FLC MISIDENTED TFC ON APCH WHEN CLRED FOR VISUAL. WHEN SIGHTING ACTUAL INTERVAL ACFT; EVADED AND SLOWED FOR UNEVENTFUL LNDG.

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FLYING HDG 030 DEGS AT 5000 FT IN IMC (THE ACTON 4 ARR) APCH ASSIGNED US HDG 010 DEGS AND DSCNT TO 3000 FT. OUT OF APPROX 4200 FT WE BROKE OUT BENEATH THE OVCST. I TOLD THE FO I HAD THE ARPT IN SIGHT AT 3 O'CLOCK. HE CONFIRMED HE HAD THE FIELD AND ADVISED ATC. THE CTLR ASKED IF WE HAD THE MD80 AT OUR 2 O'CLOCK IN SIGHT. THE FO (PNF) RESPONDED WE HAD THE MD80 AT 2:30 O'CLOCK IN SIGHT. THE CTLR THEN ASSIGNED US A HDG OF 310 DEGS AND CLRED US FOR A VISUAL APCH RWY 13R BEHIND THE MD80; CONTACT THE TWR AT THE OM. IT WAS OUR PERCEPTION THE TURN WAS TO WIDEN OUT ON THE MD80. IN THE TURN TO 310 DEGS I LOST THE TFC AND ASKED THE FO WHERE THE MD80 WAS. HE SAID 'HE IS BEHIND US AND LOW.' I CONTINUED A DSCNT (DIVIDING ATTN INSIDE TO REF INSTS AND REMAIN ABOVE THE GS UNTIL ESTABLISHED ON FINAL TO AVOID THE MD80'S WAKE) AND BEGAN AN E TURN FOR THE BASE LEG. IN THE TURN I SAW AN MD80 AT MY 12 O'CLOCK POS. THE MD80 WE WERE FOLLOWING WAS THE WRONG ACFT; ALTHOUGH IT WAS THE ACFT WE ORIGINALLY IDENTED AT OUR 2:30 O'CLOCK POS AND TURNED TO FOLLOW; WHEN WE ACCEPTED THE VISUAL APCH. TO AVOID REDUCING SEPARATION FURTHER ON THE 12 O'CLOCK ACFT; I CONTINUED A R TURN TO PARALLEL THE TFC. THE FO ADVISED THE CTLR WE HAD THE ACR MD80 IN SIGHT. ATC RESPONDED SLOW AS NEEDED; CLRED VISUAL RWY 13R. REFLECTING ON HOW TO AVOID THIS SIT IN THE FUTURE MANY CONTRIBUTING FACTORS BECOME APPARENT: THE TRANSITION FROM INST TO VISUAL CONDITIONS REQUIRES CONSIDERABLE COORD BTWN CREW MEMBERS. OUR ACCEPTANCE OF A VISUAL APCH DURING A PEAK ARR TIME AT A BUSY HUB ARPT. WHILE THE MD80 WAS TCASII EQUIPPED; OUR CLASS OF ACFT IS OPERATING UNDER AN FAA APPROVED EXTENSION TO OPERATE WITHOUT TCASII. BEGINNING A TURN WITHOUT MY HAVING TFC IN CONSTANT SIGHT. WHILE THE ABOVE ITEMS MAY SEEM OBVIOUS; THE ENVIRONMENT UNDER WHICH WE OPERATE DAILY LENDS ITSELF TO HUMAN ERROR. SCHEDULED REDUCED REST; NO AUTOPLT; MULTIPLE LEGS AT HUB ARPTS DURING HEAVILY TFC TIMES; LESS THAN INDUSTRY STANDARD TECHNOLOGY IN OUR COCKPITS.