Narrative:

We were cleared for the ILS approach to runway 27L in VMC and had the airport in sight from approximately 10 mi away. I was hand flying the aircraft and we were established on the ILS and were stabilized at 170 KTS to willt as requested by approach control. As we crossed willt; I asked for gear down and flaps 20 degrees to begin configuring for our final approach. I looked away from the ADI for a couple of seconds and when I looked back; the GS showed full deflection low (we were showing 2+ dots high). I began to nose the aircraft down to try to recapture the GS but had a gut feeling that we were where we were supposed to be and that the ADI was giving me a false indication. I looked out the window at the PAPI and saw that we were indeed in the slot. Then I noticed that a foreign cargo DC10 was parked at the end of runway 27L and landed without further incident. I know that during certain WX conditions that the ILS area is unprotected; but to park a large aircraft in front of the GS antenna and then clear aircraft for the ILS seems unsafe somehow. I know that we were not the only ones to experience this anomaly because I heard other aircraft query the ground controller about the gross deviations in the GS signal. From what the ground controller said; it sounded like this is a normal practice. I can only imagine what the aircraft might have done had the autoplt still been hooked up when the GS decided to go full scale down.

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Original NASA ASRS Text

Title: B757 ON ILS RWY 27L ORD EXPERIENCED FULL SCALE DEFLECTION ON GS OVER WILLT. RWY ACQUIRED VISUALLY AND PAPI INDICTED ON GLIDEPATH. DC10 OBSERVED ON TXWY POSSIBLY BLOCKING GS ANTENNA.

Narrative: WE WERE CLRED FOR THE ILS APCH TO RWY 27L IN VMC AND HAD THE ARPT IN SIGHT FROM APPROX 10 MI AWAY. I WAS HAND FLYING THE ACFT AND WE WERE ESTABLISHED ON THE ILS AND WERE STABILIZED AT 170 KTS TO WILLT AS REQUESTED BY APCH CTL. AS WE CROSSED WILLT; I ASKED FOR GEAR DOWN AND FLAPS 20 DEGS TO BEGIN CONFIGURING FOR OUR FINAL APCH. I LOOKED AWAY FROM THE ADI FOR A COUPLE OF SECONDS AND WHEN I LOOKED BACK; THE GS SHOWED FULL DEFLECTION LOW (WE WERE SHOWING 2+ DOTS HIGH). I BEGAN TO NOSE THE ACFT DOWN TO TRY TO RECAPTURE THE GS BUT HAD A GUT FEELING THAT WE WERE WHERE WE WERE SUPPOSED TO BE AND THAT THE ADI WAS GIVING ME A FALSE INDICATION. I LOOKED OUT THE WINDOW AT THE PAPI AND SAW THAT WE WERE INDEED IN THE SLOT. THEN I NOTICED THAT A FOREIGN CARGO DC10 WAS PARKED AT THE END OF RWY 27L AND LANDED WITHOUT FURTHER INCIDENT. I KNOW THAT DURING CERTAIN WX CONDITIONS THAT THE ILS AREA IS UNPROTECTED; BUT TO PARK A LARGE ACFT IN FRONT OF THE GS ANTENNA AND THEN CLR ACFT FOR THE ILS SEEMS UNSAFE SOMEHOW. I KNOW THAT WE WERE NOT THE ONLY ONES TO EXPERIENCE THIS ANOMALY BECAUSE I HEARD OTHER ACFT QUERY THE GND CTLR ABOUT THE GROSS DEVS IN THE GS SIGNAL. FROM WHAT THE GND CTLR SAID; IT SOUNDED LIKE THIS IS A NORMAL PRACTICE. I CAN ONLY IMAGINE WHAT THE ACFT MIGHT HAVE DONE HAD THE AUTOPLT STILL BEEN HOOKED UP WHEN THE GS DECIDED TO GO FULL SCALE DOWN.

Data retrieved from NASA's ASRS site as of January 2009 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.