FLC OF ACR MLG RESPOND TO A TCASII RA WITH A VFR ACFT THAT DSNDED WITHOUT CLRNC.

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SITUATION OCCURRED DURING INITIAL VECTORING FOR APCH AFTER DEPARTING BUCKS INTXN ARR FIX. WE WERE ON VECTORS FOR AN APCH TO 27R AT PHL. WHILE LEVEL AT 6000 FT HDG 120 DEG APPROX 15 MI NW OF THE ARPT. WE WERE TOLD TO TURN R HDG 180. CTLR CALLED OUT VFR TFC Y AT 1 O'CLOCK; 5 MI; NWBOUND AT 6500 FT ENRTE TO N PHILADELPHIA ARPT. OUR TCASII WAS ON THE 10 MI RANGE WITH NUMEROUS TARGETS IN THAT DIRECTION (WHICH WAS TOWARD PHL INTL). ALL WERE WHITE TARGETS. TCASII WAS THEN SWITCHED TO 5 MI RANGE. TARGET Y JUST APPEARED ON EDGE OF TCASII PICTURE AS YELLOW. TCASII WAS SWITCHED TO THE 10 MI RANGE TO SEE THE REST OF THE TARGET INFO. IT SHOWED TFC Y NOW AT 4 MI AND CLOSING; IN A DSCNT; AND AT 6300 FT TCASII GAVE A TA AURALLY. WE QUERIED THE CTLR THAT THE TFC WAS DSNDING. CTLR GAVE US A TURN TO HDG 090 DEG AND REPLIED THAT THE TFC Y HAD APPARENTLY BUSTED ITS ALT. TARGET Y NOW TURNED TO A RED TARGET INDICATE AT OUR ALT STILL DSNDING. TCASII COMMANDED 'MONITOR VERT SPD.' WE WERE NOW IN A TURN TOWARD 090 DEG (L TURN). TCASII NOW COMMANDED APPROX 2000 FPM CLB. TARGET Y WAS INSIDE THE 2 1/2 MI RANGE RING. THE COPLT (WHO WAS AT THE CTLS); STARTED THE CLB AND CONTINUED TO TURN. I INFORMED THE CTLR THAT THE TARGET Y WAS AT OUR ALT AND WE WERE CLBING CLR OF THE TFC. WE CLBED TO 7000 FT AND WENT TO 7300 FT BEFORE STOPPING THE CLB. IN THE MEANTIME THE TARGET Y INDICATED 5500 FT AND BEHIND US AS IT HAD PASSED OFF OUR R. BECAUSE WE WERE IN A TURN; WE DID NOT SEE THE TFC. AFTER HAVING INFORMED CTLR WE HAD A TCASII RA AND HAD BEGUN TO CLB; HE DECLARED HE WASN'T WORKING THE TFC BUT ANOTHER CTLR WAS. AFTER TFC Y WAS BEHIND US HE TOLD US TO DSND BACK DOWN TO 6000 FT AND CHANGE TO THE FINAL CTLR'S FREQ. AFTER LNDG I CALLED THE SUPERVISING CTLR TO DISCUSS THE INCIDENT AND WAS TOLD THE TFC Y WAS ABOUT 3 MI AWAY AND WE HAD ADEQUATE SEPARATION. I INFORMED HIM WE WERE REQUIRED TO FOLLOW THE TCASII RA COMMANDS AND NEVER DID HAVE THE TFC Y IN SIGHT.