AS A BE1900 IS CLBING TO ALT UNDER THE CTL OF N90; A B747 IS SEEN APCHING FROM BELOW AND BEHIND. THE BE1900 IS COMMANDED TO DSND TO AVOID THE B747 BUT THE FLC CAN SEE THAT A DSCNT WOULD BE INTO THE B747. BE1900 FLC COMPLAINS TO N90 THAT IT WOULD HAVE BEEN BETTER IF THEY CLBED AND THE LOSS OF SEPARATION WAS THE FAULT OF ATC.

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ON JUN/XX/97 OPERATING A SCHEDULED FLT FROM LGA TO ACK; NEW YORK APCH CLRED US TO BDR VOR AND ALSO CLRED US TO CLB TO 15000 FT. AT XA37Z; CLBING OUT OF 14200 FT; AND 7 MI SW OF BDR; OUR TCASII ALERTED US OF IMPENDING TFC AT OUR 5 O'CLOCK POS (2-3 MI AND 700-800 FT BELOW US AND CLBING). WE WERE THEN ADVISED OF THE TFC BY NEW YORK; AND TOLD TO DSND BACK TO 11000 FT. WE THEN TOLD NEW YORK WE WERE GOING BACK DOWN TO 11000 FT. MY FO THEN NOTICED A B747 TYPE ACFT ON OUR 5 O'CLOCK POS. HE ALSO NOTICED THAT THE CLOSURE RATE FROM THE B747 WAS TOO GREAT AND REALIZED THAT IF WE DSNDED TO 11000 FT; WE WOULD DSND INTO THE ACFT. THE CLOSURE RATE FROM THE B747 FORCED US TO TAKE EVASIVE ACTION; BY TURNING TO THE L TO AVOID A POSSIBLE MIDAIR COLLISION. THE FO TOLD ME THAT ACR FLT RR (THE B747) PASSED AT OUR ALT AND WAS 1/4 - 1/2 MI FROM US VERTLY. THE CREW OF ACR TOLD APCH THAT THEY PASSED US WITH 400 METERS OF SEPARATION. I ALSO TOLD APCH WE HAD A CLOSE CALL. UPON REACHING ACK I CALLED NEW YORK APCH AND TOLD THEM OF THE SIT; AND HOW IF WE DSNDED TO 11000 FT WE WOULD HAVE POSSIBLY DSNDED INTO THE B747. THEY TOLD ME THAT THEY WERE INVESTIGATING THE SIT. I BELIEVE APCH SHOULD HAVE DSNDED ACR BACK TO 11000 FT; BECAUSE THEY WERE BEHIND AND BELOW US; AND SHOULD HAVE HAD US TO CONTINUE OUR CLB TO 15000 FT.