstacles on the climbout path. One other operational problem should be mentioned in connection with the longitudinal component of wind shear. This is the ATC problem of spacing aircraft on the final approach path in order to maintain a maximum landing rote, but without violating the existing separation

-----,--1

\ \

'' Figure 3

Decreasing headwind component

...

WINO PROF/Lf;'

D€CREASIN6 H~f.,DW/Nf)I

INCRFA,ING TAILWIND

1-I-. __

CL.I~

- ti

-----I

-------k Figure 4

Climb wilh increasing loilwind

_.'

I I◄ or decreasing headwind component

standard. As shown in Figure 5, wind shear adds another variable to the function of determining how much separation to allow between successive aircraft at the start of the common path. As the exact point at which each aircraft may encounter the shear line cannot always be predicted, some additional separation should be allowed initially in order to insure adequate spacing throughout the common path.

Time becomes the critical factor here because of the inevitable time logs in pilot and aircraft response. When the pilat emerges from the overcast, he must see where he is, decide what to do, and actuate the controls accordingly. Under ideal conditions, this see/think/act cycle re-

Lateral (Crosswind) Component. One of the most disturbing factors in the problem of completing approaches in extremely low ceiling/visibility conditions is a changing crosswind on the final approach path. As shown in Figure 6 a lateral shear -requires the aircraft to change heading, in order to stay on the centerline of the approach course. In visual flight, the pilot con quickly detect and compensate for displacements caused by changes in the crosswind component. When flying on instruments, however, greater displacement may occur before detection, requiring larger corrections and more time to get bock on course.

A typico) low-ceiling situation involves a shallow layer of cold oi•r on the surface, being overrun by a worm moist air moss with a different wind direction and velocity. The greater the direction and velocity differences between the two layers, the greater the horizontal accelerations which will be experienced by the aircraft when it crosses the shear line at the bottom of the overcast. The lower the ceiling, the later in the approach the shear is encountered - and the less time ·is available in which to correct the approach path.

Figure 5

Spece-time plot showing effect of wind shear (increasing headwind) on final approach spacing

RUNWAY TRANSITION THROU6H SHEAR ZONI:

WIND IN UPPER Figure 6

UIYl:R

WINO IN

LOW£R

LAYER

Heading change required to compensate for lateral component of wind shear

quires about 11/, seconds. At a typical ground speed of 130 knots, the aircraft travels 330 feet (l 00 meters) during this period. But the situation still isn't necessarily squared away, as the aircraft will require additional time to respond to the controls and to fly through the desired moneuver. Figure 7 shows how much time is required to complete a runway-alignment-correction maneuver, from various offside distances. This data was obtained in flight tests by the Royal Aircraft Establishment several years ago, using a number of widely-different types of transport aircraft flown by groups of civil and military pilots. Surprisingly, the time required to correct a given displacement was nearly the some for all the types of aircraft tested, and for the two groups of pilots.

20 18 16 V)

~ 14

1.:

12 < ..... l.jJ

In the past, abnormally strong wind shear hos sometimes been suggested as o possible factor in specific landing accidents. So for as we know, however, wind shear has never been cited officially as a contributing cause of on accident. Perhaps the most important reason why the blame was usually placed elsewhere was that no official data wos available lo prove that the shear condition was present at the time. This data gap need not continue, however. Airport control towers at some major airports presently extend up into the 150-to-200-foot zone. At such locations it should be a simple matter to install on anemometer on a roof most, to provide upper-level wind data for comparison with the surface wind. For airports which do not have a structure extending into the desired height zone, it might be possible to install anemometers at appropriate heights on an existing radio or television tower within a few miles of the airport. Except during local thunderstorms or frontal passages, such on arrangement probably could provide sufficiently representt.tive data for determining the wind shear conditions which would affect airport operations in the terminal area.

For those readers who are interested in digging deeper into the details of this fascinating subject, we would recommend the reports listed below.

Lf'6£ND W- WINOS LEYfL AT THIS POINT

i::: ~ 6 Q: tic 0

It is believed that the reporting of wind shear information lo pilots would be useful in reducing some of the operational problems, as the pilot would then be better prepared for the situation ahead.

Bibliography

~ C)

Suggestions

1. I. A. Singer, G. S. Raynor, "Analysis of Meteorological

There ore those who say that for approach operations down to the proposed ICAO Category 2 minima (100-foot ceiling, 1300-foot RVR) a human pilot should not be expected to provide the last-moment flight path corrections which have heretofore been such an important part of every instrument approach procedure; instead, they say, this function should be turned over to the automatic landing system. This philosophy would imply that the automatic pilot should be capable of compensating for any lastmoment wind shear which con occur. Some previous designs of automatic landing systems have not been able to cope adequately with extreme shear conditions. Further study is being given to this problem.

Tower Data April 1950 - Morch 1952 Brookhaven Notional Laboratory", ASTIA No. AD 133806, June 1957. 2. D. L. Markusen, "lnvestig,ition of Wind Shear at Low Altitudes", RTCA 31-63/DO 118, Morch 1963. 3. C. F. Roberts, "A Preliminary Analysis of Some Observations of Wind Shear in the Lowest 100 Feet of the Atmosphere, for Application to the Problem of the Control of Aircraft on Approach", U.S. Dept. of Commerce, Weather Bureau, Sept. 1964. 4. Capt. C. M. Ramsey, "Vertical Wind Shear", Memorandum No. 10, ICAO All-Weather Operations Panel, June 1964. 5. R. C. Gerber, "Status Report, ALPA All-Weather Flying Committee", Air Line Pilots Association, Chicago, October, 1964. 6. "Pilot Reports of Wind Shear experienced on Take-off or Landing", U. K. Ministry of Aviation, Civil Aviation Information Circular 84/1965, August 1965. 7. D. H. Rieger, "Some Airline Schedule Reliability Problems", Air Line Pilot's Association, Washington D. C., October 1965. 8. G. B. Litchford, "The 100-Foot Barrier", Astronautics and Aeronautics Magazine, July 1964. 9. E. S. Colvert & J. W. Sporke, "The Effect on Weather Minima of Approach Speed, Cockpit Cut-Off Angle and Type of Approach Coupler for a Given Landing Success Rote and Level of Safety", Aeronautical Research Council Current Papers, CP No. 378, 1958.

First published in t·he April 1966 issue of THE CONTROLLER.

-{)

2 0

100

D - !NIT/Ill Fig. 7

300 200 DISPLACEMENT,

Correction time required manoeuver

for

400 IN FEE

SOO

runway-alignment-correction

Automation

The Improvement of Wet-Runway Operation by Tirey K. Vickers

One factor which can limit the acceptance rate of an airport is the time required for a landing aircraft to decelerate after touchdown to a speed at which it can safely make a turn and exit from the active runway. This factor, which is sensitive to runway surface conditions, becomes particularly important at locations where takeoffs and landings must share the same runway. When runways are wet, rollout distances and runway occupancy times tend to increase. One reason is that the presence of water on the runway can lead to a condition known as hydroplaning, which can greatly degrade the braking capability and the directional control of the aircraft during the landing rollout. There are three different types of hydroplaning, as described below:

Viscous Hydroplaning (thin-film lubrication) can occur if the runway surface has been polished smooth by repeated landings and the coincidental buildup of repeated layers of burned rubber, as well as other contaminants such as soot and oil. In this case a very thin film of water, less than .001 inch deep, can keep the tire from contacting the runway. Instead, the tire skims along the top of this film, where it cannot contribute either to the braking action or the directional control or stability of the aircraft. This type of hydroplaning can occur down to relatively low taxi speeds. Reverted Rubber Hydroplaning can occur during a prolonged skid with a locked wheel. The resulting friction generates heat at the point where the tire contacts the runway. When the rubber reaches a temperature between 400 and 600 degrees F, it reverts back to its uncured (sticky) state. Until recently it was believed that if there was water on the runway, the reverted rubber could form a seal which delayed exit of the water from the tire footprint area and that the high temperature whitin this pocket changed the water instantly to high-pressure steam which lifted the tire a microscopic distance off the runway surface.

shown in Figure 1. Figure 2 shows how this wedge of water reduces the footprint (runway contact) area of the tire, thereby degrading the braking action and increasing the stopping distance of the aircraft. Test data incidates that the minimum dynamic hydroplaning speed of conventional aircraft tires, in knots, is about 8.6 times the square root of the tire pressure, in pounds per square inch. For example, a typical executive jet aircraft uses 135 pounds pressure in the main tires and 45 pounds pressure in the nose wheel tire. The calculated hydroplaning speed for the main tires is approximately 100 knots. However, if a high-speed turnoff is anticipated, it should be noted that the calculated hydroplaning speed of the nose wheel tire is slightly less than 60 knots. If the runway is wet, a tire can hydroplane at any speed above the value calculated above, which simply represents the lowest speed at which dynamic hydroplaning can start. Once it has started, however, the condition can be sustained on down to a speed somewhat lower than the minimum starting speed. Hydroplaning has been the direct cause of a number of accidents in which aircraft have run off the end of the runway. Although there have been relatively few fatalities from this cause, aircraft damage has been extensive. In instrument weather conditions an overrun of this type often wipes out the localizer antenna, which in turn may close the airport for a long period. During hydroplaning, the reduced traction reduces the pilot's directional control of the aircraft. As a result, a hydroplaning aircraft tends to weathercock into any appreciable crosswind, skidding sidewise down the runway - or drifting off the downwind side into the mud, a situation which may require closing of the runway to other traffic, until the unfortunate aircraft can be retrieved. Erecting barriers or lengthening runways is not neces-

Recent research at the University of Michigan provides a completely different explanation for reverted rubber hydroplaning: If the runway is wet, the combination of the wet film on the runway with the reverted rubber forms a highly flexible bearing surface which changes shape to "flow" over and around irregularities in the runway surface, with very little friction. Even if the reverted rubber subsequently is cooled down to the ambient temperature, the tire may continue to skid smoothly, down to a speed of five to ten knots. A tire can produce a cornering or side force far lateral stability or steering only when it is rotating; thus a skid with a locked wheel reduces its cornering or steering capability to zero. Dynamic Hydroplaning can occur when there is a layer of standing water on the runway surface. If the water cannot get out of the way of the speeding tire fast enough, it farms a liquid wedge which lifts the tire off the surface, as

-o

D• DIRECTION OF AIRCRAFT; W• WATER LAYER ON RUNWAY; L • LIFT FORCE FROM WEDGE OF WATER (SHADED AREA I ; H • HEIGHT OF TIRE OFF SURFACE (NOTE: HEIGHT EXAGGERATED FOR CLARITY)

Figure 1 Dynamic hydroplaning

sarily the answer to the wet-runway problem, as mast hydroplaning incidents occur off the side of the runway. It would appear more rewarding to develop a means of preventing hydroplaning, by obtaining more positive drainage of surface water from the pavement itself. Most present runways are designed with a transverse grade {slope) up to l ½ per cent, to drain surface water to the sides, as shown in Figure 3. To obtain faster drainage, FAA engineers are now considering the possibility of increasing the maximum transverse grade to two per cent. One of the most effective means of reducing hydroplaning problems is the use of runway grooving. The grooving consists of small slots cut across the runway to facilitate water drainage and to improve tire traction. First tried in England in 1956, the concept has been implemented at several major U.S. airports with excellent results. In the United States, most of the research on this subject has been conducted or sponsored by NASA. Figure 4 shows NASA's experimental grooved runway at Wallops Island, Virginia. Here a number of different groove patterns were tested under damp, flooded, and slushy conditions, and over a speed ronge up to slightly over l 00 knots. Figure 5 shows a typical test. It has been found that transverse grooving tends to reduce the amount of time in which any type of hydroplan-

ing can occur, as the grooving expedites drainage of the surface water off the runway surface. This effect is quite noticeable in comparing the amount of standing water remaining on grooved and ungrooved portions of a runway after a shower; the grooved portions tend to dry off immediately. This greatly reduces the amount of spray thrown up by aircraft during takeoff or landing. Runway grooving can prevent viscous hydroplaning by providing a continuous series of edges on which the tire can grip, in order to secure positive traction, even though the runway surface may be covered by a thin film of water as well as other contaminants. Grooving con also reduce reverted rubber hydroplaning, as a locked wheel condition is less likely to occur on a runway which has a uniformly hig coefficient of friction. The provision of additional gripping surfaces also tends to start the tire rolling again. Runway grooving can eliminate dynamic hydroplaning by providing multiple escape paths for the water beneath the tire, thus preventing buildup of the type of high-pressure wedge shown in Figure l. NASA tests made with a Convair 990 jet transport on grooved portions of a flooded runway, with ½ inch of water above the grooves, and using smooth tires, indicate that braking is identical to dry-runway conditions at runway speeds below l 06 knots. These results hove been con-

Figure 2 Photos looking up through a glass runway, showing the footprint area of a 20 x 4.4 aircraft tire, under partial and total hydroplaning conditions. Vertical load = 500 pounds, tire pressure 30 pounds per

square inch, water depth = ½ inch. Tire motion is from left to right; tufts show direction of water displacement. Speeds in knots: A = 28, B = 56, C = 71, D = 88. - NASA Photos

firmed by aircraft users at Washington Notional Airport, Kansas City Municipal Airport, J. F. Kennedy Airport, and Chicago Midway Airport. Comparative tests made on grooved and ungrooved runway surfaces have shown that the grooving tends to produce more uniform friction (less variation in the friction coefficient) than ungrooved surfaces. This is particularly important in the operation of heavy aircraft, since most of them utilize anti-skid systems based on the principle of reducing the brake pressure (to prevent wheel lock) whenever the system senses an incipient skid condition. The more uniform friction characteristic of the grooved surface allows the braking system to maintain a constant braking torque, as opposed to the intermittent torque required for a smooth stop on an ungrooved wet runway. The ability to use constant torque tends further to reduce the stopping distance. The greater the cross-sectional area of the groove, the more water it can hold before overflowing. If the groove is too wide, however, it will trap stones and small debris,

Figure 3 Cross-section of (slope) G for drainage. (Drawing not to scale)

fypicol

runway

showing

transverse

grade

erecting a housekeeping problem. The narrower the spacing between adjacent grooves, the faster the runway surface will be dried off. If the spacing between grooves is too narrow, however, its top surface is more subject to shear damage. Of the various groove configurations tested so for, the ¼ inch wide and ¼ inch deep, spaced one inch between centers, as shown in Figure 6. Figure 7 compares various operational grooving installations. The cost of grooving existing runways varies over a

Figure 4 Experimental grooved runway at Wallops Island, Virginia, USA. -NASA Photo

Figure 5 Surf's up!

wide range. One variable is the total volume of material which hos to be removed. Another variable relates to the restrictions imposed by aircraft operations. For example, if the work hos to be done during night or early morning hours, in order to ovoid traffic peaks, the lobar cost is likely to be considerably higher than if the work could be done during regular daytime hours. Similarly, if the grooving equipment hos to be pulled off the runway periodically, in order to permit resumption of aircraft operations, the additional lobor and waiting time will increase the cost.

