Narrative:

Atl to ord was delayed due to ATC ground stop and thunderstorm over the departure airport. The departure, cruise and initial descent were routine. The first officer was the PF. Ord approach controller assigned runway 27L for arrival. We were vectored to intercept the localizer to the south of the course at or above 4000 ft. The localizer was captured below the GS. The airplane started what appeared to be a normal final descent with the flight director on and the autoplt engaged. At some point during the final descent, the first officer disengaged the autoplt. I noticed that the PAPI lights were showing 4 red bars, however, the FMA was showing all the normal (captured) indications. There was a growing sense of discomfort with the correctness of GS data as the colors on the PAPI bars never changed from solid 4 red bars. At about 700 ft AGL, the GS pointer suddenly jumped to indicate 2 dots low. It was apparent that the GS data was unreliable. I commanded the first officer to immediately stop the descent by shouting 'don't get any lower, the GS is unreliable.' after a short hesitation, the first officer advanced the throttles to arrest the rate of descent and re-establish the GS by reference to the PAPI. I immediately reported the unreliability of the GS to the controller. In response he said 'roger.' there was no attempt made by the controller to warn the ensuing aircraft on the same approach. This attitude from the controller was alarming, as we were cleared for the ILS and therefore our primary reliance for guidance was the localizer and the electronic GS. Contributing elements were fatigue due to prolonged delay in atl. Over reliance on aircraft automation resulted in ignoring the visual signs of being low in favor of the electronic guidance.

Related reports:

Google
 

Original NASA ASRS Text

Title: B737 PLT IS CONCERNED WITH CTLR RESPONSE TO AN APPARENT GS SIGNAL PROBLEM.

Narrative: ATL TO ORD WAS DELAYED DUE TO ATC GND STOP AND TSTM OVER THE DEP ARPT. THE DEP, CRUISE AND INITIAL DSCNT WERE ROUTINE. THE FO WAS THE PF. ORD APCH CTLR ASSIGNED RWY 27L FOR ARR. WE WERE VECTORED TO INTERCEPT THE LOC TO THE S OF THE COURSE AT OR ABOVE 4000 FT. THE LOC WAS CAPTURED BELOW THE GS. THE AIRPLANE STARTED WHAT APPEARED TO BE A NORMAL FINAL DSCNT WITH THE FLT DIRECTOR ON AND THE AUTOPLT ENGAGED. AT SOME POINT DURING THE FINAL DSCNT, THE FO DISENGAGED THE AUTOPLT. I NOTICED THAT THE PAPI LIGHTS WERE SHOWING 4 RED BARS, HOWEVER, THE FMA WAS SHOWING ALL THE NORMAL (CAPTURED) INDICATIONS. THERE WAS A GROWING SENSE OF DISCOMFORT WITH THE CORRECTNESS OF GS DATA AS THE COLORS ON THE PAPI BARS NEVER CHANGED FROM SOLID 4 RED BARS. AT ABOUT 700 FT AGL, THE GS POINTER SUDDENLY JUMPED TO INDICATE 2 DOTS LOW. IT WAS APPARENT THAT THE GS DATA WAS UNRELIABLE. I COMMANDED THE FO TO IMMEDIATELY STOP THE DSCNT BY SHOUTING 'DON'T GET ANY LOWER, THE GS IS UNRELIABLE.' AFTER A SHORT HESITATION, THE FO ADVANCED THE THROTTLES TO ARREST THE RATE OF DSCNT AND RE-ESTABLISH THE GS BY REF TO THE PAPI. I IMMEDIATELY RPTED THE UNRELIABILITY OF THE GS TO THE CTLR. IN RESPONSE HE SAID 'ROGER.' THERE WAS NO ATTEMPT MADE BY THE CTLR TO WARN THE ENSUING ACFT ON THE SAME APCH. THIS ATTITUDE FROM THE CTLR WAS ALARMING, AS WE WERE CLRED FOR THE ILS AND THEREFORE OUR PRIMARY RELIANCE FOR GUIDANCE WAS THE LOC AND THE ELECTRONIC GS. CONTRIBUTING ELEMENTS WERE FATIGUE DUE TO PROLONGED DELAY IN ATL. OVER RELIANCE ON ACFT AUTOMATION RESULTED IN IGNORING THE VISUAL SIGNS OF BEING LOW IN FAVOR OF THE ELECTRONIC GUIDANCE.

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.