FAA Powerplant Written Practice Test

If a propeller vibration occurs only at cruise RPM ranges, what is the most likely cause?

1. Imbalanced propeller

2. Incorrect propeller for the engine

3. Worn bearings

4. Loose propeller bolts

The correct answer is: Incorrect propeller for the engine

When a propeller vibration occurs specifically at cruise RPM ranges, the most likely cause is related to the propeller’s compatibility with the engine. The correct choice points to the likelihood that the propeller is not the correct type or size for the engine’s operating characteristics. Propellers are designed to operate efficiently at certain RPM ranges, which are matched to the engine's power output and flight conditions. If the propeller is unsuitable for the engine—be it through incorrect pitch, diameter, or design—it can lead to performance issues, including abnormal vibrations during certain phases of flight, such as cruising. This is because the propeller may not be able to efficiently manage the airflow or translate engine power into thrust under cruise conditions, resulting in vibrations that wouldn't manifest at other RPM ranges. In contrast, while an imbalanced propeller can cause vibrations, it tends to affect performance across all RPM ranges rather than just during cruising. Worn bearings and loose propeller bolts can also lead to vibrations but generally would not be isolated to cruise RPMs either; these issues typically present through a broader range of operating conditions. Thus, the specific occurrence of vibrations at cruise RPM highlights the probable mismatch between the propeller and engine configuration as the root cause.