What happened
While performing cruise flight at 2,800 feet mean sea level with an airspeed of approximately 100 knots, the pilot experienced an uncommanded left yaw. Upon attempting to use the rotor pedals, the pilot found they were not functioning. The pilot subsequently declared an emergency and turned back toward the airport to attempt a run-on landing.
As the aircraft's airspeed decreased during the landing, the helicopter exhibited a tendency to turn to the left. During this process, the helicopter spun to the left twice before coming to rest upright, positioned partially on the runway and the adjacent grass. There were no injuries reported in the incident.
The investigation
The investigation determined that the loss of tail rotor control was caused by a loose lower support yoke arm bushing within the lower support yoke arm. During the operation of the tail rotor controls, this looseness transferred cyclic operational stresses through the expansion pin assembly to the upper support yoke arm, which ultimately failed due to fatigue.
As the failure progressed, the pitch control input rod moved upward into the path of the rotating tail rotor gearbox input flange. The rotation caused the pitch control rod to wear down until it could no longer transmit operational forces, resulting in its separation and the sequential loss of tail rotor control.
Regarding maintenance history, a visual check of the expansible pin had been performed during a recent 500-hour inspection; however, only one end of the expansion pin assembly had been visually inspected. The manufacturer's maintenance manual lacks specific instructions on how the installation check should be conducted.