Accident Overview
On September 15, 2017, at approximately 0915 central daylight time, an Air Tractor AT-502, registration N4531K, was substantially damaged when it collided with terrain during a forced landing. The accident occurred near Union City, Tennessee, following a total loss of engine power while in cruise flight. The commercial pilot, who was the sole occupant, reported no injuries. Visual meteorological conditions prevailed at the time, and the flight was conducted under 14 CFR Part 137 without a flight plan.
Pilot and Aircraft Information
The pilot held a commercial pilot certificate with a single-engine land rating and reported 18,000 total flight hours, including 3,034 hours in the accident aircraft make and model.
The aircraft, manufactured in 1990, had accrued 5,490 total hours. Its engine had been replaced from a PWC PT6A-15AG to a PWC PT6A-34 engine under an FAA supplemental type certificate. The engine featured two data plates.
Events Leading to the Accident
According to the pilot's account to an FAA inspector, he was in cruise flight en route to an application site when he heard a "pop" and the engine lost all power. He maneuvered the airplane toward a roadway for a forced landing but changed course to a cornfield adjacent to the road to avoid vehicle traffic. After touchdown, the main landing gear separated, resulting in substantial damage to the fuselage.
Weather Conditions
Weather recorded at Everett-Stewart Regional Airport (UCY), located about 6 miles east of the accident site, at 0915 included clear skies, wind from 160° at 4 knots, temperature 23°C, dew point 20°C, and an altimeter setting of 30.06 inches of mercury.
Engine Examination
A Pratt & Whitney Canada accident investigator, under FAA supervision, conducted a field examination of the engine. The examination revealed catastrophic internal damage. Parts of the compressor turbine wheel with blade fragments were sent to the NTSB Materials Laboratory for further analysis.
The NTSB Senior Powerplants Investigator reviewed Pratt & Whitney's report and maintenance records, concurring with several findings. The engine was found to be an assembled compilation of PWC and aftermarket (PMA) parts. Some PMA parts were not authorized for the application, and serial numbers in maintenance records did not match the installed components. Additionally, a compressor turbine stator weld repair was not PWC-approved. The factory original compressor turbine disk was installed, while the logbook indicated a different disk. The installed disk was a time-limited component, and its total operating hours could not be verified.
Metallurgical examination of the compressor turbine blades showed fracture features consistent with high-velocity impact and high-temperature deformation, such as "creep." All blades were fractured, and the primary fracture could not be identified; thus, the sequence of fracture occurrence could not be determined.