How Clean Is Your Wing? Contributed by Captain (Ret'd) Stephen Klubi

How Clean Is Your Wing? Anecdotes from an Aging Aviator Contributed by Captain (Ret'd) Stephen Klubi, Cathay Pacific Airways

Editor's note: Many thanks to VFC alumnus (1973) Stephen Klubi for sharing his expertise after decades in the civil, military, and commercial aviation space.

The outside air temperature had been very cold during your flight, about as forecast and about what you would expect flying throughout the cooler months. The destination weather is not too bad, 2°C with the potential for some precipitation. You land, taxi in, shutdown and organize the refueling. While the Refueller is topping up the tanks, you decide to do a quick walkaround and after the refueling is complete, check the fuel caps are on tight before heading into the office to pay for the fuel. While in the office you happen across an old friend who talks you into a quick cup of coffee. While catching up you notice rain starts to fall, nothing too serious. After 40 minutes you head back out to the aircraft and get ready to continue onto your next destination. Do you and should you have a check of the wings before launching off? Is there some phenomenon at work that could cause ice or frost to form on the upper wing surfaces when you may not expect it?

Every aircraft operating manual for every aircraft that I’ve flown and Transport Canada’s AIM "Airmanship" section stress that take off is prohibited when frost, ice or snow is adhering to any critical surface of the aircraft. This is referred to as the "Clean Aircraft Concept." The AIM and the aircraft operating manuals define clearly what a ‘critical surface’ is. It is important to note that it can be different on different aircraft. For example, on some of the larger aircraft that I have operated, we were allowed to take off with a layer of thin hoar frost on the upper fuselage as long as marking could be seen through this frost and up to 3mm of frost on the underside of the wing. Frost or rarely frozen ridges of ice forming on the underside of the wings were very common even in the tropical areas I operated and frost forming on the upper surfaces of the wing in the higher northerly or southerly latitudes during the northern or southern hemispheres fall, winter or spring. This phenomenon called "cold soaking" is responsible for this frost and ice formation and is described in detail in Transport Canada’s AIM.

This information was not widely understood by pilots or widely disseminated until a rather tragic aircraft accident occurred in Dryden, Ontario on March 10, 1989. An Air Ontario Fokker F28 crashed 49 seconds after take off after failing to gain altitude due to ice formation on the wings. Of the 69 passengers and crew onboard, 24 lost their lives including both pilots. The investigation and the Commission of Inquiry into the crash by the Honourable Virgil P. Moshansky was a watershed moment in Canadian aviation. His 400+ page final report was the product of an exhaustive investigation not only into that particular crash but the aviation system that allowed it to occur. His report was published in 1992. In the section of the report titled "Aircraft Performance and Flight Dynamics," the Commission of Inquiry obtained the assistance of highly qualified experts who, at that time, were not normally involved in aircraft accident investigations. One such expert was Dr. Myron Oleskiw, a Research Meteorologist with expertise in studying ice accretion on airfoils from the Low Temperature Laboratory at the National Research Council.

Quoting from the report starting on Page 376:

""Cold Soaking" is a term used to indicate that an object has been in a cold temperature long enough for its temperature to drop to, or near to, the ambient temperature. Temperature at altitude is almost always colder than at ground level, and, although the outer skin of an aircraft in flight will cool quickly, the fuel in the wing tanks, because of its latent heat properties, will cool more slowly. The longer the aircraft remains at altitude, the closer the temperature of the fuel will be to the ambient temperature. On landing, the reverse occurs. The skin of the aircraft will warm quickly to ambient temperature, while the fuel will warm more slowly. However, the aircraft skin that is touched by the cold soaked fuel will remain close to the temperature of the fuel touching it."

"A well known phenomenon frequently occurs on an aircraft that

has landed with cold-soaked fuel in the wing tanks: moisture from the air deposits in the form of frost on the surfaces that are touched by the cold fuel. These frost deposits form under the wing tanks. On landing, the fuel in the wing tanks is normally depleted. Since there is no tank fuel to touch the skin at the top of the wings, there usually will not be a frost deposit on the upper wing surface."

"On occasion, however, there will still be enough cold fuel in the tanks on landing to touch the skin on the top of the wings. Addition of fuel at a warmer temperature will raise the level of the fuel to touch the upper surface of the wing but may not bring the resultant temperature of the fuel above the freezing level. Frost can then form on the upper surface of the wing that is touched by the cold fuel. Rain can freeze to the upper wing surface in the form of a smooth, transparent sheet of ice, often virtually invisible. Falling wet snow can also freeze to the upper wing surface, and the resulting ice surface may not be smooth."

Transport Canada’s AIM "Airmanship" section has a lot of excellent information that discusses this phenomenon in detail, highlighting the fact that heavy freezing has been reported during drizzle or rain, even in a temperature range between +8 to +14 degrees C.

Another aspect of cold weather flying which may not be well known to pilots concerns the stall warning system on those aircraft which have systems installed.

The Commission Report includes a great deal of research and data derived from flight simulation concerning loss of lift associated with varying amounts of contamination on the wing as they tried to recreate the profile flown by the accident aircraft. Quoting from the report page 320:

"The wind tunnel tests also demonstrated that with severe roughness, the wing can be stalled before it reaches the angle of attack that would normally activate the aircraft’s stall warning system."

The AIM makes reference to this in the "Airmanship" section titled "Aircraft Contamination in Flight - In-Flight Airframe Icing," but I really think that this fact really deserves to be emphasized.

As the hot hazy days of summer started to give way to the cooler days of autumn, I always took the opportunity to review the general and aircraft specific procedures for cold weather operations. I believe this is a great habit to get into. These procedures could get rather complicated on large aircraft especially when you are planning a departure in falling snow. Not only should the correct application of all procedures required for de-icing and anti-icing the aircraft be adhered to, but take off performance has to be carefully and correctly calculated on a runway that is contaminated.

Winter flying can be great fun, but it can present us with more challenges. Often there is much less traffic, less convective activity and the visibility can be spectacular. By following the guidance in the Cold Weather Procedures chapter (published by the aircraft manufacturer) and Transport Canada's AIM "Airmanship" section, you will be well prepared to operate during this particular time of the year.

Image below, from https:// aircrafticing.grc.nasa.gov/ Feature Article