Narrative:

Level at 6000 ft MSL on a vector to base leg. ATC called traffic at 2 O'clock (I think) at 5500 ft. We had him on TCASII at -500 ft at 2 O'clock. First officer called traffic in sight. About 10-15 seconds later we got RA to descend, descend now crossing traffic. I reacted immediately and aggressively descended to 4000 ft MSL. TCASII was screaming the whole time. Ivsi was all red with about 1/4 inch green at the bottom. Level at 4000 ft MSL. I asked the controller what had happened. He said the aircraft that we called in sight had climbed into us and he had him at 5800 ft. He saw us react to RA and I asked him if he was sure it was the same aircraft he called earlier because he seemed to come out of nowhere. He said it was. We called him on the ground. He said the intruder aircraft was in class B airspace and not talking to anyone. It would have been nice to have had this information when he called the traffic at 5500 ft earlier. I assumed the guy was assigned 5500 ft. I asked him where the guy went and he said he didn't know. I suspect he landed at one of the local small airports. This was close -- very close. TCASII worked -- don't leave home without it. This is the second event that I know of this yr between my airline and a smaller aircraft. TCASII saved us both.

Related reports:

Google
 

Original NASA ASRS Text

Title: FLC OF AN MLG TOOK EVASIVE ACTION BY DSNDING FROM ASSIGNED APCH ALT IN RESPONSE TO TCASII RA.

Narrative: LEVEL AT 6000 FT MSL ON A VECTOR TO BASE LEG. ATC CALLED TFC AT 2 O'CLOCK (I THINK) AT 5500 FT. WE HAD HIM ON TCASII AT -500 FT AT 2 O'CLOCK. FO CALLED TFC IN SIGHT. ABOUT 10-15 SECONDS LATER WE GOT RA TO DSND, DSND NOW XING TFC. I REACTED IMMEDIATELY AND AGGRESSIVELY DSNDED TO 4000 FT MSL. TCASII WAS SCREAMING THE WHOLE TIME. IVSI WAS ALL RED WITH ABOUT 1/4 INCH GREEN AT THE BOTTOM. LEVEL AT 4000 FT MSL. I ASKED THE CTLR WHAT HAD HAPPENED. HE SAID THE ACFT THAT WE CALLED IN SIGHT HAD CLBED INTO US AND HE HAD HIM AT 5800 FT. HE SAW US REACT TO RA AND I ASKED HIM IF HE WAS SURE IT WAS THE SAME ACFT HE CALLED EARLIER BECAUSE HE SEEMED TO COME OUT OF NOWHERE. HE SAID IT WAS. WE CALLED HIM ON THE GND. HE SAID THE INTRUDER ACFT WAS IN CLASS B AIRSPACE AND NOT TALKING TO ANYONE. IT WOULD HAVE BEEN NICE TO HAVE HAD THIS INFO WHEN HE CALLED THE TFC AT 5500 FT EARLIER. I ASSUMED THE GUY WAS ASSIGNED 5500 FT. I ASKED HIM WHERE THE GUY WENT AND HE SAID HE DIDN'T KNOW. I SUSPECT HE LANDED AT ONE OF THE LCL SMALL ARPTS. THIS WAS CLOSE -- VERY CLOSE. TCASII WORKED -- DON'T LEAVE HOME WITHOUT IT. THIS IS THE SECOND EVENT THAT I KNOW OF THIS YR BTWN MY AIRLINE AND A SMALLER ACFT. TCASII SAVED US BOTH.

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.