What happened
During a high-power takeoff from a confined area, an Robinson R44, registration VH-NBY, experienced a sudden and catastrophic structural failure. Following a period of unusual vibration felt through the tail rotor pedals during previous flights, the aircraft's tail section underwent a rapid breakup. The tail rotor gearbox (TRGB) separated from the airframe, causing the trailing edge of the lower vertical stabilizer to be struck by a tail rotor blade. This impact triggered a chain reaction that fractured the aft tail cone bulkhead, leading to the separation of the stabilizer and gearbox from the tail cone. The loss of control resulted in two fatalities and two serious injuries among the pilot and three passengers.
The investigation
Investigators examined the wreckage and components to determine the cause of the failure. While the TRGB was found to be manufactured to specification and functioning correctly prior to the event, evidence suggested the failure originated at the bolted connection between the TRGB and the aft tail cone bulkhead. It is believed that dynamic loads during takeoff overstressed the attachment lugs of the input cartridge. This overstress caused the internal threads within the cast alloy bulkhead to shear, allowing the gearbox to dislodge.
Engineers had previously attempted to troubleshoot the reported pedal vibrations through ground runs and dynamic balancing, but could not replicate the defect. While the exact source of the vibration remains unconfirmed, minor fretting was noted on the TRGB input cartridge hardware, which may have contributed to the instability.
Findings
- The overstress fracture of the tail rotor gearbox input cartridge attachment lugs was the primary mechanical failure.
- The pilot failed to follow the Pilot's Operating Handbook (POH) recommendation to hover and reinspect the aircraft after detecting unusual vibrations.
- The decision to carry three passengers during a high-power takeoff from a confined area increased the severity of the accident.
- The maintenance release failed to effectively communicate the vibration issue, as pilots and engineers did not formally endorse the defect.
- The pilot was operating without a valid flight review or a current medical certificate.
- The registered operator lacked a conservative defect resolution process and had a history of unreported incidents and previous tail rotor strikes.