FAA Private Pilot Airplane (PAR)Aerodynamics and Aircraft SystemsMedium
A pilot climbs a normally aspirated, carbureted engine from sea level to 8,000 feet MSL without adjusting the mixture control. What is the most likely result?
- AThe mixture becomes too lean, causing detonation at high power settings
- BThere will be no effect since fuel flow is unaffected by air density
- CThe engine will automatically compensate through the carburetor's altitude compensation system
- DThe mixture becomes too rich, resulting in rough running, reduced power, and fouled spark plugs
Show answer & explanationAnswer & explanation
Correct answer: D. The mixture becomes too rich, resulting in rough running, reduced power, and fouled spark plugs
As altitude increases, air density decreases, but a fixed carburetor setting continues to meter fuel based on sea-level density. This causes the fuel/air mixture to become progressively richer with altitude, leading to rough operation, power loss, and possible plug fouling. The pilot must lean the mixture to restore the proper fuel/air ratio.
Why the other options are wrong
- A. An unadjusted mixture becomes too rich at altitude, not too lean.
- B. Fuel flow set for sea-level air density becomes excessive relative to the thinner air at altitude.
- C. Most trainer carburetors have no automatic altitude compensation; the pilot must lean manually.
Mixture Leaning with Altitude
As altitude increases, air density decreases; without leaning, the fuel/air mixture becomes progressively richer, requiring the pilot to lean the mixture to maintain proper engine operation.
- Air density decreases with altitude
- Unadjusted mixture becomes richer at altitude
- Leaning restores proper fuel/air ratio and engine performance
Memory trick: Higher you climb, richer it gets — lean to compensate.