NASA Photo

Runway 18-36 at Washington National Airport was the first operational runway to be grooved by the FAA. The job required thirty-five days of work during the off-peak hours between 2300 and 0700 local time. Diamond sow machines were used; each machine cut thirteen grooves simultaneously, into the asphalt surface, at a cost of about !j; 0.09 per square foot. Although the number of aircraft operations which hove been made on this grooved runways is now approaching one million, so far there has been surprisingly little de-

Figure 6 Maximum-traction grooving, spacing 1" (25 mm), width ¼" (6 mm), depth ¼" (6 mm). - NASA Photo

terioration of the groove pattern. In the touchdown zones, some of the patterns have been distorted slightly, but this has been due to shifting between the upper and lower layers of the pavement, rather than to movement of the grooved portion relative to the upper layer. One problem which is involved in cutting grooves into on existing runway is the need to flish out and dispose of the large amount of abrasive waste which is produced by this process. Otherwise, this material can create a dust (or slurry) problem which can be damaging to jet engines and aircraft wheel-well components. One of the newest types of grooving machines has an automatic clean-up feature, to take core of this problem. Ultimately, it may be desirable to develop a means of casting or rolling the grooves into new runway surfaces instead of having to grind them out ot some later date. High-speed photographs of tires encountering grooves indicate that under heavy loads the tire tread pushes down into the grooves. This allows the tire to get a bite on the runway, for traciion purposes; however, it also gives the runway a chance to bite back. As a result, some tires which have been subjected to prolonged operation on grooved runways have shown a pattern of tiny transverse or chevron-shaped cuts on the tread. It is conceivable that certain grooved configurations will result in less rire wear than others. Additional research moy be desirable, particularly for those airports where runway length is not a problem, to determine the optimum compromise between tire traction and tire wear. Meanwhile, it appears to be the general consensus of the U.S. airline operators that a slight amount of additional tire wear is more than justified by the protection which the grooving provides against a catastrophic hydroplaning accident. Because an aircraft tire may be recapped ten or more times during its useful life, there was some initial concern as to whether the vibrations produced in riding over the grooves would create a fatigue problem with the tire cords. The preliminary data from recent tests indicate that there is no appreciable difference between the stresses produced in the cords of tires running on grooved and ungrooved runways. Initially, some of the aircraft manufacturers were concerned about another aspect of runway grooving whether the touchdown contact of a non-rotating wheel with a grooved runway during the landing would produce o much greater spin-up drag load on the landing gear, than if the aircraft were landing on an ungrooved runway (Spin-up drag is the rearward force transmitted to the axle when the wheel touches down and accelerates to the landing speed). The preliminary data from a recent test program indicates that the difference in the runway surface (grooved or ungrooved) makes no appreciable difference in the spin-up drag. The overall effect of runway grooving is to permit a wet runway to approach the broking capability of a dry runway. Up until the present time, the FAA has mode no allowance for the presence of grooved runways, as far as runway length is concerned. The present regulations require a runway length adequate to allow a full-stop landing (based on the aircraft type certification tests) within 60 percent of the effective length of the runway. Beginning in 1966, Federal Air Regulation 121.195 (d) hos required an additional 15 percent runway lenght for operation into wet or slippery runways. There is now con-

AIRPORT Woshin;tonNationol Kansas City Municipal Charleston W.Va. Municipal and S.ale AFB

Pitch o, Spocin9

Groove Width

Groove Depth

Angle of Cut

,·

1/8"

90"

1/8"

1/4"

90"

1/4'

90"

,·

Chic090 Midway

1-114•

114•

1/4"

90"

JFKennedy International

I ·318"

318"

1/8" 1

4:1"

r-3/8"

1·318"

Figure 7 Groove patterns in use ot various US airports.

siderable evidence that the original 15 percent allowance was not realistic,. and should be increased for turbojet transport aircraft landing on smooth wet runways. If this increase is made, it is expected that it will apply to ungrooved runways, but not to adequately grooved runways under wet conditions. Because the elimination of hydroplaning restores the ability of aircraft tires to provide side forces for lateral stability and steering, it is hoped that the installation of grooving will permit the authorized maximum crosswind component for wet runways to be increased to the limit allowed for the some runways under dry conditions. Runway grooving offers definite advantages from the standpoint of airport capacity. With ungrooved runways, braking distances are increased significantly in wet weather. High speed runway exits which are ideally placed for dry weather operation may be 1,000 feet or more too close to the touchdown point to be used by the same aircraft in wet weather, because the reduced braking action. In addition, if pilots anticipate hydroplaning conditions, they will not start any turnoff until well below their minimum hydroplaning speed. All this naturally increases runway occupancy time, so air traffic controllers have to increase approach intervals accordingly, when such problems are anticipated. This reduces airport capacity and tends to increase traffic delays. In surveys made at Washington, Kansas City and Kennedy Airports, the majority of the controllers who were polled, definitely felt that runway grooving aided most pilots in controlling their landing run, and that the turnoff point from a wet grooved runway was identical in most instances to that for dry operations on the same runway. Controllers reported that this definitely improved runway traffic management, and increased the acceptance rote over that possible with the original ungrooved runway in wet conditions. First published in the January 1970 issue of THE CONTROLLER.

by Tirey K. Vickers

Living with Vortices

On this article from o letter of the author to the editor " ... The vortex article is o case in point as this problem is worse in the USA today (than in other countries) because of our high traffic density. But it will show up in other countries soon with the constant increase in traffic density there. And I suppose my motive was portly humonitorion, as I felt that if my article could prevent o single accident somewhere, it would be worth the effort ... "

Introduction

Figure 1

Insecticide discharged from underwing nozzles rolls up into vortices generated by this converted BT-13 (left) and Ford trimotor (right), during forest spray operations in western United States.

U.S. FORESTSERVICEPHOTOS

Streaming across o clear blue sky in sharply-outlined pristine whiteness, o jet contrail is a lovely sight. Yet it is also o reminder of o growing hazard in aviation - the menace of trailing vortices. Every winged aircraft which ever flew hos generated vortices in its woke. The growing danger of this inherent characteristic is due mostly to the increased weight of some modern aircraft. The long jump from the 24,000pound DC-3 to the 300,000 pound intercontinental jet hos brought o large increase in the amount of energy which con be wrapped up in the resulting wakes. Usually invisible, these wakes con be encountered by other aircraft without warning. Because of the growing number of accidents which hove been attributed to such encounters during the past few years, on increasing amount of research is being focussed on this problem. The purpose of this article is to summarize the most important facts which hove been learned about the generation and behovior of trailing vortices. It is hoped that this knowledge will give airport traffic controllers and pilots o better understanding of vortex phenomena, and thereby enable them to ovoid the most serious effects, in their doily operations.

A little Theory In the expanding jargon of space technology, the term "spin-off" is now used as a synonym for the more prosaic term "by-product". Trailing vortices are the inevitable byproduct or spin-off of the lift generation process. The new word is particularly apt in this case, as the vortices literally spin off the wingtips continuously in flight, as shown in Fig. 1. What causes this spin-off? To gain a better feel for the factors involved, it may be desirable to review some of the fundamentals of the lift generation process. Every winged aircraft must deflect a continuous stream of air downwards, in order to generate the lift necessary to sustain itself in flight. This process is diagrammed in Fig. 2, where we see a wing of span b generating a lift L by imparting to the air a downward velocity v. The amount of air which the wing can deflect is equivalent to the amount which can pass through an imaginary circle with the span b as the diameter. The area of this circle is :reb 2/4. The volume of air which the wing con deflect in one second is equivalent to a cylinder of diameter b and lengh V, where V is the airspeed of the aircraft, expressed in distance per second. The volume of air in this cylinder is (:reb 2/4) V. The mass of air in this volume is (n b2/4) V p, where p is the air density.

The lift L is equal to the mass times the downwash velocity, so L = (:reb2/4) V p v. Let's call this Equation 1. The amount of lift being generated at any given moment can also be expressed as the gross weight of the aircraft W, times the load factor g, or L = W g. In straight and level flight, g = 1, so the lift is equal to the weight of the aircraft. In a sharp pullup or a steep bank, the load factor (and consequently the lift) is increased appreciably. For example, in a 60-degree banked turn, g = 2 and the lift is doubled. Substituting W g in place of L in Equation 1, we obtain: W g = (:reb 2/4) V p v. Transposing this equation, we get: Wg V

Let's call this Equation 2. It is a very useful little package, as it can show us how a change in any of the basic factors will affect the downwash velocity, and ultimately the rotational velocity of the vortices.

FI ight Path

Figure 2

Lift Generation.

Factors affecting Vortex Intensity Going bock to Equation 2, we find that the average downwash velocity v, which determines the rotational velocities encountered in the vortices, is directly proportional to the aircraft weight and the load factor, but is inversely proportional to the airspeed, the air density, and the square of the wing span. These factors are discussed in the following paragraphs. Weight. The higher the gross weight, the more lift is required, and the stronger are the vortices. If other factors stay the same, a long-range jet will generate a more violent wake on departure, than it will on approach a few hours later, due to the difference in fuel load (and consequently, gross weight). As the strength of the vortices depends on the amount of weight being lifted by the wings, a tricycle-geared aircraft will not generate significant wingtip vortices on the tokeoff run, until the aircraft is rotated for the lift-off. On landing, the generation of vortices will stop as soon as the full weight of the aircraft is being carried by the landing gear.

Load Factor. The higher the load factor, the higher the lift, and the stronger are the vortices. Certain local noiseabatement procedures which force all departures to make a sharp turn immediately after takeoff can create a dangerous situation if an aircraft gets banked up for the turn and flies into the extra strong outside vortex of a preceding aircraft which had made a turn at the some place. Wing Span. As vortex velocity is inversely proportional to the square of the span, a small reduction in wing span can cause a large increase in the intensity of the woke. For years, the tendency in aircraft design has bee·n toward higher span loadings, which means either higher weight for the same span, or shorter spans for the some weight class of aircraft. This factor becomes very important with the advent of slender (very low aspect ratio) delta wings for supersonic aircraft. The Concorde is a case in point. With high weight and very low span, such aircraft are expected to generate much higher vortex velocities than those produced by present jet transports.

Airspeed. It may come as o surprise lo many people that the lower the airspeed, the stronger are the vortices. The reason is apparent in Equation 2. The slower the aircraft is flying, the less air it encounters each second. In order to generote the necessary lift, it must give this air a greater downwosh velocity. The kinetic energy per second required to generate the lift is equal to half the product of the lift and the deflection velocity (L v/2). Low airspeeds require high deflection velocities which in turn require increased power. In simple terms, we could soy that the trailing vortices result from slippage of the air during the lift generation process. The lower the airspeed, the more slippage occurs, and the more power is required to maintain altitude. For this reason, helicopters require maximum power lo hover at zero airspeed. Some idea of the high downwash velocities which are required for hovering flight, may be gained from Fig. 3. Around airports, aircraft are more likely to be flying

at lower airspeeds, and thus churning up stronger vortices. This factor undoubtedly contributes to the increased rote of vortex-induced accidents in this area. However, there ore other contributing factors. Traffic density is highest around airport traffic patterns; this raises the probability of vortex encounters. In addition, aircraft ore more likely to be at low altitudes around airports; if a vortex upsets an aircraft, less altitude may be available for recovery. Air Density_ The lower the air density, the higher the vortex velocity. If other factors were unchanged, vortex velocity would be highest at high altitude. Fortunately, airspeeds ore usually much higher up there, while traffic density is relatively low. Summary. If Equation 2 leaves you cold, an easy way to remember the effects of the various governing factors, is that any change which would force the aircraft to fly at a higher angle of attack would increase the downwash velocity, and thereby increase the strength of the trailing vortices.

