Incident Sequence
During a cruise flight, the engine RPM momentarily decreased. The pilot landed and a mechanic inspected the engine, finding no anomalies. A post-maintenance flight was then flown in the landing pattern to verify engine operation. On final approach, the engine quit, forcing a landing in rough terrain short of the runway.
Examination Findings
Examination of the engine revealed that the number 2 check counterweight blade was fractured. Metallurgical examination discovered corrosion pitting on the inner diameter of the counterweight pin bushings, as well as excessive inner diameter wear. The counterweight blade fracture exhibited evidence of fatigue.
Maintenance History
A log book entry documented that the engine had been previously taken apart for inspection after a lightning strike. No record of an engine overhaul existed.
Probable Cause
According to the safety board, the fatigue failure of the counterweight resulted from prior pitting damage to the bushings, the source of which remains unknown.