FTW04LA111 · Brantly Helicopter B-2B

The 14,600-hour flight instructor was providing the 3,600-hour commercial pilot flight instruction when the helicopter began a slow, uncommanded right turn that could not be arrested with left anti-torque pedal. As the flight instructor leveled the helicopter to land, he closed the throttle, added more collective, and contacted the ground in a sideward motion and rolled over. Examination of the helicopter revealed that the upper tail rotor shaft had completely sheared off at the upper bevel gear bolt hole. A review of the maintenance records revealed the helicopter had previously experienced two hard landings and this was the second time the shaft had failed. In a separate incident, another B-2B owner reported an upper tail rotor shaft failure while in a low hover. However, there was no history of any hard landings. Both failed upper tail rotor shafts were sent to the Safety Board's Materials Laboratory, Washington DC, for examination. Results of the examination revealed that both upper shafts failed due to fatigue at the bolt hole where the shaft attached to the bevel gear. The area outside both bolt holes exhibited fretting, gouging and pitting, which resulted in a fatigue crack and subsequent torsional shear of the shafts. The manufacturer conducted an engineering study and concluded that the most probable cause for the upper tail rotor shafts failing was excessive misalignment due to external forces or improper maintenance. A review of the maintenance manual revealed there was information for inspections after a hard landing but there were no specific procedures for checking the alignment of the upper tail rotor shaft. As a result of the investigation, the manufacturer issued Service Bulletin Number 105: Tail Rotor Drive System Inspection, which directed owners to inspect the condition of the upper tail rotor shaft and parts in that assembly. In addition, they issued Service Instruction Number 10, which described maintenance procedures to remove, disassemble and then reassemble and install the upper tail rotor gearbox. At the time of this report, the manufacturer was scheduled to release Service Instructions Number 11 and 12, and also Service Bulletin Number 106. They have requested the FAA to upgrade Service Bulletin Number 106 to an Airworthiness Directive (AD), which would require all owners to inspect the dimensions that affect the alignment of the upper tail rotor shaft. If any of the shafts were found to have operated outside of the alignment tolerances they would have to be replaced.

Verbatim NTSB analysis and probable cause from the NTSB dataset