What happened
The helicopter was in cruise flight at 500 feet AGL when the engine quit without warning. The pilot executed an autorotation into an open field. During the landing sequence, the main rotor blades severed the tailboom.
The investigation
Post-accident examination of the engine revealed that the no. 1 cylinder exhaust valve failed. Further investigation determined that the engine had sustained damage from an overspeed approximately 60 hours prior to the exhaust valve failure.
Review of the engine maintenance records showed no evidence that an inspection was performed following the overspeed event. The records did indicate that the no. 2 cylinder had been removed due to oil fouling. The engine had been operated for only two-tenths of an hour following the replacement of that cylinder before the failure of the no. 1 cylinder exhaust valve occurred.