<< >> NASA ASRS Report 813316, November 2008

B777 flight crew reports capturing a false glide slope during ILS runway 28 approach to ord. Aircraft descends 500 ft before error is detected.

Attributes
ACN	813316
Time
Date	200811
Local Time Of Day	1801 To 2400
Place
Locale Reference	airport : ord.airport
State Reference	IL
Altitude	msl single value : 2500
Environment
Flight Conditions	Marginal
Aircraft 1
Controlling Facilities	tracon : c90.tracon
Operator	common carrier : air carrier
Make Model Name	B777-200
Operating Under FAR Part	Part 121
Navigation In Use	ils localizer & glide slope : 28 other
Flight Phase	descent : approach
Route In Use	approach : instrument precision
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Experience	flight time last 90 days : 60 flight time total : 15199 flight time type : 1000
ASRS Report	813316
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Experience	flight time last 90 days : 150 flight time total : 25000 flight time type : 6000
ASRS Report	813477
Events
Anomaly	altitude deviation : excursion from assigned altitude non adherence : clearance non adherence : published procedure other anomaly other
Independent Detector	atc equipment : msaw other flight crewa
Resolutory Action	controller : issued alert flight crew : returned to assigned altitude
Supplementary
Problem Areas	Navigational Facility Flight Crew Human Performance
Primary Problem	Navigational Facility

Narrative:

We were turned onto the ILS for runway 28. Intercepted the localizer. After we were established on the localizer the glide slope signal falsely captured the glide slope approximately 3 miles prior to where it should have. This led to a low altitude alert from ATC. The aircraft was flown to ZZZ later in the day where the localizer/GS function worked fine.supplemental information from acn 813477: approach descended us to 2500 ft just outside of willt. We captured a false glide slope and stated descending. At 2000 ft I realized we were on a false glide slope and as I was informing my copilot to level off ATC issued us a low altitude alert. We climbed back to 2200 ft. We reset our flight directors and intercepted the true glide slope. First time in 37 years of flying that I encountered this. Contradictory flight guidance is harder to resolve than it seems.

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Original NASA ASRS Text

Title: B777 FLIGHT CREW REPORTS CAPTURING A FALSE GLIDE SLOPE DURING ILS RWY 28 APPROACH TO ORD. AIRCRAFT DESCENDS 500 FT BEFORE ERROR IS DETECTED.

Narrative: WE WERE TURNED ONTO THE ILS FOR RUNWAY 28. INTERCEPTED THE LOCALIZER. AFTER WE WERE ESTABLISHED ON THE LOCALIZER THE GLIDE SLOPE SIGNAL FALSELY CAPTURED THE GLIDE SLOPE APPROXIMATELY 3 MILES PRIOR TO WHERE IT SHOULD HAVE. THIS LED TO A LOW ALTITUDE ALERT FROM ATC. THE AIRCRAFT WAS FLOWN TO ZZZ LATER IN THE DAY WHERE THE LOC/GS FUNCTION WORKED FINE.SUPPLEMENTAL INFO FROM ACN 813477: APPROACH DESCENDED US TO 2500 FT JUST OUTSIDE OF WILLT. WE CAPTURED A FALSE GLIDE SLOPE AND STATED DESCENDING. AT 2000 FT I REALIZED WE WERE ON A FALSE GLIDE SLOPE AND AS I WAS INFORMING MY COPILOT TO LEVEL OFF ATC ISSUED US A LOW ALT ALERT. WE CLIMBED BACK TO 2200 FT. WE RESET OUR FLIGHT DIRECTORS AND INTERCEPTED THE TRUE GLIDE SLOPE. FIRST TIME IN 37 YEARS OF FLYING THAT I ENCOUNTERED THIS. CONTRADICTORY FLIGHT GUIDANCE IS HARDER TO RESOLVE THAN IT SEEMS.

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.