FAA Powerplant Written Practice Test

On a smaller aircraft engine with a keyed propeller shaft, where is the keyway aligned during installation of the propeller?

1. At the bottom of the crankcase

2. Number 1 @ TDC

3. At the midpoint of the shaft

4. One quarter turn from the rear

The correct answer is: Number 1 @ TDC

The keyway is aligned at the number 1 cylinder at top dead center (TDC) during the installation of the propeller on a smaller aircraft engine with a keyed propeller shaft. This positioning is crucial because it ensures that the propeller is in the correct phase with the engine’s firing sequence, which is particularly important for proper engine balance and operation. Having the keyway aligned at TDC helps to provide a reference point for the timing of the propeller, as the moments of maximum combustion pressure occur when the piston is at TDC. Thus, synchronizing the propeller’s position with this critical point in the engine cycle helps to optimize performance, minimize vibration, and ensure that the engine runs smoothly. Other positioning options, such as placing the keyway at the bottom of the crankcase, the midpoint of the shaft, or a quarter turn from the rear, do not align with the engine's firing cycle, potentially leading to inefficiencies or increased wear on components due to misalignment with the engine's operating phases. This understanding is essential for ensuring that propeller attachment supports optimal engine performance and longevity.