How Clean Is Your Wing? Anecdotes from an Aging Aviator
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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
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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
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