Narrative:

Location, ILS approach to runway 17C. On an ILS approach to runway 17C with the WX approximately 1500 ft overcast, 900 ft AGL, while tracking the localizer and GS with flight director and autoplt engaged. The GS appeared to dip and the aircraft followed it as we crosschecked wind versus altitude versus sink rate and the distance to the runway. We came to the conclusion that something was seriously wrong, so the PNF, first officer, punched off the autoplt and leveled off just as we exited the cloud alert. We reported that the GS was erratic and that the identify situation occurred yesterday exactly as this one did. Flew a visual to a uneventful landing. We observed an air cargo DC10 on the east side of runway 17C threshold, and as he crossed in front of us, the GS fluctuated about 2 dots up and 2 dots down as we were on very short final though VFR. After we reported this, the preceding and following jets confirmed this also. This exact scenario occurred yesterday morning to us but the WX was good, VFR. I strongly recommend that the practice of allowing a large aircraft to situation in this location be stopped and if done in VFR, that all pilots be advised of this. Supplemental information from acn 339836: broke out of 900 ft AGL ceiling and noticed that we were low for the approach but GS showed right on. Tower advised of the altitude alert at same time. We leveled off and flew remainder of approach in visual conditions. Landing uneventful.

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Original NASA ASRS Text

Title: FLC OF AN MLG FOUND THE GS TO BE ERRATIC DURING AN ILS AUTOPLT COUPLED APCH. WHEN BREAKING OUT OF THE OVCST, THE FLC CONTINUED ON WITH A VISUAL APCH WHILE NOTING A WDB ACFT TAXIING ACROSS THE APCH END OF THE RWY AND BEYOND THE ILS HOLD SHORT POINT LINE.

Narrative: LOCATION, ILS APCH TO RWY 17C. ON AN ILS APCH TO RWY 17C WITH THE WX APPROX 1500 FT OVCST, 900 FT AGL, WHILE TRACKING THE LOC AND GS WITH FLT DIRECTOR AND AUTOPLT ENGAGED. THE GS APPEARED TO DIP AND THE ACFT FOLLOWED IT AS WE XCHKED WIND VERSUS ALT VERSUS SINK RATE AND THE DISTANCE TO THE RWY. WE CAME TO THE CONCLUSION THAT SOMETHING WAS SERIOUSLY WRONG, SO THE PNF, FO, PUNCHED OFF THE AUTOPLT AND LEVELED OFF JUST AS WE EXITED THE CLOUD ALERT. WE RPTED THAT THE GS WAS ERRATIC AND THAT THE IDENT SIT OCCURRED YESTERDAY EXACTLY AS THIS ONE DID. FLEW A VISUAL TO A UNEVENTFUL LNDG. WE OBSERVED AN AIR CARGO DC10 ON THE E SIDE OF RWY 17C THRESHOLD, AND AS HE CROSSED IN FRONT OF US, THE GS FLUCTUATED ABOUT 2 DOTS UP AND 2 DOTS DOWN AS WE WERE ON VERY SHORT FINAL THOUGH VFR. AFTER WE RPTED THIS, THE PRECEDING AND FOLLOWING JETS CONFIRMED THIS ALSO. THIS EXACT SCENARIO OCCURRED YESTERDAY MORNING TO US BUT THE WX WAS GOOD, VFR. I STRONGLY RECOMMEND THAT THE PRACTICE OF ALLOWING A LARGE ACFT TO SIT IN THIS LOCATION BE STOPPED AND IF DONE IN VFR, THAT ALL PLTS BE ADVISED OF THIS. SUPPLEMENTAL INFO FROM ACN 339836: BROKE OUT OF 900 FT AGL CEILING AND NOTICED THAT WE WERE LOW FOR THE APCH BUT GS SHOWED RIGHT ON. TWR ADVISED OF THE ALT ALERT AT SAME TIME. WE LEVELED OFF AND FLEW REMAINDER OF APCH IN VISUAL CONDITIONS. LNDG UNEVENTFUL.

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.