Frosty Peril

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With the arrival of Fall comes morning frost in many parts of the country. Frost is dangerous when it adheres to airfoils such as wings, control surfaces, propeller and rotor blades because even a thin and nearly invisible layer of frost can degrade lift catastrophically. In this timely article, AVweb's safety editor Brian Jacobson discusses how you can protect yourself from this serious threat.

91.527 Operating in icing conditions.

   (a) No pilot may takeoff an airplane that has -

      (1) Frost, snow, or ice adhering to any propeller, windshield, or powerplant installation or to an airspeed, altimeter, rate of climb, or flight attitude instrument system;
      (3) Any frost adhering to the wings or stabilizing or control surfaces, unless that frost has been polished to make it smooth.

Frost will occur when the surface temperature of an airplane is below freezing and below that of the dewpoint (which must also be less than 32 F). It usually forms early in the morning, and melts away as the temperature rises with the heating of the day and exposure to sunlight. However, if the temperature remains below freezing, and sunlight fails to heat the surface of the airplane, it is up to the pilot to be certain that steps are taken to clear the frost from the aircraft before flight is attempted.

Many pilots look at the wing surface and say there is no appreciable change in its shape, or that the amount of frost that has gathered is not appreciable. What they don't understand is that frost, because of the surface roughness created by its jagged crystals, may disrupt the flow of air across an airfoil enough to destroy a great deal of its lift. That makes flight at slow speeds, during landing and takeoff, hazardous, as the pilot of a Cessna T210 found out one November morning in 1994.

An Ill-Fated Frosty Departure

The airplane was parked overnight at the Isabel Valley Airport at Mount Hamilton, California. The pilot reported to investigators that he began his preflight activity around 0700 (PST) in preparation for flying his three passengers back to San Jose.

The weather was clear and cold. The pilot stated that he found nothing unusual during his walk-around inspection. He said there was no ice on the leading or trailing edges of the aircraft, the flaps, horizontal stabilizer or on the fuselage, though there was some on the windshield.

The pilot started the engine for a few minutes, and while he was warming it up his passengers arrived. He shut the engine down and boarded his passengers. At this time he tried to clear the frost from the windshield, but was not successful. A passenger offered some hot tea to pour on the window, and the pilot used it. The pilot then reboarded the airplane, restarted the engine, allowed it to warm up for ten minutes, then taxied out. He said there was no frost on the windshield at that time.

The dirt runway at the private airport was 3,000 feet long and 35 feet wide. The pilot began his takeoff roll at 0720. He said he rotated at 80 miles per hour and established a normal rate of climb. At that time the windshield began to "glaze over," and at 100 feet AGL the pilot could no longer see out of it. He raised the nose for a few seconds to be certain he climbed over the obstacles he knew were at the end of the runway.

By the time he reached 200-250 feet AGL he felt the engine was only at about 60 percent of its power output. The pilot dropped the nose to prevent a stall from occurring and began to descend while trying to increase airspeed. The aircraft dropped to the left, but he was able to keep it from spinning by using right rudder and leaving the power on. A high nose attitude was maintained to keep the airplane from dropping to the ground nose first. The aircraft's left main gear was ripped off when the aircraft touched down to the left of the runway on hard terrain. The airplane skidded about 100 feet before coming to a stop. All aboard suffered minor injuries.

At least one of the passengers did not agree with what the pilot told investigators. He said that the airplane had frost on all of its exposed upper surfaces, and that the only effort that was made to clean it off was the hot tea poured on the windshield. He felt that the takeoff was normal until the airplane climbed to about 100 feet AGL, and that the engine sound did not change from the time full power was applied until the airplane descended back to the ground.

What probably happened is that the pilot, unable to see through the frost covered windshield rotated the airplane to too high a nose attitude, and the frost on the airframe caused the airplane to stall at a higher than normal airspeed. The attitude that was required to clear the obstacles at the end of the runway did not help matters. All aboard were lucky that the airplane did not spin into the ground, because stall spin accidents from that altitude are normally fatal.

Had the pilot attempted to keep the airplane climbing by rotating the nose even higher, a typical response in a situation like that, the accident probably would have been more devastating for those aboard. No doubt that the use of the right rudder and the continued application of full power helped to minimize the injuries.

How Much Frost Is Too Much?

How much frost was on that aircraft? There is no consensus on that. The passenger did not offer any comment on how thick it was, and the pilot claimed it wasn't there. But it doesn't take much to change the aerodynamic affects of the airplane, to the point where it is not controllable.

So, before you go blasting into the sky this fall and winter be sure your airplane is clear of all frost, snow, and ice. The best way to do that is to move the airplane into a warm hangar and let it melt off. But before the airplane is pulled back outside it must be dried thoroughly or ice will form on the wet surfaces. And be certain that no water is left anywhere around the moveable control surfaces that could freeze and restrict their movement when the airplane is taken outdoors.

Another way to deal with frost is with deicing solutions such as alcohol or glycol. But keep in mind that these liquids offer protection that is short-lived, and that they provide no protection at all from frost that may form where the fluids cannot reach.

One winter I had a Cessna 402 defrosted in a warm hangar, and we dried the fuselage and wings thoroughly before pulling the airplane back out into the sub-zero temperatures. After loading my passengers we departed Lincoln, Rhode Island for Teterboro, New Jersey, but after takeoff as I tried to raise the landing gear they would not retract. The green lights remained on, so I returned to the airport for an uneventful landing. Water had condensed inside a faulty "squat switch" on one of the main gear then froze when the airplane was pulled back outside. The frozen switch would not let the gear retract.

FAR 91.527(a)(e) suggests that it's okay to take off with frost on an airplane so long as it has been "polished smooth." But what exactly does that mean? Is it possible for a pilot to insure that as he or she "polishes" the frost that the shape and smoothness of the airfoil is exactly the same as if there were nothing on it? I find that hard to believe. The best way to be sure there will be no unusual handling characteristics that could lead to any kind of incident is to remove all ice, frost or snow from the aircraft before any attempt to fly is made.

Frost You Can't See

The same conditions that cause frost to form on the outer surfaces of the aircraft can cause internal problems as well. If moisture condenses inside partially filled fuel tanks then freezes as the outside temperature drops, the resulting ice can block the flow of fuel to the engine. If your airplane is kept outdoors take precautions to avoid condensation. Top the fuel tanks, and be certain you drain a sufficient quantity of fuel from the tanks before flight that will assure you that there are no problems with the fuel flowing from the tanks.

Any moisture that is inside the fuel tanks, no matter its source, can freeze once an airplane is towed back outdoors. Leave the aircraft inside long enough for its temperature to modify then drain all of the fuel tanks until no water comes from the tanks. If a large amount of water is found get a mechanic to check the entire fuel system before departure. On airplanes with rubber bladder tanks water can be trapped in wrinkles at the bottom of the tank, and may slosh around when the airplane is being moved or is in flight. If that water freezes in the wrong part of the tank you might experience fuel starvation.

So, before you fly an aircraft that has a covering of frost on it, be sure it is cleaned thoroughly. Don't take any chances, no matter how little frost is on the airplane.