Thrush S2R-H80 N6216K incident near Donalsonville, Georgia

Probable cause

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.

NTSB analysis

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

Text from the NTSB. Open the NTSB docket for ENG20LA046 for the full record.