FAA Aviation Mechanic Powerplant (AMP)Reciprocating EnginesHard
A pilot is leaning the fuel/air mixture during cruise flight using an exhaust gas temperature (EGT) gauge and wants to set the mixture for best fuel economy rather than best power. According to standard leaning technique, the mixture should be set to:
- APeak EGT
- BApproximately 50 to 125°F rich of peak EGT
- CFull rich regardless of altitude
- DApproximately 200°F lean of peak EGT
Show answer & explanationAnswer & explanation
Correct answer: A. Peak EGT
Best economy mixture occurs at peak EGT, where the fuel/air ratio produces the most complete combustion per unit of fuel burned, minimizing fuel consumption. Best power mixture, by contrast, is achieved slightly rich of peak EGT (typically 50-125°F rich of peak), which sacrifices some economy for greater power output.
Why the other options are wrong
- B. This range describes best power mixture, not best economy.
- C. Full rich is used for takeoff/climb power and high-altitude cooling needs, not economy cruise.
- D. Leaning significantly beyond peak risks engine roughness and does not represent standard best-economy technique.
Best Power vs Best Economy Mixture
Best power mixture is set slightly rich of peak EGT for maximum power output; best economy mixture is set at peak EGT for minimum fuel consumption per unit of power.
- Best economy = peak EGT
- Best power = 50-125°F rich of peak EGT
- Leaning too far past peak can cause roughness or excessive CHT
Memory trick: Peak EGT is the sweet spot for sipping fuel; rich of peak is for muscle!