NTSB Identification: FTW95LA239.
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Accident occurred Tuesday, May 30, 1995 in FALFURRIAS, TX
Probable Cause Approval Date: 02/27/1996
Aircraft: BELL 47G-5, registration: N1502L
Injuries: 1 Uninjured.

NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.

AS THE HELICOPTER WAS BEING MANEUVERED AT LOW ALTITUDE TO HERD CATTLE, THE TAILROTOR DRIVE LOST CONTINUITY. THE PILOT REPORTED THAT DIRECTIONAL CONTROL WAS LOST, AND THAT HE EXECUTED AN AUTOROTATION INTO A CONFINED AREA. DURING DESCENT/LANDING, THE MAIN ROTOR BLADE STRUCK A TREE, AND THE HELICOPTER ROLLED ONTO ITS RIGHT SIDE. AN INVESTIGATION REVEALED EXCESSIVE WEAR ON THE TAIL ROTOR GEAR/DRIVE ASSEMBLY (TRANSMISSION OUTPUT QUILL GEAR). A METALLURGICAL EXAMINATION OF THE ASSEMBLY REVEALED THAT THE GEAR'S SHAFT (WHICH CONSTITUTED THE INNER RACEWAY OF AN ASSOCIATED CYLINDRICAL BEARING) HAD NOT BEEN CARBURIZED AS REQUIRED BY THE DESIGN MANUFACTURER. ROTATION OF THE BEARING ROLLERS OVER THE RELATIVELY SOFT SURFACE OF THE SHAFT HAD WORN A GROOVE IN THE SHAFT. THE ASSOCIATED BEARING (TAIL ROTOR DRIVE ASSEMBLY BEARING) SUBSEQUENTLY FAILED. THE GEAR AND BEARING WERE UNAPPROVED PARTS AND DID NOT CONFORM TO THE MANUFACTURER'S DESIGN SPECIFICATIONS. THE SOURCE OF THE UNAPPROVED PARTS WAS NOT DETERMINED, ALTHOUGH PACKAGING OF THE PARTS WAS REPORTED TO RESEMBLE THAT OF THE MANUFACTURER. PART NUMBERS WERE FOUND ON THE GEAR AND BEARING, SIMILAR TO AUTHORIZED PARTS, ALTHOUGH A DIFFERENT METHOD OF MARKING (ETCHING) WAS USED TO MARK THE GEAR.

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

INADVERTENT INSTALLATION OF AN UNAPPROVED PART (TAIL ROTOR GEAR/DRIVE ASSEMBLY), THAT BECAME EXCESSIVELY WORN SINCE IT DID NOT CONFORM TO THE MANUFACTURER'S SPECIFICATION, RESULTING IN FAILURE OF THE ASSOCIATED TAIL ROTOR BEARING (ALSO AN UNAPPROVED PART), LOSS OF CONTINUITY (DISCONNECT) OF THE TAIL ROTOR DRIVE SYSTEM, AND A FORCED AUTOROTATION INTO AN AREA WITH TREES.

Full narrative available

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