Piper PA-28-235 N8608W accident near San Luis Obispo, California

Probable cause

The loss of engine power due to the fatigue fracture of one attachment bolt for the left idler gear shaft, which resulted from an insufficient preload on both attachment bolts.

NTSB analysis

The pilot reported that, while on approach to the runway, the vacuum pump failed, then the engine lost power. The pilot initiated a forced landing to a field, and the airplane impacted multiple metal poles. Postaccident examination of the engine revealed that the left magneto drive gear and the left idler gear shaft were separated from their mount. Both of the attachment bolts for the left idler gear shaft were separated from the crankcase, and their respective bolt bores were elongated. One of the attachment bolts for the left idler gear shaft was fractured, and metallurgical examination revealed that the fracture surface was consistent with fatigue. Metallurgical analysis determined that the fatigue was due to a lack of sufficient preload on the bolts. Maintenance records showed that the last logged maintenance activity in which the bolts could have been manipulated was performed about 20 years before the accident. The engine had accumulated about 1,400 operating hours since that time.

Text from the NTSB. Open the NTSB docket for WPR11LA102 for the full record.