On June 28, 1996, a Bell UH-1B (registration N9846F) was involved in an aviation accident near Lorida, FL. Investigators recorded the probable cause as: The mounting bolts were not torqued properly, leading to bending fatigue failure of the gearbox mounting studs, which caused the tail rotor gearbox, hub, and blades to separate in flight. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
A helicopter experienced tail rotor separation in flight, causing a spin. The pilot managed to level the aircraft but touched down sideways, resulting in main rotor ground contact. Investigation revealed improperly torqued mounting bolts led to fatigue failure of gearbox mounting studs.
Accident Overview
During flight, the tail rotor gearbox, hub, and blades separated from the helicopter. The loss of the tail rotor caused the helicopter to spin to the right. The pilot responded by reducing engine power and leveling the helicopter. However, the helicopter touched down moving sideways, and the main rotor blades contacted the ground.
Investigation Findings
Postcrash examination of the gearbox revealed that the mounting studs had failed due to bending fatigue. The fatigue failure was a result of the mounting bolts not being torqued properly. Logbook records indicated that the gearbox had been replaced 95 flight hours before the accident.
Conclusion
The accident was caused by the separation of the tail rotor assembly due to fatigue failure of the gearbox mounting studs, which resulted from improper torque of the mounting bolts. No other contributing factors were identified.
Probable cause
The mounting bolts were not torqued properly, leading to bending fatigue failure of the gearbox mounting studs, which caused the tail rotor gearbox, hub, and blades to separate in flight.