C90 CHANGES ORD RWY APCH WHICH IS CHALLENGED BY TWR. NO PROB WITH FLC SPECIFIED DURING TWR SUPVR CALL.

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THE FO WAS FLYING THE ACFT WITH THE AUTOPLT ON. AN FAA INSPECTOR WAS CONDUCTING AN ENRTE CHK AND WAS MONITORING ATC COMS FROM THE JUMP SEAT. WE HAD PREVIOUSLY BRIEFED A VISUAL APCH TO RWY 27R; BACKED UP BY THE ILS; AS THIS SEEMED THE MOST LIKELY RWY FROM OUR ARR (THE PULLMAN 4 ARR ORD). UPON FIRST CALL TO ORD APCH; THE CTLR TOLD US OUR LNDG RWY WOULD BE RWY 27L. I READ BACK THE RWY AND FO SET UP AND BRIEFED THE APCH TO RWY 27L. APCH THEN QUERIED US TO SEE IF WE HAD VISUAL CONTACT WITH THE ARPT. I RESPONDED 'NEGATIVE.' APCH SAID 'NO PROB; DSND TO 3000 FT HDG 190 DEGS.' THIS HDG WAS A LOGICAL HDG; AS IT LOOKED TO PUT US ON ABOUT A 5 MI INTERCEPT TO RWY 27L. APCH AGAIN ASKED US IF WE HAD THE RWY. I AGAIN RESPONDED 'NEGATIVE.' APCH THEN SAID 'INTERCEPT THE LOC ON YOUR PRESENT HDG.' I RESPONDED 'ROGER' AND WE DID SO. APCH THEN TOLD US TO CONTACT TWR. I CHKED ON WITH TWR AND TOLD HIM WE HAD THE RWY IN SIGHT. TWR RESPONDED 'IT APPEARS YOU ARE LINED UP FOR RWY 27L.' I RESPONDED THAT IT WAS OUR CLRED RWY. TWR RESPONDED TO THE NEGATIVE AND GAVE US A 90 DEG R TURN AT ABOUT 1800-2000 FT. I THEN ASKED TWR IF WE WERE LNDG OR GOING AROUND. HE RESPONDED BY CLRING US FOR THE VISUAL TO RWY 27R. THE REST OF THE APCH AND LNDG WERE NORMAL AND THE APCH WAS STABLE WELL PRIOR TO 500 FT. UPON LNDG; I SPOKE TO THE TRACON SUPVR BY TELEPHONE. TRACON CONFIRMED THAT THERE WAS NO ONE ON RWY 27L OR RWY 27R AND THERE HAD BEEN NO LOSS OF SEPARATION. I ATTRIBUTE THIS PROB TO MISCOM BTWN APCH CTL AND TWR; PUSHING OF THE VISUAL APCH BY APCH CTL AND FAILURE TO CONFIRM THE ASSIGNED RWY ON EACH RADIO CALL. (ON AT LEAST ONE OCCASION I WAS TOLD TO INTERCEPT THE LOC WITHOUT A RWY STATED AND I RESPONDED TO THE AFFIRMATIVE WITHOUT RECONFIRMING THE ASSIGNED RWY.)