Private Pilot Private Exam

What occurs if a pilot descends from 9,500 feet MSL to 4,500 feet MSL without adjusting the mixture?

1. The mixture may become excessively rich

2. The mixture may become excessively lean

3. No change in the mixture

4. The engine will misfire

The correct answer is: The mixture may become excessively lean

When a pilot descends from a higher altitude, such as 9,500 feet MSL (Mean Sea Level) to a lower altitude like 4,500 feet MSL, the atmospheric pressure increases, which affects the air density. As the density of the air increases during descent, there is more oxygen available for combustion in the engine. If the pilot does not adjust the mixture control, the existing fuel-air mixture becomes richer than optimal because the engine is still drawing the same amount of fuel as it did at the higher altitude, but with more air now being drawn in due to the increased atmospheric pressure. This situation can lead to an excessively rich mixture, causing the engine to run less efficiently. While the engine may continue to operate without immediate issues, relying on a mixture that has not been adjusted can lead to performance problems over time, such as a decrease in power or efficiency, especially as the aircraft transitions through varying altitudes. Therefore, understanding the need to enrich or lean out the mixture based on altitude changes is crucial for maintaining optimal engine performance.