On July 25, 2010, about 1430 central daylight time, an Air Tractor AT-602, N8521L, sustained substantial damage when it impacted the terrain after a loss of engine power while cruising at low altitude near Yates City, Illinois. The commercial pilot received minor injuries. The airplane was registered to South Delta Aviation and was operated by Har-Mor Ag Air under the provisions of the 14 Code of Federal Regulations Part 137 as an agricultural flight. Visual meteorological conditions prevailed at the time of the accident. No flight plan was filed. The airplane departed Monmouth Municipal Airport (C66), Monmouth, Illinois, about 1255 on a local flight. The pilot reported that he flew to the northwest of Yates City, Illinois, to spray some cornfields. After he completed spraying the fields, he climbed to 500 to 600 feet above ground level and checked on his map where the next fields that he was going to spray were located. He planned to fly over the fields before he refueled and loaded more chemicals. He reported that he heard a loud sound and the engine lost power. He executed a forced landing to a cornfield that required clearing a power line prior to landing. The wings and fuselage sustained substantial damage during the landing roll when the landing gear collapsed. The pilot reported that he had about 50 to 75 gallons of fuel remaining in the wing tanks, and no chemical remaining in the chemical tank. He reported that the propeller did not go into the feathered position, and that he did not pull the propeller into feather manually. A Federal Aviation Administration (FAA) airworthiness inspector examined the airplane at the accident site on July 26, 2010. He checked the fuel tanks and no fuel was visible in the tanks, but the inside of the tanks were wet with fuel. When the airplane was lifted from the ground to remove it from the field, it was noted that the left fuel drain for the left wing tank was pushed up into the wing. Fuel was observed coming out of the right wing root. The engine was removed from the airframe and fuel was found in the primary fuel line to the engine. The fuel line from the high pressure pump to the fuel/oil cooler had about 1 cup of fuel in it. The fuel filter was full of fuel, and the fuel and filter were clear. A teardown inspection of the Pratt & Whitney PT6A-60AG engine was conducted at Jet Air in Galesburg, Illinois, on August 18, 2010. The FAA airworthiness inspector provided oversight for the teardown inspection. The examination revealed that the engine exhibited minimal impact damage. The power section was seized due to impact deformation of the exhaust duct. The gas generator rotated freely and exhibited continuity to the accessory gearbox and all accessory drives. The power turbine control (PY) line was removed and pressure checked and no leaks were noted. The compressor discharge air (P3) lines were visually inspected on the engine and no cracks or breaks were noted. Upon disassembly, the compressor turbine, 1st stage power turbine vane ring, 1st stage power turbine shroud, and the 1st stage power turbine exhibited light circumferential rubbing that was consistent with impact loads and external housing deformation. The following components were removed for functional testing: 1) High pressure fuel pump 2) Fuel control unit (FCU) 3) Propeller governor, and 4) Propeller overspeed governor. The inspection of the high pressure fuel pump revealed that the drive shaft was intact. The pump gearing rotated by hand. The functional test was satisfactory. The inspection of the FCU revealed that the housing was cracked at the throttle shaft mounting boss, and the fracture was consistent with impact damage. The drive shaft was intact. The FCU governor rotated by hand with normal resistance. The FCU passed the functional test with the variations attributable to field adjustments and to the impact damage of the throttle shaft mounting boss. The inspection of the propeller governor revealed that it had impact damage to the upper housing that precluded functional testing. The drive shaft was intact. Disassembly and inspection of the propeller governor showed no preimpact anomalies that would preclude normal operation. The inspection of the propeller overspeed governor revealed that the drive shaft was intact. It passed the functional test with the variations attributable to field adjustment. The airplane held 216 gallons of fuel and had about 3.2 hour endurance at a high power setting. The “Fuel Usage” form used by the operator indicated that the airplane was fueled with 125 gallons of fuel at 1255. The lineman who fueled the airplane reported that the right fuel tank was full of fuel. The left wing tank was filled 1/2 way between the 3/4 tab and the top of the tank. The flight was about one hour and thirty-five minutes in length.