The basic purpose of adjusting the fuel/air mixture at altitude is to

• A. Increase fuel flow to prevent lean conditions
• B. Alter ignition timing
• C. Enrich the mixture to improve power
• D. Decrease the fuel flow in order to compensate for decreased air density

Answer: (D)

The main idea is that air becomes thinner with altitude, so the amount of oxygen available for combustion drops. If you keep the same fuel flow, the fuel/air ratio becomes too rich for the reduced air, which can cause rough running, detonation risk, and poorer efficiency. To keep combustion near the optimal ratio, you decrease the fuel flow accordingly. Ignition timing and simply enriching the mixture aren’t the primary goals here, and enriching at altitude would make the mixture even richer, not better. So the best approach is to reduce fuel flow to compensate for the decreased air density.