What happened
During an agricultural operation involving the application of insecticide to a farm, the pilot experienced a loss of engine power. The pilot performed a forced landing in a plowed field. As the aircraft moved across the soft terrain, it encountered a small ditch, which caused the airplane to nose over.
The investigation
Post-accident examination of the engine identified a fractured crankshaft, with multiple fracture origin sites located on the surface of the journal radius. Maintenance records indicated that a top overhaul of the engine had been completed seven days prior to the accident.
Further mechanical analysis revealed that the #2 main bearing had moved fore and aft within the bearing bore and had rotated within the crankcase. This movement resulted in heavy fretting at the parting surfaces of the #2 and #3 main bearing bosses. Additionally, evidence of severe pounding was found, caused by a lack of support that initiated the breakup of the #2 bearing.
Findings
Investigators found evidence that inadequate torque had been applied to the crankcase through-bolts.