Accident Overview
On January 20, 2011, at approximately 1317 Pacific standard time, a Piper PA-28-235, registration N8608W, sustained substantial damage during a forced landing near the San Luis County Regional Airport (SBP) in San Luis Obispo, California. The airplane was operated under Title 14 Code of Federal Regulations Part 91. The private pilot, who was the sole occupant, was not injured. Visual meteorological conditions prevailed, and no flight plan was filed. The local flight had departed SBP at 1130.
The pilot reported that while on approach to runway 29, he noticed a vacuum pump failure. He continued toward SBP and was instructed by air traffic control to slow for traffic ahead. After reducing power, the pilot later advanced the throttle and found the engine did not respond. Realizing a complete power loss, he initiated a forced landing to a nearby field. During the landing, the airplane struck multiple metal poles in a vineyard and came to rest upright.
Engine and Component Examination
An FAA inspector examined the aircraft and found structural damage to both wings, and all four fuel tanks (auxiliary and main) were breached. Fuel odor was present at the scene. The engine was recovered for further analysis.
Upon disassembly, the left magneto drive gear and left idle gear shaft were found loose from their mounts. Both mounting bolts were separated from the crankcase, and their bolt bores were elongated. Heavy fretting, gouging, and smearing were noted around the mounting surface. Fragments of the safety locking plate and bolts were found in the oil sump. The oil filter contained metallic fragments, while the oil suction screen was clean.
The left crankshaft idler gear shaft, locking plate fragments, and two bolts were sent to the NTSB Materials Laboratory. Examination revealed impact wear and deformation consistent with loose bolts. The locking plate showed multiple fractures, heavy distortion, and elongated holes. Both bolt heads exhibited impact wear, with one bolt showing necking and a fatigue crack that propagated approximately 95% through its diameter. The other bolt had a visible crack that extended nearly 200 degrees around its circumference, with fatigue cracking covering about 10% of its cross-section. Both failures were consistent with overstress separation after fatigue.
Maintenance History
Maintenance records indicated the engine was overhauled on April 7, 1990, at a tachometer time of 248.8 hours. The most recent 100-hour inspection occurred on March 23, 2010, at a tachometer time of 1,609.5 hours, with an engine total time of 3,504.4 hours and 1,373.77 hours since major overhaul. No logbook entries addressed the idler gear shaft mounting bolts or lock plate.