On June 27, 2018, about 2000 central daylight time, an Air Tractor 502B, N502S, impacted vegetation and the terrain following a reported partial loss of engine power during takeoff from a private airstrip near Tappen, North Dakota. The pilot was not injured, and the airplane was destroyed. The airplane was registered to and operated by Ross Seed Company, Inc. under the provisions of Title 14 Code of Federal Regulations Part 137 as an aerial application flight. Visual meteorological conditions prevailed for the flight, which was not on a flight plan The local flight was originating at the time of the accident. The pilot reported that he had performed a full day of spraying in the accident airplane, which had performed normally all day with no abnormal sounds, vibrations, or indications. He stated that during the accident takeoff, the engine torque was initially set to 1,700 ft-lbs. When the airplane was just past the halfway point of the runway, he noticed that the tail of the airplane was not coming up as expected. He looked at the torque indicator and it was reading 1,500 ft-lbs of torque. He determined that he would not be able to abort the takeoff and stop the airplane on the remaining runway, so he elected to push the power lever full forward and dump the chemical load. He said that he believed the engine continued to lose power. The airplane continued off the end of the runway into a cattail patch and nosed over. The airplane engine was examined by Federal Aviation Administration inspectors and a representative of Pratt & Whitney Canada, the engine manufacturer. The exhaust duct was impact fractured adjacent to the exhaust duct ports. This separated the majority of the power section components and the propeller from the engine and the aircraft. The rotational signatures that were found on the compressor turbine vane, compressor turbine, power turbine vane, and the power turbine were consistent with the engine producing power during the impact with the terrain. Static impact marks were noted on downstream side of the power turbine vane baffle from contact with the power turbine. The power turbine had static impact marks on the downstream side from contact with the power turbine shaft housing. The static impact marks indicated that the power turbine stopped rotating during the impact sequence. There were no indications of any pre-impact distress found to any of the examined components. The starting flow control unit (SFCU), fuel pump, fuel control unit (FCU), propeller governor (CSU), and over-speed governor (OSG) were removed for independent testing. Observations recorded during testing of the SFCU, CSU, FCU and OSG were consistent with field adjustments or impact damage. There were no defects or damage evident that would have prevented normal operation prior to the accident.