Narrative:

Approaching lax on the julian 3 arrival just prior to paradise VOR, were given heading of 340 degrees to intercept runway 25L localizer. Just prior to intercept, TCASII TA sounded. Target aircraft was approaching from above and right of our position. Crew did not visually acquire other aircraft but could see it on TCASII display as it was stepped from 25 to 10 mi range. At approximately 5 mi range the first officer, who was the PF, visually acquired the target aircraft, an light transport. Simultaneously the descend warning was directing a descent, the first officer disengaged the autoplt and initiated descent to approximately 15300 ft MSL. During entire descent the light transport was kept in sight and he passed overhead our aircraft at 16500 ft MSL. Descent was initiated due to TCASII direction and visual assessment of another aircraft descending directly into our proposed position. We did not know he was stopping his descent at 16500 ft. Our crew was never informed of the other aircraft's position or intention until we initiated the inquiry well into our descent. Controller's response was to the effect of reporting 'another aircraft VFR at 16500 ft.' los angeles has busy airspace. See and avoid if possible. Heads up from the controller would have helped. The TCASII system worked perfectly throughout.

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Original NASA ASRS Text

Title: ACR HAS TCASII TA THEN RA, THEN SIGHTS TFC. DSNDS AS INDICATED.

Narrative: APCHING LAX ON THE JULIAN 3 ARR JUST PRIOR TO PARADISE VOR, WERE GIVEN HDG OF 340 DEGS TO INTERCEPT RWY 25L LOC. JUST PRIOR TO INTERCEPT, TCASII TA SOUNDED. TARGET ACFT WAS APCHING FROM ABOVE AND R OF OUR POS. CREW DID NOT VISUALLY ACQUIRE OTHER ACFT BUT COULD SEE IT ON TCASII DISPLAY AS IT WAS STEPPED FROM 25 TO 10 MI RANGE. AT APPROX 5 MI RANGE THE FO, WHO WAS THE PF, VISUALLY ACQUIRED THE TARGET ACFT, AN LTT. SIMULTANEOUSLY THE DSND WARNING WAS DIRECTING A DSCNT, THE FO DISENGAGED THE AUTOPLT AND INITIATED DSCNT TO APPROX 15300 FT MSL. DURING ENTIRE DSCNT THE LTT WAS KEPT IN SIGHT AND HE PASSED OVERHEAD OUR ACFT AT 16500 FT MSL. DSCNT WAS INITIATED DUE TO TCASII DIRECTION AND VISUAL ASSESSMENT OF ANOTHER ACFT DSNDING DIRECTLY INTO OUR PROPOSED POS. WE DID NOT KNOW HE WAS STOPPING HIS DSCNT AT 16500 FT. OUR CREW WAS NEVER INFORMED OF THE OTHER ACFT'S POS OR INTENTION UNTIL WE INITIATED THE INQUIRY WELL INTO OUR DSCNT. CTLR'S RESPONSE WAS TO THE EFFECT OF RPTING 'ANOTHER ACFT VFR AT 16500 FT.' LOS ANGELES HAS BUSY AIRSPACE. SEE AND AVOID IF POSSIBLE. HEADS UP FROM THE CTLR WOULD HAVE HELPED. THE TCASII SYS WORKED PERFECTLY THROUGHOUT.

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.