On August 19, 1991, a Cessna U206F (registration N35831) operated by Evergreen Helicopters was involved in an aviation accident near Nimrod, OR. Investigators recorded the probable cause as: A fatigue crack that generated into a fracture of the crankshaft, originating from the oil tube passage hole in the number four short cheek. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive; 3 related events involving the same aircraft type or operator are linked below.
During cruise, an engine began running rough and seized. The pilot performed a forced landing at night on rough terrain, resulting in substantial damage. Investigation found a fatigue crack in the crankshaft that originated from an oil passage hole.
Incident
While in cruise flight, the engine began running rough and ultimately seized. The pilot executed a forced landing during dark night conditions onto rough and uneven terrain within a logging clearcut area. During the landing rollout, the nose gear was sheared off, and the aircraft nosed over to an inverted position, resulting in substantial damage to the aircraft.
Investigation
Subsequent disassembly of the engine revealed a fatigue crack that had generated into a fracture of the crankshaft. The crack was observed to emanate from the oil tube passage hole in the number four short cheek of the crankshaft. This crack was the cause of the engine failure that led to the forced landing.
Probable cause
A fatigue crack that generated into a fracture of the crankshaft, originating from the oil tube passage hole in the number four short cheek.