Narrative:

Aircraft #1 was level at 3000 ft on a 180 degree heading for right downwind to runway 34 approximately 7 NM east of hpn. I observed unknown traffic eastbound in the hpn class D airspace. I issued the traffic twice to aircraft #1; the first one when the traffic was at the 2 O'clock position and about 5 NM away indicating between 2000-2500 ft. The second call was when the unknown traffic was at the 2 O'clock position and about 3 NM away indicating 2700 ft. The pilot of aircraft #1 said that he was receiving a TCASII alert and was responding by climbing. I advised him to reconsider since there was a B733 just off his left level at 4000 ft (this was an lga inbound; also sbound; and being worked by another controller). The pilot advised that he had traffic 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 climbing and had reached 3400 ft or 3500 ft. I advised him to maintain visual separation with both aircraft and; when able; to descend and maintain 2000 ft. Shortly thereafter; the pilot complied. A comment: this is not the first time I have observed a TCASII alert in response to one aircraft instructing the pilot to respond in such a way that it places the aircraft in very close proximity to a third aircraft. 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 aircraft in the area.

Related reports:

Google
 

Original NASA ASRS Text

Title: 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.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.