On May 13, 2011, about 1020 central daylight time, a Thrush Aircraft S2R-T660 agricultural airplane, N660KP, operated by AG Concepts Inc., was substantially damaged during a forced landing to a field, following a partial loss of engine power or propeller thrust while maneuvering at low altitude near Greenwood, Mississippi. The certificated commercial pilot was not injured. The aerial application flight was conducted under the provisions of 14 Code of Federal Regulations Part 137. Visual meteorological conditions prevailed and no flight plan was filed for the local flight. The flight originated from a private airstrip in Morgan City, Mississippi, about 1000. The pilot stated that he departed with 140 gallons of Jet A fuel and flew about 5 miles to the spray area. He then completed about 12 passes uneventfully. After the last pass, the pilot flew north and initiated a short climb to clear a transmission line. At that time, the airplane seemed to experience a partial loss of engine power. The pilot immediately lowered the nose of the airplane and began to look for a field; however, the power then seemed to "spool up again" and the pilot thought he might be able to turn back to the field he was just spraying. The engine seemed to lose power again; the pilot then released the remaining chemicals and decided to land straight ahead, into a bean field. During the landing, the airplane struck a ditch and came to rest upright. The pilot added that the entire accident sequence lasted about 15 to 20 seconds. During the forced landing, the airplane never experienced a total loss of engine power and the power fluctuated approximately three times. The pilot further stated that he was not sure if the engine was surging or the propeller was surging. The two-seat, low-wing, tailwheel airplane was manufactured in 2006 and certificated in the restricted category. It was equipped with a Pratt and Whitney PT6A-65AG, 1,325-horsepower turboprop engine, with a five-blade Hartzell propeller. The airplane was maintained under a manufacturer's inspection program. The airplane's most recent 200-hour inspection was completed on January 27, 2011. At that time, the airplane had accumulated approximately 4,553 total hours of operation and the engine had been operated for about 4,863 hours since major overhaul. The airplane had flown about 102 additional hours, from the time of the last inspection, until the time of the accident. The pilot, age 62, held a commercial pilot certificate, with ratings for airplane single-engine land and helicopter. His most recent Federal Aviation Administration (FAA) second-class medical certificate was issued on June 22, 2010. The pilot reported a total flight experience of 28,780 hours; of which, 17,680 hours were in various models of Thrush Aircraft. He had flown 309 hours in the same make and model as the accident airplane during the 90-day period preceding the accident. Examination of the wreckage by an FAA inspector and a representative from the engine manufacturer revealed substantial damage to both wings. The propeller blades were observed in the feather position, exhibiting damage consistent with rotation during impact. The pilot had stated that he was not sure if he positioned the propeller to feather during shutdown, or if it went to feather upon ground contact. External inspection of the engine did not reveal any evidence of wrinkles or impact damage to the cases or exhaust duct. All basic engine accessories were intact, undamaged, and attached per maintenance manual criteria. Py and reversing linkage between the fuel control unit and propeller governor was intact and exhibited continuity. The air line B-nut was tight and safetied. The P3 line between the gas generator and fuel control unit was intact and secure. Borescope examination of the 1st stage compressor rotor disk revealed no evidence of foreign object debris, erosion, or impact damage to the airfoils. The exhaust stack was then removed for access to the power turbine blades, which also did not display any damage. Additionally, the compressor turbine rotated freely. Inspection of the fuel pump and high pressure filter bowl revealed that both were clean with no contamination noted. The high pressure filter bowl contained fuel consistent with Jet A, from which two fuel samples were taken by the engine manufacturer representative and forwarded to a fuel laboratory. Subsequent laboratory analysis of the two samples did not reveal any contamination. After the accident, the operator had reported difficulty removing the propeller due to a "stuck" Beta mechanism. The propeller was subsequently examined by a representative from the propeller manufacturer, under the supervision of an FAA inspector. All five blades were bent slightly aft, with no twisting or other damage noted. The damage was consistent with rotation at the time of impact, with the blades at or near the feather position. The propeller examination also revealed interference between two blade clamps and link arms when the Beta mechanism was being actuated; however, the interference was the result of bent link arms due to impact damage. All other components of the Beta mechanism were found to be in good condition with no evidence of wear or binding. The propeller examination did not reveal any anomalies that would explain the power surging as reported by the pilot. The engine was subsequently transported to an overhaul facility for repair due to the propeller strike. According to maintenance personnel at the overhaul facility, during repair of the engine, no anomalies were noted that would explain a loss of engine power or surge. During the engine repair, the fuel control unit, propeller governor, and overspeed governor were sent to an overhaul facility. Those components were tested and examined under the supervision of a manufacturer representative and FAA inspector. The testing and examination of those components did not reveal any anomalies that would have causes an engine power loss, engine surge, or propeller surge.