On March 12, 1991, a Bell UH-1B (registration N9687R) operated by Colony Services, Inc. was involved in an aviation accident near Vero Beach, FL. Investigators recorded the probable cause as: The upper right tailboom attach bolt failed due to being overtorqued. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive; 4 related events involving the same aircraft type or operator are linked below.
Shortly after takeoff, a helicopter experienced an uncommanded right yaw and entered autorotation. Examination revealed a stripped tailboom attach bolt that failed due to overtorquing, causing tail rotor drive shaft separation.
Incident Description
Shortly after takeoff at approximately 40 feet above ground level, the pilot observed that the high/low rotor/engine warning light was illuminated. The helicopter subsequently entered an uncommanded right yaw. The pilot initiated an autorotation. During the descent, the main rotor blades contacted trees. The pilot pulled collective pitch before impact with terrain.
Examination Findings
Post-incident examination of the helicopter revealed that the upper right tailboom attach bolt was stripped. This stripped bolt allowed the tailboom to shift, which resulted in the separation of the number 1 and number 2 tail rotor drive shaft sections at the coupling. Further examination of the stripped bolt by a qualified individual revealed that the bolt failed due to being overtorqued.
Maintenance History
According to the company mechanic, the tailboom attach bolts are checked for proper torque every 100 hours. The last such check had been accomplished approximately 55 hours prior to the incident.
Probable cause
The upper right tailboom attach bolt failed due to being overtorqued.