ACR-MLG THOUGHT POTENTIAL CONFLICT WITH GA-SMA BOTH UNDER CTLR.

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WE HAD BEEN ISSUED CLRNC FROM COAST APCH (121.3) TO FLY HDG 280 DEGS AT 6000' AND HAD XED THE LOC EXPECTING A RIGHT DOWNWIND VECTOR TO THE APCH. WE WERE ON TOP OF A SOLID CLOUD LAYER; CLR ABOVE WITH UNRESTRICTED FLT VIS. APCH CTL CLRED US TO DSND TO 5000'. AS I ACKNOWLEDGED THE CLRNC; I SAW AN SMT AT OUR 10:30 POS; APPROX AT OUR ALT. I WAS SURPRISED AT HOW CLOSE HE WAS--POINTED STRAIGHT AT US WITH HIS WING TIP LNDG LIGHTS EXTENDED AND ILLUMINATED. (SURPRISED BECAUSE HE WAS SO CLOSE; I HADN'T SEEN HIM BEFORE AND APCH HADN'T MENTIONED HIM.) THE F/O HAD SELECTED 5000' AND LEVEL CHANGE ON THE AFDS MODE CONTROL PANEL; SO THE AUTOPLT WAS STARTING TO PITCH DOWN AND RETARD THE PWR LEVERS. I ESTIMATE THE SMT WAS 1 MI OR LESS HORIZ AND; AS I DIDN'T IMMEDIATELY SEE ANY CHANGE IN RELATIVE POS; I DISCONNECTED THE AUTOPLT WITH THE CONTROL YOKE SWITCH AND INCREASED THE RATE OF DSCNT UNTIL RELATIVE MOVEMENT BTWN US WAS APPARENT AND ANNOUNCED; 'I HAVE THE AIRPLANE.' AT THAT TIME; APCH ISSUED THE TFC TO US; BUT THE DETAILS OF THE XMISSION WERE NOT CLEARLY AUDIBLE BECAUSE OUR AUTOPLT DISENGAGE WARNING TONE WAS ACTIVATED. AS WE APCHED 5500'; IT WAS OBVIOUS THAT THE SMT WOULD PASS ABOVE AND BEHIND US; SO I RE-ENGAGED THE AUTOPLT. I TALKED TO THE APCH CTL SUPVR LATER WHO HAD REVIEWED THE DATA AND VOICE TAPES AND ALSO TALKED TO THE SMT PLT BY TELEPHONE. HE TOLD ME THE DATA SHOWED .9 MI HORIZ AND 600' VERT SEP AT OUR CLOSEST POINT. IT CERTAINLY LOOKED CLOSER THAN THAT TO ME. HE SAID THE SMT HAD BEEN ISSUED TFC (US) AND HAD RPTED US IN SIGHT AT ABOUT 5-7 MI; AND HAD MADE A SLIGHT RIGHT TURN TO MAINTAIN VISUAL SEP. SUPPLEMENTAL INFO FROM ACN 88786: ON CLB WITH LNDG LIGHTS AND STROBES AND ANTI-COLLISION LIGHT ON; WAS GIVEN TFC 6 O'CLOCK; MLG; WHICH SOON PASSED OFF THE RIGHT. THIS TFC WAS IN SIGHT. SUBSEQUENTLY GIVEN TFC MLG RIGHT TO LEFT DSNDING. MAINTAINED VISUAL CONTACT WITH THIS TFC UNTIL HE PASSED OFF TO THE LEFT. IN ORDER TO INCREASE SEP; A SLIGHT TURN TO THE RIGHT WAS MADE. SEP FROM MLG WAS NEVER LESS THAN 2 MI HORIZ AND 1000' VERT. I WAS IN CONTACT WITH COAST APCH ENTIRE TIME OF INCIDENT AND FOLLOWED ALL REQUESTS AND RESTRICTIONS. I SPOTTED TFC AT LEAST 5-7 MI AWAY.