On August 27, 2015, about 1200 central daylight time, an Air Tractor, AT-502B, single-engine turboprop airplane, N61267, had a partial loss of engine power during low-altitude maneuvering flight and impacted terrain near Monroe, Nebraska. The pilot sustained minor injuries and the airplane sustained substantial damage to both wings, the horizontal stabilizer, and elevator. The airplane was registered to, and was operated by Red Willow Aviation Spraying, Inc.; McCook, Nebraska, as a 14 Code of Federal Regulations Part 137 aerial application flight. Day visual meteorological conditions prevailed, and a flight plan had not been filed for the local flight which had departed about 1130 from Genoa Municipal Airport (97Y), Genoa, Nebraska.The pilot was seeding a cover crop into corn when he experienced a partial loss of engine power. The engine instruments showed the engine was operating, but was producing less than idle power. The pilot pushed the throttle to full power, with no effect. At that point, he knew he had to land. The pilot turned the airplane into the wind and lowered the flaps. As he heard corn start hitting the bottom of the airplane, the pilot pulled the control stick full aft to stall the airplane. The left wing dropped, the airplane turned 90 degrees, and impacted the ground sideways. The right wing subsequently struck the ground and the airplane came to rest upright. The engine continued to run until the pilot shut it down and exited the airplane. An examination of the wreckage showed there was adequate fuel on-board, there was no fuel spill, and no postimpact fire. The airplane's engine and propeller governor were examined and no anomalies were found. The fuel control unit and fuel pump were removed and sent to the manufacturer in Prague, Czech Republic, for examination. A Turbine Start Limit and Monitor (TSLM) and a USB thumb drive, which contained Global Positioning Satellite (GPS) data from a GPS unit on board the airplane, were sent to the NTSB Vehicle Recorder's Laboratory for examination. The examination of the fuel control unit and fuel pump revealed no anomalies. The GPS data on the USB thumb drive was examined and did not include the accident flight. The TSLM, which monitors internal turbine temperature, N1, N2, torque, oil pressure, and bus voltage, and records at engine startup and when the parameters are exceeded, recorded 74 events. Event 74 was triggered by the N1 parameter being set above the user set upper limit. The exceedance ended 1.5 seconds into the event. Following the exceedance, the internal turbine temperature, N1, and N2 (propeller rpm) decreased over time, consistent with a lower output by the engine. The TSLM data was reviewed by the engine manufacturer who concluded that the propeller went into feather either by an incorrect pilot command or a governor problem. The manufacturer also noted the engine was being operated above the manual temperature limits at the time of the event and the internal turbine temperature and core speed did not correlate correctly. The manufacturer could only attribute that to scaling of the data or improper engine operation.