Pilot Debrief

AIR TRACTOR INC AT-301A near New Miner, WI — 2009-08-22

Final reportCEN09LA602
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Date
2009-08-22
Location
New Miner, WI, USA
Aircraft
AIR TRACTOR INC AT-301A
Registration
N3651R
Category
Airplane
Highest injury
Minor
Fatalities
0
Phase of flight
Initial climb

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 narrative

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.

Analysis

Primary failure mode
Engine power loss
First missed decision gate
Pilot could have aborted the flight after engine torque anomaly was noted.

NTSB coding

Evidence available

  • 9 docket documents
View NTSB final reportView NTSB docket

Docket documents9

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