Somerville, TN, USA
N31580
AIR TRACTOR INC AT-301
The pilot purchased the agricultural airplane the day before the accident. During the engine run up before takeoff to return the airplane to his home airport, the engine popped and backfired during the magneto check. The seller and his helper cleaned the magnetos and replaced a couple of spark plugs, and the airplane returned to pilot’s home airport and operated during several subsequent flights with no anomalies. On the day of the accident, the pilot was performing aerial application when, during the fourth load of the day, his legs started to get warm and he saw a fire on the left side of the engine. The pilot performed an emergency landing and the airplane nosed over during the landing roll. When the pilot egressed, the airplane was still burning. The airplane was powered by an air-cooled radial engine. Examination revealed that the fire had initiated just aft of the engine, in the forward lower left portion of the cowling, near one of the segments of the exhaust manifold. The presence of an exhaust leak where that segment of the exhaust manifold was not secured by a clamp was discovered. Areas of exhaust impingement, with associated thermal and fire damage were visible, as well as an area of burn-through, beneath the front of the cockpit floor. Fire-damaged and fire-severed hoses were present, one of which was the oil tank outlet hose, which resulted in an oil-fed fire. Additional examination revealed evidence of gaps and holes in other areas of the exhaust manifold as well as evidence of other leaks. According to the airplane manufacturer’s written guidance, the exhaust system should be checked for cracks and signs of burning every 50 hours. Additionally, regulatory requirements for annual and 100-hour inspections included inspecting exhaust stacks for cracks, defects, and improper attachment. The airplane’s most recent annual inspection was completed about 7 days before the accident. It is likely that the popping and backfiring the pilot experienced during the engine run-up before the first flight was an indication of an exhaust manifold problem, and given the areas of exhaust staining and impingement observed during postaccident examination, it is likely that the exhaust system had been leaking for some time before the accident.
On May 11, 2022, about 1857 central daylight time, an Air Tractor AT-301 airplane, N31580, was substantially damaged when it was involved in an accident near Somerville, Tennessee. The pilot received minor injuries. The airplane was operated as a Title 14 Code of Federal Regulations (CFR) Part 137 aerial application flight. The pilot purchased the airplane the day before the accident and flew it from Hawkinsville-Pulaski County Airport (51A), Hawkinsville, Georgia, to Dyersburg Regional Airport (DYR), Dyersburg, Tennessee. During the engine run up before departing 51A, the left side of the engine popped and backfired during the magneto check. The seller and his helper cleaned the magnetos and replaced a couple of spark plugs. The pilot then did another run up, checked the magnetos several times, and everything seemed good. The pilot returned to his home airport and conducted several subsequent flights with no anomalies. The pilot was performing aerial application flights on the day of the accident. During the fourth load, his legs started to get warm and he caught a glimpse of what he thought was a flame on the left side of the engine. He “rocked” his wings and verified that it was a flame. The pilot then reduced power and fully extended the airplane’s wing flaps. He took a quick look around and did not see a good place to land. He then saw truck tracks on what might have been a turnrow (a headland used for turning farm implements around during field operations) and flew towards it. He made one circle to slow the airplane, then entered a right slip toward the area he wanted to land on. He leveled the wings before touchdown, but touched down on the main landing gear “kind of fast.” During the landing roll, the airplane hit a “rough patch” and nosed over. The fire continued to burn as the pilot egressed. The airplane was powered by a 600 horsepower, Pratt & Whitney R-1340-61, single row, nine-cylinder, air-cooled, radial engine driving a 3-bladed Hamilton Standard propeller. The engine’s exhaust manifold was constructed of stainless steel and was attached to the cylinders with spacer washers so that all segments of the exhaust manifold would line up with each other before the clamps attaching the segments were put in place. According to the airplane manufacturer, the clamps should not be over-tightened, and it should be possible to rotate them after the bolts are snug by tapping with a small hammer. The engine oil tank for the rested on top of the engine mount structure. The oil tank held 8 gallons of oil and an additional 2.5 gallons was required to fill the oil lines and engine crankcase. The oil lines consisted of 1.0" aluminum tubing from the oil tank to the "Y" drain, and from the "Y" drain to the engine inlet. Connections consisted of MIL-H-6000 1'' hose, which featured an oil-resistant tube reinforced with a high strength mildew resistant yarn. Lines from the engine outlet to the cooler, and from the cooler to the return port of the tank, were also of MIL-H-6000 1" hose. The firewall-mounted "Y" drain would drain the tank and inlet lines. The engine was provided with three drain points. Examination of the wreckage 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 one of the segments of the exhaust manifold. Further examination revealed the presence of an exhaust leak where that segment of the exhaust manifold was not secured by a clamp. Areas of exhaust impingement, with associated thermal and fire damage, were visible as well as an area of burn-through beneath the front of the cockpit floor. The carburetor heat valve was found separated from the fuselage, partially destroyed by heat/fire. Fire-damaged hoses with related evidence of an oil-fed fire were also present. These included the oil tank outlet hose, which was destroyed by heat/fire; however, its ends were found still attached to their appropriate locations. The fuel pressure line between the firewall and the carburetor was also found separated from the fuselage. The hose was fire damaged and one of the hose ends was separated from the hose. Additional examination of the exhaust manifold revealed evidence of gaps and holes in other areas, and other segments that displayed evidence of leaking. According to the airplane manufacturer’s written guidance, the exhaust system should be checked for cracks and signs of burning every 50 hours. 14 CFR Part 43, Appendix D, which describes the scope and detail of items to be included in annual and 100-hour inspections, requires exhaust stacks to be inspected for cracks, defects, and improper attachment. The airplane’s last annual inspection was completed on May 4, 2022.
The inadequate inspection and maintenance of the engine exhaust system, which resulted in an in-flight fire.
Source: NTSB Aviation Accident Database
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