A GLF4 CREW ON FINAL TO IAD PREMATURELY DEPARTED THEIR ASSIGNED ALT WITHOUT ATC APPROVAL.

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CONTACTED DULLES APCH AND REQUESTED RWY 1R FOR LNDG AND WAS TOLD THAT IT WAS NOT AVAILABLE THIS NIGHT. WE WERE SET UP TO USE THE R RWY AS IT IS THE MOST CONVENIENT ONE FOR OUR USE. WE QUICKLY SWITCHED ALL THE DATA TO THE L RWY AND DID A FAST BRIEF ON DOWNWIND LEG. WE WERE GIVEN AN ALT OF 4000 FT AND A VECTOR TO INTERCEPT THE LOC. AFTER INTERCEPTING THE LOC; WE HAD A TCASII FAIL MESSAGE WITH AN ACCOMPANYING 'BONG' EVERY 5-10 SECONDS THAT WE WERE NOT ABLE TO CANCEL. WE NOTICED THAT THE GS WAS MOVING AND I ASKED THE CTLR IF WE WERE CLRED FOR THE APCH. THE CTLR ASKED IF WE COULD MAINTAIN 4000 FT UNTIL THE NEXT FIX BEFORE COMMENCING WITH THE APCH. I DETERMINED THAT THIS WOULD PUT US 800 FT HIGHER THAN NORMAL; BUT ACCEPTABLE CONSIDERING WX AND RWY LENGTH. AT THIS TIME; THE NON CANCELABLE TCASII FAIL 'BONG' WAS GOING ON AND OFF EVERY 2-3 SECONDS. I LOOKED DOWN AT THE XPONDER HEAD AND SELECTED THE STAND-BY XPONDER IN HOPES OF CANCELING THIS ANNOYING DISTR. WHEN I LOOKED UP; WE WERE AT 3600 FT AND I REALIZED THAT THE FLYING FO HAD SELECTED THE APCH MODE ON THE AUTOPLT; ANTICIPATING A CLRNC FOR THE APCH. THE AUTOPLT HAD CAPTURED THE GS AND STARTED DOWN. I RESELECTED FLT LEVEL CHANGE TO GET BACK TO 4000 FT AND AT THE SAME TIME THE CTLR ASKED IF WE HAD STARTED DOWN ON OUR OWN. I SAID NO; WE HAD A GS CAPTURE; AND A SHORT TIME LATER SHE CLRED US FOR THE APCH AND WE STARTED DOWN WITH A CONSTANT 'BONG' GOING ON AND OFF. AT 3000 FT AND 5 MI; WE DECIDED THAT WITH ALL THE NONSTANDARD APCH AND DISTRS TO NOT CONTINUE AND REQUESTED A GAR FOR ANOTHER APCH. THE BONG STILL COULD NOT BE CANCELED AND WAS GOING OFF AND ON CONTINUOUSLY. THE SECOND APCH WAS ACCOMPLISHED AT THE NORMAL EXPECTED ALT AND CLRNCS WITH THE CONSTANT ALERTING MESSAGE OF THE TCASII FAIL AND BONGING THE ENTIRE TIME; AND DID NOT STOP UNTIL WE WERE ON THE GND AND ABLE TO TURN OFF THE XPONDER.