Primary finding
Probable cause
The mechanic’s improper maintenance, which resulted in the crankshaft expansion plug dislodging in flight and a subsequent forced landing.
Investigator assessment
Analysis narrative
Shortly after takeoff, the engine crankshaft expansion plug dislodged, and oil started to cover the airplane’s windscreen. The pilot attempted to return to, and land on the departure runway; however, the airplane subsequently touched down more than halfway down the runway. The pilot was unable to stop the airplane and it went off the runway and struck a chain link fence, which resulted in substantial damage to the fuselage, vertical stabilizer, and both wings. The crankshaft expansion plug was installed during maintenance that was performed 3 days prior to the accident. The mechanic who performed the maintenance stated that he installed the crankshaft plug using a ball peen hammer; instead of following the procedures outlined by the engine manufacturer; which called for the use of a special tool to properly deform and seat the plug. The mechanic said that he has used a ball peen hammer in the past to install crankshaft plugs with success; however, in this instance, the plug did not seat properly, dislodged, and allowed oil to exit the end of the crankshaft in flight.