NTSB Factual Report

On August 22, 2009, about 1615 central daylight time, an Air Tractor AT-301A, N3651R, piloted by a commercial pilot, was substantially damaged during a forced landing following a loss of engine power near New Miner, Wisconsin. The agricultural application flight was being conducted under 14 Code of Federal Regulations Part 137 without a flight plan. Visual meteorological conditions prevailed. The pilot sustained minor injuries. The local flight departed a private airstrip located near Bancroft, Wisconsin, about 1600.

The pilot reported the engine failure occurred about 15 minutes after takeoff during an agricultural application pass. He noted that the airplane was approximately 10 feet above ground level (agl) at the time. The pilot recalled that the engine torque indication was 80 psi, instead of a normal 42 – 45 psi. The other engine readings were normal. He stated, “Literally at that moment the engine suddenly lost power.” The pilot commented that the agricultural field he was spraying was completely surrounded by pine tree plantations. He attempted to stretch the glide to land on a narrow logging road; however, the airplane did not have enough "energy" to reach the road. He set-up for a landing into the trees. He noted that “just before impact [the] right wing and nose began dropping.” The right wing impacted the trees first, rotating the airplane from a northeast heading to a southeast heading as it came to rest. The pilot noted that the airplane was loaded with spray material and “topped off” with fuel prior to the flight. He commented that given the airplane’s flight and loading condition at the time of the engine failure, a 180-degree turn to return to the corn field would have been “impossible.”

A post accident examination of the engine revealed that the torsion shaft (p/n 868605-5, s/n 5K091) was separated approximately 7 inches aft of the forward face. Metallurgical examination noted that the shaft fractured circumferentially at the forward end of the forward bushing journal. In addition, a longitudinal crack (0.25 inch long) and a spiral crack (0.40 inch long) were also 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 of the circumferential fracture surface exhibited characteristics consistent with overstress separation. The surface of the torsion shaft exhibited spalling in the area of the spiral crack. The longitudinal fracture surface exhibited arrest and ratchet marks consistent with fatigue progression initiating at multiple locations. Further examination determined that the forward journal was chrome plated with the fatigue origins in the shaft base metal at the interface with the chrome plating. The examination also noted chrome plating at the other two bearing journal diameters. The plating itself was porous with radial cracks at all journal locations.

Maintenance records indicated that the engine (TPE331-1-151A, s/n P90094C) was disassembled for a hot section inspection, a compressor section inspection, and a gearbox repair in April 2005. At that time, the torsion shaft (p/n 868605-5, s/n 5K091) was installed in overhauled condition. According to a representative of the engine repair facility, the torsion shaft was in parts storage since initial receipt and it was installed in the accident engine as a newly overhauled part. Records indicated that the torsion shaft was overhauled in December 1997. However, the component overhaul facility has since ceased operations and additional information was not available.

A review of the manufacturer’s engineering documentation and repair instructions (72-IR-10 / Dec. 1995) for the torsion shaft revealed that chrome plating was not specified for production parts, nor is it permitted during overhaul. The manufacturer reported that there was no record of the accident part serial number in their production records. In addition, the serial number etched on the fractured torsion shaft did not correspond to either of two serial number formats used by the manufacturer.

Service Bulletin TPE331-72-0119, originally issued by the engine manufacturer in 1977, recommended replacement of the original (long) torsion shafts (p/n 868605-5) with short torsion shafts. 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. Compliance was recommended at the next scheduled inspection.

NTSB 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.