FAA Sport Pilot Airplane (SPA)WeatherEasy
During a cold-morning preflight, a pilot notices a thin layer of white crystalline deposit covering the upper wing surfaces, though the metal is dry underneath. What effect will this deposit have on takeoff performance if not removed?
- AIt disrupts the smooth airflow over the wing, decreasing lift and increasing stall speed
- BIt has no aerodynamic effect on light aircraft
- CIt only affects control surface hinges, not the wing's lifting ability
- DIt increases lift by adding surface roughness that energizes the boundary layer
Show answer & explanationAnswer & explanation
Correct answer: A. It disrupts the smooth airflow over the wing, decreasing lift and increasing stall speed
Frost, though thin and dry, disrupts the smooth laminar airflow over the wing, causing early boundary layer separation. This decreases the lift produced at a given angle of attack and increases the stall speed, so all frost must be removed before flight.
Why the other options are wrong
- B. Frost is a well-documented hazard even on small aircraft.
- C. Frost affects the entire lifting surface, not just hinges.
- D. Frost degrades rather than improves aerodynamic performance.
Frost Formation
Frost is ice crystals that form directly from water vapor on a surface below freezing, disrupting smooth airflow over lifting surfaces.
- Forms on clear, cold nights with high humidity
- Must be completely removed before flight, even if thin
- Increases stall speed and decreases lift
- Can also affect pitot/static ports if not cleared
Memory trick: Frost is Fine Sandpaper on the Wing — smooth flow breaks down.