N90 CTLR DESCRIBES INCIDENT AS ACFT RESPONDING TO TCASII CLB INSTRUCTION FOR TFC BELOW RESULTED IN CONFLICT WITH TFC ABOVE.

Narrative

ACFT #1 WAS LEVEL AT 3000 FT ON A 180 DEG HDG FOR R DOWNWIND TO RWY 34 APPROX 7 NM E OF HPN. I OBSERVED UNKNOWN TFC EBOUND IN THE HPN CLASS D AIRSPACE. I ISSUED THE TFC TWICE TO ACFT #1; THE FIRST ONE WHEN THE TFC WAS AT THE 2 O'CLOCK POS AND ABOUT 5 NM AWAY INDICATING BTWN 2000-2500 FT. THE SECOND CALL WAS WHEN THE UNKNOWN TFC WAS AT THE 2 O'CLOCK POS AND ABOUT 3 NM AWAY INDICATING 2700 FT. THE PLT OF ACFT #1 SAID THAT HE WAS RECEIVING A TCASII ALERT AND WAS RESPONDING BY CLBING. I ADVISED HIM TO RECONSIDER SINCE THERE WAS A B733 JUST OFF HIS L LEVEL AT 4000 FT (THIS WAS AN LGA INBOUND; ALSO SBOUND; AND BEING WORKED BY ANOTHER CTLR). THE PLT ADVISED THAT HE HAD TFC IN SIGHT. I ASKED WHICH ONE HE WAS REFERRING TO. HE SAID THAT HE NOW HAD BOTH IN SIGHT. DURING THIS EXCHANGE; HE WAS ALREADY CLBING AND HAD REACHED 3400 FT OR 3500 FT. I ADVISED HIM TO MAINTAIN VISUAL SEPARATION WITH BOTH ACFT AND; WHEN ABLE; TO DSND AND MAINTAIN 2000 FT. SHORTLY THEREAFTER; THE PLT COMPLIED. A COMMENT: THIS IS NOT THE FIRST TIME I HAVE OBSERVED A TCASII ALERT IN RESPONSE TO ONE ACFT INSTRUCTING THE PLT TO RESPOND IN SUCH A WAY THAT IT PLACES THE ACFT IN VERY CLOSE PROX TO A THIRD ACFT. IT APPEARS THAT THE TCASII RESPONDS TO ONLY ONE 'THREAT' AT A TIME WITHOUT TAKING INTO ACCOUNT THE RAMIFICATIONS OF THE RESPONSE/ACTION SHOULD THERE BE OTHER ACFT IN THE AREA.