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

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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.