CLOSE PROX ACR MLG ACR LTT BOTH ON VISUAL APCH TO JAX.

Narrative

OUR FLT HAD BEEN VECTORED BY JAX APCH CTL TO A HDG OF 130 DEG AND CLRED TO DSCNT TO 5000'. AT 5000' AND ON A HDG OF 130 DEG WITH A SPD OF 200 KTS; APCH CTL ASKED US; 'DO YOU HAVE THE FIELD IN SIGHT?' OUR ANSWER; 'YES' APCH CTL; 'THEIR IS A ACR Y AT YOUR 9 O'CLOCK AND 4 MI. DO YOU SEE HIM?' OUR ANSWER; 'YES.' APCH CTL; 'OK; YOU ARE CLRED FOR A VIS TO 25; ACR Y IS GOING TO 31; HE HAS YOU INSIGHT AND WILL KEEP VIS SEP ON YOU.' WE SWITCHED TO JAX TWR AND WERE CLRED TO LAND. THE F/O WHO WAS FLYING DEPLOYED SPD BRAKES TO SLOW AND DSCNT AS HE STARTED HIS TURN TOWARD RWY 25. OUR FMC AND VIS OBSERVATION SHOWED THAT OUR PROJECTED FLT PATH WOULD BRING US TO INTERCEPT THE LOC AT PECAN AT 1700'. DURING OUR TURNING DSCNT THE F/O ASKED ABOUT ACR Y'S POS. I SAW THAT HE WAS AT OUR 9:30 POS AND NOT PARALLELING OUR POS IN ORDER TO REMAIN CLR AS APCH SAID HE WOULD. INSTEAD HE WAS CONVERGING AT ABOUT A 45 DEG ANGLE; AT LESS THAN A 1/2 MI DISTANCE. I TOLD THE F/O TO STOP HIS DSCNT AND HOLD HIS HDG; 'ACR Y IS GOING TO PASS UNDER US.' MY F/O RETRACTED THE SPD BRAKES; STOPPED OUR DSCNT; AND STOPPED THE TURN. THE ACR Y PASSED UNDER US IN FRONT OF OUR L WING EMERGING ON OUR R SIDE FROM UNDER THE F/O WINDOW. HIS HDG WAS TAKING HIM TOWARD PECAN; O.M. FOR RWY 25. AS ACR Y PASSED BELOW US BOTH OUR RADIO ALT JUMPED TO 500'. AFTER PASSING BELOW US ACR Y CONTINUED ON ITS PREVIOUS COURSE FOR ABOUT 1/4 MI; TOWARD THE ARPT; THEN STARTED A L TURN TO THE SE; RECROSSING OUR TRACK ABOUT 1/2 MI AHEAD OF US. WE RESUMED OUR TURN TO RWY 25 AND CONTINUED OUR DSCNT PASSING OVER PECAN AT 2000'. BOTH ACFT HAD THEIR LNDG LIGHTS ON. A SUBSEQUENT CALL TO THE TWR CHIEF AFTER LNDG GOT THE RESPONSE THAT THE CAPT HAD SAID IN A PHONE CONVERSATION THAT HIS F/O HAD ERRED IN NOT MAINTAINING PROPER SEP FROM OUR ACFT SINCE THEY HAD US ALWAYS IN SIGHT. THE ON DUTY TWR CHIEF TOLD ME THAT HE HAD ADVISED THE ACR Y CAPT THAT THE INCIDENT WOULD BE RPTED IN THE SHIFT LOG AND THAT A COPY WOULD BE SENT TO COMPANY'S MAIN OFFICES FOR ANY ACTION THEY MAY FEEL IS APPROPRIATE.