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.

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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.