Narrative:

Flying from lax to san at 13000', ATC called traffic at 9 O'clock, 5 mi, same direction 9000'. I observed the traffic as called (so I thought) and we advised ATC that we had it in sight. They cleared us to 7000', maintain visibility sep. As we started down, our traffic receded to 8 O'clock, then disappeared behind us. I set the airplane up in a cruise descent confign of 246 KIAS (redline) and about 800 FPM rate of descent. We completed the descent checklist and other cockpit chores. As we passed through about 9000' ATC called our traffic at 9 O'clock, 1 mi, 9000'. My first officer responded, traffic in sight, based on my earlier gall, I, however, was confused for a moment, thinking I had left my traffic well behind. A quick scan at 9 O'clock revealed a green position light against the horizon. The green light was substantially brighter tan I expected, and when I saw a white light moving rearward relative to the horizon I realized the aircraft was very close and converging. I abruptly arrested the descent and cbed back to 9300' as the other aircraft passed directly beneath us at an identical speed. While this was occurring we heard ATC was advising them that we had them in sight. I am curious to hear how they responded as they saw us flash by. We continued to an uneventful landing. I was distressed by several factors: the other aircraft had only red, green and white position lights. No strobes or recognition or logo lights or beacon. Very difficult to detect when not moving relative to night time urban backgnd clutter. I had earlier seen another aircraft which approximately the location of the conflict aircraft when it was originally called by ATC. This immediately brought to mind memories of the 1978 psa accident which had occurred under similar circumstances. I suppose our misident could have been detected earlier if we had tried to verify with ATC that our traffic was no longer a factor. We were using VHF. They were using UHF. If we had heard them on our radio we might have detected the problem earlier.

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Original NASA ASRS Text

Title: FLT CREW OF LTT ERRONEOUSLY IDENTIFIED TRAFFIC THEY WERE TO MAINTAIN VISUAL WITH AND HAD NMAC WITH OTHER TRAFFIC.

Narrative: FLYING FROM LAX TO SAN AT 13000', ATC CALLED TFC AT 9 O'CLOCK, 5 MI, SAME DIRECTION 9000'. I OBSERVED THE TFC AS CALLED (SO I THOUGHT) AND WE ADVISED ATC THAT WE HAD IT IN SIGHT. THEY CLRED US TO 7000', MAINTAIN VIS SEP. AS WE STARTED DOWN, OUR TFC RECEDED TO 8 O'CLOCK, THEN DISAPPEARED BEHIND US. I SET THE AIRPLANE UP IN A CRUISE DSNT CONFIGN OF 246 KIAS (REDLINE) AND ABOUT 800 FPM RATE OF DSNT. WE COMPLETED THE DSNT CHKLIST AND OTHER COCKPIT CHORES. AS WE PASSED THROUGH ABOUT 9000' ATC CALLED OUR TFC AT 9 O'CLOCK, 1 MI, 9000'. MY F/O RESPONDED, TFC IN SIGHT, BASED ON MY EARLIER GALL, I, HOWEVER, WAS CONFUSED FOR A MOMENT, THINKING I HAD L MY TFC WELL BEHIND. A QUICK SCAN AT 9 O'CLOCK REVEALED A GREEN POS LIGHT AGAINST THE HORIZON. THE GREEN LIGHT WAS SUBSTANTIALLY BRIGHTER TAN I EXPECTED, AND WHEN I SAW A WHITE LIGHT MOVING REARWARD RELATIVE TO THE HORIZON I REALIZED THE ACFT WAS VERY CLOSE AND CONVERGING. I ABRUPTLY ARRESTED THE DSNT AND CBED BACK TO 9300' AS THE OTHER ACFT PASSED DIRECTLY BENEATH US AT AN IDENTICAL SPD. WHILE THIS WAS OCCURRING WE HEARD ATC WAS ADVISING THEM THAT WE HAD THEM IN SIGHT. I AM CURIOUS TO HEAR HOW THEY RESPONDED AS THEY SAW US FLASH BY. WE CONTINUED TO AN UNEVENTFUL LNDG. I WAS DISTRESSED BY SEVERAL FACTORS: THE OTHER ACFT HAD ONLY RED, GREEN AND WHITE POS LIGHTS. NO STROBES OR RECOGNITION OR LOGO LIGHTS OR BEACON. VERY DIFFICULT TO DETECT WHEN NOT MOVING RELATIVE TO NIGHT TIME URBAN BACKGND CLUTTER. I HAD EARLIER SEEN ANOTHER ACFT WHICH APPROX THE LOCATION OF THE CONFLICT ACFT WHEN IT WAS ORIGINALLY CALLED BY ATC. THIS IMMEDIATELY BROUGHT TO MIND MEMORIES OF THE 1978 PSA ACCIDENT WHICH HAD OCCURRED UNDER SIMILAR CIRCUMSTANCES. I SUPPOSE OUR MISIDENT COULD HAVE BEEN DETECTED EARLIER IF WE HAD TRIED TO VERIFY WITH ATC THAT OUR TFC WAS NO LONGER A FACTOR. WE WERE USING VHF. THEY WERE USING UHF. IF WE HAD HEARD THEM ON OUR RADIO WE MIGHT HAVE DETECTED THE PROB EARLIER.

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.