FAA Flight Instructor Airplane (FIA)Weather and Weather ServicesMedium
A pilot is planning a flight with an outside air temperature of 55°F and visible moisture is not present, though the relative humidity is high. During a long descent at reduced power, what hazard should the pilot be most alert for?
- AFog formation in the cockpit
- BStructural icing on the wings
- CReduced fuel vapor pressure
- DCarburetor icing due to the temperature/humidity combination
Show answer & explanationAnswer & explanation
Correct answer: D. Carburetor icing due to the temperature/humidity combination
Carburetor icing can occur in temperatures well above freezing (roughly 20°F to 70°F) whenever relative humidity is high, even without visible moisture, because of the cooling effect of fuel vaporization and pressure drop in the venturi. Reduced power settings, as in a descent, increase this risk further.
Why the other options are wrong
- A. Cockpit fog is unrelated to engine induction system icing.
- B. Structural icing requires visible moisture (clouds/precipitation), which is not present here.
- C. Fuel vapor pressure is not the relevant hazard in this scenario.
Carburetor Icing
Ice can form in a carburetor's venturi and throttle valve area due to fuel vaporization cooling and pressure drop, even in above-freezing outside air temperatures with high relative humidity.
- Can occur between roughly 20°F and 70°F OAT
- High relative humidity increases risk even without visible moisture
- Most likely at reduced power/glide/descent settings
Memory trick: Even warm damp air can freeze the carb's air