B767-300 captures false GS and descends below cleared altitude while cleared for the ILS to runway 2...

Attributes
ACN	814945
Time
Date	200812
Local Time Of Day	1801 To 2400
Place
Locale Reference	airport : ord.airport
State Reference	IL
Altitude	msl single value : 5000
Environment
Weather Elements	Rain
Aircraft 1
Controlling Facilities	tracon : c90.tracon
Operator	common carrier : air carrier
Make Model Name	B767-300 and 300 ER
Operating Under FAR Part	Part 121
Navigation In Use	ils localizer & glide slope : 28
Flight Phase	descent : approach
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Experience	flight time last 90 days : 150 flight time total : 7000 flight time type : 4000
ASRS Report	814945
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude non adherence : clearance
Independent Detector	other flight crewa
Resolutory Action	aircraft : equipment problem dissipated controller : issued advisory flight crew : returned to assigned altitude
Supplementary
Problem Areas	Aircraft Navigational Facility
Primary Problem	Navigational Facility

Narrative:

At 5000 ft MSL inbound on the ord ILS runway 28 approach and about 8 miles outside wavie; we were cleared for the approach. After capturing the localizer; approach mode was selected. Shortly afterward we had a false GS capture. The GS captured and the airplane started a steep descent -- 1;500-2;000 fpm. I clicked off the autothrottle and increased the power and started to click off the autopilot when the line went through the GS on the ADI. I started pulling up; but we went down to about 4;600 ft MSL before climbing back to 5;000 ft and continuing the approach to a normal landing. The controller reiterated 5;000 ft to wavie and we told him we had a false GS capture.

Original NASA ASRS Text

Title: B767-300 captures false GS and descends below cleared altitude while cleared for the ILS to Runway 28 at ORD.

Narrative: At 5000 ft MSL inbound on the ORD ILS Runway 28 approach and about 8 miles outside WAVIE; we were cleared for the approach. After capturing the LOC; approach mode was selected. Shortly afterward we had a false GS capture. The GS captured and the airplane started a steep descent -- 1;500-2;000 fpm. I clicked off the autothrottle and increased the power and started to click off the autopilot when the line went through the GS on the ADI. I started pulling up; but we went down to about 4;600 ft MSL before climbing back to 5;000 ft and continuing the approach to a normal landing. The Controller reiterated 5;000 ft to WAVIE and we told him we had a false GS capture.

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

B767-300 captures false GS and descends below cleared altitude while cleared for the ILS to runway 28 at ord.