Narrative:

We were in process of identifying navaids for runway 18R approach when we were cleared to descend to 3000 MSL, intercept ILS 13R, the localizer, maintain 3000 MSL until established, then cleared for the approach. I selected the approach mode of the AFDS panel. I announced localizer armed and localizer captured and glide slope captured. As I started descending down the glide slope something looked wrong with the visual picture I was seeing outside and the glide slope indications. The actual visibility was 5-6 mi. Rechecking my ADI, I had glide slope captured indications, flight director glide slope captured indications and raw data glide slope captured indications. I then cycled off and on, the flight directors, and reengaged the approach mode because I still felt something was wrong. All indications remained the same, both glide slope and localizer centered. The captain then recycled off then on, the flight directors, and reengaged the approach mode. I had the same indications, localizer centered glide slope centered. I followed the apparent localizer and glide slope for an additional 400-500' down, then the visual picture confirmed the glide slope was wrong so I leveled off at approximately 1650' MSL. Approximately 10 second later, tower called for us to check our altitude which we responded that we were level. We visually took over the reestablished a descent now from visually above the glide slope holding approximately 1000 FPM vsi. However, the cockpit indications were still glide slope centered both raw data and flight directors. I disregarded the indications and landed as I was flying. The captain also expressed this concern for an erroneous glide slope indication. He rechecked the ILS frequency tuned, the expanded ILS display frequency and the ILS frequency. All correct. He cycled the first officer switches off then on, as previously stated, still all producing centered glide slope indications. The captain asked the tower after landing if there had been any complaints about 13R glide slope of equipment problems. The tower never answered. The captain tried calling the tower supervisor after gate arrival and no answer after 10 rings. Due to a short on ground time we could not attempt further calls to the tower and proceeded with the next leg. Callback conversation with reporter revealed the following: reporter states that all systems and indications on the approach seemed normal but both crew members felt the altitude was low. They based their feelings on the captain locating a ground reference that he felt was too far from runway for the aircraft altitude. ATC also mentioned to the crew to check altitude. However no ATC indication of any ILS component malfunction. Crew was unable to contact tower for confirmation due to time available. Aircraft was checked by maintenance and no malfunction was found. Reporter was unable to determine source of any problem.

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Original NASA ASRS Text

Title: REPORTER ACFT MAY OR MAY NOT HAVE ACQUIRED A FALSE GLIDE SLOPE SIGNAL.

Narrative: WE WERE IN PROCESS OF IDENTIFYING NAVAIDS FOR RWY 18R APCH WHEN WE WERE CLRED TO DSND TO 3000 MSL, INTERCEPT ILS 13R, THE LOC, MAINTAIN 3000 MSL UNTIL ESTABLISHED, THEN CLRED FOR THE APCH. I SELECTED THE APCH MODE OF THE AFDS PANEL. I ANNOUNCED LOC ARMED AND LOC CAPTURED AND GLIDE SLOPE CAPTURED. AS I STARTED DESCENDING DOWN THE GLIDE SLOPE SOMETHING LOOKED WRONG WITH THE VISUAL PICTURE I WAS SEEING OUTSIDE AND THE GLIDE SLOPE INDICATIONS. THE ACTUAL VISIBILITY WAS 5-6 MI. RECHECKING MY ADI, I HAD GLIDE SLOPE CAPTURED INDICATIONS, FLT DIRECTOR GLIDE SLOPE CAPTURED INDICATIONS AND RAW DATA GLIDE SLOPE CAPTURED INDICATIONS. I THEN CYCLED OFF AND ON, THE FLT DIRECTORS, AND REENGAGED THE APCH MODE BECAUSE I STILL FELT SOMETHING WAS WRONG. ALL INDICATIONS REMAINED THE SAME, BOTH GLIDE SLOPE AND LOC CENTERED. THE CAPT THEN RECYCLED OFF THEN ON, THE FLT DIRECTORS, AND REENGAGED THE APCH MODE. I HAD THE SAME INDICATIONS, LOC CENTERED GLIDE SLOPE CENTERED. I FOLLOWED THE APPARENT LOC AND GLIDE SLOPE FOR AN ADDITIONAL 400-500' DOWN, THEN THE VISUAL PICTURE CONFIRMED THE GLIDE SLOPE WAS WRONG SO I LEVELED OFF AT APPROX 1650' MSL. APPROX 10 SEC LATER, TWR CALLED FOR US TO CHECK OUR ALT WHICH WE RESPONDED THAT WE WERE LEVEL. WE VISUALLY TOOK OVER THE REESTABLISHED A DSCNT NOW FROM VISUALLY ABOVE THE GLIDE SLOPE HOLDING APPROX 1000 FPM VSI. HOWEVER, THE COCKPIT INDICATIONS WERE STILL GLIDE SLOPE CENTERED BOTH RAW DATA AND FLT DIRECTORS. I DISREGARDED THE INDICATIONS AND LANDED AS I WAS FLYING. THE CAPT ALSO EXPRESSED THIS CONCERN FOR AN ERRONEOUS GLIDE SLOPE INDICATION. HE RECHECKED THE ILS FREQ TUNED, THE EXPANDED ILS DISPLAY FREQ AND THE ILS FREQ. ALL CORRECT. HE CYCLED THE F/O SWITCHES OFF THEN ON, AS PREVIOUSLY STATED, STILL ALL PRODUCING CENTERED GLIDE SLOPE INDICATIONS. THE CAPT ASKED THE TWR AFTER LNDG IF THERE HAD BEEN ANY COMPLAINTS ABOUT 13R GLIDE SLOPE OF EQUIPMENT PROBLEMS. THE TWR NEVER ANSWERED. THE CAPT TRIED CALLING THE TWR SUPVR AFTER GATE ARRIVAL AND NO ANSWER AFTER 10 RINGS. DUE TO A SHORT ON GND TIME WE COULD NOT ATTEMPT FURTHER CALLS TO THE TWR AND PROCEEDED WITH THE NEXT LEG. CALLBACK CONVERSATION WITH REPORTER REVEALED THE FOLLOWING: REPORTER STATES THAT ALL SYSTEMS AND INDICATIONS ON THE APCH SEEMED NORMAL BUT BOTH CREW MEMBERS FELT THE ALT WAS LOW. THEY BASED THEIR FEELINGS ON THE CAPT LOCATING A GND REFERENCE THAT HE FELT WAS TOO FAR FROM RWY FOR THE ACFT ALT. ATC ALSO MENTIONED TO THE CREW TO CHECK ALT. HOWEVER NO ATC INDICATION OF ANY ILS COMPONENT MALFUNCTION. CREW WAS UNABLE TO CONTACT TWR FOR CONFIRMATION DUE TO TIME AVAILABLE. ACFT WAS CHECKED BY MAINT AND NO MALFUNCTION WAS FOUND. REPORTER WAS UNABLE TO DETERMINE SOURCE OF ANY PROBLEM.

Data retrieved from NASA's ASRS site as of August 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.