What happened
While in cruise flight, the pilot of the helicopter heard an audible "bang," which was immediately followed by a loss of tailrotor authority. The pilot initiated maneuvers to land and reduced airspeed in preparation for the touchdown. During this process, the aircraft began to rotate. As the helicopter reached an altitude of approximately 30 to 50 feet above the ground, the pilot pulled the collective to cushion the landing. However, the helicopter impacted the ground and rolled onto its side.
The investigation
The aircraft had undergone an annual inspection 11 months and 23.3 flight hours before the accident. During that inspection, maintenance records indicated compliance with airworthiness directive (AD) 76-18-01.
Investigators examined the left aft cluster fitting and discovered that the tailboom attaching point had separated, which severed the tailrotor drive shaft. The fracture surfaces of the cluster fitting were relatively flat with smooth curving boundaries, which are characteristics typical of fatigue. Additionally, the rougher area in the lower lug contained curving arrest lines, indicating features typical of low-cycle fatigue.
AD 76-18-01 required that tailboom support strut aluminum end fittings be visually inspected for damage or deformation, followed by a dye penetrant inspection to check for cracking. These inspections were to occur within 50 flight hours of the AD's effective date and at intervals not exceeding 50 flight hours unless modifications were made. The directive specified that the dye penetrant inspection follow Service Information Notice N-82.3. While the service notice also required a daily visual inspection, the pilot stated he had performed this check prior to the flight.