NTSB Identification: CEN09LA602
14 CFR Part 137: Agricultural
Accident occurred Saturday, August 22, 2009 in New Miner, WI
Probable Cause Approval Date: 07/15/2010
Aircraft: AIR TRACTOR INC AT-301A, registration: N3651R
Injuries: 1 Minor.

NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.

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.

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

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.

Full narrative available

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