Vortex Rotation When the cylinder of air shown in Fig. 2 is deflected downward by the wing, the static pressure of the air immediately under the wing is increased, while the static pressure of the air above the wing is reduced. Because of the natural tendency for these pressures to equalize again, the air in the high-pressure area under the wing tends lo

Figure 3

spill outwards around the wingtips, and then converge into the low-pressure area above. This lateral spillage of the air sets up two oppositely-rotating horizontal whirlwinds, or vortices (one spinning off each wingtip), which quickly involve the entire moss of air deflected by the wing.

Radial wind-streaks, concentric whitecaps, and a temporary dimple on the face of the ocean, indicate the intensity of the downwash which supports this 31,000-pound Sikorsky HR-2S, as it hovers long enough to snatch an astronaut from a floating Mercury space capsule.

NASA PHOTO

In the case of a conventional wing, the vortices roll up behind the wing, as shown in Fig. l. In the case of a slender delta wing (which aerodynamically is simply a couple of giant wingtips), the high-pressure air under the wing spills out around the outer edges, which in this case are also the leading edges. The air then converges into the low-pressure area above, forming the vortices at the same time. As shown in Fig. 4, the vortices are already rolled up before they pass the trailing edge of the wing. In either case, the cylinder of deflected air (as shown in Fig. 2) is shoved downwards through the surrounding air mass. On the underside of this cylinder, the surrounding air is pushed sideways, around the periphery of the cylinder. This fact is strikingly evident, if you ever get a direct head-on view of the smoke trail from a tail-engined jet, as shown in Fig. 5. Looking down the tube, as it were, you can visualize its cylindrical surface as the smoke spirals out behind the aircraft. Caught in the initial downwash, the smoke sweeps down to the bottom of the tube, where it splits into two strands which follow semicircular paths outwards, upwards, and inwards to the top of the tube. Here the strands merge and start downwards again, perhaps a thousand feet or more behind the aircraft. So much for the outer periphery, but what goes on inside the cylinder? Each of the two vortices has a core, or a circle of maximum rotational velocity. These cores are quite visible in Fig. 1. The air within each core rotates around the core axis; thus the ta n g e n t i a I velocity decreases to zero at the center of the core, as shown by the velocity graph of Fig. 6. This rotating air inside the core is also moving forward at high speed, toward the retreating aircraft. Outside the core of the vortex, the tangential velocity of the air decreases with distance from the core surface, as shown in Fig. 6. The lateral distance beween the centers of the twin vortices depends on the lift distribution of the wing, but in most cases is approximately :n/4 (about .78) times the wing span.

Figure 4

Cross-section of vortex flow past trailing edge of Concorde wing, in model tests in Onera Water Tunnel.

SUD-AVIATION PHOTO

:··

•••• .. ... ·•. •.

Figure 5

Head-on view of smoke trail from Boeing 727.

Up 40 ~

.. 20 .c

~ 20 0u ·..:: ... 40

-J;

60 -----------------------------------Down figure 6

Typical distribution

of vertical velocities shortly after rollup of vortices.

When partial-span wing flaps are extended, an additional set of vortices forms at the ends of the flaps. Although these vortices finally get rolled up into the wingtip vortices, the flaps have several effects worth noting. By shifting more of the lift inboard (away from the wingtips) they tend to move the main cores closer together, and to weaken the tip vortices. The flop vortices tend to diffuse the resulting main cores. Observations indicate that when partial-span flaps are used, the vortices tend to rotate slower and die out faster than when no flaps are used.

Wake Movement As shown in Fig. l, the size of the cores gradually increases with time. Their rotational speed gradually decreases, due to the internal friction of the air. Unless torn apart by shear, turbulence, convection, or interference by some external object, the vortices will continue to spin for several minutes. Meanwhile, they will drift with the wind. However, they ore also subject to another very important

motion - a downward thrust which is proportional to their rotational velocity. Both the downward thrust and the rotation ore caused by the initial downward deflection of the air by the wing. Consequently, the wake will be propelled steadily downward either until the vortices dissipate or until they reach the surface of the earth. The sinking speed of the woke from a typical four engine jet transport is initially around 350 feet per minute (3½ knots). If the woke reaches the surface, the cores level off at on altitude of half the wing span, and immediately spread apart laterally, as shown in Fig. 7. Their horizontal velocity in still air is approximately equal to their previous sinking speed. As the progress of each vortex is subject to the wind, a moderate crosswind will blow both vortices awoy. However, as shown in Fig. 8, a light crosswind component can hold the windward vortex in a steady position. If the aircraft is near the surface of the ground when the wake is generated, as shown in Fig. 9, the wakes will spread apart at a faster rate than when generated at a higher altitude.

Wind

Calm

1111 111111111111111111111 •••

+ Figure 7

Sinking vortices spread laterally

as soon as they reach the surface.

Wind

Direction

\ Figure 8

A light crosswind can counteract horizontal movement of windward vortex and create a hazardous situation for other aircraft.

Figure 9

Curling clouds of dust show the lateral paved airstrip.

spread of the wingtip

vortices, as lhis Lockheed C-130 •Hercules" nears touchdown on an unLOCKHEED-GEORGIA PHOTO

Helicopter Vortices Many years ago, when midget automobiles were first introduced in the United States, a favorite bit of parking advice was "Don't park under a horse!" As downwash problems have changed considerably since then, the remark should now be paraphrased to read: ,,Don't fly under a helicopter!" Each rotor blade tip of a helicopter sheds a separate vortex. The resulting vortex trail is a highly complex, intertwined wake. However, in forward flight, these vortices soon rearrange themselves to form the cores of twin vortices which trail the rotor path like wing-tip vortices trail an airplane wing. The intensity of helicopter vortices varies (like airplane vortices) directly with the amount of lift being developed, and inversely with the forward airspeed, the air density, and the square of the wing span (which in this case is the rotor diameter). In hovering flight, the airspeed approaches zero, and the wake intensity reaches very high values, as illustrated in Fig. 3. In hovering flight, the wing-tips describe a closed loop. Consequently, the downwash immediately below the helicopter is squeezed into a circular area one-half the area of the rotor disc. Within the lower area, the downwash velocity is twice the velocity at the rotor plane. A typical downwash velocity under an S-58 helicopter in hovering flight is about 60 feet per second (35 knots). Helicopter vortices settle toward the ground, and expand rapidly when they get there. Vortices shed in cruising flight tend to sink more slowly than those shed in hovering flight.

Vortex Dissipation Airplane vortices come in symmetrical pairs. When one core is broken, the other--.cor.e·usually breaks at the same place. At low altitude, any wind of more than five knots usually has enough velocity gradient and turbulence to shear the cores apart in a minute or two. In sunny days, thermal (convection) currents play havoc with the cores, tearing them apart vertically. When a temperature inversion exists, however, convection currents are suppressed in the area below the inversion level. When the inversion is accompanied by a calm or extremely light wind, as is often the case between sunset and sunrise, the vortices can roll along undisturbed for many minutes, subject only to a very gradual decay due to internal friction, until they reach a particular stage where they brake up rather abruptly and become diffused in the surrounding air mass. How do vortices break up, in smooth air? Perhaps jet contrails can give us a clue. We have recently had the opportunity to observe a number of high-altitude jet vortices (in contrail form), at close range, flying parallel to the trails in a direction opposite to the generating aircraft. From these observations, it would appear that the life of any part of the trail depends on maintaining unbroken vortex cores, all the way from the generating aircraft; and that once a break occurs, that portion of the trail behind the break soon loses its identity as a unified rotating mass. This is how it looks: In smooth air, continuity of the cores con be maintained for many miles, and the twin contrails stream out like fluffy white cylinders. Gradually, however, perhaps due to precession or instability as the cores slow down, they take

on a slight helical (corkscrew) motion. The amplitude of the wiggles gradually increases until suddenly, if you are watching very closely, you will see thin radial spikes of vapor protruding from each core. Within seconds, both cores shear apart vertically at this point. Almost instantly, the spikes blossom out into twin puffs, and the cores behind break into a series of short segments, with puffs at each break. Soon the segments are absorbed into the puffs, and the puffs evaporate in a wispy moss of turbulence. There may still be a considerable amount of energy left in the woke, but it is now greatly diffused and dis-' organized.

Penetration When one considers the millions of tokeoffs and landings which are made each year, and compares this with the number of accidents and near-accidents which have been due to vortex encounters, it may appear that the actual hazard is insignificant. The relatively low number of encounters is due to the fact that any encounter requires that on aircraft be in a certain limited volume of airspace at a certain time and under certain atmospheric conditions. Yet, when these contingencies occur, the results can be sudden, violent, and sometimes disastrous. The actual effects of a vortex encounter will vary widely, depending or. a vast range of possible factors. These factors include all those which have been discussed so far on the generation and behavior of the vortices. They also include such factors as the relative size of the generating aircraft and the penetrating aircraft, the elapsed time between generation and penetration, the angle and speed at which the vortex is encountered, as well as the control characteristics and the structural design limits of the penetrating aircraft. Aircraft Size. The chances of a small aircraft becoming completely immersed in the rotating wake of a large aircraft ore obviously greater than the opposite case. Consequently, in a mixed aircraft environment, small aircraft ore more likely than large aircraft to become involved in a dangerous vortex encounter. Aircraft Separation Time. The time between vortex generation and penetration hos a significant effect on the forces involved in the encounter. Tests indicate that the original intensity decays very little during the first thirty seconds, but then drops off at a faster rate, depending on atmospheric conditions. Pilots have reported vortex encounters with separation times of as much as 5 minutes; however, few if any measurements of vortex intensity have been made at separation times of more than 160 seconds, largely because of the difficulty of locating the actual wake after that time. The woke of a large jet aircraft con be dangerous to any other aircraft during the first minute. In any case, the greater the aircraft separation time, the lower the intensity which is likely to be encountered. Penetration Angle and Speed. If the penetrating aircraft is flying essentially parallel ta the woke and enters the downwosh area between the cores, as shown in Fig. 10, the result will be either a rapid settling or a significant reduction in the rate of climb. This type of situation is most likely to occur if the penetrating aircraft descends or climbs into the woke, or is overtaken by another aircraft passing overhead. Because of the strong downdraft, the

Figure 10

Parallel

Penetration

between cores.

settling can be dangerous at low altitude. There is olso the possibility that the pilot may stall the aircraft in trying to compensate for the downwash. In any case, a lower airspeed will worsen the situation. If the penetrating aircraft is flying essentially parallel to the wake and flies into one of the cores, as shown in Fig. 11, it will be subjected to a rolling motion induced by a downward airflow on one wing and an upward airflow on the other. If the rolling forces are greater than the maximum control force which can be applied by the ailerons, the aircraft will roll over in spite of anything the pilot can do. The slower the airspeed, the less control force will be available to counteract the induced roll. What usually happens is that the aircraft is flipped to an inverted position and thrown out of the-bottom of the wake. If the penetrating aircraft runs into the wake crosswise, as shown in Fig. 12, it will fly first into the outer upwash; then the airflow is reversed instantly as it passes the center of the core and the aircraft hits the downwash area; passing the center of the other core the flow is again reversed as the aircraft hits the upwash area on the far side. The effect, which is similar to a rapid series of sharp-edged vertical gusts, can produce violent pitching and vertical motions, as well as severe strains on the aircraft structure. The higher the speed, the higher the structural loads. We know of at least one instance where a light aircraft broke up in flight, as a result of this type of encounter.

Figure 12

Tronsverse Penetration.

Figure 11

Parallel

Penetration

into core.

How to avoid upsetting your Clients The two known ways of avoiding encounters with vortices may be stated briefly in six words: 1. Give them time. 2. Give them room. Giving the vortices time to dissipate is not always practical. The almost unlimited number of possible combinations of variables which can affect vortex intensity and decay rates will make it very difficult for any regulatory agency to tailor a set of time separation standards which would: 1. Guarantee that every wake would be harmless at the end of the prescribed separation period; 2. Be simple enough for easy application tions; 3. Never unduly restrict airport flow.

utilization

in daily operaor air traffic

Giving the vortices plenty of room implies that one be able to visualize !he location of these normally-invisible disturbances at any moment of their active lives, and then control any subsequent flight path so as to remain clear of the active area. Here is where a working knowledge of vortex behavior can pay off, in lowering the possibility of exposing aircraft to the most dangerous effects.

