Incident Overview
A pilot engaged in agricultural application of insecticide experienced a sudden loss of engine power. In response, the pilot executed a forced landing in a plowed field. During the landing roll, the aircraft encountered a small ditch in soft terrain, causing the airplane to nose over.
Maintenance History
Examination of the maintenance records revealed that a top overhaul of the engine had been completed seven days prior to the accident.
Postaccident Examination
A detailed examination of the engine after the accident disclosed a fractured crankshaft. Multiple origin sites of the fracture were observed, originating from the surface of the journal radius. Investigation also found evidence that inadequate torque had been applied to the crankcase through-bolts.
Further inspection revealed that the #2 main bearing had moved both fore and aft within the bearing bore and had rotated within the crankcase. Heavy fretting was present at the parting surfaces of the #2 and #3 main bearing bosses. Additionally, there was evidence of severe pounding due to a lack of support, which had initiated the breakup of the #2 bearing.
Findings
The combination of the fractured crankshaft, inadequate torque on through-bolts, and bearing movement and fretting indicates significant mechanical issues that likely contributed to the loss of engine power. However, the investigation did not specify a single probable cause for the accident.