Narrative:

While being vectored for ILS wy 16 approach at hpn at 2000 ft, at a location nne of airfield, a TCASII target appeared between us and the aircraft -- approximately 9-10 mi in front of us. The new target was initially 500 ft above the descending on a converging path from our 11 O'clock position. As his descent continued to 300 ft above and he closed to within 5 mi while still converging, I selected the 5 mi TCASII scale and disconnected the autoplt. His rate of closure on this scale was alarming and I initiated a descent to keep some altitude between us. I descended approximately 400-450 ft and turned away. The controller called and asked us to confirm altitude. I responded that I had maneuvered to avoid a collision. He responded, 'there's nobody out there.' by this time, the aircraft passed and I was back up to 2000 ft. Total time from beginning of descent to reaching 2000 ft was approximately 10 seconds. Conflict aircraft continued descent when I did and maintained 200-300 ft above, even as we were turning away at 1600 ft. We received a GPWS warning at approximately 1700 ft and climbed in response when clear of traffic.

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Original NASA ASRS Text

Title: ALT EXCURSION WHEN THE FLT CREW OF AN FA50 RECEIVE A TCASII RA FROM CLOSING TFC TO DSND AND THEN, AT 1500 FT AGL, RECEIVE A GPWS ALERT WHEN 9 NM NE OF HPN, NY.

Narrative: WHILE BEING VECTORED FOR ILS WY 16 APCH AT HPN AT 2000 FT, AT A LOCATION NNE OF AIRFIELD, A TCASII TARGET APPEARED BTWN US AND THE ACFT -- APPROX 9-10 MI IN FRONT OF US. THE NEW TARGET WAS INITIALLY 500 FT ABOVE THE DSNDING ON A CONVERGING PATH FROM OUR 11 O'CLOCK POS. AS HIS DSCNT CONTINUED TO 300 FT ABOVE AND HE CLOSED TO WITHIN 5 MI WHILE STILL CONVERGING, I SELECTED THE 5 MI TCASII SCALE AND DISCONNECTED THE AUTOPLT. HIS RATE OF CLOSURE ON THIS SCALE WAS ALARMING AND I INITIATED A DSCNT TO KEEP SOME ALT BTWN US. I DSNDED APPROX 400-450 FT AND TURNED AWAY. THE CTLR CALLED AND ASKED US TO CONFIRM ALT. I RESPONDED THAT I HAD MANEUVERED TO AVOID A COLLISION. HE RESPONDED, 'THERE'S NOBODY OUT THERE.' BY THIS TIME, THE ACFT PASSED AND I WAS BACK UP TO 2000 FT. TOTAL TIME FROM BEGINNING OF DSCNT TO REACHING 2000 FT WAS APPROX 10 SECONDS. CONFLICT ACFT CONTINUED DSCNT WHEN I DID AND MAINTAINED 200-300 FT ABOVE, EVEN AS WE WERE TURNING AWAY AT 1600 FT. WE RECEIVED A GPWS WARNING AT APPROX 1700 FT AND CLBED IN RESPONSE WHEN CLR OF TFC.

Data retrieved from NASA's ASRS site as of July 2007 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.