What happened
While returning from a mission involving a power line patrol, the engine of the aircraft lost power. The pilot was forced to perform an autorotation, which resulted in the aircraft impacting trees.
The investigation
Metallurgical examination conducted by the Safety Board identified fatigue beach markings on most of the fracture surface of the #4 connecting rod. This component had fractured through the bearing strap on one side, which subsequently punctured a hole in the crankcase.
Further inspection revealed deterioration within the center main bearings and the failure of the top ring on the #1 piston. The failure of this ring had created a groove in the piston that extended almost entirely through to the second ring groove. While most spark plugs appeared normal, the spark plugs for the #1 cylinder exhibited oil deposits. Additionally, the pilot reported adding 3 quarts of oil during refueling on the day of the accident.