Any of the points brought out in the preceding sections of this orticle may have application at one time or another. However, in our opinion, the one most important characteristic to remember at all times is the basic "downand-out" motion of the vortices - downwards until they reach the surface, then spreading laterally outwords as shown in Fig. 7. Some important applications of this characteristic are listed below: 1. In crossing the flight poth of a preceding aircroft, it is better to cross at a slightly higher, rather than a slightly lower, altitude. 2. In following a larger aircroft on approach it is desirable to fly the same, or a slightly higher, path, but never a lower path, unless a strong cross-wind is present to blow the preceding wake offside. The use of a common ILS or VASI glide slope by all aircraft is a desirable practice, for this reason.

as It Is on a common course. If this is not possible, a takeoff interval of at least two minutes would be desirable. We would not close this article without a last reminder that vortices are likely to be most dangerous when the air is most peaceful, during calm or very light wind conditions, between late afternoon and early morning. Wind direction is more unpredict-able then. Vortices will stay stronger longer, and may occasionally show up in some unexpected places.

Bibliography We hope that the foregoing information will be of practical use to controllers ond pilots. Meanwhile, if any readers are interested in digging deeper into the details of this fascinating subject, we would recommend the reports listed below.

3. Flight directly under, and parallel to, the wake of another aircraft should be avoided, because of the inherent sinking characteristic of the wake.

1. "Big Plane Turbulence Can Cause a Flight Hazard", Safety Suggestion No. 8, Beech Aircraft Corp., 1952.

4. Takeoff or landing by a light aircraft should be avoided, immediately after a heavy aircraft has made a low pass (or missed approach) down the runway in use.

2. D.R. Andrews, "A Flight Investigation of the Wake Behind a Meteor Aircraft, With Some Theoretical Analysis", RAE Technical Note Aero 2283, ARC CP 282, 1954.

5. When a light crosswind exists, controllers and pilots should anticipate possible vortex encounters on a runway behind a larger aircraft taking off or landing; this situation is illustrated in Fig. 8.

3. C. C. Kraft, "Flight Measurements of the Velocity Distribution and Persistence of the Trailing Vortices of an Airplane", NACA Technical Note 3377, 1955.

6. It should also be anticipated that the ultra-high-velocity vortices shed by slender-delta-wing interceptors and SST's will sink quickly out of the way of following aircraft on the same flight path, but that such vortices probably will travel faster and farther laterally (ogainst stronger crosswinds) when they reach the surface. 7. Whenever possible, operations of light and heovy aircraft should be segregated on different runways. To avoid problems with vortices from one runway interfering with operations on a parallel runway during crosswind or calm conditions, it has been recommended that parallel runwoys be spaced at least 2 500 feet (800 meters) apart, if both are used for takeoffs and landings. However, if one runway is used exclusively for takeoffs and the other for landings, a lateral spacing of only 1 000 feet (320 meters) will still keep the hazard at a very low level. 8. Except for the condition described in Item 5 above, it is usually safe for a light aircraft to follow the takeoff of a heavy aircraft on the same runway, if the light aircraft lifts off after a shorter ground run and remains above the climbout path of the heavy aircraft as long

4. Report of Project NR AVN 2656 "Effect of Wing-Tip Vortices and Sonic Shock on Army Aircraft in Flight", U.S. Army Aviation Board, 1957. 5. T. H. Kerr and F. Dee, "A Flight Investigation into the Persistence of Trailing Vortices Behind Large Aircraft", RAE Technical Note Aero 2649, 1959. 6. J. W. Wetmo,e and J.P. Reeder, "Aircraft Wakes in Relation to Terminal Operations", Technical Note D-1777, 1963. 7. Leighton W. Collins, "Caution Magazine, May 1964.

Advised",

Air

Vortex NASA Facts

8. "Evaluation of the Wake of an S-58 Helicopter", final report, Project 348 011 01V, Federal Aviation Agency, July, 1963. 9. R. Rose & F. W. Dee, "Aircraft Vortex Wakes and Their Effects on Aircraft", RAE Technical Note No. Aero 2934, December, 1963. 10. A. B. Connor & T. C. O'Bryan, "A Brief Evaluation of Helicopter Wake as a Potential Hazard to Aircraft", NASA Technical Note D-1227, March, 1962.

First published in the January 1965 issue of THE CONTROLLER.

We're learning more about Clear AirTurbulence by Tirey K. Vickers

Significance Clear Air Turbulence (CAT for short) costs the US airlines over $ 18 million per year in personal injuries, flight diversions, extra training ond aircraft inspections. And not so long ogo, the CAT problem came very close to grounding o large portion of the USAF deterrent fleet. Consequently, o large amount of money ond effort is being poured into CAT research projects. On February 23 ond 24, 1966, in Washington, the Institute of Navigation ond the Society of Automotive Engineers sponsored o Notional Air Meeting on Cleor Air Turbulence, to review the progress which hos been mode in this field. The meeting attracted o total attendance of 335, including 23 visitors from 10 other countries. In the following orticle we will summarize the information which wos presented ot, this meeting.

Characteristics CAT is presently defined os ony atmospheric turbulence above 20,000 feet MSL which is not associated with convective-type clouds or thunderstorms. More CAT is found in winter than in summer, more over land than over seo, ond relatively more over mountain oreos than over flat country. The latter characteristic implies that terrain irregularities con trigger off the turbulence; this is certainly the case with the most violent form of CAT, the mountain wove, which is shown in Figures 1, 2 ond 3. Here the initial disturbance and the additional waves which form downwind resemble the eddies ond ripples which form downstream from o submerged rock in o swiftly flowing river. Nearly o million miles of high-altitude flying by U-2 aircraft indicates that o greater amount of CAT is found between 30,000 ond 40,000 MSL, than in ony other layer above or below. The probable reason is that the jet

Fig. 1 Mountain waves (also known as lee waves or gravity Legend: W = Wove length (normally

1 ta 30 miles);

stream, o potent generator of CAT, operates within this bond of altitudes. The U-2 doto indicates that turbulence decreases both in intensity and amount ot the higher altitudes. For example, above 50,000 feet, less than 2% of the U-2 flight distance wos in turbulence. Scientists of Douglas Aircraft believe that CAT is o· direct result of wind shear (differences of wind direction or velocity in adjacent layers of the atmosphere); when the shear reaches o criticol value, the flow becomes turbulent in the form of cells or eddies which drift owoy from the source region ond gradually dissipate downstream. However, some so-called CAT moy not be turbulence ot oil, but moy simply be the result of o flight path which happens to skim through on undulating or wavelike boundary between two slightly dissimilar air layers, os shown in Figure 4. If the two mosses ore moving ot different speeds or in different directions, the aircraft encounters on abrupt change in airspeed each time it crosses the boundary. An increased airspeed produces more lift and a positive (upward) acceleration, while a decreased airspeed produces the opposite effects. If there is a difference in wind direction between the layers, the aircraft will experience loterol accelerations also. CAT is an elusive thing, impossible to detect from the ground, using present radiosonde techniques. When CAT occurs, it is often scattered through areas 50 to 100 miles long. Within these areas it is patchy and highly localized in nature. In most cases the individual patches are seldom·· more than 2000 feet in depth, and a fairly small change in flight level moy enable the aircraft to ovoid the disturbance entirely. Many air traffic controllers hove noticed that during periods when CAT is present, there is often o wide variation in the CAT intensity reports from successive aircraft passing through the same airspace. Part of this difference is due to the patchy nature of the disturbance itself, but there are other reasons as well.

waves)

L = Lenticular

(lens•shapd) cloud;

Rotor zone (often marked by rotor cloud)

fig. 2 Lenticular

cloud of mountain wave, with small rotor cloud beneath

Just as a Rolls-Royce, a Volkswagen, and a Honda will respond differently to the chuckholes und undulations of a given roadway, different types of aircraft will respond differently to the same CAT conditions. Also, aircraft response may vary with different airspeeds, and with different gross-weight conditions. Sometimes individual crew members do not agree on the intensity encountered. Each

U. S. Weather

Bureau Photo

pilot tends to judge the intensity on the basis of his training, experience, and individual mental reaction. As the first step toward a more standardized method of reporting CAT, the National Aeronautics and Space Administration (NASA) has suggested the criteria listed in Table 1.

Fig. 3

Televised photo from Tires 5 satellite showing mountain waves generated in lee of Appolochion Mountains. Lake Erie ond Loke Ontario ore visible near top of pie• lure. Coasts of New Jersey ond Morylond ore visible just below cenler crossmark. Dots show position of Washington (W), Pittsburgh (P) ond Huntington (H). Wovelength of the mountain waves wos about 12 nautical miles. U. S. Weather Bureau Photo

Table l -

about 50 miles for SST aircraft. Meeting this requirement is on extremely difficult scientific problem; clear air does not contain the concentrations of particulate matter (rain, hail, and snow) which enable present radars to detect the most turbulent areas of thunderstorms. Consequently, CAT is as invisible to a conventional radar as it is to the naked eye.

CAT Intensity Criteria Airspeed Fluctuation in Knots

Description

Symptom

Very light

Perceptible

Lessthan 5

Light

Slight discomfort

5 to 15

Moderate

Difficulty

15 to 25

Severe

Loose objects dislodged

Extreme

Aircraft violently tossed Over 25 with around, impossible to rapid changes control, possible structural damage

in walking

Side effects or clues which might be used to differentiate turbulent clear air from non-turbulent clear air are difficult enough to detect at close range; to be able to do this from 20 to 50 miles away is fantastically difficult. But even if adequate detection range can someday be attained, there comes another problem equally important - to be able to determine whether or not this turbulence will affect the intended flight path of the aircraft.

Over 25

Detection Problems A large percentage of the present CAT research effort is being directed toward the goal of developing an airborne device which will detect CAT for enough ahead of the aircraft to give the pilot time to initiate action in evading the disturbance ar in alleviating its effects. This warning function implies that the device needs o detection range of at least 20 miles for present subsonic jets and

Warm

~--~--

- - -----

Fig. 4 Warped

interlace

between air layers

,.,

Here is o significant difference between the problems of thunderstorm avoidance and CAT avoidance: Thunderstorms may extend through 30,000 feet of altitude, while CAT is usually limited to less than 3000. Unless the aircraft passes through the same strata, the disturbance will not affect it. The point here is that pilots cannot be expected to rely on an airborne warning system unless it can demonstrate a high rate of succesful warning a n d a low rate of false alarms.

Air

- - --~"---~

Cold

- -

~F11·ght

Path

Air

CAT Sniffers Perhaps the simplest technical approach to CAT detection is the one being tried by Eastern Air Lines. This approach exploits the fact that jet streams (a primary source of CAT) are usually accompanied by a temperature change of the surrounding air. In this approach, the pilot uses o very sensitive, compensated free-air thermometer system coiled TRAPCAT (Temperature Rote Alarm for Predicting Clear Air Turbulence). At constant altitude and cruising speed, o temperature change of one degree Centigrade in one minute is a fairly accurate indication that the aircraft is headed for o CAT area. Eastern Air Lines characteristically flies north-south routes which tend to cross the jet stream. It wil I be interesting to see whether the TRAPCAT principle will prove sufficiently accurate in determining the proximity of other forms of CAT as well. A number of CAT detector designs employ some form of electronic radiation to scan the airspace ahead of the aircraft. Various bands of the frequency spectrum ore being tried, including VHF, UHF, microwaves, infra-red, and visible light waves. Such systems emit a pulse of radiation and then hopefully search the return for some evidence of CAT. Some of these systems monitor the intensity of the return, some measure the Doppler shift of the reflected signal, while other systems depend on such exotic techniques as measuring the absorption properties of certain types of molecules in the atmosphere to determine the temperature of the air ahead of the aircraft; detected changes in air temperature are then used far CAT prediction.

At the high end of the frequency spectrum, optor (optical radar) systems scan the airspace ahead of the aircraft, with o powerful laser beam. However, the proponents of this technique apparently have given little consideration to the fact that the output of a high-powered loser is a baby Death Ray which can permanently damage the eyes of anyone with in range who happens to be looking directly into the beam. Spraying this kind of energy around in today's traffic environment appears to be a highly questionable practice. All CAT detectors which scan the airspace ahead of the aircraft to determine air temperature probably will require a highly stabilized antenna to insure that the scanning beam will remain horizontal at all times. A slight deviation from the horizontal would cause the beam to scan through other altitude layers which could hove a total temperature gradient much larger than the tiny gradients which the system depends upon far detecting CAT. Stanford Research Institute hos discovered that electrical discharges from the aircraft's wingtips and tailtips sometimes occur near CAT areas, particularly in the vicinity of the jet stream. It remains to be seen whether this principle can provide a reliable remote indication of CAT, and a sufficiently low record of false alarms. One particularly far-out technical approach to CAT detection would use an automatic star-tracker to monitor the scintillation of a star on the horizon ahead of the aircraft, on the theory that any intervening CAT area will change the refraction of the light waves and cause the

of sight

Flight Fig. 5 Star•tracking

to star

path

CAT detection concept

star to twinkle. As shown in Figure 5, however, the tracker inevitably is looking a vast distance through the atmosphere; it is also looking through a vast range of altitude levels besides its own. Conceivably, air turbulence at any point along the line of sight could cause the star image to scintillate. Thus it appears that the false-olarm rote of such a system would be very high indeed. (Twinkle, twinkle, little CAT, I wonder where you're re a 11y at?)

