LORIDA, FL, USA
N9846F
Bell UH-1B
The tail rotor gearbox, hub, and blades separated from the helicopter during flight. The helicopter then began to spin to the right. The pilot reduced engine power and leveled the helicopter; however, it touched down moving sideways, and the main rotor contacted the ground. Postcrash examination of the gearbox showed the gear box mounting studs had failed due to bending fatigue, which was a result of the mounting bolts not being torqued properly. Logbook records showed the gearbox had been replaced 95 flight hours before the accident.
On June 28, 1996, about 0815 eastern daylight time, a Bell UH-1B, N9846F, registered to Colony Services, Inc., crashed in a field after loss of the tail rotor, near Lorida, Florida, while on a 14 CFR Part 137 aerial application flight. Visual meteorological conditions prevailed at the time and no flight plan was filed. The helicopter received substantial damage and the commercial-rated pilot received minor injuries. The passenger was not injured. The flight originated from a field adjacent to the crash site a few minutes before the accident. The pilot stated he was spraying a canal. He heard a "grinding" noise coming from the tail rotor area and maneuvered towards a field for a precautionary landing. As the helicopter slowed he checked the tail rotor with the anti-torque pedals. As he did, the tail rotor and gearbox separated. The helicopter began to spin to the right and the nose dropped. He reduced engine power to idle and leveled the helicopter. The helicopter then touched down moving sideways and the main rotor contacted the ground. Examination of the helicopter by FAA inspectors showed the 90 degree tail rotor gear box, hub, and blades had separated from the helicopter and were found about 100 feet from the main wreckage, along the flight path of the helicopter. Examination of the separated tail rotor 90 degree gearbox, hub, and blades was performed by Bell Helicopter Engineering Laboratories, Fort Worth, Texas, under FAA supervision. The examination concluded "all six of the 90 degree gearbox case assembly studs, used to secure the case and pinion sleeve assembly to the support, fractured due to bending fatigue. The mating surfaces of the 90 degree gearbox support, pinion sleeve assembly, and the unthreaded portion of the case assembly studs all exhibited wear and fretting. The wear and fretting are indications of a loose clamp-up. The loose clamp-up would have allowed bending of the studs resulting in the bending fatigue fractures." (See attached Bell Helicopter report) Logbook records showed the tail rotor 90 degree gearbox had been installed 95 flight hours before the accident. (See attached logbook pages)
failure of company maintenance personnel to properly torque the tail rotor 90 degree gear box mounting bolts during installation, which resulted in failure of the mounting studs due to bending fatigue and subsequent separation of the gearbox.
Source: NTSB Aviation Accident Database
Aviation Accidents App
In-Depth Access to Aviation Accident Reports
