FAA Sport Pilot Airplane (SPA)Aircraft Performance and SystemsMedium
A pilot is flying on a day with an outside air temperature of 70°F and high relative humidity, though there is no visible precipitation. Carburetor heat is not being used. What is the primary concern under these conditions?
- ACarburetor icing is unlikely because the temperature is too warm
- BCarburetor icing can occur even in relatively warm temperatures when humidity is high, due to cooling from fuel vaporization
- CIcing can only occur when the outside air temperature is at or below freezing
- DHigh humidity reduces the risk of carburetor icing
Show answer & explanationAnswer & explanation
Correct answer: B. Carburetor icing can occur even in relatively warm temperatures when humidity is high, due to cooling from fuel vaporization
Carburetor icing can occur at temperatures well above freezing, even into the 70s°F, when relative humidity is high. This happens because fuel vaporization and the pressure drop across the venturi cause a significant temperature drop inside the carburetor, sometimes 60-70°F below ambient.
Why the other options are wrong
- A. Incorrect — carb ice is a serious risk even at temperatures this warm.
- C. Incorrect — icing frequently occurs well above freezing temperatures.
- D. Incorrect — high humidity actually increases the risk of carb icing.
Carburetor Icing
Ice formation inside the carburetor venturi caused by the temperature drop from fuel vaporization and pressure reduction, which can occur even in warm, humid conditions.
- Can occur with OAT as high as 70-100°F given high humidity
- Temperature drop inside carburetor can be 60-70°F
- Carb heat should be applied when icing is suspected, especially during descent/low power
Memory trick: Warm and wet is the sweet spot for surprise carb ice