Narrative:

Vectors and clearance for ILS runway 27L at ord. With autoflt engaged, approach mode armed, we were cleared to maintain 4000 ft until 'adame' and cleared for approach at 'adame.' aircraft appeared well above GS. Disconnected autoplt and attempted to capture the GS. Passing 'wiltt' on the approach, we contacted tower. Xchking altitude with our position and with the left side PAPI to runway 27L, we appeared low on the approach. At this time, we still indicated above GS. We questioned with tower the status of GS. Tower responded that a B747 was holding short of runway 27L. Apparently, the B747 was blocking an accurate GS signal. Tower did not report any 'low altitude alert' on our aircraft or make any further comments on the GS. This problem arose from the aircraft on the ground not honoring the ILS hold short point with the ATIS reporting approachs to runway 27L. However, their ground clearance may have been simply to hold short of the runway for arriving traffic. Preventative action: altitude xchks along with PAPI's help validate erroneous GS position.

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Original NASA ASRS Text

Title: MD10 ON APCH INTO ORD RWY 27L HAS ERRATIC GS SIGNAL ON FINAL AND IS ADVISED BY THE LCL CTLR THAT A B747 IS HOLDING SHORT OF RWY 27L.

Narrative: VECTORS AND CLRNC FOR ILS RWY 27L AT ORD. WITH AUTOFLT ENGAGED, APCH MODE ARMED, WE WERE CLRED TO MAINTAIN 4000 FT UNTIL 'ADAME' AND CLRED FOR APCH AT 'ADAME.' ACFT APPEARED WELL ABOVE GS. DISCONNECTED AUTOPLT AND ATTEMPTED TO CAPTURE THE GS. PASSING 'WILTT' ON THE APCH, WE CONTACTED TWR. XCHKING ALT WITH OUR POS AND WITH THE L SIDE PAPI TO RWY 27L, WE APPEARED LOW ON THE APCH. AT THIS TIME, WE STILL INDICATED ABOVE GS. WE QUESTIONED WITH TWR THE STATUS OF GS. TWR RESPONDED THAT A B747 WAS HOLDING SHORT OF RWY 27L. APPARENTLY, THE B747 WAS BLOCKING AN ACCURATE GS SIGNAL. TWR DID NOT RPT ANY 'LOW ALT ALERT' ON OUR ACFT OR MAKE ANY FURTHER COMMENTS ON THE GS. THIS PROB AROSE FROM THE ACFT ON THE GND NOT HONORING THE ILS HOLD SHORT POINT WITH THE ATIS RPTING APCHS TO RWY 27L. HOWEVER, THEIR GND CLRNC MAY HAVE BEEN SIMPLY TO HOLD SHORT OF THE RWY FOR ARRIVING TFC. PREVENTATIVE ACTION: ALT XCHKS ALONG WITH PAPI'S HELP VALIDATE ERRONEOUS GS POS.

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.