Narrative:

Ord runway 14R ILS glide path deviation. During a stabilized approach to runway 14R ord, flown by the first officer, I noticed the GS indication start to move to the bottom of the indicator, even though our actual glide path did not vary, beginning about 200 ft AGL. It varied slightly but did not come back up to the proper indication during the landing. No railroad equipment was noticed on the tracks under us as we approached the runway which has been mentioned as a possible factor in this type of situation. I notified the ord tower controller of this occurrence as we exited the runway so that any video monitoring of this area could be scrutinized for possible interference. Callback conversation with reporter revealed the following information: the reporter has flown the runway 14R ILS many times. This is the first time he has had a problem with the GS signal. The WX was day VMC insuring that the proper approach profile was flown. The aircraft did not have any GS flags, and because of radio altimeter based GS attenuation, the flight directors did not show a fly down indication. The pilot further stated that since an air carrier accident at this runway several yrs ago, pilots at his company have been sensitive to this type of problem. The pilot reports that boeing thinks that some type of RF interference may be coming from a new type of railroad locomotive that has an AC to dc to AC power conversion. The analyst also contacted the quality assurance manager at ord tower. There have been 17 problems reported with the runway 14R GS. There have been 13 flight checks, all of which were normal. Ord has replaced the entire runway 14R ILS system -- all electronics, cable, and antennas -- yet the problem seems to continue.

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Original NASA ASRS Text

Title: MD80 CREW HAD THE GS SIGNAL SHOW HIGH ON THE GS AT 200 FT. THE ACFT WAS ACTUALLY ON THE GS.

Narrative: ORD RWY 14R ILS GLIDE PATH DEV. DURING A STABILIZED APCH TO RWY 14R ORD, FLOWN BY THE FO, I NOTICED THE GS INDICATION START TO MOVE TO THE BOTTOM OF THE INDICATOR, EVEN THOUGH OUR ACTUAL GLIDE PATH DID NOT VARY, BEGINNING ABOUT 200 FT AGL. IT VARIED SLIGHTLY BUT DID NOT COME BACK UP TO THE PROPER INDICATION DURING THE LNDG. NO RAILROAD EQUIP WAS NOTICED ON THE TRACKS UNDER US AS WE APCHED THE RWY WHICH HAS BEEN MENTIONED AS A POSSIBLE FACTOR IN THIS TYPE OF SIT. I NOTIFIED THE ORD TWR CTLR OF THIS OCCURRENCE AS WE EXITED THE RWY SO THAT ANY VIDEO MONITORING OF THIS AREA COULD BE SCRUTINIZED FOR POSSIBLE INTERFERENCE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR HAS FLOWN THE RWY 14R ILS MANY TIMES. THIS IS THE FIRST TIME HE HAS HAD A PROB WITH THE GS SIGNAL. THE WX WAS DAY VMC INSURING THAT THE PROPER APCH PROFILE WAS FLOWN. THE ACFT DID NOT HAVE ANY GS FLAGS, AND BECAUSE OF RADIO ALTIMETER BASED GS ATTENUATION, THE FLT DIRECTORS DID NOT SHOW A FLY DOWN INDICATION. THE PLT FURTHER STATED THAT SINCE AN ACR ACCIDENT AT THIS RWY SEVERAL YRS AGO, PLTS AT HIS COMPANY HAVE BEEN SENSITIVE TO THIS TYPE OF PROB. THE PLT RPTS THAT BOEING THINKS THAT SOME TYPE OF RF INTERFERENCE MAY BE COMING FROM A NEW TYPE OF RAILROAD LOCOMOTIVE THAT HAS AN AC TO DC TO AC PWR CONVERSION. THE ANALYST ALSO CONTACTED THE QUALITY ASSURANCE MGR AT ORD TWR. THERE HAVE BEEN 17 PROBS RPTED WITH THE RWY 14R GS. THERE HAVE BEEN 13 FLT CHKS, ALL OF WHICH WERE NORMAL. ORD HAS REPLACED THE ENTIRE RWY 14R ILS SYS -- ALL ELECTRONICS, CABLE, AND ANTENNAS -- YET THE PROB SEEMS TO CONTINUE.

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.