FAA Ground School Practice Test

What happens to the fuel/air mixture when descending from 9500 feet to 4500 feet MSL without adjustment?

1. The mixture may become excessively rich

2. The mixture may become excessively lean

3. The mixture remains unchanged

4. The mixture may cause engine stall

The correct answer is: The mixture may become excessively lean

As an aircraft descends from a higher altitude, the atmospheric pressure increases, which affects the fuel/air mixture in a carbureted engine. At higher altitudes, the air density is lower, resulting in a leaner mixture, as the engine receives less air for the same amount of fuel. When descending from 9500 feet to 4500 feet MSL, if the fuel/air mixture is not adjusted, the increase in air density at the lower altitude means that the engine will draw in more air compared to the fuel being supplied. This will result in a leaner mixture than what is optimal for efficient combustion. If the mixture becomes excessively lean, it can lead to decreased engine performance and the risk of engine roughness or even stalling due to insufficient fuel for the amount of air being delivered. Therefore, without any adjustment to the mixture during descent, the correct answer refers to the potential for the mixture to become excessively lean as the aircraft transitions to a denser air environment.