On May 9, 1985, a Bell 47G (registration N951B) was involved in an aviation accident near Calwood, MO. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
Helicopter N951B crashed in a wooded area during a powerline patrol flight. Examination revealed separation of tail rotor components, and metallurgical analysis identified a fatigue fracture in the broken yoke.
Accident Overview
Helicopter N951B was conducting a powerline patrol flight that included photography of the powerline when it crashed in a wooded area. According to witnesses, the helicopter was flying parallel to the powerline in a westerly direction at the time of the accident.
Wreckage Examination
Examination of the wreckage revealed that the tail rotor gear box and hub, along with one tail rotor blade, had separated in flight. These components were located approximately 50 yards from the main wreckage. The other tail rotor blade was found approximately 160 yards away from the main wreckage.
Metallurgical Findings
Metallurgical examination of the broken yoke revealed a fatigue fracture that had progressed from multiple origins at the root of the internal threads of the spline. The examination also showed that the internal threads had been treated with nitride. The part in question was not manufactured by Bell Helicopter. According to the Parts Manufacturer Approval (PMA) under which the part was manufactured, nitride treatment of the spindle bore and threads is prohibited.