Minnesota Flyer - Serving Midwest Aviation Since 1960

Be careful during the frost of Fall


Dan McDowell

When the warm(ish) days and cool crisp nights of fall are present, many things begin to change. The kids are back in school, football season is on again and hockey season isn’t that far away either! The colors of the leaves and the damp, musky smells of fallen leaves on the ground begin to stir thoughts of the coming holidays and celebrations with family and friends. But for pilots, there is another very important thing that begins again. That thing is frost, specifically Hoar frost that is sometimes referred to as “Fret Frost” (no doubt for obvious reasons).

Hoar frost can develop in clear air on an aircraft or automobile for instance when the night time temperature is at or below freezing and the metal surface of the vehicle or aircraft comes in contact with the moist air of the night. A NASA definition of Hoar Frost reads; A uniform, thin white deposit of fine crystalline texture that forms on exposed surfaces during calm, cloudless nights when the temperature falls below freezing and the humidity of the air at the surface is close to the saturation point. It is not associated with precipitation. The deposit is thin enough that the underlying surface features, such as paint lines, markings or lettering can be distinguished.

While Hoar frost may not be as immediately dangerous as clear ice as it impacts the wings shape and lift capabilities but it cannot be left on wings and control surfaces as it is especially dangerous at take-off. In fact Federal Aviation Regulation (FAR) part 91.527 says in part:

§91.527 – Operating in icing conditions.

(a) No pilot may take off 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;

(2) Snow or ice adhering to the wings or stabilizing or control surfaces; or

(3) Any frost adhering to the wings or stabilizing or control surfaces, unless that frost has been polished to make it smooth.

A safety bulletin distributed by the National Transportation Safety Board (NTSB) on Aircraft Ground Icing says in part: Fine particles of frost or ice, the size of a grain of table salt and distributed as sparsely as one per square centimeter over and airplane wing’s upper surface, can destroy enough lift to prevent a plane from taking off. Almost virtually imperceptible amounts of ice on an aircraft wing’s upper surface during takeoff can result in significant performance degradation. Small patches of ice or frost can result in localized, asymmetrical stalls on the wing, which can result in roll control problems during lift off.

Given the previous information, what should pilots know and do about airplane icing before takeoff? The NTSB recommends:

• Pilots should be aware that no amount of snow, ice or frost accumulation on the wing upper surface should be considered safe for takeoff. It is critically important to ensure, by any means necessary, that the upper wing surface is clear of contamination before takeoff.

• The NTSB believes strongly that the only way to ensure that the wing is free from critical contamination is to touch it.

• With a careful and thorough preflight inspection, including tactile inspections and proper and liberal use of deicing processes and techniques, airplanes can be operated safely inspite of the adversities encountered during winter months.

• Pilots may observe what they perceive to be an insignificant amount of ice on the airplane’s surface and be unaware that they may still be at risk because of reduced stall margins resulting from icing-related degraded airplane performance.

• Frost, snow, and rime ice may be very difficult to detect on a white upper wing surface and clear ice can be difficult to detect on an upper wing surface of any color.

• Many pilots may believe that if they have sufficient engine power available, they can simply “power through” any performance degradation that might result from almost imperceptible amounts of upper wing surface ice accumulation. However, engine power will not prevent a stall and loss of control at lift off, where the highest angles of attack are normally achieved.

It is the pilot’s responsibility for the safe operation of that aircraft from the start to the finish of the flight . Be safe. Stay aware of Fall’s frost.


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