ACR WDB TRACK HDG DEV MAKING A COUPLED APCH TO ATL.

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WE RECEIVED A TURN TO INTERCEPT THE LOC FOR RWY 27L ATLANTA. WE WERE AT 3500 FT MSL 20 NM FROM ARPT. LNAV WAS USED TO TRACK RWY CENTERLINE INBOUND DUE TO FLUCTUATING LOC 20 MI FROM THE RWY. WHEN LOC WAS SELECTED ON THE MODE CTL PANEL THE ACFT BANKED L. SINCE THIS WAS AN INCORRECT TURN BASED ON WHERE WE WERE; I DISENGAGED THE AUTOPLT AND INITIATED A TURN TO THE R TO REINTERCEPT THE LOC. IT WAS THEN I NOTICED THAT 172 DEG WAS SET IN THE HDG SELECT WINDOW NOT 272 DEG WHICH IS THE CORRECT SETTING. WE RECEIVED A DSCNT TO 2800 FT AND A SWITCH FREQ TO TWR ALL IN THE VICINITY OF THE OM. WHEN WE RESET THE HDG WINDOW TO 272 DEG; PROPER ILS INFO WAS RECEIVED AND A LNDG WAS MADE WITHOUT INCIDENT. THE QUESTION REMAINS AS TO HOW 172 DEG INSTEAD OF 272 DEG WAS SET INTO THE HDG SELECT WINDOW. THE NUMBERS ARE MANUALLY SET AND ELECTROMAGNETICALLY ROTATED INSIDE THE ILS SELECT PANEL. WHEN THE APCH WAS BRIEFED (ABOUT 17000 FT MSL IN THE DSCNT) THE CAPT SET 108.5 FREQ AND 272 DEG HDG; I XCHKED THESE NUMBERS AND WAS SATISFIED THAT THESE WERE CORRECT. HOW 172 DEG ARRIVED IN THE WINDOW IS STILL IN QUESTION IN MY MIND.