Incident Description
A helicopter pilot reported experiencing a high frequency vibration through the antitorque pedals while flying at an altitude of about 500 feet, with an indicated airspeed of 100 knots and torque of 65 to 70 pounds per square inch. The vibration initially dissipated but soon returned and became excessive, with strong pedal forces. Subsequently, a loud bang was heard, and the aircraft yawed violently to the right with the nose pitching down. The antitorque pedals became ineffective. The pilot responded by lowering the collective control and applying aft cyclic input to arrest the yaw and level the aircraft. The yaw rate slowed but did not stop; at about 70 to 80 degrees of right yaw, the pilot reduced the throttle, which stopped the yaw. The pilot then performed an autorotative landing to an open field.
Examination Findings
On-scene examination revealed that the stabilizer and an 18-inch section of the hollow portion of the tail had separated from the aircraft in flight. The 90-degree gearbox was partially separated, held by only one remaining bolt. Contact marks and paint transfer signatures consistent with multiple tail rotor blade strikes were observed at the fracture point. The aft 6 inches of the tailboom, along with the tail rotor gearbox and tail rotor assembly, were retained for metallurgical examination. The examination determined that the fractured tail rotor gearbox studs had failed due to fatigue. Additionally, both elastomer bearings in the tail rotor drive fork were found to have failed due to fatigue.