Air Tractor Inc AT-301 N31580 accident near Somerville, Tennessee

Probable cause

The inadequate inspection and maintenance of the engine exhaust system, which resulted in an in-flight fire.

NTSB analysis

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.

Text from the NTSB. Open the NTSB docket for ERA22LA224 for the full record.