ZNY ARTCC CTLR INSTRUCTED AN MD80 CREW TO MAINTAIN VISUAL SEPARATION FROM A DH8 THAT RESULTED IN A TCASII EVENT.

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THIS SIT OCCURRED ON DEC/SUN/02 ON ARR TO LGA. ZNY ISSUED US A DSCNT TO 11000 FT AND INFORMED US OF TFC (A DASH 8 CALL SIGN X AT 11 O'CLOCK). THE CTLR THEN ASKED US IF WE HAD THE DASH 8 IN SIGHT AND IF WE COULD MAINTAIN VISUAL SEPARATION. WE COULD SEE ACFT Y CLRLY AND DETERMINED THAT WE WOULD PASS WELL BEHIND AND TO THE L OF HIM. WE TOLD ZNY THAT WE COULD MAINTAIN SEPARATION AND HE GAVE US A FURTHER DSCNT TO 10000 FT TELLING US THAT WE WOULD BE OVERTAKING THE DASH 8 ON THE WAY TO LGA (UNCLEAR IF THAT MEANT HIM OR US). 10000 FT HAPPENED TO BE THE SAME ALT THE DASH 8 WAS CLRED TO. A FEW MOMENTS LATER CTR TOLD US THAT HE HAD GIVEN ACFT Y A SLIGHT L TURN TO HELP US OUT WITH THE SEPARATION; AND HE THEN GAVE US A FREQ CHANGE TO CALL NY APCH. AS WE CHANGED FREQS AND CHKED IN; WE NOTICED THAT THE TURN GIVEN TO ACFT Y PLACED HIM DIRECTLY IN FRONT OF US ON THE SAME COURSE. I ASKED APCH IF THERE WAS ANY PREFERENCE TO THE DIRECTION WE TURNED TO AVOID HIM AND WHERE HE WAS HEADED. I DON'T RECALL ANY RESPONSE FROM APCH; BUT I THEN TOLD THE CAPT THAT WE NEEDED TO TURN. HE INITIATED A SMALL; SMOOTH TURN TO THE L PLACING US ON A DIVERGENT HDG FROM THE DASH 8. ABOUT THE SAME TIME; THE TCASII GAVE US A TA FOLLOWED QUICKLY BY AN RA. THE CAPT INITIATED A SLIGHT CLB IN ADDITION TO THE TURN ALREADY IN PROGRESS. THE DASH 8 DSNDED RAPIDLY AND ASKED APCH WHAT WAS HAPPENING. ACFT Y TOLD NY APCH THAT HE WAS UNAWARE THAT WE WERE MAINTAINING VISUAL SEPARATION FROM HIM. THIS MAY HAVE BEEN DUE TO THE FREQ CHANGE OR HE MAY HAVE JUST SIMPLY NOT HEARD OR REALIZED WHAT WAS HAPPENING. AT ANY RATE; ACFT Y WAS UNDERSTANDABLY CONCERNED ABOUT THE OCCURRENCE. I REMEMBER LOOKING AT THE TCASII DISPLAY; AND SEEING ACFT Y AT ABOUT 4 MI OFF OUR R AS WE PASSED HIM. OF COURSE; ANY TCASII RA IS A BIT UNNERVING; HOWEVER; I WOULD ESTIMATE THAT THE CLOSEST WE CAME TO THE DASH 8 WAS ABOUT 3-4 MI. SUPPLEMENTAL INFO FROM ACN #567860: WHILE DSNDING TO 11000 FT AT 320 KTS ON THE MILTON 3 ARR INTO LGA APCHING BEUTY INTXN; ZNY ASKED IF WE HAD A DASH 8 AT 10:30-11 O'CLOCK POS DSNDING TO 10000 FT HEADING E. WE REPLIED YES. HE THEN ASKED IF WE COULD REMAIN IN VISUAL CONTACT AND DSND TO 10000 FT AS WE DSNDED INTO LGA. WE REPLIED YES; RECEIVED CLRNC AND BEGAN DSCNT SO AS TO PASS BEHIND HIM. SHORTLY THEREAFTER; THE CTLR TOLD THE DASH 8 TO TURN 15 DEGS L FOR ADDITIONAL SPACING ON US. THIS TURN PUT HIM DEAD AHEAD ON THE SAME HDG AS US WITH BOTH OF US APCHING 10000. AS WE REALIZED WHAT WAS HAPPENING; WE STARTED TO SLOW AND TRIED TO ASK CTR WHERE THE DASH 8 WAS GOING; BUT WE COULD NOT GET IN. JUST THEN THE DASH 8 CALLED THAT HE WAS RESPONDING TO A TCASII RA AND WAS DSNDING. WE TRIED TO CALL CTR AGAIN; WHEN OUR TCASII GAVE US AN RA TO CLB. AS WE WATCHED THE DASH 8 QUICKLY DSND BENEATH US; WE STARTED A CLB; BUT STOPPED IT AFTER ONLY 100-200 FT AND RETURNED TO 10000. ONLY AFTER THE FACT DID WE LEARN THAT THE DASH 8 WAS TURNING TO JOIN OUR ARR. LUCKILY WE HAD HIM EASILY IN SIGHT THE ENTIRE INCIDENT.