Event Description
While in cruise flight at 2,800 feet mean sea level (msl) and approximately 100 knots airspeed, the pilot experienced an uncommanded left yaw. The pilot pressed each rotor pedal and found they were not functioning. The pilot turned back to the airport and declared an emergency. The pilot attempted a run-on landing. During the landing, the helicopter "wanted to turn left as the airspeed slowed." Subsequently, the helicopter twice spun to the left and came to rest upright, partially on the runway and adjacent grass.
Findings
The loss of tail rotor control was due to the lower support yoke arm bushing being loose in the lower support yoke arm. During operation of the tail rotor controls, the looseness transferred the cyclic operational stresses, via the expansion pin assembly, to the upper support yoke arm, which failed in fatigue. Under continued operation, the pitch control input rod then translated upwards into the path of the rotating tail rotor gearbox input flange. The pitch control rod wore down until the input rod no longer transmitted operational forces and it separated with the sequential loss of tail rotor control.
Maintenance Issues
During a recent 500-hour inspection, a visual check on the expansible pin was performed; however, only one end of the expansion pin assembly appeared to have been visually checked. The manufacturer's maintenance manual does not specify how the installation check should be performed.