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.