During a compression check of a reciprocating engine, what does it indicate if the propeller moves in the direction of engine rotation?

When a compression check is conducted on a reciprocating engine and the propeller moves in the direction of engine rotation, it indicates that the piston has moved past top dead center. During the compression phase of the engine cycle, the piston travels upward, compressing the air-fuel mixture. If the propeller is turning in the direction of engine rotation, it means the engine is in the process of completing the compression stroke and has passed the point of maximum compression, which is top dead center.

This movement signifies that the piston has moved beyond the top dead center position, as the propeller will rotate due to the force created when combustion begins or during starter engagement. Typically, when the piston is at top dead center, the engine is on the verge of igniting the air-fuel mixture, and there wouldn't be significant rotational movement of the propeller until combustion occurs.

Understanding this motion is essential when diagnosing engine issues, as it provides insight into the engine's current position in its operational cycle and indicates whether all components are functioning appropriately.