Narrative:

OJT was being performed during this time by controller a. Air carrier X (medium large transport) was being vectored on a north downwind to lax by controller a to runway 25L. Air carrier Y (large transport) was cleared for a straight in visibility approach to runway 24L, following air carrier Z (widebody transport) on runway 24R, by controller B. Air carrier Y was advised 20 mi east of lax that 'an aircraft downwind north will pass above you. Maintain visibility sep landing runway 25.' controller a called traffic for air carrier X, which initially was for air carrier Z. Air carrier X said, I'm not sure if traffic is in sight.' controller a reissued traffic to air carrier X when turned to a base leg. Air carrier X had the widebody transport in sight, but no air carrier Y large transport (approximately 3 mi in trail, 24R and 24L). Air carrier X was cleared for a visibility approach to runway 25L and told to maintain visibility sep from air carrier, and proceeded to descend on the base leg for runway 25. Air carrier Y asked controller B, 'who is the guy crossing in front of me?' controller B reiterated that air carrier X was instructed to maintain visibility sep above air carrier Y. Several factors caused this to happen: 1) the instrument believed in his mind that the deviation was issuing traffic for the air carrier and not the air carrier . When the pilot of air carrier X reported the traffic, he was satisfied that everything had been done. 2) on all 3 aircraft involved, the mode C portion on the data blocks were masked by 'XXX' because the system could not give the correct altitude. Controller B also saw this and assumed controller a was restr above his aircraft. Neither controller was alerted to any problems, because they could not see the mode C. The instrument for controller a should not have assumed anything and should have questioned the pilot for altitude information if targets were to merge. Supplemental information from acn 121797: air carrier X was on a runway 24/25 profile descent at lax. Prior to being cleared for a visibility approach to runway 25L (downwind to base), ATC asked if we had 2 aircraft in sight in our approximately 2 O'clock position. We indicated that we had one, then both aircraft in sight. We followed them and made a visibility approach to runway 25L and landed. Later that evening, ATC advised us that a near miss had been reported with our aircraft. We had not seen any aircraft in our close proximity.

Related reports:

Google
 

Original NASA ASRS Text

Title: APCH CTLR FAILED TO HAVE ACR X MAINTAIN VISUAL SEPARATION FROM ACR Y ON VISUAL APCH. LESS THAN STANDARD SEPARATION. SYSTEM ERROR. NMAC.

Narrative: OJT WAS BEING PERFORMED DURING THIS TIME BY CTLR A. ACR X (MLG) WAS BEING VECTORED ON A N DOWNWIND TO LAX BY CTLR A TO RWY 25L. ACR Y (LGT) WAS CLRED FOR A STRAIGHT IN VIS APCH TO RWY 24L, FOLLOWING ACR Z (WDB) ON RWY 24R, BY CTLR B. ACR Y WAS ADVISED 20 MI E OF LAX THAT 'AN ACFT DOWNWIND N WILL PASS ABOVE YOU. MAINTAIN VIS SEP LNDG RWY 25.' CTLR A CALLED TFC FOR ACR X, WHICH INITIALLY WAS FOR ACR Z. ACR X SAID, I'M NOT SURE IF TFC IS IN SIGHT.' CTLR A REISSUED TFC TO ACR X WHEN TURNED TO A BASE LEG. ACR X HAD THE WDB IN SIGHT, BUT NO ACR Y LGT (APPROX 3 MI IN TRAIL, 24R AND 24L). ACR X WAS CLRED FOR A VIS APCH TO RWY 25L AND TOLD TO MAINTAIN VIS SEP FROM ACR, AND PROCEEDED TO DSND ON THE BASE LEG FOR RWY 25. ACR Y ASKED CTLR B, 'WHO IS THE GUY XING IN FRONT OF ME?' CTLR B REITERATED THAT ACR X WAS INSTRUCTED TO MAINTAIN VIS SEP ABOVE ACR Y. SEVERAL FACTORS CAUSED THIS TO HAPPEN: 1) THE INSTR BELIEVED IN HIS MIND THAT THE DEV WAS ISSUING TFC FOR THE ACR AND NOT THE ACR . WHEN THE PLT OF ACR X RPTED THE TFC, HE WAS SATISFIED THAT EVERYTHING HAD BEEN DONE. 2) ON ALL 3 ACFT INVOLVED, THE MODE C PORTION ON THE DATA BLOCKS WERE MASKED BY 'XXX' BECAUSE THE SYS COULD NOT GIVE THE CORRECT ALT. CTLR B ALSO SAW THIS AND ASSUMED CTLR A WAS RESTR ABOVE HIS ACFT. NEITHER CTLR WAS ALERTED TO ANY PROBS, BECAUSE THEY COULD NOT SEE THE MODE C. THE INSTR FOR CTLR A SHOULD NOT HAVE ASSUMED ANYTHING AND SHOULD HAVE QUESTIONED THE PLT FOR ALT INFO IF TARGETS WERE TO MERGE. SUPPLEMENTAL INFO FROM ACN 121797: ACR X WAS ON A RWY 24/25 PROFILE DSCNT AT LAX. PRIOR TO BEING CLRED FOR A VIS APCH TO RWY 25L (DOWNWIND TO BASE), ATC ASKED IF WE HAD 2 ACFT IN SIGHT IN OUR APPROX 2 O'CLOCK POS. WE INDICATED THAT WE HAD ONE, THEN BOTH ACFT IN SIGHT. WE FOLLOWED THEM AND MADE A VIS APCH TO RWY 25L AND LANDED. LATER THAT EVENING, ATC ADVISED US THAT A NEAR MISS HAD BEEN RPTED WITH OUR ACFT. WE HAD NOT SEEN ANY ACFT IN OUR CLOSE PROX.

Data retrieved from NASA's ASRS site as of August 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.