NTSB Identification: DEN90FA172.
The docket is stored on NTSB microfiche number 43816.
Nonscheduled 14 CFR
Accident occurred Sunday, August 19, 1990 in MEETEETSE, WY
Probable Cause Approval Date: 12/14/1992
Aircraft: AEROSPATIALE SA316B, registration: N83776
Injuries: 3 Serious,4 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.

APPROXIMATELY 2 MINUTES INTO THE FLIGHT, THE PILOT EXPERIENCED A SLIGHT 1 TO 1 VIBRATION THROUGH THE CYCLIC. THIS TYPE VIBRATION IS ASSOCIATED WITH MAIN ROTOR DAMPERS AND THE PILOT 'STIRRED' THE CYCLIC IN AN EFFORT TO PURGE AIR OUT OF THE BLADE DAMPERS. THIS ELIMINATED THE PROBLEM, HOWEVER, IT RECURRED AND COULD NOT BE ELIMINATED. THE VIBRATION INCREASED TO THE POINT OF LOSS OF CONTROL. AN EMERGENCY LANDING WAS ATTEMPTED WITH TOUCHDOWN OCCURRING AT ABOUT 60 KNOTS. THE HELICOPTER CAME TO REST INVERTED AND WAS LARGELY CONSUMED BY POST-IMPACT FIRE. METALLURGICAL ANALYSIS ON A FAILED BLADE RESTRAINING CABLE REVEALED A CATASTROPHIC FAILURE IN A LOW CYCLE FATIGUE MODE CONSISTENT WITH A SECONDARY CAUSE, AND DUE TO SHORT TERM VIBRATIONS. IN ADDITION TO FATIGUE, THERE WAS EVIDENCE OF PLASTIC DEFORMATION DUE TO VIBRATORY LOADS FROM THE DAMPER SYSTEMS. FIRE DAMAGE PREVENTED ADDITIONAL FAILURE ANALYSIS.

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

A MAIN ROTOR HEAD BLADE DAMPENER FAILURE. A FACTOR IN THE ACCIDENT WAS THE SUBSEQUENT FAILURE OF A BLADE RESTRAINING CABLE.

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