What happened
During a flight engaged in cattle herding, the aircraft experienced a total loss of engine power. In response to the engine failure, the pilot executed an autorotation. The aircraft subsequently landed in an area constrained by trees.
The investigation
Mechanical examination of the engine revealed a broken crankshaft and a broken camshaft. The camshaft showed deformation that was consistent with an overload condition. Further inspection of the internal components found that all piston rings exhibited excessive wear, and the top compression rings for the #1 and #4 cylinders were broken. Additionally, all four cylinder mounting flanges showed signs of fretting, and the main bearing saddle faces of the crankcase exhibited heavy fretting.
The engine manufacturer's Materials Laboratory conducted an analysis of the crankshaft and determined that the mode of fracture was fatigue. The fatigue crack originated at the oil hole located between the #3 main and #3 crankpin journals in the cheek, as well as near the rear fillet radius of the #3 main bearing journal. The specific cause for the initiation of this fatigue was not determined.
Records indicated that during the previous overhaul, the crankshaft had undergone a magnaflux inspection and re-nitriding at an inspection facility. Following that overhaul, the engine was reassembled and returned to service by company maintenance personnel. At the time of the accident, the engine had accumulated 1150.6 hours of operation since its last overhaul.