What happened
The flight crew reported that they were initiating a descent into final approach for a landing when the right engine gearbox chip detector light illuminated. This was immediately followed by a loud bang and severe vibration throughout the airframe. The crew identified the source of the disturbance as the right powerplant and proceeded with emergency procedures.
They feathered the propeller on the affected side and shut down the engine using the condition lever. Upon completion of these actions, the rotation and vibration ceased. The flight crew continued the approach and landed the aircraft without further incident. No injuries were reported among the occupants.
The investigation
A post-incident teardown inspection of the powerplant was conducted at the manufacturer's facility. The examination revealed a sudden and catastrophic failure of the propeller reduction gearbox assembly. The root cause was identified as a high-cycle metal fatigue fracture on one of two helical gears within the unit.
Additionally, teardown and inspection of the propeller assembly showed severe damage to the composite plastic blade retainer rings and metal actuator yokes. This damage was attributed to the sudden deceleration and stoppage resulting from the gearbox failure.