FAA Powerplant Written Practice Test

What is the main function of the automatic mixture control on a pressure injection carburetor?

1. To manually adjust fuel flow

2. To compensate for changes in air density due to temperature and altitude

3. To enhance the acceleration during takeoff

4. To maintain a constant fuel-air mixture

The correct answer is: To compensate for changes in air density due to temperature and altitude

The main function of the automatic mixture control on a pressure injection carburetor is indeed to compensate for changes in air density due to temperature and altitude. As an aircraft ascends, the air density decreases, which can affect the performance of the engine. The automatic mixture control adjusts the fuel-air mixture based on these variations, ensuring that the engine continues to receive the optimal amount of fuel relative to the amount of incoming air. This adjustment is crucial for maintaining engine performance and efficiency as external conditions change. In contrast, the other options focus on different aspects of engine operation. Manual adjustment of fuel flow is typically not the role of the automatic mixture control, as it is designed to automate this process. Enhancing acceleration during takeoff might require rich mixtures, but it is not the primary function of the automatic control system. Maintaining a constant fuel-air mixture can be a general goal of mixture controls, but the specific role of the automatic mixture control is particularly about adapting to changing air densities, making option B the most accurate.