S80 flight crew encounter aircraft and ground equipment erroneous fluctuations.

Attributes
ACN	495475
Time
Date	200012
Day	Thu
Local Time Of Day	1801 To 2400
Place
Locale Reference	airport : ord.airport
State Reference	IL
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	tower : ord.tower
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
Experience	flight time last 90 days : 250 flight time total : 18000 flight time type : 7000
ASRS Report	495475
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : multi engine pilot : instrument
Events
Anomaly	altitude deviation : crossing restriction not met altitude deviation : overshoot non adherence : clearance non adherence : published procedure other anomaly
Independent Detector	atc equipment other atc equipment : gpws / tower other controllera other flight crewa other flight crewb
Resolutory Action	controller : issued alert flight crew : became reoriented flight crew : returned to original clearance
Consequence	faa : reviewed incident with flight crew
Supplementary
Problem Areas	Flight Crew Human Performance Navigational Facility Airport
Primary Problem	Navigational Facility

Narrative:

On VFR night ILS runway 27L at ord inside willt, we noticed altitude seemed low, PAPI was all red. Correcting when ord tower called to give altimeter and check we had runway in sight as they had gotten low altitude alert on us. We held altitude and then GS seemed to get better closer to runway. No off flags, showed on GS, but really low, with both captain and first officer showing on GS. Callback conversation with reporter revealed the following information: reporter advised that GS deflection problem had been encountered recently twice before. Reporter did not observe any other aircraft activity operating in the immediate vicinity of the runway. Shortly after the tower issued a 'low altitude alert,' and asking if they had the runway in sight, the GS stabilized, which was confirmed by both pilots with an 'on GS' PAPI (precision approach path indicator) visual indication. Company was notified about the encountered situation.

Original NASA ASRS Text

Title: S80 FLC ENCOUNTER ACFT AND GND EQUIP ERRONEOUS FLUCTUATIONS.

Narrative: ON VFR NIGHT ILS RWY 27L AT ORD INSIDE WILLT, WE NOTICED ALT SEEMED LOW, PAPI WAS ALL RED. CORRECTING WHEN ORD TWR CALLED TO GIVE ALTIMETER AND CHK WE HAD RWY IN SIGHT AS THEY HAD GOTTEN LOW ALT ALERT ON US. WE HELD ALT AND THEN GS SEEMED TO GET BETTER CLOSER TO RWY. NO OFF FLAGS, SHOWED ON GS, BUT REALLY LOW, WITH BOTH CAPT AND FO SHOWING ON GS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR ADVISED THAT GS DEFLECTION PROB HAD BEEN ENCOUNTERED RECENTLY TWICE BEFORE. RPTR DID NOT OBSERVE ANY OTHER ACFT ACTIVITY OPERATING IN THE IMMEDIATE VICINITY OF THE RWY. SHORTLY AFTER THE TWR ISSUED A 'LOW ALT ALERT,' AND ASKING IF THEY HAD THE RWY IN SIGHT, THE GS STABILIZED, WHICH WAS CONFIRMED BY BOTH PLTS WITH AN 'ON GS' PAPI (PRECISION APCH PATH INDICATOR) VISUAL INDICATION. COMPANY WAS NOTIFIED ABOUT THE ENCOUNTERED SIT.

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.