B747-200 flight crew has altitude deviation during ILS approach to ord.

Attributes
ACN	711823
Time
Date	200609
Place
Locale Reference	atc facility : c90.tracon
State Reference	IL
Altitude	msl single value : 1700
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	tracon : c90.tracon
Operator	common carrier : air carrier
Make Model Name	B747-200
Operating Under FAR Part	Part 121
Navigation In Use	ils localizer only : 27l
Flight Phase	descent : approach
Route In Use	approach : charted visual
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : departure controller : radar controller : approach
Qualification	controller : radar controller : non radar pilot : instrument
Experience	controller non radar : 5 controller radar : 24.5 controller time certified in position1 : 18 controller time certified in position2 : 6 flight time total : 2200
ASRS Report	711823
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Events
Anomaly	altitude deviation : excursion from assigned altitude non adherence : clearance non adherence : published procedure other spatial deviation
Independent Detector	atc equipment : msaw other controllera
Resolutory Action	controller : issued alert
Consequence	faa : reviewed incident with flight crew
Supplementary
Problem Areas	Navigational Facility ATC Human Performance Airport Flight Crew Human Performance
Primary Problem	Navigational Facility

Narrative:

Cleared for ILS runway 27L; 4000 ft until established. Aircraft descended to 1700 ft MSL; MSAW activated. I issued safety alert and immediate climb. Aircraft reported runway 27L in sight and was cleared for visual approach. Air carrier reported anomalies in GS. We considered GS OTS; and upon advising ord tower; we were told that they had parked aircraft in an area that made the runway 27L GS unreliable. We ran for about 40 mins without a usable GS.

Original NASA ASRS Text

Title: B747-200 FLT CREW HAS ALTDEV DURING ILS APCH TO ORD.

Narrative: CLRED FOR ILS RWY 27L; 4000 FT UNTIL ESTABLISHED. ACFT DSNDED TO 1700 FT MSL; MSAW ACTIVATED. I ISSUED SAFETY ALERT AND IMMEDIATE CLB. ACFT RPTED RWY 27L IN SIGHT AND WAS CLRED FOR VISUAL APCH. ACR RPTED ANOMALIES IN GS. WE CONSIDERED GS OTS; AND UPON ADVISING ORD TWR; WE WERE TOLD THAT THEY HAD PARKED ACFT IN AN AREA THAT MADE THE RWY 27L GS UNRELIABLE. WE RAN FOR ABOUT 40 MINS WITHOUT A USABLE GS.

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.