On September 15, 2017, about 0915 central daylight time, an Air Tractor AT-502, N4531K, was substantially damaged during collision with terrain following a forced landing after a total loss of engine power while in cruise flight near Union City, Tennessee. The commercial pilot was not injured. Visual meteorological conditions prevailed, and no flight plan was filed for the aerial application flight which was conducted under the provisions of 14 Code of Federal Regulations Part 137.In an interview with a Federal Aviation Administration (FAA) inspector, the pilot reported he was in cruise flight on his way to an application site when he heard a 'pop' and the engine stopped producing power. The pilot maneuvered the airplane for a forced landing on a roadway but had to amend his forced landing site to a cornfield adjacent to the road due to vehicle traffic. After touchdown, the main landing gear separated which resulted in substantial damage to the fuselage. At 0915, the weather recorded at Everett-Stewart Regional Airport (UCY), about 6 miles east of the accident site, included clear skies and wind from 160° at 4 knots. The temperature was 23°C, and the dew point was 20°C. The altimeter setting was 30.06 inches of mercury. The pilot held a commercial pilot certificate with a rating for airplane single-engine land. The pilot reported 18,000 total hours of flight experience, of which 3,034 hours were in the accident airplane make and model. According to FAA records, the airplane was manufactured in 1990 and had accrued 5,490 total aircraft hours. The engine had two data plates and had been changed from a PWC PT6A-15AG engine to a PWC PT6A-34 engine under an FAA supplemental type certificate (STC). A Pratt & Whitney Canada (PWC) accident investigator performed a field examination of the accident airplane's engine under the supervision of an FAA aviation safety inspector. The examination revealed catastrophic internal damage, and parts of the compressor turbine wheel with blade fragments were harvested and forwarded to the NTSB Materials Laboratory for examination. An NTSB Senior Powerplants Investigator reviewed the report prepared by Pratt & Whitney following the field examination of the engine and maintenance records. He concurred with the report and commented on some of its findings. In summary, the engine was an assembled compilation of PWC and aftermarket (Parts Manufacturer Approval – PMA) parts. Some PMA parts installed were not authorized for the application, and the serial numbers reflected in the maintenance records did not match the components installed and reflected a compressor turbine stator (weld) repair that was not PWC approved. For example, the factory original compressor turbine disk was installed while the logbook indicated another disk was installed. The compressor turbine disk was a time-limited item, and the total hours on the installed disk could not be verified. Metallurgical examination of the compressor turbine blades revealed fracture features consistent with high-velocity impact and high-temperature deformation such as "creep." All blades were fractured, the primary fracture could not be identified, and therefore the sequence in which the fractures occurred could not be determined.