PLEDGER, TX, USA
N33PW
BELL 206B
WHILE CRUISING AT 900' AGL, THE PLT NOTED A HVY SHUDDER FLWD BY A GRINDING NOISE, LOUD 'BANG' & A LOW ROTOR RPM SIGNAL. HE MADE AN AUTOROTATION TO A ROAD, BUT DRG TOUCHDOWN, THE MAIN ROTOR BLADES FLEXED DWN & SEVERED THE TAIL BOOM. AN EXAM REVEALED THE MAIN PWR (SHORT) SHAFT HAD FAILED DUE TO MISALIGNMENT & HI TEMPS RESULTING FROM THE MISALIGNMENT. FURTHER INV SHOWED THAT THE MISALIGNMENT WAS THE RESULT OF A FAILURE OF THE PYLON ISOLATOR MOUNT DUE TO DEBONDING BETWEEN THE UPPER PLATE & THE RUBBER DAMPER ELEMENT. DEBONDING OCCURRED AFTER PROLONGED EXPOSURE OF THE ISOLATOR MOUNT TO OIL & GREASE WHICH CHEMICALLY DETERIORATED THE BOND. THERE WAS EVIDENCE THE DETERIORATION HAD OCCURRED OVER A LONG PERIOD OF TIME. THE UNIT HAD BEEN INSPECTED PERIODICALLY, BUT THE IMPENDING FAILURE OF THE BOND HAD NOT BEEN DETECTED.
Source: NTSB Aviation Accident Database
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