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.