Air Tractor Inc AT-301A N3651R accident near New Miner, Wisconsin

Probable cause

A total loss of engine power due to the fatigue failure of the engine torsion shaft. Contributing to the accident was the chrome plating of the shaft by an undetermined maintenance/overhaul facility.

NTSB analysis

The pilot reported a complete loss of engine power about 15 minutes after takeoff during an agricultural application pass. He executed a forced landing into a tree plantation adjacent to the agricultural field. A postaccident inspection of the engine revealed that the torsion shaft had separated near the forward bushing journal. The fracture surfaces exhibited features consistent with fatigue failure. In addition, chromium plating was observed at all three bearing journals. Further examination by NTSB materials engineers determined that the torsion shaft fractured due to fatigue that was likely initiated by cracks in the chromium plate repair of the shaft. The plating itself was porous with radial cracks at all journal locations. Records indicated that the torsion shaft was newly overhauled when it was installed in the engine, about 3 1/2 years prior to the accident. However, detailed records related to the overhaul work were not available and the source of the chrome plating could not be determined. The engine manufacturer stated that chrome plating was not specified for production parts, nor was it permitted during component overhaul. A service bulletin recommending replacement of the torsion shaft had not been completed. The bulletin recommended replacement of the original (long) torsion shaft with a shorter shaft. The bulletin noted that the long torsion shafts were susceptible to bushing and bushing journal wear, which would be reduced by installation of the short torsion shafts.

Text from the NTSB. Open the NTSB docket for CEN09LA602 for the full record.