NTSB Identification: ANC01IA058.
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Incident occurred Thursday, May 17, 2001 in Quinhagak, AK
Probable Cause Approval Date: 02/20/2002
Aircraft: Cessna 207, registration: N917AC
Injuries: 2 Uninjured.
NTSB investigators used data provided by various sources and may not have traveled in support of this investigation to prepare this aircraft incident report.
The certificated commercial pilot reported that during a flight earlier on the day of the incident, he had difficulty controlling the propeller rpm, so he elected to make a precautionary landing. The pilot related that after a mechanic replaced the propeller governor, the airplane's engine and governor appeared to be operating normally, so they began their flight back to the operator's base. He said that about 10 minutes after departure, while in cruise, level flight, he noted a light sheen of oil forming on the windshield, and 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. He said that he was able to glide the airplane to the beach, and land without further incident. The airplane was equipped with a Teledyne Continental Motors (TCM) IO-520-F engine. The propeller and engine crankshaft flange were eventually recovered, and sent to the NTSB metallurgical laboratory for examination. A Safety Board metallurgist examined the fractured crankshaft flange, and reported that his 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 A postincident investigation revealed that the engine crankshaft had been replaced by TCM, 94.3 hours before the crankshaft separation.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 that 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 reportedly was 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 two forward most bearings, just aft of the fractured propeller shaft, indicating a localized loss of lubrication. He added that the oil transfer collar assembly was fractured in one location.
The National Transportation Safety Board determines the probable cause(s) of this incident to be: An in-flight fracture of the engine crankshaft during descent, precipitated by restricted oil flow through an internal engine oil passage/port. Full narrative available
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