Narrative:

We intercepted the runway 10 localizer about 23 miles from ord. The localizer needle was swinging all over the HSI (as is sometimes normal at this distance from the airport). Around pratt (15 miles out); the localizer was still swinging left then center momentarily; then right then center; then back to the left and so on. We were expecting the localizer needle to steady by this point; even with the 30+ knot cross wind from the north; so we asked if there was any problem with the signal. Before the controller could reply; another flight said that they were having problems with the signal also. We were in VMC but above the clouds and couldn't see the airport. As all this was happening; the aircraft followed the needle to the north and we received a TA for traffic on the runway 9R approach. At the same time that the TA went away; we lost the signal immediately after a turn command to the south. We were already executing a missed approach at the same time ATC issued us a heading and altitude for the missed approach. The ATC instructions for the missed were to re-intercept the localizer. As the signal was still erratic; we did the best we could. We were given vectors for the runway 4R approach and the instruments behaved properly. We also accomplished the ILS approach to another airport and the aircraft and instruments worked perfectly. I think the ATC approach signal was erroneous. The ATC controller I talked to after the incident said that there were issues with 'one or two pieces of equipment.' the quality control specialist I talked to also said that there may have been problems because of all the equipment and construction or possible aircraft interference with the radio facility.

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Original NASA ASRS Text

Title: EMB 145 FLT CREW EXPERIENCED AN ERRATIC LOC WHILE EXECUTING THE ILS APCH TO RWY 10; ORD. MISSED APCH WAS EXECUTED.

Narrative: WE INTERCEPTED THE RWY 10 LOCALIZER ABOUT 23 MILES FROM ORD. THE LOCALIZER NEEDLE WAS SWINGING ALL OVER THE HSI (AS IS SOMETIMES NORMAL AT THIS DISTANCE FROM THE AIRPORT). AROUND PRATT (15 MILES OUT); THE LOCALIZER WAS STILL SWINGING LEFT THEN CENTER MOMENTARILY; THEN RIGHT THEN CENTER; THEN BACK TO THE LEFT AND SO ON. WE WERE EXPECTING THE LOCALIZER NEEDLE TO STEADY BY THIS POINT; EVEN WITH THE 30+ KNOT CROSS WIND FROM THE NORTH; SO WE ASKED IF THERE WAS ANY PROBLEM WITH THE SIGNAL. BEFORE THE CONTROLLER COULD REPLY; ANOTHER FLIGHT SAID THAT THEY WERE HAVING PROBLEMS WITH THE SIGNAL ALSO. WE WERE IN VMC BUT ABOVE THE CLOUDS AND COULDN'T SEE THE AIRPORT. AS ALL THIS WAS HAPPENING; THE AIRCRAFT FOLLOWED THE NEEDLE TO THE NORTH AND WE RECEIVED A TA FOR TRAFFIC ON THE RWY 9R APPROACH. AT THE SAME TIME THAT THE TA WENT AWAY; WE LOST THE SIGNAL IMMEDIATELY AFTER A TURN COMMAND TO THE SOUTH. WE WERE ALREADY EXECUTING A MISSED APPROACH AT THE SAME TIME ATC ISSUED US A HEADING AND ALTITUDE FOR THE MISSED APPROACH. THE ATC INSTRUCTIONS FOR THE MISSED WERE TO RE-INTERCEPT THE LOCALIZER. AS THE SIGNAL WAS STILL ERRATIC; WE DID THE BEST WE COULD. WE WERE GIVEN VECTORS FOR THE RWY 4R APPROACH AND THE INSTRUMENTS BEHAVED PROPERLY. WE ALSO ACCOMPLISHED THE ILS APPROACH TO ANOTHER ARPT AND THE AIRCRAFT AND INSTRUMENTS WORKED PERFECTLY. I THINK THE ATC APPROACH SIGNAL WAS ERRONEOUS. THE ATC CONTROLLER I TALKED TO AFTER THE INCIDENT SAID THAT THERE WERE ISSUES WITH 'ONE OR TWO PIECES OF EQUIPMENT.' THE QUALITY CONTROL SPECIALIST I TALKED TO ALSO SAID THAT THERE MAY HAVE BEEN PROBLEMS BECAUSE OF ALL THE EQUIPMENT AND CONSTRUCTION OR POSSIBLE AIRCRAFT INTERFERENCE WITH THE RADIO FACILITY.

Data retrieved from NASA's ASRS site as of May 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.