FAA Powerplant Written Practice Test

What adjustment should a pilot make if they notice the engine running too rich at high altitude?

1. Lean the mixture

2. Enrich the mixture

3. Increase throttle gradually

4. Change fuel type

The correct answer is: Lean the mixture

When an engine is running too rich at high altitude, the appropriate adjustment is to lean the mixture. At higher elevations, the air density decreases, which means there is less oxygen available for combustion. An overly rich mixture, which has an excess of fuel relative to the amount of air, can lead to incomplete combustion, resulting in performance issues, increased fuel consumption, and potential engine fouling. Leaning the mixture reduces the amount of fuel being delivered to the engine, which helps maintain the proper fuel-to-air ratio. This adjustment optimizes engine performance by allowing it to burn a mixture that is more suitable for the less-dense air found at high altitudes. This practice is crucial for maximizing efficiency, maintaining power output, and ensuring the engine runs smoothly under these conditions. The other choices do not adequately address the need to adjust for lower atmospheric pressure. Enriching the mixture would further exacerbate the rich condition, increasing fuel consumption and potentially causing engine troubles. Gradually increasing the throttle might not solve richness and could worsen engine performance by adding more fuel without correcting the air-to-fuel ratio. Changing the fuel type is typically unnecessary and impractical for a simple altitude-related issue. Therefore, leaning the mixture is the best response to correct the situation