Incident Overview
A helicopter was in cruise flight at approximately 600 feet above ground level when it began to experience "moderate to heavy" vibration. The pilot reported recognizing a tail rotor failure because the anti-torque pedals went to the floor with no effect. The failure was further confirmed when the pilot attempted a run-on landing.
Loss of Control
As the helicopter neared the ground, the check pilot "got on the controls" and refused to release them. The check pilot pulled back on the cyclic, slowing the helicopter, which immediately started "spinning violently to the right." In response, the pilot-in-command "slammed the collective down" in an attempt to hit the ground hard and tip the helicopter over to stop the rotation. He tried to roll off the throttle, which is the proper procedure for loss of tail rotor thrust according to the Pilot Operating Handbook, but could not override the check pilot who was on the controls. The pilot then turned off the main fuel control in an effort to shut down the engine and halt the spin.
Impact and Examination
The helicopter impacted terrain and rolled over on its side. A post-accident examination revealed that both tail rotor blades were missing. The 90-degree gearbox remained attached, and the tail rotor drive shaft showed no damage. Despite an extensive search, the tail rotor blades were not recovered. No other anomalies were found.