ASN Wikibase Occurrence # 358941
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| Date: | Tuesday 30 May 1995 |
| Time: | 14:10 LT |
| Type: |  Bell 47G-5 |
| Owner/operator: | Private |
| Registration: | N1502L |
| MSN: | 7945 |
| Year of manufacture: | 1964 |
| Total airframe hrs: | 11078 hours |
| Engine model: | LYCOMING VO-435-B1A |
| Fatalities: | Fatalities: 0 / Occupants: 1 |
| Aircraft damage: | Substantial |
| Category: | Accident |
| Location: | Falfurrias, TX -
 United States of America
|
| Phase: | Manoeuvring (airshow, firefighting, ag.ops.) |
| Nature: | Unknown |
| Departure airport: | |
| Investigating agency: | NTSB |
| Confidence Rating: |  Accident investigation report completed and information captured |
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.
Probable Cause: 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.
Accident investigation:
 |
|
Sources:
NTSB FTW95LA239
Location
Revision history:
| Date/time | Contributor | Updates |
|---|---|---|
| 14-Mar-2024 19:54 | ASN Update Bot | Added |
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