Incident Summary
A helicopter operating in cruise flight at an altitude of 500 feet above ground level experienced a sudden loss of engine power without prior warning. The pilot executed an autorotation to an open field. During the landing, the main rotor blades severed the tailboom.
Post-Accident Examination
Inspection of the engine after the accident identified a failure of the No. 1 cylinder exhaust valve. Further investigation revealed that the engine had been damaged during an overspeed event approximately 60 hours before the valve failure.
Maintenance Records Review
Examination of the engine maintenance records showed no documentation of an overspeed inspection following the previous over-speed incident. However, the records indicated that the No. 2 cylinder had been removed due to oil fouling. After replacement of that cylinder, the engine was operated for 0.2 hours before the failure of the No. 1 cylinder exhaust valve occurred.
Conclusion
The incident resulted from an engine failure during cruise flight, with the subsequent autorotation leading to structural damage to the tailboom. The failure of the exhaust valve and the history of an unaddressed overspeed event were noted in the investigation.