FLC OF AN MDT ACR ACFT DSNDED PREMATURELY DURING A VISUAL APCH RESULTING IN EVASIVE ACTION IN RESPONDING TO A TCASII ADVISORY ONLY.

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WE RECEIVED RADAR VECTORS FROM RYMES INTXN TOWARD THE LONG ISLAND SOUND. WHEN WE TOLD NY APCH WE HAD WHITE PLAINS IN SIGHT; HE CLRED US FOR THE SOUND VISUAL (34) APCH; MAINTAIN 3000. ONCE WE WERE OFF SHORE; ON BASE WE STARTED DSCNT TO 2000 FT (GS INTERCEPT FOR ILS 34 USED AS REF). AS WE STARTED OUR DSCNT WE PICKED UP A TCASII TA AT 11-11:30 AT 500 FT BELOW US. WE HAD NOT BEEN ADVISED OF TFC BY APCH BUT WE STOPPED DSCNT AT ABOUT 2700 FT BEFORE WE FLEW INTO A RA SIT. I TOLD APCH CTL THAT WE WERE CLBING BACK TO 3000 FT. HE SAID WE SHOULD STILL BE AT 3000 FT. BOTH PLTS AGREED THAT; ACCORDING TO THE PLATE; ONCE CLRED FOR APCH ON BASE OFF SHORE WE COULD DSND. I TALKED TO THE NY APCH SUPVR AND HE AGREED WITH US; BUT SAID THE RADAR PLOT MAY NOT HAVE SHOWN US WHERE WE THOUGHT WE WERE AND HE WOULD TALK TO THE CTLR. PLATES LIKE THIS ARE VAGUE AT BEST AND OPEN TO INTERP OF POS. CAN THEY BE MADE MORE SPECIFIC; I.E. DME AND RADIAL OF GREENWICH POINT AND GREAT CAPT ISLAND. SHOULDN'T WE HAVE BEEN ADVISED OF VFR TFC? NO RPTS WERE FILED. TCASII RA DID NOT TAKE PLACE SO NO TCASII RPT FILED.