Narrative:

Visual conditions, coupled approach, intending to complete an autoland for currency requirements. B747 departed on runway 14R and tower said that ILS critical area was not clear as we passed inside the FAF. By about 500-1000 ft AGL the B747 was well clear of the air traffic area. As I wanted to get the autoland (if possible) I was looking for other aircraft/cars/trucks, etc in the ILS critical area, none in sight. We continued. Near the 110 ft AGL RA altitude, the first officer and I spotted the large skid marks left by the B727 a couple of weeks before. I was watching the intended point of landing with concern for ILS zone intrusion. I noticed the intended point of landing moving upward on the windshield and 'felt' an increased sink rate. I glanced at the pfd and saw the GS dropping rapidly with the aircraft following it. Altitude must have been less than 110 ft AGL and greater than 65 ft AGL. I disconnected the autoplt and autothrottles, added power, raised the nose, and re- established a normal landing profile to a normal landing -- somewhat shorter than usual. Ground control notified and said 2 subsequent aircraft had the same encounter. I'm not sure how I'd have interpreted the EFIS had I been CAT III conditions. VMC was easy to spot the negative trend. Callback conversation with reporter revealed the following information: reporter captain has no idea what caused the signal interference if that is what it was and tower controller reported the ILS signal had been normal. When reporter aircraft was 10 mi out on the ILS the tower reported the ILS not clean due to a B747 taking off, but by the time reporter was at 1000 ft, everything was stable. Reporter states that the B737-300 is notorious for having localizer deflections on ILS approachs, but has never seen a GS deflection. In his case he picked up visual clues below 100 ft of increasing sink and checking the pfd noted GS dropping rapidly with autoplt following it. Captain was keyed to disconnecting the autoplt, not only because of the autoland, but also because of busy traffic conditions at ord, being that at any time he may have to deviation because of ATC instructions. He says that had they been in instrument conditions with this occurrence, he is sure they would have hit short of the runway and very hard. He notified his air carrier chief pilot who notified other air carrier's.

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Original NASA ASRS Text

Title: B737-300 ACFT ON AUTOLAND APCH IN GOOD WX, RPTR CAPT SAW AND FELT INCREASED SINK RATE BELOW 100 FT. GS SIGNAL WAS DROPPING RAPIDLY AND THE AUTOPLT WAS FOLLOWING IT. RPTR DISCONNECTED AUTOPLT, ADDED PWR, RAISED NOSE AND LANDED. CAUSE OF SIGNAL VARIATION IS UNKNOWN.

Narrative: VISUAL CONDITIONS, COUPLED APCH, INTENDING TO COMPLETE AN AUTOLAND FOR CURRENCY REQUIREMENTS. B747 DEPARTED ON RWY 14R AND TWR SAID THAT ILS CRITICAL AREA WAS NOT CLR AS WE PASSED INSIDE THE FAF. BY ABOUT 500-1000 FT AGL THE B747 WAS WELL CLR OF THE ATA. AS I WANTED TO GET THE AUTOLAND (IF POSSIBLE) I WAS LOOKING FOR OTHER ACFT/CARS/TRUCKS, ETC IN THE ILS CRITICAL AREA, NONE IN SIGHT. WE CONTINUED. NEAR THE 110 FT AGL RA ALT, THE FO AND I SPOTTED THE LARGE SKID MARKS LEFT BY THE B727 A COUPLE OF WKS BEFORE. I WAS WATCHING THE INTENDED POINT OF LNDG WITH CONCERN FOR ILS ZONE INTRUSION. I NOTICED THE INTENDED POINT OF LNDG MOVING UPWARD ON THE WINDSHIELD AND 'FELT' AN INCREASED SINK RATE. I GLANCED AT THE PFD AND SAW THE GS DROPPING RAPIDLY WITH THE ACFT FOLLOWING IT. ALT MUST HAVE BEEN LESS THAN 110 FT AGL AND GREATER THAN 65 FT AGL. I DISCONNECTED THE AUTOPLT AND AUTOTHROTTLES, ADDED PWR, RAISED THE NOSE, AND RE- ESTABLISHED A NORMAL LNDG PROFILE TO A NORMAL LNDG -- SOMEWHAT SHORTER THAN USUAL. GND CTL NOTIFIED AND SAID 2 SUBSEQUENT ACFT HAD THE SAME ENCOUNTER. I'M NOT SURE HOW I'D HAVE INTERPED THE EFIS HAD I BEEN CAT III CONDITIONS. VMC WAS EASY TO SPOT THE NEGATIVE TREND. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR CAPT HAS NO IDEA WHAT CAUSED THE SIGNAL INTERFERENCE IF THAT IS WHAT IT WAS AND TWR CTLR RPTED THE ILS SIGNAL HAD BEEN NORMAL. WHEN RPTR ACFT WAS 10 MI OUT ON THE ILS THE TWR RPTED THE ILS NOT CLEAN DUE TO A B747 TAKING OFF, BUT BY THE TIME RPTR WAS AT 1000 FT, EVERYTHING WAS STABLE. RPTR STATES THAT THE B737-300 IS NOTORIOUS FOR HAVING LOC DEFLECTIONS ON ILS APCHS, BUT HAS NEVER SEEN A GS DEFLECTION. IN HIS CASE HE PICKED UP VISUAL CLUES BELOW 100 FT OF INCREASING SINK AND CHKING THE PFD NOTED GS DROPPING RAPIDLY WITH AUTOPLT FOLLOWING IT. CAPT WAS KEYED TO DISCONNECTING THE AUTOPLT, NOT ONLY BECAUSE OF THE AUTOLAND, BUT ALSO BECAUSE OF BUSY TFC CONDITIONS AT ORD, BEING THAT AT ANY TIME HE MAY HAVE TO DEV BECAUSE OF ATC INSTRUCTIONS. HE SAYS THAT HAD THEY BEEN IN INST CONDITIONS WITH THIS OCCURRENCE, HE IS SURE THEY WOULD HAVE HIT SHORT OF THE RWY AND VERY HARD. HE NOTIFIED HIS ACR CHIEF PLT WHO NOTIFIED OTHER ACR'S.

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.