Forecasting Most of the CAT detection schemes under development appear to be tremendously complicated and expensive to implement. Individually, they may be so limited in application that it would be necessary to combine two or more different sensing schemes, with their outputs properly weighted by a computer, in order to maintain an adequate score of successful warnings versus false alarms. Because of this inherent complexity, the most practical approach to CAT avoidance may lie in the field of improved forecasting, rather than inflight detection. For example, Eastern Air Lines ·hos found that the reliability of their CAT forecasts con be improved considerably by taking into consideration any trough of minimum temperature which appears on the 200 to 300 millibar charts. Where such a trough intersects either the jet stream, or a pressure trough, or the edge of a large shield of high cloud cover, CAT is highly probable; in most coses its altitude will coincide with the altitude level which hos the greatest amount of wind shear. In recent years, United Air lines and Northwest Air Lines hove mode extensive studies of the mountain wove conditions which prevail on some of their routes. This work has culminated in the publication of a new report, which contains on analysis of 169 different mountainwove generating sites. It is expected that the mountain wove situation con now be predicted accurately, over a wide range of meteorological conditions, so that flights con be given altitude and route revisions which will keep them out of the most turbulent areas. A familiarity with this report, by pilots, dispatchers, and air route traffic controllers, should help greatly in reducing the exposure of aircraft to mountain wave turbulence. Before long it may be practical to confirm mountain wove forecasts through the use of satellite photographs similar to the one shown in Figure 3. This technique is not used operationally yet, because 6 hours ore presently required to process and distribute the satellite data in a

form suitable for forecasting purposes. Within a year, however, it is expected that the delay con be cut to 1-½ hours.

Flight Techniques As the result of a series of jet upsets in turbulence a few years ago, piloting techniques were modified to cope more adequately with all types of turbulence. The higher the airspeed, the greater the strain on the aircraft when it encounters rough air. Thus it is prudent to reduce airspeed when on encounter with turbulence is imminent, or in progress. An analysis of early jet upsets showed that pilots were occasionally getting the aircraft into a stall buffet condition in rough air, particularly at high altitudes, where there is relatively little spread between the high-speed buffet and the stallspeed buffet condition. Subsequently, the recommended turbulence penetration speeds were revised slightly upward. They now form what should be the best compromise between the following factors: High speed Good control Maximum stress

Versus

Low speed Poor control (Danger of stall) Minimum stress

In a strong updroft as shown in Figure 6, the wingtips of a swept-wing jet tend to bend and twist to a lower angle of incidence, thus producing less lift. Meanwhile the center section of the wing (which is farther forward than the swept-bock tips) continues to lift normally. Thus, the aircraft temporarily becomes toil-heavy, and starts to pitch up. In early encounters with this phenomenon, pilots used full forward stick and then tried to bring the nose down by retrimming the aircraft with the stabilizer. When the nose finally started to come down, the now nose-heavy aircraft accelerated into a screaming dive before the pilot could re-trim the stabilizer. Some of the wrecks were found with the stabilizer still in the full-nose-down-trim position. Consequently, pilots are now taught to ovoid any use of the stabilizer trim switch in such encounters; and some jet aircraft hove been modified to limit the amount of stabilizer control available to the pilot, when using the handy "pickle switch" on his control column. It was found that in severe turbulence, pilots could receive false pitch clues if they concentrated too much 85

on the altimeter. As a result, pilots are now taught to fly the attitude indicator as the primary instrument during turbulence encounters. They are also taught that, under such conditions, the "Three A's" of instrument flying should be considered in the following order af importance: l. Attitude 2. Airspeed 3. Altitude

Wingtip

Section

Center Section

Updraft

Conclusion So far, no CAT detector has demonstrated a confidence level sufficient ta justify its operational implementation. The physics of CAT are still poorly understood. Much has yet to be learned about the basic parameters (there are at least 55 of them) which influence the generation of CAT.

Fig. 6 Deflection of swept wing in updralt

However, if we consider how much has been learned about the subject during the past three or four years, we can still be optimistic. Maybe we'll never get a foolproof CAT detector. But meanwhile, further improvements in forecasting and piloting techniques could greatly reduce the operational hazard of clear air turbulence.

First published in the April 1966 issue of THE CONTROLLER.

CONVEX 72 The U. K. Guild's bi-annual Convention-CONVEX 72-was held at Bournemouth in the Maison Royale Hotel complex, from October 25th to 27th, 1972. Expedition was the theme af CONVEX 72, dealing with the present and future problems of maintaining both a safe and expeditious flow of air traffic. Coupled with the Convention was a large and impressive exhibition of ATC equipment. Some 400 participants attended the Convention, including 44 delegates from overseas. A well-balanced mix of professional and activities was provided during the three days of the conference, with many interesting papers on the Convention theme. Presenting the U. K. Guild's first paper, Mr. Alan Rackham spoke on "The Management of the Airspace" and developed a number of pertinent points: Differing types of airspace, user requirements, flight rules and procedures has resulted in an increasing number of restrictions and loss of expedition; Mixed Visual and Instrument Flight Rules within U. K. controlled airspace should be discontinued; - All public transport aircraft carrying fare-paying passengers over the United Kingdom should be obliged to fly in controlled airspace for their entire flight. Further papers were presented by:G. N. S. Taylor, Marconi Radar Systems Ltd., "The Beamwidth Illusion"; R. M. Berry, Plessey Company Ltd., "Instrument Landing Systems in the modern ATC environment"; R. W. B. Smith, British Airport Authority, "Expedition-the South East Area over the Next Ten Years";

Herbert C. Scott MBE, Cessor Electronics Ltd., "SSR-the Way Ahead"; Dr. V. D. Hopkin, Stress in Air Traffic Control Research Association, "Human Factors vs. Expedition"; Hugo Schmid, Eurocontrol, "The Control of the SST"; H. P. Jeffreys, British Overseas Aircraft Corporation, "Airline Considerations of ATC Requirements for SST Operations"; R. N. Harrison and D. L. Stoddart, Ferranti Digital Systems Division, "The SST and the North Atlantic"; I. Cochrane, British Aircraft Corporation, "Concorde Operational Experience and Prospects"; C. V. Stephens BA, DFC, Guild of Air Traffic Control Officers, "Management of the Flow"; Daniel Gorin, French Air Traffic Controllers Association, on behalf of IFATCA, "Sector Capacity and Air Route Layout"; I. R. Adderley, Software Sciences Ltd., "Plan Ahead"; Capt. Angus Caesar-Gordon, Guild of Airline Pilots and Air Navigators, "Expedition and ATC"; W. E. J. Groves, National Air Traffic Services, .,Expedition and the Flow in Relation to Airspace Availability". On the last day of the Convention an open forum was held and a wide-ranging list of topics discussed. Highlight of the social events was a Gala Dinner in the Maison Royale. Some 270 guests attended this function, including the Mayor of Bournemouth. CONVEX 72, well organised by the U. K. Guild's Wessex Lodge, was one of THE ATC events of 1972. or

The Role of the Touch Displayin Air Traffic Control By N. W. Orr and V. D. Hopkin About the Authors N. W. Orr is an Operations Officer I at the United Kingdom Board of Trade. He gathered first hand experience with the touch display as project manager in the computer (EUCLID) group of the U. K. Air Traffic Control Evaluation Unit, Hurn. Mr. Orr is now Head of Section CP 7 (Planning of Future ATC Systems) in the Directorate of Control Plans, National Air Traffic Control Services. V. D. Hopkin is a Principal Psychologist at the Institute of Aviation Medicine, RAF Farnborough. One of his main areas of interest is Human Engineering Problems in Air Traffic Control and Air Defence Systems; this has included work on the evaluation of Touch Display principles. He hos also prepared a study of Human Factors in Ground Control of Aircraft for AGARD and is a major promoter of the Stress in Air Traffic Control Research Association (SATCRA).

Introduction At the present stage of its evolution the application of automation to Air Traffic Control is, in large measure, a matter of using computers to drive or print tabular traffic displays. The current state of the art is such that much of the updating and amending of these displays has to be done by the controller or his assistant, through the computer, so that the success of any automated system depends to a considerable degree on the effectiveness of the communications between the controller and the computer. This man/machine interfoce is generally regarded as one of the weakest points in the Controller-Aircraft-Computer Loop.

Communication between controller and computer has hitherto relied heavily on the electro-mechanical keyboard in one form or another - a medium which may set the maximum pace at which information con be handled by the system. R/T messages normally cannot be passed to the computer in the form in which they are received but have to be converted, by the controller, into terms acceptable to a particular keyboard format. For example, items of ATC data such as aircraft col lsigns may have to be coded by designating them with a letter or numeral. This conversion process is time consuming, odds to the Controller's workload and is subject to human error. Additionally, the characters/symbols available on on orthodox keyboard ore limited and inflexible. These characters and the key-

Figure 1 Operator using Touch Display.

board layout hove to be learned by the operator and it may take a considerable time to acquire operating proficiency. Another limitation is that the feedback from the computer, which allows the operator to check the accuracy of his input, hos to be presented on a display remote from the keyboard and this may lead to problems of head movement and visual occomodotion, and introduce ambient lighting difficulties. The Touch Display principle offers the means of overcoming, or at least reducing, oil of the above limitations. The Touch Display, in its original form, was invented at the Royal Rodar Establishment by Mr. E. A. Johnson, and the first evaluation of its use in on air traffic control context was carried out at the Boord of Trade's ATC Evaluation Unit at Hurn in 1965/66. Since then several further evaluations in various ATC environments hove been conducted at the ATCEU in conjunction with the RAF Institute of Aviation Medicine. The advantages of the Touch Display as on input device for some ATC tasks appeared so self-evident that certain plans for including it in future systems were formulated before the evaluation trials hod confirmed its worth. Fortunately these decisions hove largely proved sound as the evaluations hove confirmed that the Touch Display functions very efficiently as on input device and, in the main, is preferable to the conventional keyboard for most inputting tasks.

Description of the Touch Display A Touch Display is a touch sensitive electronic data display designed to exploit the operators' body capacity in such a manner that a finger tot:Jch on an eletronicol contact - a "touch-wire" - produces a capacitance and resistance to earth which unbalances on inductance capacitance bridge. T!1is produces a signal which can be used

to activate a digital computer. In the version illustrated here (Figs. 1 to 3), twenty-four touchwires are arranged in four rows of six wires per row. A Touch Display coupled to a computer con be regarded as a keybord, the "key" labelling of which may be varied by computer program to suit any particular requirement. In addition to the key labels, further information may be presented on the display, by the computer, as and when required. A Touch Display therefore combines the functions of keyboard and dis p I a y. In other words - the input and output functions ore integrated. A simple example of its use in a suggested ATC role is illustrated in Figs. 2 and 3, and the touch display function involved are described in the captions.

Advantages of the Touch Display over the Conventional Keyboard The first evaluation conducted by the ATCEU compared results obtained by operators using the Touch Display with those obtained in an earlier trial in which a conventional keyboard had been used in an otherwise identical operational role ("en route" Sector Radar Controller). Among the findings of these trials were the following: 1. The training time necessary for operators to reach an acceptable level of proficiency using the Touch Display is significently less than that for the conventional keyboard. 2. Most of the searching and coding problems hitherto associated with locating displayed information are avoided. Only information essential to the operator's immediate requirements need be displayed - the rest is stored but can be made instantly available when required. Information on touchwire labels can be written in full or with only partial abbreviation. This dis-

Figure 2 and Figure 3 Suggested Use of the Touch Display for Aerodrome Control. {A method of meeting the requirement to inform the computer in o central processing complex, of the "airborne" times of aircraft deporting from on aerodrome.) In Fig. 2 {the "REST" picture) on Aerodrome Controller is presented with the collsigns of aircraft about to deport from his airfield. These callsigns hove been "fed" lo the display by the Ground Movement Controller shortly before they reached the runway holding point. The lake-off direction is also displayed and a digital clock appears in the top right hand corner of the display. II the controller touches the touchwire under any collsign he will be presented with the aircraft's relevant flight data {Fig. 3, GARPZ). When the aircraft takes off he touches the wire under "ENTER" and this

action immediately informs the main processing computer of the time of take-off and initiates the necessary action to notify the "en-route" con• trollers of this event. The aerodrome controller's display then returns to the "REST" position - but the aircraft which hos just deported will now be omitted from the list of callsigns {which will all move up to the left to "fill the gap"). Should the controller at any time wish to return to the "REST" position from the "Flight Pion" display {Fig. 3), before the oircroft concerned becomes airborne, he moy do this by touching the wire under "BACKTRK·. It will be seen that this Touch Display format olso provides the aerodrome controller with the means of notifying the computer of changes of lake-off direction {Fig. 2).

penses with the clumsy data line and field identification procedures necessary in some systems which use orthodox keyboards in conjunction with independent displays. Both iiiese features effectively contribute to lightening the operator's workload by substantially reducing the search aspect of the task. The operator con be "led" through any desired sequency of displays by appropriate computer programming - by presenting him with the set of touchwire labels which must be used next (This is one of the factors which helps to reduce operator training time). By showing only information directly relevant to a given input both on the display proper and on the touchwire labels, it is possible to present on identical layout on each. This avoids the problems associated with a fixed keyboard of making its layout logically compatible with the other displayed data. Because the orthodox keyboard requires mechanical depression of the keys it is slower and demands more effort to operate than a touchwire system. Potentially therefore the Touch Display can provide a foster rate of input. The Touch Display is silent in operation.

