Accident Overview
On May 11, 2022, at approximately 1857 central daylight time, an Air Tractor AT-301 airplane, registration N31580, was substantially damaged in an accident near Somerville, Tennessee. The pilot, who was the sole occupant, received minor injuries. The flight was conducted under Title 14 Code of Federal Regulations Part 137 as an aerial application operation.
The pilot had purchased the airplane the day prior and ferried it from Hawkinsville-Pulaski County Airport (51A), Georgia, to Dyersburg Regional Airport (DYR), Tennessee.
Sequence of Events
During the pre-departure engine run-up at 51A, the left side of the engine popped and backfired during the magneto check. The seller and a helper cleaned the magnetos and replaced several spark plugs. Subsequent run-ups showed no anomalies. The pilot then flew the airplane to his home base and conducted several flights without issues.
On the accident day, while on his fourth load, the pilot felt his legs getting warm and saw what appeared to be a flame emanating from the left side of the engine. He confirmed the flame by rocking the wings. He reduced power, fully extended the flaps, and searched for a landing site. Seeing truck tracks on what appeared to be a turnrow, he maneuvered toward it, executing a single circle to slow down and then entering a right slip. He leveled the wings before touchdown but landed at a relatively high speed on the main landing gear. During the landing roll, the airplane encountered a rough patch and nosed over. The fire continued to burn as the pilot evacuated the aircraft.
Post-Accident Examination
The wreckage examination revealed substantial damage to the vertical stabilizer, rudder, and wings. A fire had initiated just aft of the engine, forward of the dish pan assembly, in the forward lower left portion of the cowling near an exhaust manifold segment. An exhaust leak was found where that segment was not secured by a clamp. Areas of exhaust impingement with thermal and fire damage were observed, including a burn-through beneath the front of the cockpit floor. The carburetor heat valve had separated from the fuselage, partially destroyed by heat. Fire-damaged hoses, including the oil tank outlet hose, showed evidence of an oil-fed fire. The fuel pressure line between the firewall and carburetor was also separated and fire-damaged.
Further examination of the exhaust manifold revealed additional gaps and holes in other segments, indicating multiple leaks.
The airplane was powered by a Pratt & Whitney R-1340-61 radial engine. The exhaust manifold was constructed of stainless steel with spacer washers, and according to the manufacturer, clamps should not be overtightened. The oil tank held 8 gallons of oil, with an additional 2.5 gallons required for lines and crankcase. Oil lines included aluminum tubing and MIL-H-6000 hose.
Maintenance History
The airplane’s last annual inspection was completed on May 4, 2022. The airplane manufacturer’s guidance recommends checking the exhaust system for cracks and signs of burning every 50 hours. Title 14 CFR Part 43, Appendix D, requires inspection of exhaust stacks for cracks, defects, and improper attachment during annual and 100-hour inspections.