Incident Summary
A pilot reported that while flying at an altitude of approximately 75 to 80 feet above ground level, the engine lost power. The pilot executed an autorotative landing in an open pasture. During the landing, one of the helicopter's skids dug into the ground, causing the helicopter to roll over.
Engine Examination
Following the accident, the engine was disassembled and examined. The teardown revealed evidence that the engine had been operated in an overspeed condition over a prolonged period. Specific findings included high valve seating velocities and valve bounce. The upper spring seats had been striking the tops of the valve guides, leading to fatigue failure of the keepers. Subsequently, a valve failed from fatigue.
Findings
The investigation determined that the engine had been subjected to prolonged overspeed operation, which resulted in fatigue damage to valve components and eventual valve failure, leading to the loss of engine power.