Incident Overview
On August 24, 2017, at approximately 1000 central daylight time, an Air Tractor AT-401 (registration N4532F) conducted a forced landing after a partial loss of engine power. The aircraft was operating under Title 14 Code of Federal Regulations Part 137 as an agricultural application flight, having departed from McKinley Field Airport (T30) in Pearsall, Texas, at 0900. Day visual meteorological conditions prevailed, and no flight plan was filed. The commercial pilot was not injured, but the airplane sustained substantial damage.
Pilot Actions
The pilot reported that the engine power loss occurred during a low-altitude spray run over a field. He noted the engine was surging and that he could not restore normal operation. He climbed to 100 feet above ground level and jettisoned the chemical load, but engine performance did not improve. Subsequently, he executed a forced landing to a nearby field. The airplane decelerated rapidly and nosed over after descending into tall vegetation and landing on soft, sandy soil. The vertical stabilizer and rudder were substantially damaged.
Postaccident Examination
Examination of the engine, a Pratt & Whitney R-1340AN-1 (serial number 41-12807), revealed a circumferential fracture of the No. 9 cylinder. The cylinder had accumulated 8,899.2 hours since new and 1,310.1 hours since the last overhaul on March 5, 2009. The No. 9 cylinder was not replaced or repaired during that overhaul, and available documentation indicated it had not been removed or repaired since. A logbook entry from August 1, 2017—about 64 hours before the accident—recorded an inspection per airworthiness directive No. 99-11-02, with "no cracks found."
Laboratory Analysis
The fractured cylinder (serial number 4 4HN52 L215) was sent to the National Transportation Safety Board's Materials Laboratory. A large circumferential crack measuring 26.3 inches long was found through the cylinder housing, located at the valley between two air cooling fins and through the forward spark plug hole. Approximately 5.6 inches of the cylinder circumference remained uncracked. The widest crack opening was about 8.5 inches from the bottom side of the inboard flange and 10.5 inches from the bottom opening of the cylinder.
Fracture faces exhibited features consistent with prolonged exposure to exhaust and combustion gasses. Crack arrest and ratchet marks indicated progressive cracking from multiple initiation sites. No corrosion pits, wear marks, or machining marks were observed at these sites. The crack propagated from the outer cylinder surfaces inward. Scanning electron microscopy revealed fatigue striations consistent with fatigue cracking. Energy dispersive X-ray spectroscopy and X-ray fluorescence confirmed the material composition matched an AA 200-series aluminum casting alloy. Hardness measurements were also consistent with that alloy.