WPR18LA234

The pilot was maneuvering the airplane during an aerial application over a field. A witness noticed that the airplane’s final turn was different than previous turns. Just before the airplane impacted the ground, the witness noticed a puff of smoke or liquid coming from the airplane. As the witness was a distance away, she was not certain. During the impact, the airplane sustained substantial damage. Examination of the engine revealed circumferential rubbing marks due to contact between the compressor turbine disk assembly and power turbine (PT) disk assembly. Of the 41 PT blades, 18 were recovered, and those blades exhibited fracture surfaces consistent with overstress fracture under elevated temperatures. Impact marks were identified inside the exhaust duct. The fracture of these blades likely occurred at ground impact when the engine was operational. It is likely that one or more of the unrecovered PT blades failed for an undetermined reason during the flight and impacted adjacent PT blades and components, including the exhaust duct. The internal engine damage from the released blades caused the PT disc assembly to fail, resulting in a loss of engine power.

On August 21, 2018, about 0850 central daylight time, an Air Tractor AT-400, N7300M, sustained substantial damage when it was involved in an accident near Grand Forks, North Dakota. The pilot was seriously injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 137 aerial application flight. The Federal Aviation Administration (FAA) inspector interviewed the pilot in the hospital. The pilot stated that he does not remember the accident or anything leading up to it. A witnessreported that she watched the accident airplane as it sprayed a field east of her position. The witness stated that the pilot’s final turn did not seem as “graceful” as the other turns. The witness further stated that, during the final turn, she did not notice any abnormal engine noises and that the engine sounded about the same as it did during all the previous turns. The witness added that, just before the airplane impacted the ground, she noticed something that appeared to be “a puff of smoke or liquid” coming from the airplane. She was some distance away and could not be certain. The airplane impacted an open field and came to rest upright. The forward fuselage, empennage and both wings were substantially damaged. The exhaust duct displayed compressional bending due to impact with the terrain and showed mechanical damages from power turbine (PT) blade release. Examination of the turbine engine revealed circumferential rubbing marks that were consistent with contact between the compressor turbine disk assembly and the adjacent static parts (CT vane ring & PT baffle) and contact between the power turbine disk assembly with the adjacent static parts (PT vane ring & PT shaft housing). Most of the PT blades were separated and missing; 18 of the 41 PT blades were recovered. The PT shroud was mechanically deformed and exhibited circumferential rub marks and multiple dents that were consistent with PT blade release. Metallurgical examination of the 18 recovered PT blades revealed that the fracture surfaces exhibited areas of dark coloring, along with light gray-to-dark orange areas, consistent with surface oxidation and combustion deposits. These deposits were typical of exposure to high-temperature environments. In addition, the fracture surfaces exhibited a general rough and twisted surface texture with a dull and dark luster. Postfracture smearing and overstress fractures were also noted on the surfaces.