A pilot reports that a carbureted engine ran rough during descent from cruise altitude on a warm, humid day, and power was restored after applying carburetor heat, though a slight power loss occurred initially. A mechanic later inspects the induction system and finds no discrepancies. What most likely explains the initial power loss when carburetor heat was applied?
- AThe alternate air door failed to open, causing a vacuum lock
- BThe warm air is less dense, so the fuel/air mixture becomes richer initially until adjusted, and the engine also loses ram air pressure recovery
- CApplying carburetor heat cuts fuel flow to the carburetor temporarily
- DCarburetor heat directly ignites unburned fuel in the intake manifold
Show answer & explanationAnswer & explanation
Correct answer: B. The warm air is less dense, so the fuel/air mixture becomes richer initially until adjusted, and the engine also loses ram air pressure recovery
Carburetor heat routes warm, less dense, unfiltered air into the induction system. This initially enriches the fuel/air mixture because it's now less air by mass for the same fuel flow, and the heated air source often lacks a ram air intake, causing a brief power reduction before the ice melts and the engine returns to smoother operation.
Why the other options are wrong
- A. This describes an alternate air malfunction, not the normal, expected response to applying carb heat.
- C. Carburetor heat is a valve for air routing and has no direct effect on fuel flow.
- D. Carburetor heat does not ignite fuel; it simply provides warm air to melt ice.
Carburetor Heat Effects
Applying carburetor heat routes warm, unfiltered, less dense air into the induction system, temporarily enriching the mixture and reducing power slightly before ice melts and power is restored.
- Warm air source bypasses air filter
- Less dense air causes temporary rich mixture
- Loss of ram air recovery reduces manifold pressure slightly
- Should always be used at full application, never partial except for specific procedures
Memory trick: Warm air rushes in, mixture goes rich, power dips before ice melts and clears