A SIKORSKY S76 SUFFERS A BROKEN MAIN ROTOR HUB CAUSING VIBRATION.

Narrative

EARS THAT HOLD MAIN ROTOR DAMPER TO THE MAIN ROTOR HUB BROKE ALLOWING DAMPER TO ROTATE 180 DEGS AND STRIKE THE MAIN ROTOR BLADE. THE ACFT GAVE A MODERATE SHUDDER AND BEGAN A MODERATE 1 TO 1 VIBRATION WITH VERT AND LATERAL COMPONENTS. MY COPLT AND I DISCUSSED THE POSSIBLE SITS; THERE WERE SEVERAL STRUCTURES IN SITE BUT NONE WERE KNOWN TO BE SUITABLE FOR OUR SIZE OF ACFT OR KNOWN TO BE CLR. WE WERE 8 MINS FROM OUR INTERMEDIATE STOP WHICH WE KNEW WAS SUITABLE AND CLR. WE REDUCED SPD SLIGHTLY AND BEGAN A DSCNT. THE VIBRATION DECREASED SLIGHTLY SO I DECIDED TO DSND TO 500 FT WHERE WE WOULD BE CLOSE TO THE SURFACE FOR DITCHING IF THINGS GOT WORSE. I DECIDED TO CONTINUE TO THE KNOWN STRUCTURE FOR A PRECAUTIONARY LNDG. WE LANDED WITHOUT INCIDENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THERE WAS AN ALERT BULLETIN FROM THE MANUFACTURER FOR AN INSPECTION OF THE HUB. HOWEVER THIS ACFT HAD NOT BEEN DONE YET. THE HUB ROTATED ON ITS 1 REMAINING FITTING DOING DAMAGE TO THE TRAILING EDGE OF 1 ROTOR.