FAA Powerplant Written Practice Test

As altitude increases, what general adjustment must be made to the fuel mixture to maintain performance?

1. Enrich the mixture

2. Lean the mixture

3. No adjustment is needed

4. Increase fuel flow

The correct answer is: Lean the mixture

As altitude increases, the density of the air decreases, which means there is less oxygen available for combustion in the engine. To maintain optimal performance and an efficient combustion process, the fuel mixture must be leaned out. This adjustment ensures that the ratio of air to fuel remains appropriate for the reduced oxygen levels at higher altitudes. Leaning the mixture effectively reduces the amount of fuel relative to the incoming air, which compensates for the decreased air density. If no adjustment were made, the engine could run too rich, leading to incomplete combustion, fouled spark plugs, and overall loss of power and efficiency. By properly leaning the mixture at increased altitudes, pilots help ensure better performance, smoother engine operation, and improved fuel efficiency.