Perhaps the most important finding of these trials, however, was that the operators seemed to derive genuine pleasure from using the Touch Display, ond oil of them were extremely enthusiastic about it. This contrasted markedly with the attitude encountered in the trials using the conventional keyboard. In a later project (North Atlantic Oceanic Control environment) operators worked ot their own pace rather than at the rate set by incoming messages. The superiority of the Touch Display over the keyboard in this role wos confirmed, ond the trial also showed that performance with the Touch Display could be further improved by suitable computer programming designed to reduce the number of touches reqired to input any given message. In addition to the updating of controllers' traffic displays, an important role to which the Touch Display may be applied is that of liaison between one controller and another, through the computer. Any system which might help to lessen the ever growing volume of ATC verbal communications - already near saturation point - cannot be lightly dismissed. Employment of the Touch Display in the "liaison" data transfer role hos been evaluated at the ATCEU in both Controlled Airspace and Middle Airspace environments and these evaluations hove produced encouraging results which give reasonable grounds for optimism that the Touch Disploy's contribution to this aspect of ATC will be an important one. The points which most impressed those who witnessed these projects were, once again: a) the speed, ease and accuracy with which data could be transferred and displayed; b) the relatively small amount of training and practice needed by operators to acquire proficiency with the Touch Display; c) less susceptibility to human error than a system relying on verbal communications; d) the silence of the operations room.

A Note on Operator Training Experience on a series of trials has shown that it is possible to introduce operators to the principles of using the touch display by employing books of illustrations of the

various display sequences, in which the turning of a page corresponds to the touching of a wire. By this method operators can be familiarised with the way in which the touch display works and has been programmed. It is thus possible for potential operators to become familiar with touch display formats and routines without having to use the actual device itself, thus effectively reducing the amount of costly computer time otherwise necessary during training. It has been found that after such introductory training it is possible for operators to become rapidly proficient on the touch display itself.

Ergonomics The angle of slope of the face of the Touch Display presents the most important of the ergonomic problems associated with this equipment and has been examined by the ATCEU/IAM. It would appear that on angle of between 30° and 40° to the horizontal is the optimum angle, which is a compromise between treating the Touch Display as a keyboard - when it should be nearly horizontal and treating it as a display - when the face should be perpendicular to the operator's line of sight (usually about 60° to the horizontal for a seated observer with the display at desk top height). N. B. The angle of the face of the Touch Display shown in Fig. 1 (62½ 0) although ideal for viewing purposes was found by the majority of operators to be much too steep for comfortable touching. However, any display at 30°-40° to the horinzontal normally presents considerable problems of ligthing and reflections. One possible solution to this dilemma is to detach the touchwires from the display. This would allow touchwires (keyboard) and display each to be set at its own optimum angle although retaining their same relative positions in the other planes. Thus other ergonomic problems such as lighting difficulties could be largely overcome. If such a system were to be adopted, then conventional keys might be used instead of touchwires - the layout of the keys corresponding to the layout of the labels on the display above, thus preserving most of the advantages of the Touch Display principle. However there ore some obvious operating difficulties in such an arrangement and these ore to be examined experimentally in the near future at the A TCEU.

In Conclusion This article has covered only a few of the possible applications of the Touch Display to ATC of the future, and everyone who has been concerned with the evaluation of this device is convinced that many other ways will be found of exploiting its latent potential. The evidence to dote suggests that the touch display principle represents a major advance in the field of man/computer communications, and that it is particularly suited to Air Traffic Control. The few problems associated with its use should be overcome fairly quickly. The enthusiasm commonly shown by those who hove used the Touch Display may indicate an appreciation that the application of automation to Air Traffic Control, at least in this instance of its evolutionary progress, can help to make the controller's task an easier and more congenial one. First published in the October 1968 issue of THE CONTROLLER.

Lessons learnt in nine Years SATCO

by J. S. Smit

Paper presented to the Fourth Annual IFATCA Conference, Vienna

One of the lessons learnt in the many years we have been active in the automation of air traffic control is about the controller and his attitude of mind towards automation. In an effort to moke o contribution to the discussions at the Vienna Conference, I have chosen to make some remarks on this particular aspect of automation as I believe they fit in your program more than anything else. In the years I have been concerned with the automation of ATC I hove talked, worked, discussed and argued with many controllers from all over the world. And I regret to soy that I have experienced more false notion than comprehension of the subject automation. Indeed, the controller who is afraid that automation will take away, maybe not his job, but onyway his status, does exist. Also the controller who thinks that automation will bring him heaven on earth, is not a fiction. Automation is unjustifiably condemned; there are also unjustified expectations. As a result, automation is met by many disappointed controllers: some just because it is coming, others because it does not do what was expected. What is so special about automation? Why is there more discussion about this type of equipment than there ever was about any other ATC equipment. The answer is obvious: automation penetrates into the controller's method of working more than any other equipment ever did. I am not trying to disparage the present role of equipment like radio, telephone, direc1ion finders, radar, etc. when I coll these "tools of the controller". What I am trying to say is thot he is playing the orchestra of instruments; he is playing it with o considerable omount of freedom. And the orchestra arrangements vary per controller. Automation will inevitably affect freedom. To a very small extent in the beginning, but slowly it will go further. To make this clear, let us consider a strip-printing system, a simple form of automation. In such o system strips ore printed in a standard way: the format is fixed, the typesize is fixed, the moment of printing is programmed in o computer. All this is identical to every controller: a step to a more defined working method of the controller. The introduction of a keyboard for clearance and progress entries is o further step: the keyboard hos certain input rules lo be adhered to. Etcetera, etcetera. Coming back to my figurative language: the controller will become o conductor leading a trained orchestra and he himself as an individual is no longer making the arrangements. How for and how fast will it go? I do not feel competent to answer that question. Eventually, I believe, it will go for. We may expect it will also go fast. But I am convinced that the control responsibility - i. e. the decision of how the traffic should be cleared - will in what today we coll the foreseeable future, remain on the shoulders of the human controller. Why does the controller have to be bothered with

automation now? Why does he have to get part of his work programmed by a machine? Why is his freedom going to be affected, even when it does not seem to go better or easier, or even it may be more difficult to begin with? Why does he have lo argue with computer-mad people who ask him silly questions and eventually only produce a printed strip ... ? The answer to all these questions is not obvious to everyone, although it is not difficult. Automation, automatic data processing, automatic data handling, or whatever name is given to it, is the only way which will enable ATC lo make good the arrears and catch up with aviation. The main reason why automation, and only automation, con do this is that a central data processor will become the coordination-medium. The bottleneck of controller-to-controller coordination will be replaced by controllers working with a computer as a common source of information. They hove to feed data into that computer, but also can extract data from it. And there we come to the crucial point: the data is available and con be distributed to and used by many who to-day are devoid of that information for the simple reason that the human coordination-capacity is saturated. There lies the enormous advantage of automation: the A TC system con grow, it can do more things than it ever did before. This is the important issue at stake. Automation will not necessarily make life easier for controllers. In the beginning, maybe, even the contrary is true. Especially of those controllers working in an administration which is actively involved in the development and introduction of automation in ATC, much will be demanded in terms of skill and perseverance. Automation will affect their working method and quite likely not always in a way every individual likes to see it affected. But in introducing automation in ATC - may be this is hard to say for this audience - the individual controller is not the most important issue, nor is an individual control position. The ATC s y s I em, and consequently aviation is involved and automation of ATC can only be judged from that wide point of view. This, I am afraid, is not always easy, in particular not for those faced with the daily difficulties of a particular control position. Automation will rationalize the ATC system. It may be painful that port of the controllers' freedom will disappear. In particular that port of "freedom" which 1 in fact means "no rules". But this does not mean that the con~ troller in an automated system may be less skilled than lo-day. I believe that automation will require different, but definitely not less and may be even more skill of the human controller. The controller of to-day ploys an extremely important role in the history of ATC. It is up to him to take courage and face automation with an open, positive mind. Then, I am convinced, the result will be positive for aviation for air traffic control and for the controller. ' First published in lhe October 1965 issue of THE CONTROLLER.

Falconryin the Air Command of the Royal Navy By Lieutenant Commander D. D. Fairweather, Royal Navy

Before March 1966 the average number of birds that could be counted at any time of the day on the airfield at the RN Station Lossiemouth was about 650 and bird strikes by aircraft occurred at the rate of one every two weeks. Six months later the average count had reduced to 10 but of greater significance was the fact that daylight bird strikes in the vicinity of the airfield were reduced to nil. The reason - the Peregrine Falcon. In March 1966 a trial was started in the Air Command of the Royal Navy in which Falcons were used to scare birds off the airfield. Previous attempts to scare birds from the duty runway, its approaches and overshoots using

Fig. 2 The "problem engine".

acoustic devices were at first found to be effective but the birds gradually become accustomed to these devices and latterly were observed to be sitting on the loudspeakers evidently deriving some masochistic experience by listening to the alarm call. It was therefore necessary to find other means of scaring the birds. It was for this reason that Falconry was introduced. The problem of birds on airfields within the Naval Air Command was found to be worst at Lossiemouth because of its proximity to the Moray Firth fishing grounds. As the town of Lossiemouth itself is centred around the harbour and the main industry is fishing, the types of birds which settle on the airfield ore seobirds, mostly Gulls and some of these, notably the Black Back Gull, are very large, weighing as much as 5 lbs (2.27 kgs) with a wingspan of 4 feet (1.22 m). It was with some reservation that Falconry was considered as a possible method of scaring off these large birds, for a Falcon weighs on average just under 2 lbs (0.91 kilograms) with a wingspan of about 26" (0.66 m). However, doubts were quickly dispelled when the first kills were achieved. The Peregrine unhesitatingly attacked and knocked down birds more than twice its weight. At the end of a six month trial period the records showed fewer than l O birds counted on the airfield and no bird strikes in daylight by aircraft, this situation remains unaltered. The Peregrine Falcon, and the female of the species at that, is one of the few birds which has the tenacity to attack these large Gulls, and as can perhaps be appreciated, it is not sirnply a matter of flying one Falcon for bird scaring duties, nor for that matter is it simply a case of detailing off someone who can be spared from his other duties to look after the bird. Four Petty Officers have been trained as Falconers and they ore assisted by four Naval Airmen. Two Petty Officers and two Naval Airmen are drafted lo Lossiemouth for a two year period of duty as Falconers and Assistants whilst the others employed at sea in their primary trades, maintaining and handling aircraft. They then change over at the end of two years. A mews has been built at Lossiemouth capable of ac91

commodoting up to eight birds. Six to eight is the ideol number to hove on the strength ond the Folconers ond their Assistonts are responsible for the core of the birds, the upkeep of the mews and the manufocture of folconry gear (bells, hoods etc.). The weight of each folcon is recorded doily ond it's behaviour noted. From these observotions it has been found that whenever a folcon is below it's optimum weight ond is in the process of goining weight, it will make a kill on or near to the day on which it's weight again reaches the optimum. Other foctors in the folcons behaviour hove to be taken into account; for example it cannot be flown effectively when it is in moult and so it is necessary to keep a sufficient stock to ensure that sever a I sorties can be flown throughout the day by "operational" birds. It goes without saying perhaps that a folcon is not interested in killing when it has just fed. There are several days in the year when conditions are unsuitable for flying folcons, for example in gale force winds. On these occasions shell crackers (shot gun cartridges with a double explosive charge but no shot) are fired at irregular intervals in the vicinity of the duty runway. In addition, carbide charges are placed around the airfield and these too go off at irregular intervals. The overall effect is to dissuade birds from settling in the vicinity of the runways, approaches and overshoots. Two problems remain unsolved; firstly, how to scare birds at night when the folcons cannot be flown and secondly, how to maintain the stock of falcons at the required numbers. From time to time folcons are lost, fortunately not often, but replacement is difficult. The· folcons at present in the mews have been purchased, for about £ 60 each, from pieces os for apart as the

Fig. 4 Two solutions to the problem.

Fig. 5 The mews.

Trucial Oman and Tripoli. The search for a fresh source is constant and any assistance in this matter would be greatly appreciated by the Commanding Officer, RN Air Station, Lossiemouth Morayshire, Scot:a:1d. A sum of £ 500 is allocated annually to cover the purchase of new birds, veterinary fees, the upkeep of the mews etc. This sum does not of course include the wages of the Falconers, but even if the wages of the Falconers were included, it would be a small price to pay for the saving which has been effected in damage to aircraft, which, before the introduction of folcons, was reckoned to be several hundreds of thousands of pounds per year. There has been some criticism of employing folcons to kill other birds but the answer to this must be that it is natural, death is quick compared to the suffering of some birds which have been winged by aircraft and die slowly, probably in pain and finally it is now merely sufficient to fly the folcons regularly, with an occasional kill, in order to keep the airfield clear of birds. In the last analysis the folcons may have been responsible for saving human life. This cannot be proven, fortunately, but one thing is certain, folcons have made a valuable contribution to accident prevention in the Air Command of the Royal Navy. Fig. 3 The •problem airframe".

