During a descent for landing with the engine at reduced power on a day with an outside air temperature of 65°F and visible moisture/high humidity, a pilot in a carbureted airplane should be most alert for which hazard?
- ACarburetor icing, which can occur even at relatively warm temperatures with high humidity, especially at reduced power settings
- BVapor lock in the fuel system due to high humidity
- CDetonation caused by an overly lean mixture
- DStructural icing on the wings
Show answer & explanationAnswer & explanation
Correct answer: A. Carburetor icing, which can occur even at relatively warm temperatures with high humidity, especially at reduced power settings
Carburetor icing can occur at outside air temperatures well above freezing, even into the 60s and 70s Fahrenheit, when relative humidity is high, because the temperature drop caused by fuel vaporization and pressure reduction in the carburetor venturi can bring the air below freezing internally. Reduced power settings, such as during descent, increase susceptibility to carb ice.
Why the other options are wrong
- B. Vapor lock is related to fuel line heating, not directly tied to humidity or carb icing conditions.
- C. Detonation is related to fuel/mixture and heat issues, not to humidity or carburetor icing.
- D. Structural icing requires visible moisture and airframe surface temperatures at or below freezing, unlikely at 65°F OAT.
Carburetor Icing
Carburetor icing can form when moist air passes through the carburetor venturi, where pressure and temperature drops (due to fuel vaporization) can cause ice to form even when outside air temperatures are well above freezing, especially with high relative humidity.
- Can occur at OAT up to around 70°F (21°C) with high humidity
- Most likely at reduced power settings such as during descent
- Symptoms include rough engine and gradual RPM/manifold pressure loss
- Carburetor heat should be applied at first sign of icing or per POH guidance
Memory trick: Carb ice doesn't care about warm air — it cares about moist air and low power