During a compression check, what does a propeller movement in the direction of engine rotation indicate?

A propeller movement in the direction of engine rotation during a compression check indicates that the piston has passed top dead center (TDC). As the piston travels upward in the cylinder on the compression stroke, it compresses the air-fuel mixture. Once it reaches TDC, the crankshaft continues to rotate, causing the propeller to move.

After the piston reaches TDC and begins descending, the propeller continues to rotate in the direction of engine rotation, which is typically the same direction that the engine produces thrust. This behavior illustrates that the compression stroke is complete and the piston is now descending on the power stroke. Therefore, the correct interpretation of this propeller movement is that it reflects the piston having moved past TDC.

Understanding this concept is crucial for diagnosing engine performance, as it can provide insights into the engine's operational phases during a compression check, which is a vital part of routine maintenance and troubleshooting.