ACR FK10 FLC CHALLENGES APCH CTLR'S CLRNC FOR VISUAL APCH TO RWY WHEN OBSERVING CONFLICTING TFC. CTLR RECLRS FK10 FOR VISUAL APCH TO OTHER RWY. LATER INVESTIGATION DETERMINES FK10 ACKNOWLEDGED APCH CLRNC TO WRONG RWY AND NO ONCE CAUGHT INCORRECT READBACK.

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ACR X CAME ONTO MY FREQ. I WAS WORKING THE N FINAL POS AT ATL APCH CTL. THE FLT WAS DSNDED TO 5000 FT; APPROX 12 MI NE OF ATL. I INSTRUCTED THE FLT TO TURN R HDG 180 DEGS; PLT RESPONDED. VERY SHORTLY AFTER; I ADVISED ACR X THEY WERE FOLLOWING A DC9 AT 2 O'CLOCK POS 4 MI SWBOUND LEAVING 4500 FT. ACR X'S REPLY; 'IN SIGHT.' I THEN CLRED ACR X FOR VISUAL APCH RWY 26R; FOLLOW THE DC9 MAINTAIN 170 KTS TILL THE OM. ACR X ACKNOWLEDGED THE CLRNC. I WAS BUSY AT THE TIME AND I WENT BACK TO MY OTHER TFC. A FEW MOMENTS LATER; I OBSERVED ACR X GOING THROUGH THE RWY 26R FINAL APCH COURSE. I ASKED PLT IF THEY WERE TURNING IN FOR RWY 26R. PLT INFORMED ME THAT I HAD CLRED THEM FOR RWY 27L. MY RESPONSE WAS; 'I DON'T THINK SO; BUT LEVEL OFF AT 4000 FT AND TURN R TO 290 DEGS.' I THEN ASKED PLT IF THEY HAD TFC OFF THEIR L APPROX 800 FT BELOW. PLT REPLIED YES. I INSTRUCTED ACR X TO MAINTAIN VISUAL SEPARATION ON THE B737 HDG 290 DEGS. I LATER RECLRED ACR X FOR VISUAL APCH TO RWY 26R. I THOUGHT EVERYTHING WAS FINE; THAT I HAD SAVED A POTENTIAL PROB. APPROX 2 HRS LATER; THE SUPVR RECEIVED A CALL FROM AN FAA EVALUATOR ONBOARD ACR X (IN COCKPIT) AND ADVISED WE MAY HAVE HAD A PROB. I WAS RELIEVED FROM DUTY WHILE INVESTIGATION WAS STARTED. LATER; I WAS INFORMED THAT I HAD CLRED ACR X FOR APCH TO RWY 26R TO FOLLOW DC9. PLT READ BACK RWY 27L. NEITHER I NOR THE EVALUATOR CAUGHT THE READBACK ERROR. MY PROB WITH THIS: I CLRED ACR X TO FOLLOW DC9 TO RWY 26R; WHICH WAS ACKNOWLEDGED. BUT; PLT WOUND UP TRYING TO FOLLOW A B737 ON THE RWY 27L FINAL. ACR X NEVER QUESTIONED SEPARATION OF TFC; WHICH WAS DECLINING. SUPPLEMENTAL INFO FROM ACN 408090: COMING FROM N TO ATL. CLRED FOR VISUAL RWY 27L. WE GO THROUGH RWY 26R TO INTERCEPT RWY 27L. CAPT FLYING AND WONDERS WHAT THIS B737 IS DOING BELOW AND BEHIND HIM ON RWY 27L. B737 THEN GOES IN FRONT OF US ON RWY 27L ILS. I SEE HIM FOR THE FIRST TIME AT THIS POINT; APPROX 800 FT AWAY. JUST THEN APCH ASKS IF WE ARE GOING TO CORRECT BACK TO RWY 26R. WE TELL HIM WE WERE CLRED FOR RWY 27L. HE APOLOGIZES AND WE LAND VISUALLY RWY 26R.