Narrative:

On downwind radar vector for lax runway 24R, ATC had us at 2500 ft MSL and 180 KTS. We were following a foreign air carrier's DC9. We were turned to a heading approximately 160 degrees and told of VFR traffic in turn at 2100 ft MSL. I saw traffic on TCASII at 2400 ft MSL and saw immediate potential for conflict. I told first officer to get ready for a TCASII RA as I disconnected the autoplt. At that exact moment TCASII went 'traffic, traffic' followed by 'descend!' I followed steering then picked up red and white cessna at 1 O'clock. Increased downwind maneuver as the cessna passed above and down the right side within 300 ft horizontal and vertical. Altitude bottomed out at 1900 ft MSL. If we had not followed TCASII and visually picked up this aircraft, it would have been a midair. The good news is I saw this scenario developing before it was beyond our control. ATC, I think, was not doing a very good job of keeping us informed of this 'pop up' traffic, and I feel the turn into this known traffic was not well thought out. Also, being at 2500 ft MSL at 17 mi from lax seemed a bit low. I am not sure if this was below class B airspace.

Related reports:

Google
 

Original NASA ASRS Text

Title: AN ACR MLG FLC HAVE AN NMAC WITH A CESSNA ACFT WHILE ON A VECTOR FOR APCH TO LAX. THE FLC REACTS PROPERLY TO A TCASII 'RA' THEN ACQUIRES THE CESSNA VISUALLY. IT HAD CLBED TO THE ACR'S ALT.

Narrative: ON DOWNWIND RADAR VECTOR FOR LAX RWY 24R, ATC HAD US AT 2500 FT MSL AND 180 KTS. WE WERE FOLLOWING A FOREIGN ACR'S DC9. WE WERE TURNED TO A HDG APPROX 160 DEGS AND TOLD OF VFR TFC IN TURN AT 2100 FT MSL. I SAW TFC ON TCASII AT 2400 FT MSL AND SAW IMMEDIATE POTENTIAL FOR CONFLICT. I TOLD FO TO GET READY FOR A TCASII RA AS I DISCONNECTED THE AUTOPLT. AT THAT EXACT MOMENT TCASII WENT 'TFC, TFC' FOLLOWED BY 'DSND!' I FOLLOWED STEERING THEN PICKED UP RED AND WHITE CESSNA AT 1 O'CLOCK. INCREASED DOWNWIND MANEUVER AS THE CESSNA PASSED ABOVE AND DOWN THE R SIDE WITHIN 300 FT HORIZ AND VERT. ALT BOTTOMED OUT AT 1900 FT MSL. IF WE HAD NOT FOLLOWED TCASII AND VISUALLY PICKED UP THIS ACFT, IT WOULD HAVE BEEN A MIDAIR. THE GOOD NEWS IS I SAW THIS SCENARIO DEVELOPING BEFORE IT WAS BEYOND OUR CTL. ATC, I THINK, WAS NOT DOING A VERY GOOD JOB OF KEEPING US INFORMED OF THIS 'POP UP' TFC, AND I FEEL THE TURN INTO THIS KNOWN TFC WAS NOT WELL THOUGHT OUT. ALSO, BEING AT 2500 FT MSL AT 17 MI FROM LAX SEEMED A BIT LOW. I AM NOT SURE IF THIS WAS BELOW CLASS B AIRSPACE.

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.