FAA Airline Transport Pilot (ATM)Meteorology and Weather ServicesHard
During preflight on a cold, clear morning, a pilot finds a thin, uniform layer of frost covering the upper surface of the wing. Even though the frost feels smooth to the touch and appears thin, why must it be removed before flight?
- AIt reduces the aircraft's weight slightly, which improves climb performance
- BIt disrupts smooth airflow over the wing, increasing stall speed and reducing lift at a given angle of attack
- CIt primarily affects drag but has no measurable effect on lift or stall characteristics
- DIt has negligible aerodynamic effect since it is thinner than typical sandpaper grit
Show answer & explanationAnswer & explanation
Correct answer: B. It disrupts smooth airflow over the wing, increasing stall speed and reducing lift at a given angle of attack
Even a thin, rough layer of frost disrupts the smooth laminar airflow over the wing's upper surface, causing early boundary layer separation. This increases stall speed and decreases the maximum lift coefficient, which is why FAA regulations prohibit takeoff with frost adhering to critical surfaces.
Why the other options are wrong
- A. Frost adds weight rather than reducing it, and it has no beneficial performance effect.
- C. Frost affects both lift (by disrupting airflow) and drag, not drag alone.
- D. Frost roughness comparable to fine sandpaper has been shown to significantly degrade lift, contrary to this claim.
Frost and Aerodynamic Effect
Frost, even in a thin layer, disrupts smooth airflow over lifting surfaces, causing premature boundary layer separation, increased stall speed, and reduced lift.
- 14 CFR prohibits takeoff with frost, ice, or snow adhering to critical surfaces
- Frost roughness comparable to coarse sandpaper can reduce lift up to 30%
- Must be removed by deicing or allowing it to sublimate before flight
Memory trick: Frost = frosted glass over the wing — smooth-looking but ruins clean airflow