FAA Powerplant Written Practice Test

As a pilot ascends in altitude on a carburetor without an automatic mixture control, how does the mixture change?

1. The mixture is leaned

2. The mixture remains the same

3. The mixture becomes enriched

4. The mixture is throttled

The correct answer is: The mixture becomes enriched

As a pilot ascends in altitude on a carburetor that does not have an automatic mixture control, the mixture becomes enriched due to the decrease in air density. At higher altitudes, the air is less dense, which means there is less oxygen available for combustion. However, the amount of fuel being drawn into the engine remains relatively constant, leading to a higher fuel-to-air ratio or an enriched mixture. This phenomenon occurs because carburetors operate based on the principle of airflow through the venturi, where the airflow pressure influences fuel delivery. As the aircraft ascends and air pressure decreases, the reduced density results in less air volume mixing with fuel. If the pilot does not manually adjust the mixture to compensate for the changing air density, the engine may run rich, leading to incomplete combustion and potential fouling of spark plugs. Thus, recognizing this change is vital for maintaining optimal engine performance and efficiency during ascent.