What happened
While performing water uplift operations over the Crawford-River, the pilot of a helicopter detected unusual vibrations and a buzzing noise within the airframe. In response to these symptoms, the pilot aborted the intended uplift, released the water bucket, and attempted to land in a nearby clearing.
As the aircraft entered a hover position just before touchdown, it began to yaw to the right. The pilot was unable to regain directional control, leading to a hard landing. The impact caused the main rotor blades to strike the ground, which subsequently caused the main gearbox mounts to fail. This impact resulted in the ejection of the main gearbox, mast, rotors, and the KAflex® driveshaft from the airframe.
The investigation
Investigators examined the airframe and the components involved in the mechanical failure. The inspection identified that the KAflex® driveshaft had suffered a fatigue failure. Specifically, the outer flex plate had failed at a bolt hole where it was secured to the main gearbox-end fitting.
Evidence of scoring marks on the rear transmission mount support assembly was found, which matched the pattern of the failed driveshaft flex plate striking the assembly during flight. These physical findings confirmed that the driveshaft failure had initiated prior to the hard landing, correlating with the vibrations and noises reported by the pilot during the initial approach.