Incident Description
A certificated commercial pilot reported that during a flight earlier in the day, he had difficulty controlling the propeller rpm and elected to make a precautionary landing. After a mechanic replaced the propeller governor, the engine and governor appeared to operate normally, and the pilot began the return flight to the operator's base. About 10 minutes after departure, while in cruise level flight, the pilot noted a light sheen of oil forming on the windshield. He elected to make a precautionary landing on a remote beach. As he retarded the throttle to start the descent, the propeller rpm increased for about 20 seconds, followed by the propeller detaching from the engine. The pilot was able to glide the airplane to the beach and land without further incident.
Investigation Findings
The airplane was equipped with a Teledyne Continental Motors (TCM) IO-520-F engine. The propeller and engine crankshaft flange were recovered and sent to the NTSB metallurgical laboratory for examination. A Safety Board metallurgist examined the fractured crankshaft flange and reported that initial visual examination revealed a fatigue fracture originating from a heat check site.
A postincident investigation revealed that the engine crankshaft had been replaced by TCM 94.3 hours before the crankshaft separation. An engine log book notation entered by TCM indicated the reason for removal was "for compliance with CSB 99-3A." According to a representative from TCM's in-house analytical laboratory, the crankshaft was a remanufactured part that was reportedly ground undersize and re-nitrided at TCM.
In the presence of an NTSB Investigator, the engine was disassembled. The investigator noted a significant amount of heat discoloration with modest circumferential scoring of the two forwardmost bearings, just aft of the fractured propeller shaft, indicating a localized loss of lubrication. Additionally, the oil transfer collar assembly was fractured in one location.