FALSE GS. ACFT DSNDED WELL BELOW GS WHILE IN IMC; BUT WHEN BREAKING OUT OF CLOUDS REALIZED THEY WERE TOO LOW. RPTR WONDERS WHY ATC DIDN'T ALERT THEM OF THEIR ALTDEV.

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AT ABOUT 14-15 ILS DME; RWY 27L; ORD PLATE 21B-5; JUN/XX/95; THE GS CAME OFF THE TOP OF THE CASE AS IF IT WERE STUCK. THE AUTOPLT CAPTURED IT AND BEGAN A DSCNT THAT SEEMED NORMAL. NO FLAGS. AT ABOUT 3100 FT MSL WE GOT GND CONTACT AND WE WERE TOO LOW VISUALLY BUT IT WAS NOT CERTAIN AS THE CLOUDS WERE RAGGED. AT THE SAME TIME THE GS WAS ERRATIC AND I CHKED THE DME AND PROPER ALT AND DETERMINED WE WERE LOW AND IMMEDIATELY CLBED BACK TO 4000 FT MSL. WE CAPTURED THE CORRECT GS AND LANDED NORMALLY. THIS IS THE SECOND ONE OF THESE BELOW NORMAL GS I'VE SEEN. THE OTHER WAS IN KNOXVILLE ON RWY 23; A FEW YRS AGO. IT WAS RPTED TO THE COMPANY SAFETY DEPT AT THAT TIME. I'D BE INTERESTED IN KNOWING WHEN OR IF THE APCH CTLRS WOULD HAVE CALLED US ABOUT THE ALT BEING BELOW ASSIGNED; ALTHOUGH NOT BELOW THE LOWER ALT THAT IS USED IN THAT AREA FOR THAT APCH. SUPPLEMENTAL INFO FROM ACN 352647: EVERYTHING APPEARED NORMAL UNTIL ADAME. OUR ALT WAS 3000 FT; CHARTED ALT WAS 3981 FT. AT THIS TIME; GS DISAPPEARED; WE CLBED TO REAL GS AND FLEW NORMALLY TO A LNDG. CONDITIONS AT TIME WERE TEMP MINUS 2 DEGS; IN CLOUDS; LIGHT RIME ICING. AS SOON AS WE BROKE OUT OF CLOUDS (3000 FT MSL); FALSE GS DISAPPEARED.