Event Description
During cruise flight, the pilot experienced a slight vibration and immediately adjusted the engine power, but the vibration persisted. After reducing the engine RPM from 2300 to 2100, the airplane vibrated intensely for a split second before the vibration ceased. At that moment, the propeller separated from the engine, taking part of the crankshaft with it.
Examination Findings
A metallurgical examination of the fractured crankshaft was conducted. The examination revealed that the failure was due to fatigue. The fatigue crack originated inside the crankshaft bore, located 1.7 inches forward of the crankshaft oil seal. The origin points of the fatigue were identified as two corrosion pits.
Sequence of Events
The pilot initially noted a slight vibration while in cruise. A power change was made but did not affect the vibration. Subsequently, reducing the engine RPM from 2300 to 2100 resulted in a brief, severe vibration followed by smooth operation. The propeller separation occurred at this point, with a portion of the crankshaft remaining attached to the propeller.
Conclusion
The crankshaft failed due to fatigue that initiated at corrosion pits within the bore. This failure led to the separation of the propeller in flight.