MD83 ON VECTOR FOR APCH RWY 24R TO FOLLOW ACFT ON RWY 24L. TFC IN SIGHT AND CAPT ADVISED FO; PF; HAD TCASII TARGET BUT NO TA OR RA. FO DECREASED RATE OF TURN AND ENDED S OF TRACK. NO CONFLICT OR COMMENT FROM CTLR.

Narrative

CAPT WAS PNF ON A VISUAL APCH TO RWY 24L AT LAX. DURING THE TURN FROM BASE TO FINAL; CAPT NOTICED THAT THE GND TRACK WAS PROGRESSIVELY MOVING S OF DESIRED EXTENDED LOC COURSE AND IMMEDIATELY ADVISED THE FO TO CORRECT N. FO COMPLIED AND A NON EVENTFUL LNDG WAS MADE ON RWY 24R. BOTH PLTS HAD VISUALLY ACQUIRED THE TFC TO FOLLOW AND WERE DOING SO. DURING THE TURN FROM BASE TO FINAL; CAPT NOTICED A 'POP UP' TCASII TARGET THAT WAS JUST OUTSIDE THE 2 MI RANGE RING ON THE TCASII INDICATOR. TARGET SHOWED 1000 FT BELOW AND DSNDING; WHICH DID NOT ACTIVATE ANY TA OR RA WARNINGS. CAPT VERBALLY EXPRESSED CONCERN TO THE FO REGARDING THE TARGET; WHICH APPARENTLY DISTR THE FO LONG ENOUGH TO DECREASE HIS BANK ANGLE IN THE TURN TO HELP LOOK FOR THE TFC. THE RESULTANT TURN RATE PUT THE ACFT S OF THE LOC COURSE; AND WAS CORRECTED AS SOON AS IT WAS NOTICED. THERE WERE NO TFC CONFLICTS EITHER FROM TCASII OR ATC. TOTAL TIME S OF LOC TRACK IS ESTIMATED 5-7 SECONDS. ATC MADE NO MENTION OF THE GND TRACK OR ANY TA'S DURING THE APCH. CAPT BELIEVES THAT THE 'XFER OF RESPONSIBILITY' FOR VISUAL TFC SEPARATION WAS AN ADDITIONAL FACTOR. WHETHER THE 'POP-UP' TARGET WAS ACTUALLY THERE OR NOT; WE HAVE BEEN UNABLE TO RESOLVE. SINCE THERE WAS NO ADVISORY COM FROM ATC; WE CAN ONLY ASSUME THAT IT WAS EITHER 1) A TCASII MALFUNCTION; OR 2) THE TFC WAS THERE BUT ATC ASSUMED WE HAD THEM IN SIGHT.