FAA Private Pilot Airplane (PAR)Aerodynamics and Aircraft SystemsMedium
A pilot is planning a high-altitude flight in a normally aspirated aircraft. To maintain engine performance and prevent spark plug fouling, what adjustment to the fuel-air mixture is typically required as altitude increases?
- AMaintain a full rich mixture to ensure maximum cooling.
- BLean the mixture to compensate for decreasing air density.
- CThe mixture setting is automatically adjusted and requires no pilot input.
- DEnrich the mixture to compensate for decreasing air density.
Show answer & explanationAnswer & explanation
Correct answer: B. Lean the mixture to compensate for decreasing air density.
As altitude increases, air density decreases. Without adjustment, the fixed amount of fuel in a normally aspirated engine would result in an overly rich mixture (too much fuel for the available air). Leaning the mixture reduces the fuel flow, maintaining the proper fuel-air ratio for efficient combustion and preventing issues like spark plug fouling and power loss.
Why the other options are wrong
- A. Maintaining a full rich mixture at altitude would lead to an excessively rich mixture, resulting in reduced power, rough engine operation, and spark plug fouling.
- C. Most light general aviation aircraft with normally aspirated engines require manual mixture control by the pilot; automatic adjustment is typically found in fuel-injected or turbocharged engines, or FADEC systems.
- D. Enriching the mixture at altitude would make an already rich mixture even richer, leading to poor performance and potential spark plug fouling.
Mixture Control at Altitude
The pilot's adjustment of the fuel-air ratio to compensate for changes in air density, primarily leaning for increased altitude.
- Air density decreases with altitude.
- Leaning reduces fuel flow to match less air.
- Prevents rich mixture, spark plug fouling, and power loss.
Memory trick: Lean High, Rich Low, Keep it Flow.