FAA Powerplant Written Practice Test

What does a movement of the propeller in the direction of engine rotation indicate during a compression check?

1. The piston is at bottom dead center

2. The piston is positioned past top dead center

3. The piston is in the exhaust stroke

4. The piston is in the intake stroke

The correct answer is: The piston is positioned past top dead center

A movement of the propeller in the direction of engine rotation during a compression check indicates that the piston is positioned past top dead center. This occurs because as the piston moves up toward top dead center, it compresses the air-fuel mixture in the combustion chamber. Once the piston has passed the top dead center, the propeller continues to rotate due to its connection to the crankshaft, which is moving in the same rotational direction as the engine. Understanding the relationship between the piston position and the propeller movement is crucial for diagnosing engine conditions. For example, if the propeller moves in the direction of rotation but the piston is not at the highest point (top dead center), it signifies that compression is fully established, allowing the air-fuel mixture to be compressed effectively. Recognition of this principle is essential for tracking engine performance and ensuring proper operation during maintenance checks.