On August 24, 2017, about 1000 central daylight time, an Air Tractor AT-401, N4532F, impacted terrain during a forced landing following a partial loss of engine power near Pearsall, Texas. The commercial pilot was not injured, and the airplane was substantially damaged. The airplane was registered to and operated by Fitch's Flying Service as a Title 14 Code of Federal Regulations Part 137 without a flight plan. Day visual meteorological conditions prevailed for the local agricultural application that departed from McKinley Field Airport (T30), Pearsall, Texas, at 0900.The pilot reported that the airplane experienced a partial loss of engine power during a low-altitude spray run over a field. He noted that the engine was surging and that he was unable to regain normal engine operation. He climbed to 100 ft above ground level, jettisoned the airplane's chemical load, but the engine operation still did not improve. The pilot reported that he subsequently made a forced landing to a nearby field. The pilot stated that the airplane decelerated rapidly and nosed over when it descended into tall vegetation and landed in soft sandy soil. The vertical stabilizer and rudder were substantially damaged. A postaccident examination revealed a circumferential fracture of the No. 9 cylinder. The fractured cylinder, serial number (s/n) 4 4HN52 L215, was submitted to the National Transportation Safety Board's Materials Laboratory for additional examination. The laboratory examination revealed a large circumferential crack through the cylinder housing. The crack was located at the valley between two air cooling fins and through the forward spark plug hole. The crack measured circumferentially as 26.3 inches long, with about 5.6 inches of the remaining cylinder circumference uncracked. The widest opening of the crack was located about 8.5 inches from the bottom side of the inboard flange, and 10.5 inches from the bottom opening of the cylinder. The fracture faces exhibited features consistent with prolonged exposure to exhaust and combustion gasses for different lengths of time. Further examination revealed crack arrest and ratchet marks consistent with progressive cracking from multiple crack initiation sites. There were no discernable discrepancies, such as corrosion pits, wear marks, or machining marks present at the crack initiation sites. The crack had prorogated from the outer cylinder surfaces inward. Further examination of the crack features using a scanning electron microscope revealed fatigue striations consistent with fatigue cracking. A sample of the cylinder was tested using energy dispersive x-ray spectroscopy and x-ray fluorescence, which confirmed the material composition was consistent with an AA 200-series aluminum casting alloy. Additionally, hardness measurements were consistent with a casted AA 200-series aluminum alloy material. The engine, a Pratt & Whitney R-1340AN-1, s/n 41-12807, had accumulated 8,899.2 hours since new and 1,310.1 hours since the last overhaul on March 5, 2009. A complete service history for the No. 9 cylinder could not be determined with the available logbook documentation. The No. 9 cylinder was not replaced or repaired during the 2009 overhaul. Based on available documentation, the No. 9 cylinder was not removed or repaired since the 2009 overhaul. According to the engine logbook, on August 1, 2017, about 64 hours before the accident, the engine cylinders were inspected in accordance with airworthiness directive No. 99-11-02 to verify that there were no visible cylinder cracks, and the corresponding logbook entry indicated "no cracks found."