FAA Powerplant Written Practice Test

What is most likely the cause of apparent propeller vibration occurring only at cruise RPM ranges?

1. Propeller imbalance

2. Incorrect propeller for the engine

3. Engine misalignment

4. Excessive blade wear

The correct answer is: Incorrect propeller for the engine

The most likely cause of apparent propeller vibration occurring only at cruise RPM ranges is related to the incorrect propeller for the engine. When a propeller is not matched ideally for the specific engine's characteristics—including its power output, cruise speed, and rotational speed—it can create a condition where vibrations become more pronounced at certain RPM settings, particularly during cruise. At cruise RPM, the engine operates at a more consistent speed, which can enhance or highlight the effects of any mismatch between the propeller and engine. This misalignment between the propeller’s pitch, diameter, and design to the engine's characteristics can lead to inefficiencies that manifest as vibration during specific operational ranges. This would not typically be noticeable at lower RPMs where the load and speed dynamics are more variable. On the other hand, issues such as engine misalignment, propeller imbalance, or excessive blade wear may cause vibrations, but they generally do not selectively manifest only at cruise RPMs. For instance, a propeller imbalance would usually cause vibration across a broader RPM range, not isolated to cruise speed, while engine misalignment might lead to vibrations at various operating conditions. Excessive blade wear could result in a general deterioration of performance but would not necessarily be limited to cruise RPM situations.