Incident Description
A flight instructor with 14,600 flight hours was providing instruction to a commercial pilot with 3,600 hours. During the flight, the helicopter initiated a slow, uncommanded right turn that could not be corrected with left anti-torque pedal input. The instructor leveled the helicopter for landing, closed the throttle, and increased collective pitch. The helicopter contacted the ground in a sideward motion and rolled over.
Examination Findings
Post-accident examination of the helicopter revealed that the upper tail rotor shaft had completely sheared off at the upper bevel gear bolt hole. Maintenance records indicated the helicopter had experienced two prior hard landings and that this was the second failure of the shaft. In a separate incident, another owner of a B-2B helicopter reported a similar upper tail rotor shaft failure while in a low hover, with no history of hard landings.
Both failed shafts were sent to the National Transportation Safety Board's Materials Laboratory in Washington, DC. Examination results showed that both shafts failed due to fatigue originating at the bolt hole where the shaft attaches to the bevel gear. The area around both bolt holes exhibited fretting, gouging, and pitting, which initiated fatigue cracks and led to torsional shear failure.
Manufacturer's Analysis
The manufacturer conducted an engineering study and concluded that the most probable cause for the upper tail rotor shaft failures was excessive misalignment resulting from external forces or improper maintenance. A review of the maintenance manual revealed that while it contained inspection procedures after a hard landing, it lacked specific procedures for checking the alignment of the upper tail rotor shaft.
Corrective Actions
As a result of the investigation, the manufacturer issued Service Bulletin Number 105, which directed owners to inspect the condition of the upper tail rotor shaft and associated components. Additionally, Service Instruction Number 10 was published, describing procedures to remove, disassemble, reassemble, and install the upper tail rotor gearbox. At the time of the report, the manufacturer planned to release Service Instructions Number 11 and 12, as well as Service Bulletin Number 106. The manufacturer requested the Federal Aviation Administration to upgrade Service Bulletin Number 106 to an Airworthiness Directive, which would require all owners to inspect dimensions affecting alignment of the upper tail rotor shaft. Shafts found to have operated outside alignment tolerances would need replacement.