FAA Powerplant Written Practice Test

From which source is sufficient pressure available to feed fuel into the idle circuit when idling?

1. From the fuel tank

2. From the atmosphere

3. From the engine

4. From the carburetor itself

The correct answer is: From the engine

The correct answer is derived from the understanding of how fuel systems work in an aircraft engine, specifically concerning idle conditions. While the fuel tank provides the source of fuel, it relies on gravity or a fuel pump to move the fuel, and during idling, the pressure must be sufficient to adequately supply fuel to the engine. During the idle condition, the engine is still running, which creates a negative pressure (vacuum) in the intake manifold. This vacuum draws fuel from the carburetor into the air-fuel mixture that feeds the engine. Therefore, the engine itself generates the necessary pressure to feed fuel into the idle circuit. The idle circuit is designed to operate effectively when the engine is running, ensuring that there is an adequate fuel supply to maintain optimal engine performance at low RPMs. In contrast, the other sources mentioned do not provide the required pressure during idling. The fuel tank typically relies on gravity or a mechanical/electrical pump to deliver fuel, and does not generate pressure independently. The atmosphere cannot provide pressure for fuel flow into the idle circuit. The carburetor does not produce pressure but instead facilitates the mixing of fuel and air, leveraging the negative pressure created by the engine to draw fuel in. Therefore, understanding that the engine