First published in the April 1969 issue of THE CONTROLLER.

Developements in Collision Avoidance by Tirey K. Vickers

Background

Passive Visual Enhancement

The possibility of having mid-cir collisions oil began in Dayton, Ohio, one fateful doy in 1904,when Or.ville Wright suddenly turned to his brother end soid, "Wilbur, let's build another oirplone" ! Thus, it wos altogether fitting thot Dayton should be the site of the Notional Air Meeting on Collision Avoidance, which wos held February 23-24, 1967.Sponsored by the Institute of Novigotion end the Flight Safety Foundation, the meeting provided o progress report on the entire field of ADSA (Air-Derived Seporotion Assurance). ADSA is o generic term which covers four different oreos of effort:

Point

1. Visual Copobilities (human factors for unaided visual detection), 2. P o s s i v e V i s u o I E n h o n c e m e n t (oircroft point end lights),

3. V i s u I A V O i d O n C e A i d s (pilot warning in'struments - PWI),

4. N o n - V i s u o I A v o i d o n c e S y s t e m s (collision warning systems -

CAS).

Following is o review of the developments which were reported in eoch oreo.

Visual Capabilities Douglas Aircraft psychologists hove found thot with special training, pilots ot oil experience levels can greatly improve their ability to detect other aircraft targets. This training is pointed toward two objectives: o) more efficient instrument scanning patterns, to give more time for looking outside the cockpit; b) more systematic outside scanning techniques to increase the probability of target detection. The training is done in o special oircroft simulator. Pilots ore trained to read more end more instruments in o single scan, before looking outside ogoin. When there is nothing outside to look ot except o big blank empty sky, the Douglas-trained pilots start their target scanning potterns by looking first ot o wing tip. This allows their eyes to refocus ot infinity, thereby preventing the condition known os altitude (or empty-field) myopic, which could keep them from seeing on intruder until it got dangerously close. In scanning for targets, the pilots ore·encouroged to use swivel-neck techniques. The results of this training include on increased _occurocy in instrument flying, os well os on increased ability to detect intruder targets. Follow-up tests mode several months after completion of the training show that these increased proficiencies ore retained.

FAA-sponsored studies of various oircroft color schemes show thot fluorescent point con increase oircroft conspicuity, but only when the oircroft gets close enough for the color to be detected (normally, about four miles owoy). The tests olso show that the oll-importont factor in longronge visual detection is the degree of contrast between the target end its background. Obviously, no single circroft color can provide maximum contrast under oil bockground conditions. The final recommendation of this study wos thot the upper surfaces of the oircroft be pointed o light highreflectonce color, end the undersides o dork, low-reflectance color; to provide o visual cue os to flight direction, jt was recommended that the entire toil be pointed o solid fluorescent red or orange. However, the resulting visual improvement wos not forge enough to justify compulsory use of the recommended color scheme. Lights Unquestionably, present types of rotating or fleshing anti-collision lights greatly increase the visual detection range, compared to the stondord red-green-white position lights. Unlike the letter, however, most of the anti-collision lig~ts provide no cue as to the aspect of the oircroft being encountered (end consequently its direction of motion). Perhaps someday the mony different types of anti-collision lights presently in use moy hove to be stondordi:z:ed to provide this directional cue. Altitude coding for aircraft lighting hos been tested os o possible means of providing o cue os to the relative altitude of other oircroft. One suggested fleshing-code scheme is orronged in o cycle which repeats itself every 5000 feet, in the following order: Level

Code

5000 4000 3000 2000 1000 Initial tests indicated thot the fleshing code wos more useful in showing altitude changes, rather than relative altitude, of other targets. Further modifications end tests ore planned.

Aids to Visual Avoidance Severo! years ego, FAA researchers announced the profound discovery thot o pilot hos o much better chance of spotting o distant aircraft if he knows where to look. Fol-

lowing this principle, NASA is studying the possible design of a Pilot Warning Indicator (PWI), to detect other aircraft by means of infra-red radiation, and then show the pilot where to look, in terms of relative bearing and elevation angle. However, the concept doesn't appear very promising. System capability will be handicapped by the fact that infra-red radiation fades out very rapidly in precipitation, clouds, or haze. In addition, the infra-red sensor probably will be useless whenever it is looking towards the sun.

Non-Visual Avoidance Systems Justification Higher aircraft closing speeds require correspondingly higher target detection ranges. As speeds increase, the point is reached where the relatively limited visual ranges ovo.ilable, in any visual or optical collision avoidance concept, cannot provide enough warning time in which to carry out the sequential functions of target detection, threat evaluation, moneuver selection and execution. This limitation is the reason behind

3. The system must exchange altitude data between aircraft. 4. The preferred avoidance maneuvers will be short climbs or descents, rather than turns; the desired vertical separation or miss distance (ot least ot altitudes below 29,000 feet) will be 650 feet. This distance must be great enough to provide nominal separation under the worst conditions, yet small enough to keep any normal ovoidonce .moneuver from triggering off a chain reaction with aircraft ot other assigned altitude levels. Allowing for o possible altimeter error of ± 250 feet, the actual miss distance under the worst conditions would be 150 feet, minus the height of the aircraft. The 747 jumbojet (already nicknamed the "Boeing Hilton") will be about 60 feet toll, in level flight. This leaves o "guaranteed" safety margin of at least 90 feet! 5. The CAS wi II be o Tou system. Tou (t) is the Greek letter which in CAS terminology stands for time-to-minimum-range. Tou systems measure target range (R) ond rote-of-closure (R) to determine this closure time, in accordance with the following equation: I R/R.

The desired Tou value is set into the CAS computer as o criterion for deciding whether or not any target is o threat. Fig. 1 shows the various combinations of target range and rote-of-closure which will trigger off o Tau alarm set for 40 seconds. However, when aircraft closure roles ore very low, as shown in Fig. 2, on intruder could slip in dangerously close, without violating the Tou criterion. For this reason, the Tau alarm is supplemented by o range proximity warning. Fig. 3 shows the combined criteria for a typical Tau value of 40 seconds, and o range of 3 miles.

a) the implementation of positive control procedures by the ground-based ATC system; and b) the current development effort for an airborne electronic collision avoidance system (CAS). Since 1955, the U.S. airlines have wanted o CAS. Ideally, they want it to provide on additional measure of protection against traffic not controlled by the ATC system, ond to provide o lost-ditch escape in coses of ATC system error. CASorNAS? The airlines hove stated carefully that they do not intend for CAS to be replacement or substitute for the ground-based ATC system. Also, they do not wont their interest in CAS to be regarded as o signal to reduce efforts to improve the present ATC system, nor to predicate ony new ATC system design on the possibility that some day airline aircraft might be equipped with CAS. System

16~--..----..----..----..----..-----,

Characteristics

The U.S. airlines are studying the possible characteristics for o CAS design which will meet their functional requirements. The characteristics and requirements ore also being coordinated with those of other civil and military agencies, through o committee known os Collision Prevention Advisory Group (COPAG). Ideally, the ultimate goal would be o common system design which would meet the requirements of oir carrier, military and generol o~iation users, ond thereby receive the widest possible implementation. The commitlees are still o very long way from o common system design. As far as the airlines ore concerned, however, several of the basic system characteristics ore now firmly established: 1. The present state of the electronics ort dictates that the CAS will hove to be o cooperative system. As o result, only equipped aircraft will be able to participate; unequipped aircraft will not be detected by the system. 2. The CAS must operate within the frequency bond of 1540 to 1660 MHz, which has been allocated for this use. 94

6. The CAS will be o T/F system. T/F stands for Time/ Frequency, an exotic new principle which may prove to be the most revolutionary development since radar. Not on·ty con it form the basis for CAS, but it hos the potential capability of taking over all functions which ore presently carried out by SSR and DME. You will be hearing much more about T/F applications, in the years to come.

141----+----+----+---t-----t-----1

"' 12 i

c:,

Cl) et, C c:,

c::

6 4 2

200

800 600 Rate of Closure, Knots

-100

Figure 1 Range/Rote of Closure Criteria

for

l000

40-Second Tau

1200

-------------+ ---

---

--- ---

I.ow Closin~ Speed

lniri:ition of turn bv either .i i rc.-r.'.lft<.·oUJd <.TC<HCdan~(.:rous

si1u.nrion

Figure 2 Need

for

Range Proximity

Warning

----- ---- ---+- ------- ------T/F is based on the idea of carrying o very precise caesium or rubidium (atomic) clock in each airplane. These clocks ore reset at suitable intervals to o master time signal, so that all clocks ore synchronized with each other to on accuracy of 0.2 millionths of o second! This extremely precise common time reference permits the reservation of o definite time slot for each aircraft to transmit, while all others listen. This orderly procedure prevents garbling of transmissions from two or more aircraft. The reservation technique, coiled time-multiplexing, is diagrammed in Fig.4.

16,-----r-----.------,-----,.--~------,

l4t-----+----+---t----+----+---; ·◊e,

J! 12

,<,,~"..::/i//

1 £

;~~id! System

81----1----+-----!--....✓,,

(1)

"" 6 t-----+-----t---,,".•

0::

2 ......~.......,,.

200

400

600

Rate of Closure, Figure 3 Combined Criteria

800

1000

1200

Knots

for 40-Second Tau and 3 miles Range

Operation

McDonnell Aircraft Corporation has already developed an operational T/F CAS for aircraft in their flight test area. It is expected that the proposed airline CAS will utilize many of the operating principles of the McDonnell system, as described below. The output of the stable oscillator (the heart of the atomic clock) is multiplied lo provide the UHF radio transmission frequency. At the beginning of its assigned time slot, an aircraft transmits o burst of UHF CW energy, followed immediately by the oircraft'5 encoded altitude data. When thi5 message is received by any other aircraft in the system, the range is measured by noting the time difference between the start of the time slot and the start of the CW transmission. The rote-of-closure is measured by noting the Doppler deviation from the standard CW frequency. The altitude data is decoded by the receiving aircraft, which then compares the intruder's altitude against its own altitude, as well as the altitude strata it expects to pass through within the Tau warning period. This screening procedure immediately eliminoies from consideration

.\11\t:l(,\FT

MESS,\GE

SLOT NllMllEl( 1000

JO()()

Figure 4 Time-Ordered Reporting Scheme. Shaded .boxes indicate aircraft message reservation time. Blonk boxes indicate reception time.

2 5 .,(,....·_o_o - -cc_~ _______

2 Sc,c Cy.:le (-'>00 aircr.tft

rc·ports

Tirn"

-1,~-----•>'1'

per scconcl)

a large percentage of the irrelevont targets. All other aircraft signals are examined to determine which of them imply threats, in terms of range and rate-of-closure. The entire process is repeated for each aircraft every two seconds. When any R/R combination shows that the intruder has reached either the Tau line or the minimum range line (see Fig. 4), the CAS computer triggers off an audio alarm in the pilot's earphones, examines the altitude situation, and lights an appropriate UP or DOWN arrow in the cockpit as a command for the avoidance maneuver. The light continues to flash until the threat is eliminated. The system logic is designed so that the intruder pilot will receive the opposite indication in his cockpit. The system can handle situations involving three aircraft; in this case the middle aircraft receives a "hold altitude" signal while one airer.aft passes over and the other aircraft goes underneath. The concept of having a pre-assigned time slot for each aircraft also forms the basis for an add-on system feature which someday could become a very useful aid for ATC. Known as station-keeping, this feature would enable a pilot to maintain a preassigned separation distance behind a designated aircraft ahead. The pilot would simply dial in the time-slot number of the aircraft which ATC told him to follow. The CAS equipment would provide a direct readout to show the distance from the designated aircraft, in miles. A rather simple left-right indicator could also be provided to show the relative direction of this aircraft. With pilots able to space themselves, a long chain of similar-speed aircraft could be cleared along the same route with little more controller workload than that required for a single aircraft today. This technique could be especially useful in oceanic traffic control operations where no ATC radar coverage is available. Economic

Factors

The one big catch in T/F technology is that it is still very expensive. The clock alone costs more than a number of small aircraft on the market today, and the complete

CAS will cost initially between 30 and 50 thousand dollars per aircraft. This will prohibit its adoption by most general aviation aircraft, and may severely restrict its adoption by the military. Thus, the airlines must choose between a) the desire to obtain some protection immediately from other airline aircraft, with the proposed T/F system, or b) the desire to obtain, ultimately, protection from a much larger percentage of the entire aircraft population. The latter objective can be met only by a much-lower-cost system. Technologically, however, such a system may be many years away.

