FAA Powerplant Written Practice Test

What might a propeller moving backward during a compression check indicate?

1. The engine is stalling.

2. The piston has not yet reached top dead center.

3. The valves are currently open.

4. The engine is misfiring.

The correct answer is: The piston has not yet reached top dead center.

A propeller moving backward during a compression check suggests that the piston has not yet reached top dead center (TDC). During a compression check, the engine is turned over to assess the compressive seal within the cylinders as the piston moves up toward TDC. If the propeller is moving backward, it indicates that the piston is in the downward stroke or is in the process of compressing the air-fuel mixture in the cylinder rather than reaching the point where it would be at TDC. This situation typically occurs because the engine is being rotated in a direction that allows the piston to descend before reaching the compression phase, leading to an unusual visual indication of propeller motion. Understanding this concept is crucial as it helps mechanics diagnose issues effectively during checks and maintenance.