Incident Details
During a cattle herding flight, the pilot experienced a total loss of engine power. The pilot executed an autorotation to an area confined by trees, successfully landing the aircraft. No injuries were reported.
Engine Examination
Post-accident examination of the engine revealed significant internal damage. The crankshaft and camshaft were both broken. The camshaft exhibited deformation consistent with overload. All four cylinder mounting flanges showed fretting, and all piston rings displayed excessive wear. The #1 and #4 cylinder top compression rings were broken. The main bearing saddle faces of the crankcase showed heavy fretting.
Crankshaft Analysis
The engine manufacturer's Materials Laboratory examined the crankshaft and determined that the mode of fracture was fatigue. The fatigue crack originated from the oil hole located between the #3 main bearing journal and the #3 crankpin journal, specifically in the cheek near the rear fillet radius of the #3 main bearing journal. The underlying cause of the fatigue initiation could not be determined.
Maintenance History
During the most recent overhaul, the crankshaft received a magnaflux inspection and was re-nitrided by an inspection facility. The engine was then reassembled and returned to service by company maintenance personnel. At the time of the failure, the engine had accumulated 1,150.6 hours since the overhaul.