History of the Flight
The helicopter was on an instructional flight climbing from 1,500 to 2,500 feet QNH. Passing approximately 1,500 feet agl, a sudden, extreme vibration began. The pilot perceived high-frequency vibration through the collective lever and lower-frequency vibration through the structure. He immediately entered autorotation, transmitted two distress calls, and requested assistance, fearing component separation. The aircraft remained controllable, and an engine-off landing was performed with the throttle in the idle detent. The touch was gentle, and the helicopter came to rest in long grass with no significant ground markings. The tail-skid contacted the ground because both attachment lugs at the forward end of the tail-boom had failed, causing the boom to droop. The tail rotor drive shaft disengaged at its forward end. The leading-edge skin of the horizontal stabiliser was fractured, nearly separating the outboard section.
Examination
Examination of the tail boom attachment lugs showed no fatigue; failure was due to overload. The fork bolt (tail rotor teeter pivot bolt) had fractured near the exposed face of the threaded insert. The fracture face showed a raised angular peak and trough with a ridge dividing two independent fracture surfaces. SEM examination revealed fatigue cracking that initiated at the side of the shank opposite the peak and trough, propagating at different angles on each half, culminating in ductile overload at the angular peak. The fatigue characteristics indicated torsional shear stressing in two distinct directions, but not oriented about the bolt axis. The threaded insert end face had a brown powdery deposit, identified as wear debris from teflon-lined teetering bearings. The bolt shank showed fretting damage and evidence of rotation within the hub/insert/shoe assembly, including a sharp-edged wear band from contact with the screwdriver slot edge of the threaded insert, indicating relative rotation of about 60°. Hardness testing gave expected values; bolt diameter was not measurably different from a serviceable sample.
Maintenance Documentation
The most recent maintenance on the tail rotor assembly occurred at 632 airframe hours. The technician reported torque-tightening the securing nut per the manufacturer's Maintenance Manual (HMI). An alternative method using measured bolt stretch was also available at that time. Subsequently, the torque-tightening procedure was removed from the HMI, and the measured stretch method became the only accepted procedure. When torque-tightening was removed, a requirement for a torque check 25 hours after installation was introduced in the HMI for the hub and fork assembly (Appendix C Part IV Section 6), but this requirement was already present in the SPECIAL INSPECTIONS section (Appendix B Table B3) at the time of the maintenance.