Accident Overview
On August 22, 2009, at approximately 1615 central daylight time, an Air Tractor AT-301A (registration N3651R) experienced a loss of engine power during an agricultural application flight, resulting in a forced landing near New Miner, Wisconsin. The commercial pilot, who was the sole occupant, sustained minor injuries; the aircraft was substantially damaged. The flight was conducted under 14 Code of Federal Regulations Part 137 without a flight plan. Visual meteorological conditions prevailed at the time.
Flight Details
The flight departed from a private airstrip near Bancroft, Wisconsin, around 1600. About 15 minutes after takeoff, while performing an agricultural application pass approximately 10 feet above ground level, the pilot noted the engine torque indication was 80 psi—higher than the normal 42–45 psi—while other engine readings were normal. The pilot reported that the engine suddenly lost power. The agricultural field being sprayed was surrounded by pine tree plantations. The pilot attempted to glide to a narrow logging road but determined the airplane lacked sufficient energy to reach it. He then set up for a landing into the trees. Just before impact, the right wing and nose began dropping. The right wing struck trees first, rotating the aircraft from a northeast heading to a southeast heading as it came to rest. The pilot noted that the aircraft was loaded with spray material and fuel was topped off; he commented that a 180-degree turn back to a corn field would have been "impossible" under the conditions.
Post-Accident Examination
Examination of the engine (TPE331-1-151A, serial number P90094C) revealed that the torsion shaft (part number 868605-5, serial number 5K091) was separated approximately 7 inches aft of the forward face. Metallurgical analysis showed the shaft fractured circumferentially at the forward end of the forward bushing journal. A longitudinal crack (0.25 inch long) and a spiral crack (0.40 inch long) were present immediately aft of the circumferential crack. The circumferential fracture surface exhibited crack arrest marks consistent with fatigue emanating from the longitudinal crack; the remainder showed characteristics of overstress separation. Spalling was present on the torsion shaft surface near the spiral crack. The longitudinal fracture surface had arrest and ratchet marks consistent with fatigue progression from multiple origins. The forward journal was chrome plated, with fatigue origins in the shaft base metal at the interface with the plating. Chrome plating was also found at the other two bearing journal diameters; the plating was porous with radial cracks at all journal locations.
Maintenance History
Maintenance records indicated the engine was disassembled for a hot section inspection, compressor section inspection, and gearbox repair in April 2005, at which time the torsion shaft (serial number 5K091) was installed in overhauled condition. A representative of the engine repair facility stated the torsion shaft had been in parts storage since initial receipt and was installed as a newly overhauled part. Records showed the shaft was overhauled in December 1997, but the overhaul facility had since ceased operations, and no additional information was available.
Engineering Review
Review of the manufacturer's engineering documentation and repair instructions (72-IR-10, dated December 1995) for the torsion shaft revealed that chrome plating was neither specified for production parts nor permitted during overhaul. The manufacturer reported no record of the accident part serial number in their production records. Additionally, the serial number etched on the fractured torsion shaft did not match either of two serial number formats used by the manufacturer.
Service Bulletin
Service Bulletin TPE331-72-0119, originally issued by the engine manufacturer in 1977, recommended replacement of the original long torsion shafts (part number 868605-5) with short torsion shafts. The bulletin stated that long torsion shafts were susceptible to bushing and bushing journal wear, which would be reduced by installation of short torsion shafts. Compliance was recommended at the next scheduled inspection.