Narrative:

XX50Z, frequency 126.55, altitude: 1000 ft. ATC facility: dfw tower. Location: ILS runway 17C. We were given the ILS runway 17C at dfw. At 12 DME the flight director commanded 5 degrees nose low. The GS became very erratic but we were able to continue visually. We reported the bad GS indications to tower and another flight said they were also having problems. After landing, I contacted tower on the phone. They said a B747 waiting to cross runway 17C at taxiway Y was responsible for the problem. Now that we know B747's crossing at taxiway Y can do this, can we ask ATC to warn inbounds of possible false GS indications? Better yet, can we cross B747's somewhere else? This can be a very confusing situation in the cockpit and could really bite someone next time.

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Original NASA ASRS Text

Title: AN S80 ACFT LNDG AT DFW HAS ITS GS FLUCTUATE AT 1000 FT MSL DUE TO A B747 SITTING CLOSE BY THE ARR RWY.

Narrative: XX50Z, FREQ 126.55, ALT: 1000 FT. ATC FACILITY: DFW TWR. LOCATION: ILS RWY 17C. WE WERE GIVEN THE ILS RWY 17C AT DFW. AT 12 DME THE FLT DIRECTOR COMMANDED 5 DEGS NOSE LOW. THE GS BECAME VERY ERRATIC BUT WE WERE ABLE TO CONTINUE VISUALLY. WE RPTED THE BAD GS INDICATIONS TO TWR AND ANOTHER FLT SAID THEY WERE ALSO HAVING PROBS. AFTER LNDG, I CONTACTED TWR ON THE PHONE. THEY SAID A B747 WAITING TO CROSS RWY 17C AT TXWY Y WAS RESPONSIBLE FOR THE PROB. NOW THAT WE KNOW B747'S XING AT TXWY Y CAN DO THIS, CAN WE ASK ATC TO WARN INBOUNDS OF POSSIBLE FALSE GS INDICATIONS? BETTER YET, CAN WE CROSS B747'S SOMEWHERE ELSE? THIS CAN BE A VERY CONFUSING SIT IN THE COCKPIT AND COULD REALLY BITE SOMEONE NEXT TIME.

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.