On July 22, 2011, about 1400 central daylight time, an Air Tractor Inc, AT802/A, N396LA, experienced engine surging and a loss of engine power following the completion of an aerial application of a field near Ullin, Illinois. The pilot subsequently made a forced landing to a field. The certificated commercial pilot sustained minor injuries. The airplane sustained substantial damage. The airplane was registered to and operated by Thompson Flying Service LLC under the provisions of 14 Code of Federal Regulations Part 137 as an aerial application flight. Visual meteorological conditions prevailed at the time of the accident. The flight originated from Flora Municipal Airport, Flora, Illinois. The pilot stated that while returning from an aerial application near Flora, Illinois, the engine started to surge and “slip.” He said it felt like a “clutch was going out in a truck.” The engine power would fade away and come back. He did not hear any “popping” or “unusual” noises. The torque and propeller speed gauge indications did not fluctuate. The engine gauges displayed: torque pressure – 30 psi, propeller speed – 1,400 to 1,500 rpm, gas generator speed - 95 percent, and inter-turbine temperature – 740 to 750 degrees Celsius. The airspeed was about 150 mph during “normal” cruise. The oil temperature was hotter than normal, but it was below the upper limit of the oil temperature range. The first loss of engine power occurred about 1,500 feet above ground level at a terrain elevation about 340 feet mean sea level. The pilot tried to climb the airplane three times, but each successive climb resulted in altitude loss. He said that he performed emergency procedures by selecting the auxiliary fuel [pump] on and selecting the igniters to continuous operation. He said that the emergency procedures had no effect on engine power or the surging. The pilot said that the propeller had been rotating and that it was feathered before landing on a field covered with about 4 inches of water. The airplane was powered by a Pratt & Whitney PT6A-65AG, serial number 32254. The last maintenance logbook entry dated July 13, 2011, indicated that the engine accumulated: time since new - 5,020.8 hours; cycles since new - 5,020, time since overhaul - 2,527.7 hours; cycles since overhaul - 2,527. The last maintenance logbook entry also showed that the gas generator was partially disassembled for a suspected bearing failure and overhauled number 1 and 2 bearings were installed. Overhauled accessory gear box bearings were installed. The accessory gears, fuel pump, pressure pump gear, and associated hardware were demagnetized. The gas generator section was reassembled and one serviceable thermocouple was installed. The engine was removed from the airplane and shipped to Pratt & Whitney Canada for disassembly examination under the supervision of the Transportation Safety Board of Canada. The examination did not reveal any anomalies associated with the reported power surge. The examination also revealed that a fracture of the reduction gearbox front case at the A-flange and a fracture of the second stage carrier housing at the web were characteristic of engine power at the time of impact. The Woodward overspeed governor, part number 210960 B, serial number 16517050, underwent testing and disassembly at Woodward Governor Company under the supervision of the National Transportation Safety Board. Visual inspection of the governor revealed the presence non-Woodward safety wire and torque seal on the governor. The governor exhibited external contamination consistent with sand and/or dirt. Discoloration consistent with corrosion/rust was present on the drive gear. Testing showed that the minimum and maximum speeds were set about 0.5 percent high at both points. Disassembly showed that the governor interior exhibited corrosion/rust. The reset piston was sticking, which was attributed to internal corrosion/rust.