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The clean aircraft concept, means…


Have you ever taken off with some snow or frost on the wings? If you are still here, you are lucky. Every winter someone tries to take-off with contaminated wings and the result can be tragic.

Canadian Aviation Regulations (CARs) states that all critical surfaces of an aircraft must be clean of any surface contamination – the clean aircraft concept. The critical surfaces of an aircraft are the wings, control surfaces, rotors, propellers, horizontal stabilizers, vertical stabilizers or any other stabilizing surface. In the case of an aircraft with rear mounted engines, the upper surface of the fuselage is also a critical surface.

According to A.I.M. 2.12.2 (d), "Test data indicate frost, ice or snow formations having a thickness and surface roughness similar to medium or course sandpaper on the leading edge and upper surface of a wing can reduce lift by as much as 30% and increase drag by 40%." As well as affecting lift and drag, critical surface contamination can reduce thrust, increase stall speed, alter stall and handling characteristics and cause trim changes.

The pilot of a Murphy Rebel intended to take a local flight around the lake he was based from. Before departure the pilot brushed some of the snow off of the wings, but he did not completely clean off the wings. After becoming airborne, the right wing dropped.

He kept the aircraft flight path straight ahead with rudder and attempted to land straight ahead. The aircraft touched down on the lake in a left forward lip, causing the left float to fail. The aircraft rolled inverted and the pilot and his wife were able to egress the aircraft and swim to an island where they were later rescued.

Just after the C-172 lifted off, it appeared to bob up and down. One witness said the aircraft turned left and he heard a power reduction. The aircraft entered a dive and crashed into a wooded area and caught fire. The investigation did not reveal any mechanical or structural failures. Witnesses reported aircraft on the ramp had about 1/16 of an inch of ice on the wings at the time of the accident, and there was snow on the ramp. Investigators could not find any evidence that the pilot had de-iced the aircraft prior to departure.

Simply sweeping snow off of an aircraft may not be enough. Often there is a thin layer of frost or ice under the snow that may not even be visible to the eye. Running the fingers over a surface may detect contamination that may not be seen. This contamination must be eliminated before take-off as well.

We tend to forget about the propeller and vertical stabilizer when checking for frost or ice. We forget they are lifting or stabilizing surfaces.

The best solution is to prevent snow, frost or ice from forming on the aircraft in the first place. A hangar is best, but any shelter will do. It doesn’t have to be enclosed. #9; Wing and tail covers designed for each make and model of aircraft are available from several sources. They are not very expensive and work well. If professionally made wing and tail covers are outside the budget, cheap tarps can be used. They won’t fit as well as a professionally designed covers, but they work.

Be careful about putting covers over a wet wing. The water can freeze to clear ice under the cover and can be difficult to detect. This can also happen if the cover is poorly fitted or has tears. Anytime a cover is removed from a surface, the surface should be checked to make sure the cover was effectively doing its job.

If the aircraft does have some contamination on it, a heated hangar is the best option for eliminating the contamination.

De-icing fluids like isoproponol work well, especially if heated. Don’t rush the job. It takes a little time for the de-icing fluid to penetrate all of the way to the skin. Take care not to spray directly onto any of the aircraft windows, or the pitot or static ports, as damage may occur. The slush formed by the melted contamination during the de-icing process has a tendency to run down between the horizontal surfaces and the flaps, ailerons and elevators. The slush may re-freeze and jam a control surface. Make sure these areas are also clear of contamination before flight.

Let the aircraft sit in the sun. Even on very cold days, the sun will clean off the frost. It just takes time.

A rope rubbed gently over an aircraft surface will loosen most of the frost. A broom, a cloth or a glove can then wipe the surface clean down to the paint.

Gentle use of a plastic scraper will clean off difficult ice or frost, and in a pinch, a credit card will do. Plastic scrapers can damage the paint and deice boots if care is not taken.

Do not use hot water as a de-icing method. It just appears to work well. Hot water may leave a thin, hard to detect layer of ice on the wing surface. The water may also form ice around hinges, control rods and cables, which may cause control surfaces to jam.

High wing aircraft and float aircraft are very difficult to completely de-ice. There is a tendency to become frustrated and think a partial job is adequate.

Even after the aircraft has been de-iced, we must keep an eye on the aircraft surfaces as the atmospheric conditions that caused the problem may not have changed and the contamination may reform. A visual check just before take-off is important, and if there is any doubt, the take-off should be aborted.

The clean aircraft concept means NO snow, frost or ice on any critical aircraft surface.

Ignoring this rule is tempting fate. An occasional pilot survives, but is it worth taking the chance?

Dale Nielsen is an ex-Armed Forces pilot and aerial photography pilot. He lives in Abbotsford, B.C., and currently flies air charters. He still freelances as a flying instructor and seminar facilitator. Nielsen is also the author of seven flight training manuals published by Canuck West Holdings.

Got an aviation safety story to tell? Dale Nielsen would like to hear from pilots who have educational aviation experiences to relate. Excerpts from these stories will be used in upcoming safety articles. Dale can be contacted via e-mail: