AFTER FREQ CHANGE FROM ZDC TO N90 APCH CTL; RPTING LEVEL AT 11000 FT; AN MD82 FLC RECEIVES A TCASII TA. FOLLOWING THE TCASII ADVISORY APCH CTL QUERIES FLC'S ALT AND IS ISSUED AN IMMEDIATE DSCNT TO 8000 FT.

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THE FLT WAS CLRED THE RBV 1 ARR (ENO.RBV 1) TO EWR. PRIOR TO REACHING DAVYS INTXN; ZDC CLRED THE FLT TO PROCEED DIRECT TO HOLEY INTXN AND MAINTAIN 11000 FT. THE FLT WAS THEN HANDED OFF TO NEW YORK APCH WITH THE FO RPTING 11000 FT ON INITIAL CONTACT. NEW YORK APCH THEN CLRED THE FLT TO PROCEED DIRECT TO THE OWBIE INTXN. SHORTLY THEREAFTER; TCASII DISPLAYED A TA; AND THE CREW RECEIVED VOCAL WARNING OF 'TFC; TFC.' TCASII DISPLAY INDICATED THAT THE CONFLICTING ACFT WOULD PASS BEHIND US AND 11000 FT WAS MAINTAINED. IMMEDIATELY THEREAFTER; NEW YORK APCH QUERIED US AS TO OUR ALT. THE FO RESPONDED LEVEL AT 11000 FT. NEW YORK APCH THEN ORDERED US TO 'DSND IMMEDIATELY TO 8000 FT.' THE AUTOPLT AND AUTOTHROTTLES WERE IMMEDIATELY DISENGAGED; SPOILERS DEPLOYED; AND WE IMMEDIATELY BEGAN DSCNT TO 8000 FT AT MAX RATE OF DSCNT. NEW YORK APCH THEN QUERIED US AS TO WHY WE WERE MAINTAINING 11000 FT; AND THE FO RESPONDED THAT IT WAS THE LAST ASSIGNED ALT. NEW YORK APCH THEN REQUESTED THAT WE CHANGE XPONDER CODE TO A NEWLY ISSUED CODE; WHICH WAS COMPLIED WITH. THE FLT CONTINUED TO EWR WITHOUT FURTHER EVENT.