CL65 FLC EXECUTES A GAR DUE TO CONFLICTING B757 WHILE ON VECTOR WITH D10 TO RWYS 17 AT DFW.

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FLT FROM SAV TO DFW. EVENTS OCCURRED IN THE TFC PATTERN N OF RWYS 17L&C FINAL APCH AREAS. APCH CTL (FREQ 127.07) INSTRUCTED US TO DSND FROM 7000 FT TO 6000 FT ON THE DOWNWIND LEG. PRIOR TO REACHING 6000 FT; APCH CTL ASKED US TO 'FLY HDG 270 DEGS; DSND TO 5000 FT FOR RWY 17C AND TO KEEP THE TURN TIGHT.' ONCE WE REACHED 5000 FT THE CTLR INSTRUCTED US TO DSND TO 4000 FT ON THE BASE LEG FOR RWY 17C. PRIOR TO REACHING 4000 FT; THE CTLR STATED 'FLT XXX TURN L HDG 260 DEGS; MAINTAIN 4000 FT UNTIL ADVISED (NOTE: SLIGHT PAUSE FROM CTLR); CLRED FOR VISUAL RWY 17C.' MY READBACK WAS 'CLRED FOR VISUAL RWY 17C.' NOTE: AT THIS POINT; OUR ACFT CONFIGN WAS FLAPS 20 DEGS AND GEAR DOWN. APPROX 3 SECONDS AFTER MY READBACK; MY CAPT STATED TO ME 'WHAT IS THAT TFC OFF TO OUR R DOING?' NOTE: CAPT ADDED PWR FROM IDLE TO APPROX 50% N1 WHICH IS ABOUT 1/2 OF THE PWR AVAILABLE. ACFT PITCH WAS RAISED TO DECREASE RATE OF DSCNT. I TOLD HIM 'HE IS FLYING RIGHT BELOW US FROM R TO L.' APPROX 10 SECONDS AFTER MY READBACK FOR THE VISUAL RWY 17C; THE APCH CTLR STATED 'FLT XXX ARE YOU MAINTAINING 4000 FT?' SURPRISED BY THIS STATEMENT BECAUSE WE WERE CLRED FOR VISUAL; I READ BACK 'MAINTAINING 4000 FT' WITHOUT QUESTIONING HIM. NOTE: CAPT BROUGHT THE THROTTLES TO APPROX MAX PWR. BUT; BECAUSE OF THE ACFT CONFIGN AND THE SPOOL-UP TIME REQUIRED BY THE ENGS; WE COULD NOT PROMPTLY MAINTAIN 4000 FT. OUR ACFT STARTED TO REGAIN ALT AT 3850 FT. AT THIS POINT WE RECEIVED AN RA FROM THE TCASII. MY CAPT STATED THE GAR PROCS 'MAX PWR; FLAPS 8 DEGS; SPOILERS IN.' MY CAPT AND I SAW AN ACR Y B757 GOING FROM R TO L BELOW OUR FLT PATH AND QUERIED THE CTLR ABOUT THE B757 DIRECTION. THE CTLR ADVISED US THAT THE B757 WAS GOING TO THE L AND THAT IS WHY HE TOLD US TO MAINTAIN 4000 FT. NOTE: THE B757 WAS THE SAME ACFT MY CAPT WAS REFERRING TO PREVIOUSLY. THE B757 WAS ON TWR FREQ 127.5 FOR RWY 17L. A FEW SECONDS LATER; APCH CTL CLRED US FOR A VISUAL RWY 17C; BUT AT THIS POINT; WE WERE NO LONGER CONFIGURED FOR THE APCH AND WE WERE HIGH; APPROX 4000 FT JUST OUTSIDE THE OM. I STATED 'FLT XXX IS GOING AROUND.' IN A CONDESCENDING TONE; THE CTLR SAID 'YOU MEAN YOU CANNOT MAKE A VISUAL APCH FROM 4000 FT AT THE MARKER.' I STATED ONE MORE TIME 'FLT XXX IS TOO HIGH TO ACCEPT A VISUAL APCH; WE NEED TO GO AROUND.' APCH CTL COMPLIED. REST OF FLT WAS UNEVENTFUL.