CALWOOD, MO, USA
N951B
BELL 47G
N951B CRASHED IN A WOODED AREA DURING A POWERLINE PATROL FLT WHICH INCLUDED PHOTOGRAPHY OF THE POWERLINE. ACCORDING TO WITNESSES, THE HELICOPTER WAS FLYING PARALLEL TO THE POWERLINE IN A WESTERLY DIRECTION WHEN THE ACCIDENT OCCURRED. EXAM OF THE WRECKAGE REVEALED THE TAIL ROTOR GEAR BOX AND HUB, ALONG WITH ON TAIL ROTOR BLADE HAD SEPARATED IN FLT AND WAS LOCATED APRX 50 YARDS FROM THE MAIN WRECKAGE. THE OTHER TAIL ROTOR BLADE WAS LOCATED APRX 160 YARDS AWAY FROM THE MAIN WRECKAGE. METALLURGICAL EXAM OF THE BROKEN YOKE REVEALED A FATIGUE FRACTURE WHICH PROGRESSED FROM MULTIPLE ORIGINS AT THE ROOT OF THE INTERNAL THREADS OF THE SPLINE. THE EXAM ALSO SHOWED THAT THE INTERNAL THREADS WERE NITRIDE TREATED. THIS WAS NOT A BELL HELICOPTER PART. THE PMA, UNDER WHICH THIS PART WAS MANUFACTURED, PROHIBITS NITRIDE TREATMENT OF THE SPINDLE BORE & THREADS.
Source: NTSB Aviation Accident Database
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