Category of Aircraft

Involved

Air carrier versus air carrier Air carrier versus military Air carrier versus general aviation Total Table l

Midair Collisions -

Collisions 6

8 20 34

U.S. Carriers 1938 through 1966

Table I shows that if it had been available, a CAS used only by airlines could not have prevented more than six midair collisions during the post 27 years. Does this record carry sufficient justification for the airlines to invest up to 100 million dollars in a CAS now, knowing that the equipment con provide no protection against 100,000 other U.S. aircraft that can't afford it? The stokes are getting higher. Airline aircraft are getting larger and more expensive. A collision involving two fully-loaded SOO-passenger jumbojets over a metropolitan area could amount to a national disaster. As Mr. Lincoln Lee stated at the 1966 GATCO Convention, "The enormous number of passengers on board will make imperative not only the provision of positive control but of fail-safe positive c.ontrol ". CAS may be that fail-safe backup, especially in high-altitude operations. We think the airlines will buy it.

First published in the April 1967 issue of THE CONTROLLER.

Digital Radar Plot Extractors Translation from German

by Dr. Heinz Ebert AEG-TELEFUNKEN Ulm/Donau

Basic Functions of Radar Plot Extractors Improved methods for air traffic control are increasingly based on the information gained by radar on the air traffic situation. Thus failsafe radar control becomes more and more important. This can be achieved primarily by a multiple coverage of the respective air space with the aid of an increased number of radar systems. In order to permit evaluation at centrally located places of the information supplied by these radar systems, it is necessary to build up a closely meshed network for the transmission of such information, enabling economic transfer of radar data even over great distances. An additional requirement resulting from this fact is to more and more equip major Air Traffic Control Centers with major digital computer systems facilitating or rendering it possible to process the high rate of information received. One of the important functions which must be assumed by computers is the organization of the presentation of the air traffic situation on displays designed along a new concept for control by computers, and on which both geographic.al and alphanumerical information is presented. If the air traffic presentation is to be continuously and automatically updated, the installations primarily used for determination of the air traffic situation, primary and secondary radar, must be directly connected to the computer system at the Control Center. Radar installations, however, have not originally been designed for such linkage, their signals, therefore, must first be "digitized" and subjected to digital pre-processing. This requires a sub-system serving as a connecting link between radar and computer. Upon closer investigation of the problems arising as a result of radar and computer linkage, it is noted that several important intermediate problems normally solved by the controller in addition to his other tasks must now be solved automatically, viz. detection of the presence of valid targets, determination of target position, and tracking of target movements. The controller observing the radar screen performs all these tasks on the basis of the distribution of the radar echoes displayed, discriminating between aircraft targets and clutter caused by ground and weather returns. For target detection he utilizes his experience, for target tracking his memory, and perhaps also a grease pencil or a "shrimp-boat". Target tracking on the basis of fed-in target position data is a task which can best be performed by a generalpurpose computer. It can, therefore, be accomplished additionally by the computer provided for general information processing.

For target detection and determination of the target coordinates, however, it is preferable to utilize a system specifically designed for these purposes, and assuming the function of a connecting link between radar and computer. Th is system is ea lied "Radar Plot Extractor".

Principle and Technical Function of Radar Plot Extractors Automatic Target Detection Technique Basic

Principles

In principle, automatic target detection is subject to the same difficulties as human target detection. These difficulties are a result of the specific characteristics of radar signals: 1. Due to the decrease of the return pulse power by the 4th power of range, the return signals are always weak and subject to considerable noise interference. 2. Returns from the earth's surface, rain clouds etc., in short called "clutter returns", frequently make it very difficult to detect and process aircraft target returns (valid targets). Generally it can be said that a reasonable target detection in the case of a primary radar of great range coverage would hardly be feasible if the target would furnish but a single return pulse as the antenna beam is passing over the target. For this reason, radar systems ore designed in such a way that an aircraft target is subjected to several hits, normally 10 to 20. Target detection is then based on the detection of a correlated pulse train reflected by the target, the individual pulses of the pulse train having the same range values. Noise pulses, however, are more or less uncorrelated and have different range values. By the addition of several target returns on the display screen or in the observer's eye, respectively, the valid target is distinguished from a single noise return. Thus target detection is the result of an "integration" over several individual returns. For automatic target detection, the human capabilities are replaced by circuits having similar capabilities of integration. The process of target detection may be broken down into two steps: 1. Return signal detection. 2. Return signal correlation

(= integration). 97

decleared for a return pulse, it is definitely declared a target return, even if it might result from an occasionally occurring noise spike (Fig. l ). This means that the effectiveness of target detection greatly depends on the absolute ---,('1W......--..-+~n-n-:v~---..-.\jJ\J.-t+~----..---..-~-n-\J\J\.-THRfSHOlD value of this threshold: if the threshold setting is low, all target pulses are detected, but the probability of noise I pulses exceeding the low threshold increases. If the threI I I shold selling is high, it will not be exceeded by many noise I pulses, but target pulses of low amplitude may be lost. In practice, the threshold is adjusted so that the number QUANTIZfD HITS of noise spikes occasionally exceeding the threshold is still great compared with the number of actual target pulses. Since the chosen threshold selling must be strictly mainFigure 1 Principle of hit delection tained for the sake of uniform system operation, an automatic threshold adjusting circuit is used, adjusting the threshold in accordance with any changes of the noise level. Return Signal Detection The value to which the threshold is set is the mean value Return signal detection settles the question whether a of the number of noise pulses exceeding the first threshold; received return pulse can be declared a target pulse or this mean value is determined by the circuitry denoted simply a pulse generated due to noise. This is accomplishnoise meter (NM) in the block diagram (Fig. 2). The threed in a threshold circuit having a bivalent threshold criteshold circuit denoted "lst threshold" is followed by the rion (Fig. l): if the amplitude of the return pulse exceeds "quantizer" declaring a "O" or "I", respectively. The pula preset threshold voltage, a digital "I" is declared for ses leaving the quantizer are stored in memory Ml from that pulse. If it does not exceed this threshold, it is rejected, where they are supplied to a circuit denoted "2nd threresulting in a digital "O". Once a digital "I" has been shold" for target detection.

TARGH HIT

TARGH HIT NOi Sf HIT

nD□

1stTHRESHOLDQUANTIZER DIGITAL 2ndTHRESHOLDBUFFER MEMORY =SLIDING WINDOW MEMORY DETECTOR T1 0. M1 SWD M2 VIDEO

NM NOISE METER Figure 2

Automatic

target delection

system -

Return Signal Correlation Target Detection)

Now target detection is performed in such a woy that the quantized return pulses received within one antenna beamwidth are examined as lo their correlation, and their number is compared with o second threshold criterion, the "2nd threshold". This is accomplished by a circuit called "sliding window detector" (SWD) in accordance with its method of operation. Figure 3 illustrates the method of operation of the sliding window detector: a target is assumed of a range of 58.25 NM, and the antenna beam pattern passes over this target. Within this pattern, the target is to be hit by a total of 9 radar periods. Period P, supplies a return pulse which, after exceeding the first threshold, is written as a "I" into the first location of a register consisting of 9 memory locations. Then the· entire contents of the register is shifted by one location, thus automatically erasing the contents of the 9th location; the first location into which the "I" of period P, has just been 98

TARGET INFORMATION

block diagram

written becomes free since the "I" has been transferred lo the second location. Period P2 of the antenna beam supplies a return pulse not exceeding the first threshold. A "O" is written into the sliding window; again, the content is shifted by one location. In the third through sixth periods, 'T's are written into the sliding window; this means that by this time the target has supplied 5 quantized return pulses. This number 5 out of a total of 9 possible hits is now defined as the criterion for exceeding a second threshold, the target detection threshold. If the number of hits coming from a target attains this minimum value during one antenna sweep, start-of-target is declared. When the antenna continues to rotate, the target is gelling out of the beam. If the sliding window contents falls below a certain value, "end-of-target" is declared. (In the above example, "end-of-target" is declared if the value decreases to 4 hits.) Depending on the resolution and range of the radar, approximately 500 to 1000 sliding windows will be required in range direction in order to permit optimum target detection within the radar's range. The radar range

may then be assumed to be subdivided into range increments, each sliding window corresponding to a certain range value. The range of coverage of one range increment is determined in occordonce with the radar pulse duration; normally it will amount to 2 to 4 microseconds. In practice, the shift register and the second threshold circuit of the SWD ore installed but once and operated on o time multiplexing principle. A ferrite core memory stores the information of oil sliding windows and makes available the information of the sliding window required at o given moment. The number of sliding window locations depends on the maximum number of hits per one antenna sweep. Generally thi's number amounts to between 9 and 20 and con be computed from the antenna beomwidth, the rotation speed and the pulse repetition frequency. Together with the associated azimuth ond range values, the "start-of-target" ond "end-of-target" information furnished by the sliding window is fed to the buffer memory M2 (Fig. 2), where they ore stored until further processing takes place. Whereas the first threshold is operating with analogue quantities, the second threshold is operating by counting the available return pulses ond comparing their number with o given value, thus providing on "m out of k" criterion. For this reason, the process of target detection is also coiled "binary integration".

Detection

SSR

Targets

The principle of automatic target detection discussed above con also be utilized for SSR targets. The information supplied by on SSR decoder and obtained by detection of bracket pulse pairs is processed in on SW detector.

Processing

Detected

Targets

Basically, the process of automatic target detection is completed when the PR and SSR azimuth and range doto of detected targets hove been fed to o memory. For operational reasons, it is necessary to complete the plot extractor by provisions for further processing of detected targets. A remarkable result of automatic target detection which con be exploited operationally is the substantial reduction of the doto flow originally coming from the radar system. This finds its expression in the fact that 2 to 3 telephone lines will be sufficient for transmission of the detected targets e. g. to the control center located at some distance, while o bandwidth of approximately 10 MHz is required for transmission of the PR and SSR raw video. This makes it possible to materialize the above network for the exchange of radar information over great distances without exceeding a reasonable cost level. In some instances, radar plot extractors ore used if only for the reason thot thus the

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problem of radar data transmission can be solved in a technically simple way. For the transmission of information via telephone cables, several additional provisions ore required in the plot extractor: a) buffer memory for converting the statistical data flow into a continuous one; b) circuits for assembling the target information into socalled target messages; c) provisions for digital data transmission through telephone lines ("modem"). In the interest of a data flow reduction, it is furthermore useful to assemble the PR and SSR replies of the same target aircraft into one target message. For purposes of monitoring the overall system, defined test targets can be inserted automatically, and their correct processing can be checked continuously. Malfunctions can also be reported and switchover to the standby unit be initiated. This short discussion gives only a general outline of the possible and necessary requirements for processing detected targets which must be met by a plot extractor. For the solution of this and other problems, a so-called process computer is used which can be universally programmed. The respective tasks are then determined by a program specifically established for this computer. Utilization of such a computer for the target processing portion of a digital detector permits a high degree of flexibility with respect to the operational requirements. By program changes it will be possible at any time to make use of the experience gained during operation, and to comply with particular needs.

BlockDiagram of a Combined PR/SSRPlot Extractor Figure 4 is a block diagram of a combined PR/SSRplot extractor equipped with a process computer: The group denoted "plot detector" accommodates the circuits for target detec.tion, one separate processing channel each being provided for PR and SSR targets. Since the antennas of both radar systems are mechanically or electronically linked, a commong timing unit is sufficient for the generation of digital azimuth and range information. If a start-of-target or end-of-target is declared, the associated azimuth and range data are sent to the computer by transfer circuit. In addition to these data, the received code information is connected through to the computer if the start-of-target of an SSR target hos been declared. One of the computer tasks is then to determine the center-oftorget from the azimuth values for start-of-target and endof-torget. Additional tasks ore i. o. the previously mentioned correlation of PR and SSR-replies from the some target, buffering of the position coordinates of the detected targets and their associated additional data, organization of doto transmission and performance of test functions. The units shown in the block diagram ore duplicated in the plot extractor systems utilized in actual operation; this meons that immediate switch-over to a complete standby portion is possible if a malfunction occurs. By this measure and the almost exclusive 1:1seof integrated circuits for the individual circuitries it is possible, despite the great number of functions to be exerted, to attain such a high degree of reliability that a practically failure-free operation con be ensured. fost

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Selenia offers very advanced equipment for Air Traffic Control, including: ATC RADARS BROAD BAND AND NARROW BAND LINKS DIGITAL DISPLAY SUBSYSTEMS COMPUTERS PRIMARY AND SECONDARY RADAR EXTRACTORS SIMULATORS AND DIGITAL INTERFACE EQUIPMENT

together with wide experience in: SYSTEM DESIGN SYSTEM IMPLEMENTATION AND INTEGRATION LOGISTIC SUPPORT.

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RADAR AND SYSTEMS DIVISIONS ROME-ITALY

selenia airtraffic contro system . .,-,Seleniaoffers very advanced equipment - -•for Air Traffic Control, including:

e ATC RADARS

e BROAD BAND AND

NARROW BAND LINKS e DIGITAL DISPLAY SUBSYSTEMS e COMPUTERS e PRIMARY AND SECONDARY RADAR EXTRACTORS e SIMULATORS AND DIGITAL INTERFACE EQUIPMENT

together with wide experience in:

e SYSTEM DESIGN e SYSTEM IMPLEMENTATION AND INTEGRATION e LOGISTIC SUPPORT.