20000702008389A

(-23) In cruise flight pilot felt a tail rotor vibration (high freq) then hheard a loud bang, pilot had lost all tail rotor control pilot autorotated, after shut down and on inspections of the heli the last two feet of the tail boom including the tail rotor and vertical fin had departed the a/c. (.4) the pilot said he had been flying the helicopter at an altitude of about 500 feet, 100 knots indicated airspeed, and 65 to 70 pounds per square inch torque, when he felt a high frequency vibration through the antitorque pedals. He said the vibration dissipated but soon began again, and became excessive with 'strong' forces in the pedals. The forces were soon followed by a loud 'bang' and the aircraft yawed violently to the right, and the nose pitched down. He said the antitorque pedals had no effect, so he lowered the collective control and applied aft cyclic control input to the stop the yaw, and level the aircraft. He said the yaw rate slowed, but did not stop, and at about 70 to 80 degrees of right yaw, he reduced the throttle in an attempt to further reduce/control the yaw condition, and the aircraft yaw stopped. He was then able to execute an autorotative landing to an open fie ld. The on scene examination of the aircraft showed that the stabilizer, as well as an 18-inch section of the hollow portion of the tail had separated from the aircraft in flight. The 90-degree gearbox also partially separated from the aircraft, and was only held by one remaining bolt, and there were contact marks and paint transfer signatures, consistent with those from multiple tail rotor blade strikes at the fracture point. The aft 6 inches of the tailboom with attach points, tail rotor gearbox, and tail rotor assembly were retained for metallurgical examination. The examination revealed that the fractured tail rotor gearbox studs were fatigue failures. In addition, both elastomer bearings in the tail rotor drive fork were examined, and were found to have failed due to fatigue.