A CL65 CREW; ON A VISUAL TO CVG; WERE UNABLE TO MAINTAIN SEPARATION ON TFC AHEAD. ATC ISSUED A GAR. AN UNEVENTFUL APCH AND LNDG FOLLOWED.

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CLRED VISUAL APCH; RWY 18R; CVG. 170 KT TO LOGOZ INTXN. FOLLOWING MD88. WITH A 30 KT XWIND ALOFT; THE MD88 WENT THROUGH THE RWY 18R FINAL; S-TURNED TO REJOIN; AND SLOWED WELL BEFORE LOGOZ. WE STARTED TO LOSE OUR SEPARATION AND STARTED TO SLOW OURSELVES. ABOUT THAT TIME WE WERE HANDED OFF TO TWR. UPON CHKING IN; TWR TOLD US TO SLOW (WHICH WE WERE ALREADY DOING) AND TWR INFORMED US THAT WE WERE OVERTAKING THE TFC AHEAD. WE THEN ASKED TWR IF THEY WOULD LIKE US TO DO SOMETHING DIFFERENT. THEY SAID 'CONTINUE; AND BE PREPARED FOR THE GAR.' SHORTLY THEREAFTER; TWR INSTRUCTED US TO GAR; WHICH WE DID; AND LANDED UNEVENTFULLY FOLLOWING THE SECOND APCH. THIS EVENT ILLUSTRATES THE INHERENT PROBS WITH NIGHT VISUAL APCHS; TO FOLLOW TFC AND MAINTAIN VISUAL. FORTUNATELY; WE COULD USE THE TCASII TO APPROX THE DISTANCE; ESPECIALLY FROM BEHIND. ALSO; I'M NOT SURE WE HAD 3 MI SEPARATION. THE MD88 WAS ON BASE TURN; APPROX 3 MI AHEAD; AND EVEN IF HE HAD NOT OVERSHOT THE TURN; WE PROBABLY WOULD HAVE HAD LESS THAN 3 NM. LAST; ATC SHOULD HAVE BROKEN US OUT SOONER. IT WAS OBVIOUS THAT THIS WASN'T GOING TO WORK WHEN WE WERE STILL AT 2000 FT AND ABOUT 3 MI FROM LNDG. FOR REASONS UNKNOWN TO US; THE CTLR WANTED US TO CONTINUE. THIS DELAY RESULTED IN A GAR WITH LESS SEPARATION AND AT A LOWER ALT.