B757 flight crew reported a false GS indication on the ord runway 27L ILS.

Attributes
ACN	713905
Time
Date	200510
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : ord.airport
State Reference	IL
Altitude	msl single value : 4000
Environment
Flight Conditions	VMC
Aircraft 1
Controlling Facilities	tracon : c90.tracon
Operator	common carrier : air carrier
Make Model Name	B757-200
Operating Under FAR Part	Part 121
Navigation In Use	ils localizer & glide slope : 27l
Flight Phase	descent : approach
Route In Use	approach : instrument precision
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
ASRS Report	713905
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Events
Anomaly	altitude deviation : excursion from assigned altitude non adherence : published procedure cabin event other
Independent Detector	other flight crewa other flight crewb
Resolutory Action	flight crew : overcame equipment problem
Supplementary
Problem Areas	Airport Navigational Facility
Primary Problem	Navigational Facility
Situations
Navigational Aid	ORD.ILS

Narrative:

Cleared for ILS runway 27L in VFR conditions. GS captured at 3000-4000 ft. Outside of the marker; first officer said; 'this looks low;' I agree; check the OM GS crossing altitude and note it to be 2100 ft MSL. Over the marker; on GS: altimeter read 1500 ft and VASI clearly indicated low. We leveled off; disregarded the GS; picked up the VASI and landed uneventfully. The ILS receiver had captured a false runway 27L GS. In 25 yrs of flying; I have captured false GS before; but never at ord. If this had been low IFR; the only chance to catch the false GS would be the marker altimeter check; no other abnormal indications. No far's were violated and no pltdevs occurred.

Original NASA ASRS Text

Title: B757 FLT CREW RPTED A FALSE GS INDICATION ON THE ORD RWY 27L ILS.

Narrative: CLRED FOR ILS RWY 27L IN VFR CONDITIONS. GS CAPTURED AT 3000-4000 FT. OUTSIDE OF THE MARKER; FO SAID; 'THIS LOOKS LOW;' I AGREE; CHK THE OM GS XING ALT AND NOTE IT TO BE 2100 FT MSL. OVER THE MARKER; ON GS: ALTIMETER READ 1500 FT AND VASI CLRLY INDICATED LOW. WE LEVELED OFF; DISREGARDED THE GS; PICKED UP THE VASI AND LANDED UNEVENTFULLY. THE ILS RECEIVER HAD CAPTURED A FALSE RWY 27L GS. IN 25 YRS OF FLYING; I HAVE CAPTURED FALSE GS BEFORE; BUT NEVER AT ORD. IF THIS HAD BEEN LOW IFR; THE ONLY CHANCE TO CATCH THE FALSE GS WOULD BE THE MARKER ALTIMETER CHK; NO OTHER ABNORMAL INDICATIONS. NO FAR'S WERE VIOLATED AND NO PLTDEVS OCCURRED.

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.