On October 27, 1989, a Hughes 369D (registration N9163F) was involved in an aviation accident near Neah Bay, WA. Investigators recorded the probable cause as: Fatigue failure of all four tail rotor gearbox attachment studs. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
During an external load flight, the tail rotor gearbox separated after all four attachment studs failed from fatigue. Contaminants and fretting were found on mounting pads. The helicopter collided with terrain.
Incident Overview
A helicopter conducting an external load operation experienced a catastrophic failure while transitioning to a climb attitude after releasing the load. The tail rotor gearbox detached from the tail boom, resulting in a loss of control and subsequent collision with rough terrain.
Metallurgical Examination
Post-accident metallurgical examination of the tail rotor gearbox revealed that all four attachment studs had failed due to fatigue. Further analysis identified contaminants on the gearbox mounting pads between the tail rotor gearbox and the tail boom, with evidence of fretting noted.
Regulatory and Service Information
Two documents—AD88-17-09 and Service Information Notice DN-151/EN-39/FN-28—mandate initial and recurrent inspections (every flight) of the gearbox attachment area. These directives aim to detect potential issues before they lead to detachment.
Probable cause
Fatigue failure of all four tail rotor gearbox attachment studs.
What happened in the 27 Oct 1989 Hughes 369D accident near Neah Bay, WA?
During an external load flight, the tail rotor gearbox separated after all four attachment studs failed from fatigue. Contaminants and fretting were found on mounting pads. The helicopter collided with terrain.
What aircraft was involved and where did it happen?
The accident on 27 Oct 1989 involved a Hughes 369D, registration N9163F, at Neah Bay, WA.
What was the probable cause of the accident?
Fatigue failure of all four tail rotor gearbox attachment studs.
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