NTSB Identification: ATL91FA013.
The docket is stored on NTSB microfiche number 44609.
Accident occurred Thursday, November 01, 1990 in GATLINBURG, TN
Probable Cause Approval Date: 12/14/1992
Aircraft: BELL 47J-2, registration: N49DA
Injuries: 1 Serious,2 Minor.

NTSB investigators either traveled in support of this investigation or conducted a significant amount of investigative work without any travel, and used data obtained from various sources to prepare this aircraft accident report.

THE TWO PASSENGERS REPORTED THAT THEY HEARD A 'POP' SOUND, FOLLOWED BY THE HELICOPTER STARTING A CLOCKWISE SPIN TO THE RIGHT. THE HELICOPTER DESCENDED INTO THE TREES ON THE SIDE OF A STEEP SLOPE. INVESTIGATION REVEALED THAT THE TAIL ROTOR DRIVE SHAFT FAILED IMMEDIATELY FORWARD OF THE TAILBOOM EXTENSION TUBE. THE DRIVE SHAFT FAILED FROM FATIGUE CRACKING, PROPAGATING AT A HIGH RATE, SUGGESTING HIGH LOADS. THE T/R DRIVE SHAFT FORWARD BEARING HOUSING OUTER DIAMETER DIMENSION AND THE INNER DIAMETER OF THE TAILBOOM EXTENSION TUBE WERE WORN BEYOND REPLACEMENT LIMITS AND SHOULD HAVE BEEN REPLACED ACCORDING TO THE BELL MAINTENANCE AND OVERHAUL MANUAL. THE PASSENGER THREE-PLACE BENCH SEAT HAD BEEN ALTERED SO THAT THE INDIVIDUAL SEAT BELTS HAD BEEN MODIFIED INTO ONE SINGLE LONG BELT. THE BELT COULD BE OPENED ONLY ON THE LEFT SIDE OF THE BELT; THE BUCKLE ON THE RIGHT SIDE OF THE BELT WAS TAPED CLOSED WITH SILVER DUCT TAPE.

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

THE ACCIDENT WAS INADEQUATE MAINTENANCE OF THE HELICOPTER, ALLOWING A TAIL ROTOR DRIVE SHAFT BEARING TO EXCEED WEAR LIMITS. THIS RESULTED IN HIGH LOADS ON THE TAIL ROTOR DRIVE SHAFT, AND THE RESULTING FATIGUE FAILURE OF THE DRIVE SHAFT.

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