Engineering Matters

Including: Gyro blade cracks response, propeller up-keep, ballistic parachute recovery systems and G-INFO and the perils of animals using your aircraft for food storage…

Welcome to Engineering Matters – the section of Light Aviation that is dedicated to discussing all manner of topics concerning both technical and operational aspects of the LAA fleet. If you have anything to say that you think would benefit others, then please email words and pictures to LAA Engineering at engineering@laa.uk.com

Gyro blade cracks – Part two

Following a piece in the February edition of Engineering Matters, Gerry Speich of AutoGyro GmbH sent in the following response. In that article, I should have made it clear that the words regarding cracks in gyro blades were provided by experienced gyro Inspector Kai Barnett. LAA Engineering always values the opinion of Inspectors and their findings and views on any aircraft in the LAA fleet. With the factory-built gyros, LAA Engineering has no direct design authority input as this is provided by the CAA.

Below is Gerry Speich’s letter response to the previous gyro comment on blade cracking:

Dear Sirs

I took the time to read the magazine February edition during my flight to Germany last night, and was surprised and disappointed to read the comments made regarding our aircraft rotor blades in the Engineering Matters section.

The fourth paragraph reads: ‘The calculated safe life of the RotorSystem 2 blades is 2,500hrs hours, however it has been found that the real-world service life is significantly less. Apparently, this is because the calculated service life was based upon the rotors being in flight, and the associated stresses it causes. The calculation does not take into account the stresses applied to the system in a ‘non-flying’ state such as taxying (and not rotating) or at a lower rpm, or a combination of the two, as this is highly unpredictable and very much depends upon the surface on which the aircraft is operating from.

‘For many owners the news that their ‘2,500 hour’ rotor may only last 1,600 or 1,800 hours will be a great disappointment but in reality, reaching 1,500hr will take a considerable time.

‘Flight school aircraft used for continual training may need to figure in revised replacement costs and may even have to factor in flight cycles, touch-and-goes, pre-rotations and other operating regimes as well as flight hours’.

AutoGyro has no idea where you obtained this opinion from, but it has not been fact checked or verified with AutoGyro, misrepresents the situation, and creates drama out of fresh air. There also seems to be a misunderstanding of what a rotor safe life limit is based upon.

There are actually two different crack scenarios described within long standing AutoGyro inspection documents, and highlighted within the recently issued MPD.

The longitudinal cracks as shown in the photo, of which three affected rotors have been found in the UK in that last 16 years of operation.

A small chordwise crack in one side of one outboard bolt hole on one face of one rotor blade at some 1850hr in service, of which only this one instance in a Rotorsystem II has been reported in our entire global fleet of around 3,000 aircraft. This is the crack that could be considered to affect the safe life limit, or the means to manage the safe life, but until a proper engineering investigation as to the root cause is completed, no action will be taken. At the time of writing the MPD checks have not revealed any other instance.

The ‘safe life’ is the life at which AutoGyro (and concurred with the UK CAA, based upon the figures agreed) consider that a critical failure that risks life is very unlikely. It does not include areas of the part that are not critical to flight safety.

This safe operating life of the rotor system is based on strain gauge data gathered during ground handling, taxying, pre-rotation and flight of our aircraft, assessed with different loading conditions. It is NOT true that the data does not take into account taxying and non-rotating rotors, although, of course, the tests cannot take into account every possible ground operation loading due to the global diversity of such events. The data gathered was used to estimate an operational safe fatigue life with specific reference to the critical area of the rotor blade. The longitudinal crack shown in your photo, while undesirable, is not flight critical, and this area is not part of the safe life calculation.

Therefore, it is not true that the real-world safe life is significantly less than that published, and no reason to change this limit due to the longitudinal crack. And no reason to create market concern by suggesting this, this is drama for the sake of drama. For your information, several rotor systems in UK flight schools have already reached their limits and been scrapped at the appropriate timelines.

The cause of the longitudinal crack is under investigation, and at this time is thought to have been initiated by some unplanned or uncontrolled loading event. If and when this has been determined, we can give more information, but since the rotors in question have been in service for 10 years and with significant flight time, data availability is limited.

We take a lot of pride in our products and make enormous efforts to make a safe and robust aircraft to suit all markets around the world.

Best regards,

Gerry Speich, CEO AutoGyro GmbH

Propeller maintenance and overhaul

Whether or not a propeller fitted to an LAA-administered aircraft must be maintained to the manufacturer’s maintenance recommendations, has long been a subject of extensive conversation for owners and Inspectors alike.

The Inspectors’ procedures manual, SPARS, and latterly, the relevant propeller TADS, call for certified propellers to be maintained in accordance with the manufacturer’s maintenance recommendations. Under the certified aircraft’s world of PART-ML maintenance, owners can opt out of certain maintenance requirements, such as engine and propeller overhaul.

In order to clarify the situation with LAA-administered aircraft propeller maintenance and to realign the Permit to Fly maintenance requirements with that of the certified world (i.e. to not be more onerous), the decision has been made by LAA Engineering that the advice to LAA-administered aircraft owners (and Inspectors) for all propeller types is now as follows.

LAA Engineering recommends that all propellers are maintained in accordance with manufacturer’s recommendations and requirements This includes certified, uncertified, variable pitch, ground adjustable and fixed pitch propellers.

Ex-certified types must comply with all Airworthiness Directives under CAA MPD 1995-001. For the non-certified aircraft, Airworthiness Directives issued by the State of manufacture or registration should be technically assessed and complied with accordingly. Airworthiness Directives considered mandatory by LAA Engineering will be listed in the relevant propeller TADS.

The purpose of Engineering Matters is to provoke thought and comment and it is certainly not intended to upset anyone, be they manufacturers, dealers, suppliers or owners.

Hopefully, covering any subject such as cracks in blade roots, gets people thinking about their own aircraft (rotor blades and propeller blades, gyros and fixed-wing) and potential problems that they might encounter – regardless of the type of flying machine, its age or the material from which it has been constructed.

Owners may decide not to comply with manufacturer’s maintenance recommendations and requirements, but if this is the case, the owner (or one of the co-owners in a group-owned aircraft) must clearly state in the logbook (airframe logbook for fixed-pitch and ground adjustable propellers, propeller logbook for variable pitch propellers) that they have taken the decision not to comply with the manufacturer’s maintenance recommendations. The statement must include the propeller designation and serial number and include the owner’s signature, printed name and date.

The owner’s decision to apply (or not) a variation to the manufacturer’s recommended maintenance procedures may be based on a number of factors such as aircraft being used for remunerated use, Night/IFR operations or even that the aircraft is based outside in a coastal environment.

LAA Engineering continues to promote the LPIP inspection procedure, which owners may choose to follow where applicable, but it should be noted that extra work may be required once the propeller has been stripped down. Propeller overhaul organisations will not be able to ‘release’ a propeller to service if, in their opinion, it is not serviceable.

It should also be remembered that an LAA Inspector is not under any obligation to sign for an aircraft which they believe is not being maintained in a manner that they feel it should be.

As soon as possible, the relevant Technical Leaflets, TADS and other information will be amended to reflect the above, and LAA Engineering will issue an LAA Alert in due course.