Donalsonville, GA, USA
N6216K
Thrush S2R-H80
The incident engine suffered a release of low-energy power turbine blade debris due to a power turbine overspeed event. One or more hex nuts that secure the torque meter support within the reduction gearbox came loose and backed off from its threaded stud. A loose nut then became wedged between the mating teeth of the stage 2 planetary gears and crown wheel and the sudden torque shock caused teeth to fracture on one stage 1 planetary gear. Once these teeth fractured, mis-meshing between the stage 1 planetary gears and the sun gear occurred resulting in the 360° fracture and liberation of the sun gear teeth. The reduction gearbox then became decoupled from the power turbine, allowing for overspeed to occur
HISTORY OF FLIGHT On September 28, 2020, about 1630 eastern daylight time (EDT) a Thrush S2R-H80, N6216K, powered by one General Electric Aviation Czech (GEAC, formerly Walter Aircraft Engines a.s.) H80-100 turboshaft engine, was involved in an incident approximately seven miles northwest of Donalsonville Municipal Airport (17J), Donalsonville, Georgia. According to the pilot, he departed 17J at 1600 EDT with 350 gallons of pesticide to spray peanut fields. While maneuvering to re-enter a field, he reported that the “engine came apart” and he force landed the airplane in the peanut field. During landing, the landing gear sunk into the wet field and the airplane nosed over. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 137 aerial application flight. DAMAGE TO AIRPLANE The pilot reported that there was no damage to the airframe, but he observed propeller blade damage and a hole in the turbine engine case and nacelle consistent with an uncontained engine failure. TESTS AND RESEARCH Engine Examination and Disassembly The incident engine was disassembled by technicians at the GEAC facility in Prague, Czech Re-public between October 20-22, 2020. The engine had a hole at approximately the 7 o’clock position. The power turbine (PT) disk was found separated from the PT shaft with a fractured forward pin and all eight PT disk coupling bolts fractured. All the recovered blade attachments were found with transverse airfoil fractures near the airfoil-to-platform transition area. Two of the six studs that secure the torque meter support within the reduction gearbox were found with their nuts and washers missing. One of the stage 2 planetary gears had imprint marks transverse to both the loaded and unloaded faces of multiple gear teeth. One stage 1 planetary gear had eight consecutive fractured gear teeth. The forward end of the quill shaft, called the sun gear, had all its teeth fractured and separated. Corrective Actions GEAC released Service Bulletin SB-H80-72-00-00-0090 on February 23, 2021 defining the inspection of fasteners that secure the torque meter support to the roller bearing housing. Missing nuts, lock washers, or washer locking tabs are not permitted. Nuts and lock washers must not be free to rotate under finger pressure. Engine operation is not permitted if either of the above conditions are found.
A total loss of engine power due to a loose torque meter attachment nut in the reduction gearbox that jammed the planetary gearset causing the power turbine shaft to separate from the power turbine disk, resulting in the power turbine overspeeding and shedding blades that penetrated the compressor turbine case.
Source: NTSB Aviation Accident Database
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