CLOSE PROX ACR-MLG COMMUTER-SMT IN TFC AT CVG.

Narrative

AS WE APCHED CVG FROM LEX VOR (12000 FT AT 250 KIAS); APCH CTL TURNED US TO A 020 DEG HDG FOR DOWNWIND 18L. FO WAS FLYING THE ACFT AND BEGAN TO SLOW DOWN TO 210 DEGS KIAS TO FACILITATE DSCNT. APCH CTL CLRED US TO 7000 FT AND WHILE IN THE DSCNT FURTHER CLRED US TO 4000 FT. PASSING 5500 FT; SINK RATE APPROX 2000 FPM; 210 KIAS; FLAPS 15 DEGS; THE FO SPOTTED TFC AT 2 O'CLOCK. TFC WAS LOW AND APPEARED TO BE IN A R TURN TO THE W -- AWAY FROM US. SHORTLY THEREAFTER; A TA WAS ANNOUNCED OVER TCASII. I QUERIED THE CTLR ABOUT THE TFC AND HE STATED THAT THE TFC WAS AT 4000 FT. IT THEN BECAME EVIDENT THAT THE TFC WAS INDEED NOT TURNING AWAY FROM US AND POSED A SERIOUS CONFLICT. THE FO BROKE THE DSCNT AND INITIATED HIS OWN CLB WHEN AN RA SOUNDED OVER TCASII TO CLB. WE PASSED OVER THE TFC BY APPROX 350 FT (4350 FT MSL). THIS TFC WAS NEVER CALLED OUT UNTIL I CALLED AND THE CONFLICT WAS ALREADY ESTABLISHED. ONCE AT THE GATE IN CVG; I CALLED APCH CTL ON THE LANDLINE AND TALKED WITH THE SUPVR. HE TOLD ME THE TFC WAS A COMMUTER ACFT SMT DSNDING VFR. I THEN EXPRESSED MY CONCERN OVER BEING CLRED TO THE SAME ALT AS THE OBVIOUSLY KNOWN VFR TFC WAS AT. HAD THE VISIBILITY BEEN LESS AND/OR TCASII BEEN UNAVAILABLE; THIS WOULD'VE BEEN VERY CLOSE.