MD80 CREW HAD LOC INTERFERENCE FROM PRECEDING B747-400 WHILE LNDG ON RWY 4R AT ORD.

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IN GOOD VISIBILITY AND STD ATC SEPARATION WE FOLLOWED A B747 ON A VISUAL APCH TO RWY 4R AT ARD. ALL INSTRUMENT INDICATIONS ON APPROACH WERE NORMAL UNTIL THE B747 BEGAN TO TURN OFF RWY 4R AT THE END OF THE RUNWAY. AS THE TAIL OF THE B747 CROSSED THE RWY CENTERLINE; WE EXPERIENCED A ONE DOT CDI DEVIATION TO THE RIGHT-LEFT-RIGHT THEN STABILIZING AS THE AIRCRAFT CONTINUED TO CLEAR THE RWY AND ENTER THE RWY 27L PAD WHERE HE HELD FOR 10 MIN. OUR AUTOPILOT WAS COUPLED AND WAS SUBSEQUENTLY DISCONNECTED AND THE APPROACH WAS CONTINUED HAND FLOWN TO AN UNEVENTFUL LANDING. WE WERE READY FOR THE POSSIBILITY OF COURSE DEVIATIONS DUE TO THE PRIOR HISTORY OF RWY 4R AT ORD. THIS EVENT WAS REPORTED TO THE TOWER. THE ACFT FOLLOWING US REPORTED EXACTLY THE SAME INDICATIONS. THE BEAM OF THE LOCALIZER WAS CLEARLY AFFECTED BY THE TAIL OF THE B747. CALLBACK CONVERSATION WITH REPORTER REVEALED THE FOLLOWING INFORMATION: CAPT STATED THE AREA AROUND THE RWY 27L PAD IS DESIGNED AS AN ILS CRITICAL AREA. THE CREW WAS COMPLETING A LONG THREE DAY TRIP AND HAD AN EARLY GET UP THAT MORNING. THE AUTOPILOT WAS USED BECAUSE THE CREW WAS TIRED; AND TO REDUCE WORKLOAD.