What happened
A commercial pilot reported experiencing difficulty controlling propeller RPM during a flight earlier on the day of the incident, which led him to perform a precautionary landing. Following the replacement of the propeller governor by a mechanic, the engine and governor appeared to be operating normally, prompting a flight back to the operator's base.
Approximately 10 minutes after departure, while in cruise, level flight, the pilot observed a light sheen of oil forming on the windshield. He decided to execute a precautionary landing on a remote beach. As the pilot retarded the throttle to begin his descent, the propeller RPM increased for approximately 20 seconds, at which point the propeller detached from the engine. The pilot was able to glide the Teledyne Continental Motors (TCM) IO-520-F to the beach and land without further incident. The pilot and occupants were not injured.
The investigation
The propeller and the engine crankshaft flange were recovered and sent to the NTSB metallurgical laboratory for examination. A metallurgist's visual examination of the fractured crankshaft flange revealed a fatigue fracture originating from a heat check site.
Post-incident investigation showed that the engine crankshaft had been replaced by TCM 94.3 hours prior to the separation. Logbook entries indicated the removal was for compliance with CSB 99-3A. According to a representative from the TCM analytical laboratory, the crankshaft was a remanufactured part that had been ground undersize and re-nitrided at TCM.
During engine disassembly conducted in the presence of an NTSB investigator, significant heat discoloration and modest circumferential scoring were noted on the two forward-most bearings just aft of the fractured propeller shaft. This indicated a localized loss of lubrication. Additionally, the oil transfer collar assembly was found to be fractured in one location.