Narrative:

Air carrier X; air carrier Y; and air carrier Z were all cleared for ILS runway 27L approach at ord. All three aircraft got inaccurate glideslope indications at the same time and started descending prematurely on the approach. I was the supervisor when the vector controller told me the glideslope appeared to be out of service. I immediately called the tower and confirmed that there was a 'heavy' aircraft in the runway 27L glideslope area. Once the aircraft had taxied out of the area; the glideslope returned to operating normally. We were fortunate that ord was not utilizing simultaneous runway 27L and runway 27R ILS apches in marginal/IFR WX conditions. At ord; apches are turned on at 5000 ft and runway 27R apches at 4000 ft. If runway 27L aircraft suddenly started to descend below glideslope they would conflict with runway 27R aircraft. FAA 7110.65; 3-7-5; states that glideslope critical areas only have to be protected when WX is less than 800/2. This is an extremely unsafe practice. The WX could be 900/3 (IFR); simultaneous runway 27L and runway 27R apches; with parallel monitors and the tower does not have to protect the critical area. To rectify the problem; critical areas should be protected as indicated in 3-7-5; in addition; the critical areas should also be protected during simultaneous ILS apches.

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Original NASA ASRS Text

Title: C90 SUPVR CTLR EXPRESSED CONCERN REGARDING AUTHORIZED ACFT IN THE ILS CRITICAL ZONE; THUS INTERRUPTING THE GLIDESLOPE WHEN VARIOUS ILS PROCS ARE IN USE.

Narrative: ACR X; ACR Y; AND ACR Z WERE ALL CLRED FOR ILS RWY 27L APCH AT ORD. ALL THREE ACFT GOT INACCURATE GLIDESLOPE INDICATIONS AT THE SAME TIME AND STARTED DSNDING PREMATURELY ON THE APCH. I WAS THE SUPVR WHEN THE VECTOR CTLR TOLD ME THE GLIDESLOPE APPEARED TO BE OUT OF SVC. I IMMEDIATELY CALLED THE TWR AND CONFIRMED THAT THERE WAS A 'HVY' ACFT IN THE RWY 27L GLIDESLOPE AREA. ONCE THE ACFT HAD TAXIED OUT OF THE AREA; THE GLIDESLOPE RETURNED TO OPERATING NORMALLY. WE WERE FORTUNATE THAT ORD WAS NOT UTILIZING SIMULTANEOUS RWY 27L AND RWY 27R ILS APCHES IN MARGINAL/IFR WX CONDITIONS. AT ORD; APCHES ARE TURNED ON AT 5000 FT AND RWY 27R APCHES AT 4000 FT. IF RWY 27L ACFT SUDDENLY STARTED TO DSND BELOW GLIDESLOPE THEY WOULD CONFLICT WITH RWY 27R ACFT. FAA 7110.65; 3-7-5; STATES THAT GLIDESLOPE CRITICAL AREAS ONLY HAVE TO BE PROTECTED WHEN WX IS LESS THAN 800/2. THIS IS AN EXTREMELY UNSAFE PRACTICE. THE WX COULD BE 900/3 (IFR); SIMULTANEOUS RWY 27L AND RWY 27R APCHES; WITH PARALLEL MONITORS AND THE TWR DOES NOT HAVE TO PROTECT THE CRITICAL AREA. TO RECTIFY THE PROB; CRITICAL AREAS SHOULD BE PROTECTED AS INDICATED IN 3-7-5; IN ADDITION; THE CRITICAL AREAS SHOULD ALSO BE PROTECTED DURING SIMULTANEOUS ILS APCHES.

Data retrieved from NASA's ASRS site as of January 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.