FAA Ground School Practice Test

Why is the fuel/air mixture adjusted at altitude?

1. To maintain engine temperature

2. To decrease fuel flow in order to compensate for decreased air density

3. To increase power output

4. To stabilize engine pressure

The correct answer is: To decrease fuel flow in order to compensate for decreased air density

The adjustment of the fuel/air mixture at altitude is primarily necessary to compensate for the decreased air density. As altitude increases, the density of the air decreases, which means there are fewer oxygen molecules available for combustion in the engine. If the fuel flow remains constant while the air density decreases, the mixture would become richer (more fuel relative to air), leading to inefficient combustion and potential engine performance issues. By decreasing the fuel flow to match the reduced amount of incoming air, the correct stoichiometric mixture can be maintained, allowing for optimal combustion. This balance ensures that the engine runs efficiently and produces the desired power output even at higher altitudes. Maintaining engine temperature is important for engine performance but is a secondary effect of keeping the proper fuel/air mixture. The goal is to ensure the fuel is used efficiently, which indirectly aids in maintaining temperature. While increasing power output is a consideration, the primary purpose of mixture adjustment is to adapt to the changes in air density rather than to increase power directly. Stabilizing engine pressure is another factor, but it is not the primary reason for adjusting the fuel/air mixture in response to altitude changes.