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Frost is hazardous because it causes early airflow separation resulting in a loss of lift. Ice, snow, or frost, having a thickness and roughness similar to medium or coarse sandpaper, on the leading edge and upper surface of an airfoil can reduce lift by as much as 30% and increase drag by 40%.

When conditions favoring the formation of ice are present, pilots should check for ice accumulation prior to flight using a flashlight and watching for light reflection.

The first sign of ice accretion in flight is generally found on the pitot tube, if it is visible, or in small, narrow exposed areas. Icing can be difficult to identify on the flat upper wing surface. If you detect icing accumulation in flight, especially if the aircraft is not equipped with a deicing system, you should leave the area of precipitation, or fly to an altitude where the temperature is above freezing.

The highest accumulation rate of structural icing should be expected when the aircraft is in freezing rain. Heavy icing on the leading edge is not as bad as light icing on the upper surface. Small patches of ice sparsely distributed on the upper surface of the wing can cause asymmetrical stalls that result in roll control problems on takeoff. Should you experience uncommanded roll due to icing forward of the ailerons, the most appropriate response is to reduce the angle of attack by reducing the aircraft pitch, and if in a turn, roll wings.

Tailplane icing can be detected by pulsing, oscillations, or vibrations in the elevator controls or a sudden, uncommanded pitch down of the nose. Buffeting or vibrations after extending the flaps upon exiting or during icing conditions, probably indicate an incipient tailplane stall.

Ice pellets indicate freezing rain at a higher altitude. Glossy, transparent ice in temperatures less than +5 °C should alert a pilot to the presence of large supercooled water droplets. SLD (supercooled liquid droplets) can accrue aloft even if the droplets are not being observed at the surface. Temperatures are above freezing at some higher altitude if you fly into rain which freezes on impact. If wet snow is encountered the temperature is above freezing at your altitude.

Deicing boots should be activated at the first indication of icing. Continue cycling of the boots after leaving icing conditions to ensure any residual ice has been removed. Pneumatic deicing systems should be cycled several times to remove all ice. After deice boots are inflated and ice is shed, residual ice increases with a decrease in airspeed or temperature.

A generally recommended practice for autopilot usage during cruise flight in icing conditions is periodically disengage the autopilot and hand fly the airplane. This may alert the pilot to unusual flight characteristics indicative of icing. If you experience icing during an approach, you should retract the flaps as necessary and disengage the autopilot.

From the NTSB Safety Alert - Aircraft Ground Icing

• Fine particles of frost or ice, the size of a grain of table salt and distributed as sparsely as one per square centimeter over an airplane wing’s upper surface, can destroy enough lift to prevent a plane from taking off.
• Virtually imperceptible amounts of ice on an aircraft wing’s upper surface during takeoff can result in significant performance degradation.
• Small, almost imperceptible amounts of ice distributed on an airplane’s wing upper surface cause the same aerodynamic penalties as much larger (and more visible) ice accumulations.
• Small patches of ice or frost can result in localized, asymmetrical stalls on the wing, which can result in roll control problems during liftoff.
• It is nearly impossible to determine by visual observation alone whether a wing is wet or has a thin film of ice. A very thin film of ice or frost will degrade the aerodynamic performance of any airplane.
• Ice accumulation on the wing upper surface may be very difficult to detect from the cockpit, cabin, or front and back of the wing because it is clear/white.
• Accident history shows that nonslatted, turbojet, transport-category airplanes have been involved in a disproportionate number of takeoff accidents where undetected upper wing ice contamination has been cited as the probable cause or sole contributing factor.
• Most pilots understand that significant visible ice contamination on a wing can cause severe aerodynamic and control penalties, but it is apparent that many pilots do not recognize that minute amounts of ice adhering to a wing can result in similar penalties.
• Despite evidence to the contrary, these beliefs may still exist because many pilots have seen their aircraft operate with large amounts of ice adhering to the leading edges (including the dramatic double horn accretion) and consider a thin layer of ice or frost on the wing upper surface to be more benign.