Narrative:

ILS 36 localizer was out to maintenance. When it was returned to service, a NOTAM was issued (D10/010) runway 36R localizer unusable beyond 20 degrees from centerline. An large transport X 5 mi east of localizer began a turn because his equipment indicated he was joining localizer. An large transport Y and an mlt Z both reported indications they were on the localizer. Both were 4 mi west. Maintenance checked the equipment, and found no explanation. They said the equipment was monitoring normally. All 3 aircraft involved either saw the field, or realized the problem because it conflicted with controllers instructions. One aircraft also noted his RNAV equipment indicated he was not on localizer, but he was receiving it, and thought he was on it. Callback conversation with reporter revealed the following information. Reporter states that the problem is intermittent and still exists. Facility management aware of problem and alerted af. Af cannot find anything wrong. Nothing out to controllers from facility management to use caution when issuing ILS approachs. It appears that they are trying to get the ILS system upgraded to CAT III, so some modification probably made to existing equipment. No flight check yet, but one is due. Fortunately, all but one aircraft was on a visual approach, but using the ILS as a backup. Af claims that equipment meets requirements whenever they check it.

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Original NASA ASRS Text

Title: ILS LOC GIVES FALSE SIGNAL RESULTING IN ACFT TURNING INTO THE ARPT EARLY.

Narrative: ILS 36 LOC WAS OUT TO MAINT. WHEN IT WAS RETURNED TO SVC, A NOTAM WAS ISSUED (D10/010) RWY 36R LOC UNUSABLE BEYOND 20 DEGS FROM CTRLINE. AN LGT X 5 MI E OF LOC BEGAN A TURN BECAUSE HIS EQUIP INDICATED HE WAS JOINING LOC. AN LGT Y AND AN MLT Z BOTH RPTED INDICATIONS THEY WERE ON THE LOC. BOTH WERE 4 MI W. MAINT CHKED THE EQUIP, AND FOUND NO EXPLANATION. THEY SAID THE EQUIP WAS MONITORING NORMALLY. ALL 3 ACFT INVOLVED EITHER SAW THE FIELD, OR REALIZED THE PROBLEM BECAUSE IT CONFLICTED WITH CTLRS INSTRUCTIONS. ONE ACFT ALSO NOTED HIS RNAV EQUIP INDICATED HE WAS NOT ON LOC, BUT HE WAS RECEIVING IT, AND THOUGHT HE WAS ON IT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO. RPTR STATES THAT THE PROBLEM IS INTERMITTENT AND STILL EXISTS. FACILITY MGMNT AWARE OF PROBLEM AND ALERTED AF. AF CANNOT FIND ANYTHING WRONG. NOTHING OUT TO CTLRS FROM FACILITY MGMNT TO USE CAUTION WHEN ISSUING ILS APCHS. IT APPEARS THAT THEY ARE TRYING TO GET THE ILS SYS UPGRADED TO CAT III, SO SOME MODIFICATION PROBABLY MADE TO EXISTING EQUIP. NO FLT CHK YET, BUT ONE IS DUE. FORTUNATELY, ALL BUT ONE ACFT WAS ON A VISUAL APCH, BUT USING THE ILS AS A BACKUP. AF CLAIMS THAT EQUIP MEETS REQUIREMENTS WHENEVER THEY CHK IT.

Data retrieved from NASA's ASRS site as of July 2007 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.