A D328 CREW RESPONDED TO A TCASII RA WHILE ON APCH TO PHL.

Narrative

AFTER BEING HANDED OFF TO PHL FINAL CTLR AT 6000 FT WE WERE TOLD TO FLY A HDG OF 060 DEGS AND DSND AND MAINTAIN 3000 FT. PASSING 500 FT A SQUARE APPEARED LESS THAN 5 MI AND 1500 FT BELOW US CLBING. AFTER INVESTIGATION I ADVISED THE FO THAT IT APPEARED THIS ACFT WOULD BE IN CONFLICT WITH US AND TO BE READY. ON THE FOURTH OR FIFTH INTERROGATION WE GOT A TA WHICH WAS ALMOST IMMEDIATELY FOLLOWED BY AN RA 'MONITOR VERT SPD.' A RED ARC WAS SET AT ANY VS BELOW 0. I DISCONNECTED AUTO PLT INSTINCTIVELY AT THE SAME TIME THE FO REACHED FOR IT AND WE BOTH BROUGHT THE ACFT BACK UP TO 4000 FT FROM 3800 FT AS THE ACFT DIRECTLY BELOW US WITHIN 400 FT TO 500 FT; I ADVISED ATC OF OUR ACTION AND WE WERE TOLD TO MAINTAIN 4000 FT AND THAT THIS ACFT WAS SUPPOSED TO BE ON A DIFFERENT SECTOR. THE CTLR SAID; 'HE JUST POPPED UP OUT OF NOWHERE.' WE ADVISED HIM THAT WE CAME WITHIN 400 FT OR 500 FT. THE REST OF THE FLT WAS UNEVENTFUL. THE FO WAS THE FLYING PLT. WE NEVER OBTAINED VISUAL CONFIRMATION OF THIS ACFT. THIS HAD MID AIR COLLISION WRITTEN ALL OVER IT. ATC WAS UNAWARE AND WE NEVER SAW HIM. IF IT WEREN'T FOR TCASII AND COLLISION AVOIDANCE PROCS; THIS WOULD HAVE BEEN TRAGIC.