S80 flight crew advise C90 of GS deflections while conducting ILS runway 27L approach.

Attributes
ACN	505422
Time
Date	200103
Day	Thu
Local Time Of Day	0001 To 0600
Place
Locale Reference	airport : ord.airport
State Reference	IL
Altitude	msl single value : 4000
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	tracon : o90.tracon tracon : edff.tracon
Operator	common carrier : air carrier
Make Model Name	MD-80 Super 80
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
Qualification	pilot : atp
ASRS Report	505422
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial pilot : multi engine
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude
Independent Detector	atc equipment other atc equipment : radar aircraft equipment other aircraft equipment : glide slope other controllera other flight crewa
Resolutory Action	controller : issued advisory flight crew : took precautionary avoidance action flight crew : overrode automation flight crew : returned to original clearance
Consequence	faa : reviewed incident with flight crew other
Supplementary
Problem Areas	Aircraft ATC Facility
Primary Problem	ATC Facility

Narrative:

ILS runway 27L at ord, clear skies. Cleared at 4000 ft until established on the localizer. Intercepted localizer. GS alive at about the right time. Intercepted GS. GS started to go erratic. Then full scale defections, not pilot induced -- up and down on both receivers. Reported indications to approach and he noticed that we were low. Nobody else had any problems with the GS. Caught sight of the VASI and continued approach. Uneventful landing.

Original NASA ASRS Text

Title: S80 FLC ADVISE C90 OF GS DEFLECTIONS WHILE CONDUCTING ILS RWY 27L APCH.

Narrative: ILS RWY 27L AT ORD, CLR SKIES. CLRED AT 4000 FT UNTIL ESTABLISHED ON THE LOC. INTERCEPTED LOC. GS ALIVE AT ABOUT THE R TIME. INTERCEPTED GS. GS STARTED TO GO ERRATIC. THEN FULL SCALE DEFECTIONS, NOT PLT INDUCED -- UP AND DOWN ON BOTH RECEIVERS. RPTED INDICATIONS TO APCH AND HE NOTICED THAT WE WERE LOW. NOBODY ELSE HAD ANY PROBS WITH THE GS. CAUGHT SIGHT OF THE VASI AND CONTINUED APCH. UNEVENTFUL LNDG.

Data retrieved from NASA's ASRS site as of July 2007 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.