Narrative:

We commenced ILS runway 9 approach at cid. We were kept high on approach and needed to descend rapidly. We had captured localizer, but GS showed full scale 'above GS' indication. First officer had cid VOR tuned for DME, since ILS on runway 9 has no DME. As we began to capture GS I reduced descent rate. At this point we were configured with flaps 15 degrees, gear down. The GS then stabilized at 3/4 DOT 'above GS.' I assumed a wind shift accounted for the GS not continuing to a capture, so I reduced power to increase descent. The GS indication did not change, there were no 'off' flags or other indication of instrument malfunction. We then heard a 'GS' callout from the GPWS. Since that callout is due to being 1 1/4 DOT below GS and we still indicated 3/4 DOT above, the 3 crew members discussed the alert and the consensus was that it was erroneous. At about that time, approach control called us on the radio with a 'low altitude alert.' I increased power to reduce descent even though I still had 'above GS' indication. The first officer then tuned his radio to the ILS and had a 'below GS' indication. At that point I determined we had an erroneous and conflicting indications so I initiated a missed approach. During the maneuvering, my GS stayed at its 3/4 DOT above GS reading until we retuned the radio. The following approach to ILS runway 9 was flown by the first officer. All instrument indications were proper during the second approach, including the GS readings. All instruments tested properly on the ground, and airborne prior to approach. At the lowest point in our descent, we were approximately 900 ft AGL, approximately 5 NM outside the FAF. Because we were making an aggressive descent to capture the GS from above, we had no indications that the GS was apparently 'stuck' at '3/4 DOT above GS.' because there is no DME on this ILS frequency, the first officer was not able to switch over to ILS for backup. We needed his navigation on the VOR for DME readings. If this ILS had DME installed, my subtle GS failure would have been detected earlier with the first officer GS indication.

Related reports:

Google
 

Original NASA ASRS Text

Title: LGT ACFT ON APCH WAS HIGH WHEN CLRED FOR APCH. STEEPER THAN NORMAL DSCNT WAS REQUIRED TO INTERCEPT GS AND GS INDICATOR CONTINUED TO INDICATE ACFT WAS HIGH SO RPTR CAPT CONTINUED DSCNT. GPWS GS WARNING ALERTED FLC BUT, BECAUSE THE INDICATOR WAS STILL SHOWING HIGH, THEY ASSUMED THE ALERT WAS ERRONEOUS UNTIL THE APCH CTLR INTERVENED WITH A LOW ALT ALERT. FLC DID A MISSED APCH AND NEXT APCH ALL INDICATIONS NORMAL.

Narrative: WE COMMENCED ILS RWY 9 APCH AT CID. WE WERE KEPT HIGH ON APCH AND NEEDED TO DSND RAPIDLY. WE HAD CAPTURED LOC, BUT GS SHOWED FULL SCALE 'ABOVE GS' INDICATION. FO HAD CID VOR TUNED FOR DME, SINCE ILS ON RWY 9 HAS NO DME. AS WE BEGAN TO CAPTURE GS I REDUCED DSCNT RATE. AT THIS POINT WE WERE CONFIGURED WITH FLAPS 15 DEGS, GEAR DOWN. THE GS THEN STABILIZED AT 3/4 DOT 'ABOVE GS.' I ASSUMED A WIND SHIFT ACCOUNTED FOR THE GS NOT CONTINUING TO A CAPTURE, SO I REDUCED PWR TO INCREASE DSCNT. THE GS INDICATION DID NOT CHANGE, THERE WERE NO 'OFF' FLAGS OR OTHER INDICATION OF INST MALFUNCTION. WE THEN HEARD A 'GS' CALLOUT FROM THE GPWS. SINCE THAT CALLOUT IS DUE TO BEING 1 1/4 DOT BELOW GS AND WE STILL INDICATED 3/4 DOT ABOVE, THE 3 CREW MEMBERS DISCUSSED THE ALERT AND THE CONSENSUS WAS THAT IT WAS ERRONEOUS. AT ABOUT THAT TIME, APCH CTL CALLED US ON THE RADIO WITH A 'LOW ALT ALERT.' I INCREASED PWR TO REDUCE DSCNT EVEN THOUGH I STILL HAD 'ABOVE GS' INDICATION. THE FO THEN TUNED HIS RADIO TO THE ILS AND HAD A 'BELOW GS' INDICATION. AT THAT POINT I DETERMINED WE HAD AN ERRONEOUS AND CONFLICTING INDICATIONS SO I INITIATED A MISSED APCH. DURING THE MANEUVERING, MY GS STAYED AT ITS 3/4 DOT ABOVE GS READING UNTIL WE RETUNED THE RADIO. THE FOLLOWING APCH TO ILS RWY 9 WAS FLOWN BY THE FO. ALL INST INDICATIONS WERE PROPER DURING THE SECOND APCH, INCLUDING THE GS READINGS. ALL INSTS TESTED PROPERLY ON THE GND, AND AIRBORNE PRIOR TO APCH. AT THE LOWEST POINT IN OUR DSCNT, WE WERE APPROX 900 FT AGL, APPROX 5 NM OUTSIDE THE FAF. BECAUSE WE WERE MAKING AN AGGRESSIVE DSCNT TO CAPTURE THE GS FROM ABOVE, WE HAD NO INDICATIONS THAT THE GS WAS APPARENTLY 'STUCK' AT '3/4 DOT ABOVE GS.' BECAUSE THERE IS NO DME ON THIS ILS FREQ, THE FO WAS NOT ABLE TO SWITCH OVER TO ILS FOR BACKUP. WE NEEDED HIS NAV ON THE VOR FOR DME READINGS. IF THIS ILS HAD DME INSTALLED, MY SUBTLE GS FAILURE WOULD HAVE BEEN DETECTED EARLIER WITH THE FO GS INDICATION.

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.