Primary finding
Probable cause
A loss of engine power due to the disengagement of the crankshaft gear from it's hub due to the overload shear of the eight retaining screws.
Investigator assessment
Analysis narrative
The pilot reported that the engine quit while he was en route to a job and he landed in an alfalfa field. The aircraft collided with a ditch which was obscured by the crops. A postcrash fire erupted and consumed the aircraft. The aircraft was examined by an FAA airworthiness inspector. He reported that the engine crankshaft would rotate; however, no corresponding valve action was noted on any cylinder. The inspector observed a disassembly of the engine and reported that the eight screws which secure the crankshaft hub to the crankshaft gear had sheared. The crankshaft gear rotates all accessory drive gears, including the magnetos.
Source record
Factual narrative
On July 29, 1996, at 0615 hours Pacific daylight time, a Boeing A75-N1, N68251, collided with a ditch during a forced landing near Tracy, California. The forced landing was precipitated by a total loss of engine power. The aircraft was operated by Trinkle & Boys of Tracy, and was on an aerial application flight under 14 CFR 137 of the Federal Aviation Regulations. Visual meteorological conditions prevailed and no flight plan was filed. The aircraft was demolished in the impact and postcrash fire sequence. The certificated commercial pilot was not injured. The flight originated at a private agricultural airstrip near Tracy at 0600 on the morning of the accident and was loaded with sulfur for application to a nearby field. The pilot reported that the engine quit while he was en route to a job and he landed in an alfalfa field. The aircraft collided with a ditch which was obscured by the crops. A postcrash fire erupted and consumed the aircraft. The aircraft was examined by an FAA airworthiness inspector. He reported that the engine crankshaft would rotate; however, no corresponding valve action was noted on any cylinder. The inspector observed a disassembly of the engine and reported that the eight screws which secure the crankshaft hub to the crankshaft gear had sheared. The crankshaft gear rotates all accessory drive gears, including the magnetos. -