During a power-off descent for landing with carburetor heat OFF, a pilot in a fixed-pitch propeller airplane notices a gradual, unexplained loss of RPM. The pilot applies full carburetor heat, and the RPM initially drops further before slowly climbing back higher than before. What does this sequence confirm?
- AThe carburetor heat control is malfunctioning and should be turned off immediately
- BThe engine has suffered a magneto failure unrelated to carburetor heat
- CThe engine is now running too lean, requiring the mixture to be enriched
- DCarburetor ice had formed, and the initial RPM drop was caused by warm air momentarily enriching the mixture and melting ice, followed by recovery as the ice cleared
Show answer & explanationAnswer & explanation
Correct answer: D. Carburetor ice had formed, and the initial RPM drop was caused by warm air momentarily enriching the mixture and melting ice, followed by recovery as the ice cleared
When carburetor heat is applied to an engine with carburetor ice, the initial RPM often drops further because the warm air is less dense (temporarily richening the mixture) and because melting ice produces water vapor that further disrupts combustion momentarily. As the ice fully melts and clears, RPM recovers and typically ends up higher than before heat was applied, confirming that ice was the original cause of the RPM loss.
Why the other options are wrong
- A. This response pattern is expected and normal, not a sign of malfunction.
- B. A magneto failure would not correlate with carb heat application in this pattern.
- C. The issue was ice-induced restriction, not a lean mixture problem.
Carb Heat Response to Icing
Applying carburetor heat to an iced carburetor typically causes a brief further RPM drop (as warm, less dense air and melting ice disrupt combustion) followed by a recovery to higher RPM as the ice clears.
- Initial RPM drop after applying carb heat is normal if ice is present
- RPM recovers and often exceeds the pre-heat value once ice melts
- This response confirms ice was the cause of the original power loss
Memory trick: Dip then rise — that's ice saying goodbye.