FAA Aviation Mechanic Powerplant (AMP)Reciprocating EnginesHard
A pilot applies carburetor heat to clear suspected carburetor icing during cruise flight in a normally aspirated reciprocating engine. What immediate effect on engine operation should the pilot expect from applying carburetor heat, assuming no ice is present?
- AAn immediate increase in fuel pressure at the carburetor inlet
- BNo measurable change in engine performance
- CAn increase in RPM due to improved volumetric efficiency
- DA decrease in RPM and slightly rougher running due to less dense, richer intake air
Show answer & explanationAnswer & explanation
Correct answer: D. A decrease in RPM and slightly rougher running due to less dense, richer intake air
Carburetor heat routes warm, unfiltered air into the induction system, which is less dense than ambient air. This decreases power output (lower RPM in a fixed-pitch prop) and enriches the fuel/air mixture because the carburetor meters fuel based on airflow volume, not density, causing slightly rougher running if no ice was present.
Why the other options are wrong
- A. Carburetor heat does not directly affect fuel pressure at the inlet.
- B. There is always a measurable performance change from applying carb heat.
- C. Volumetric efficiency decreases, not increases, with heated air.
Carburetor Heat Effect
Applying carburetor heat introduces warm, less dense air into the induction system, reducing power and enriching the fuel/air mixture, useful for melting ice but causing a slight performance penalty when no ice is present.
- Warm air is less dense, reducing volumetric efficiency and power
- Mixture becomes richer since fuel metering is based on airflow volume
- Should be used cautiously since it can promote detonation if misused during high power settings
Memory trick: Hot air thins power and thickens the mix