On July 29, 1996, a Boeing A75N1 (registration N68251) was involved in an aviation accident near Tracy, CA. Investigators recorded the probable cause as: The eight screws securing the crankshaft hub to the crankshaft gear had sheared. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
A pilot made an emergency landing in an alfalfa field after the engine quit en route to a job. The aircraft struck a ditch hidden by crops, and a post-crash fire consumed the aircraft. Investigation revealed sheared screws in the engine's crankshaft assembly.
Incident Summary
A pilot reported that the engine quit while he was en route to a job. He subsequently landed the aircraft in an alfalfa field. During the landing roll, the aircraft collided with a ditch that was obscured by the crops. A postcrash fire erupted and consumed the aircraft.
Aircraft Examination
An FAA airworthiness inspector examined the aircraft after the fire. The inspector reported that the engine crankshaft could be rotated by hand; however, no corresponding valve action was noted on any cylinder. This indicated a disconnection between the crankshaft and the valvetrain.
Engine Findings
The inspector observed a disassembly of the engine and reported that the eight screws securing the crankshaft hub to the crankshaft gear had sheared. The crankshaft gear is responsible for rotating all accessory drive gears, including the magnetos. The shearing of these screws likely resulted in the loss of valve actuation and magneto operation, consistent with the reported engine power loss.
Probable cause
The eight screws securing the crankshaft hub to the crankshaft gear had sheared.