What happened
The pilot was operating a helicopter when he experienced a mechanical failure of the tail rotor drive shaft flexible coupling. This occurred at the moment he pulled collective pitch to transition into a hover prior to landing. Following the coupling failure, the aircraft entered an uncontrollable right spin.
In an effort to land the aircraft quickly, the pilot executed a hard touchdown. The spinning helicopter subsequently rolled onto its right side before coming to a complete stop.
The investigation
The investigation focused on the mechanical components of the tail rotor system. Examination revealed that the flexible coupling within the tail rotor drive shaft had failed. This failure was the primary mechanical event that led to the loss of directional control during the hover phase.
Findings
The sequence of events indicates that the failure of the tail rotor drive shaft flexible coupling directly caused the loss of anti-torque capability. This resulted in an uncontrollable right spin. The pilot's reaction to regain control involved a hard landing, which contributed to the aircraft rolling onto its side.
Safety message
Regular inspection and maintenance of tail rotor drive shaft couplings are critical for preventing in-flight failures that can lead to loss of control during low-speed flight phases such as hover and landing.