Narrative:

Runway 4R localizer deflections experienced on final approach. WX VMC and cleared for visual approach. However, if it had been IMC inside the FAF, a go around due to localizer deflections might have been required. Callback conversation with reporter revealed the following information: reporter concerned with what he perceives as constant and erratic localizer deflections for most ILS's conducted at ord. Reporter recognizes MD80S 'stray electron' anomaly. But he also has problem with the way ATC handles these types of complaints -- localizer deflections. Even though the pilot reports the problem, pilot observes that if visual approach criteria exists, TRACON still has the pilot intercept the localizer until traffic or airport is in sight. With the traffic/airport in sight, all things 'go away,' for the aircraft can proceed visually to the airport. But the pilot is concerned with that period of time when he has to depend on the localizer for course guidance (company procedures) and begins to see the deflections occur. Reporter is concerned if he 'drifts' into protected airspace of another aircraft and is violated. The pilot must weigh the probabilities in smoothing out the deflections, ask ATC for clarification for another clearance or execute a go around. Reporter 'knows' that when visual criteria exists, tower no longer protects the ILS critical area and aircraft taxi through that area. Reporter believes this may have something to do with the problem. The reporter also understands the 'movement of commerce,' and everyone must 'make do.' reporter wonders if there can be enough time to make a decision, considering all the factors, and not get into a 'violation.' on a second callback conversation, ATCT specialist advises that the positioning of the runway 4R localizer was approximately 120 ft closer to the runway than other runways due to the highway. The specialist advised that when widebody transport aircraft exit runway 4R at the end, they turn more or less perpendicular to the localizer as they taxi onto taxiway Q and D. Their size and the physical proximity to the localizer is known to cause a momentary interrupt in the localizer signal. Also, this information is allegedly disseminated to the users. The specialist also advised that due to this anomaly, some airlines have issued company bulletins not authorizing coupled approachs to runway 4R.

Google
 

Original NASA ASRS Text

Title: ACR MD80S FLC ENCOUNTER LOC FLUCTUATIONS WHILE CONDUCTING AN ORD ILS RWY 4R APCH. DEFLECTIONS ENCOUNTERED SOON AFTER INTERCEPTING LOC. VISUAL APCH ASSIGNED WHEN ARPT OR TFC WAS SIGHTED.

Narrative: RWY 4R LOC DEFLECTIONS EXPERIENCED ON FINAL APCH. WX VMC AND CLRED FOR VISUAL APCH. HOWEVER, IF IT HAD BEEN IMC INSIDE THE FAF, A GAR DUE TO LOC DEFLECTIONS MIGHT HAVE BEEN REQUIRED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR CONCERNED WITH WHAT HE PERCEIVES AS CONSTANT AND ERRATIC LOC DEFLECTIONS FOR MOST ILS'S CONDUCTED AT ORD. RPTR RECOGNIZES MD80S 'STRAY ELECTRON' ANOMALY. BUT HE ALSO HAS PROB WITH THE WAY ATC HANDLES THESE TYPES OF COMPLAINTS -- LOC DEFLECTIONS. EVEN THOUGH THE PLT RPTS THE PROB, PLT OBSERVES THAT IF VISUAL APCH CRITERIA EXISTS, TRACON STILL HAS THE PLT INTERCEPT THE LOC UNTIL TFC OR ARPT IS IN SIGHT. WITH THE TFC/ARPT IN SIGHT, ALL THINGS 'GO AWAY,' FOR THE ACFT CAN PROCEED VISUALLY TO THE ARPT. BUT THE PLT IS CONCERNED WITH THAT PERIOD OF TIME WHEN HE HAS TO DEPEND ON THE LOC FOR COURSE GUIDANCE (COMPANY PROCS) AND BEGINS TO SEE THE DEFLECTIONS OCCUR. RPTR IS CONCERNED IF HE 'DRIFTS' INTO PROTECTED AIRSPACE OF ANOTHER ACFT AND IS VIOLATED. THE PLT MUST WEIGH THE PROBABILITIES IN SMOOTHING OUT THE DEFLECTIONS, ASK ATC FOR CLARIFICATION FOR ANOTHER CLRNC OR EXECUTE A GAR. RPTR 'KNOWS' THAT WHEN VISUAL CRITERIA EXISTS, TWR NO LONGER PROTECTS THE ILS CRITICAL AREA AND ACFT TAXI THROUGH THAT AREA. RPTR BELIEVES THIS MAY HAVE SOMETHING TO DO WITH THE PROB. THE RPTR ALSO UNDERSTANDS THE 'MOVEMENT OF COMMERCE,' AND EVERYONE MUST 'MAKE DO.' RPTR WONDERS IF THERE CAN BE ENOUGH TIME TO MAKE A DECISION, CONSIDERING ALL THE FACTORS, AND NOT GET INTO A 'VIOLATION.' ON A SECOND CALLBACK CONVERSATION, ATCT SPECIALIST ADVISES THAT THE POSITIONING OF THE RWY 4R LOC WAS APPROX 120 FT CLOSER TO THE RWY THAN OTHER RWYS DUE TO THE HWY. THE SPECIALIST ADVISED THAT WHEN WDB ACFT EXIT RWY 4R AT THE END, THEY TURN MORE OR LESS PERPENDICULAR TO THE LOC AS THEY TAXI ONTO TXWY Q AND D. THEIR SIZE AND THE PHYSICAL PROX TO THE LOC IS KNOWN TO CAUSE A MOMENTARY INTERRUPT IN THE LOC SIGNAL. ALSO, THIS INFO IS ALLEGEDLY DISSEMINATED TO THE USERS. THE SPECIALIST ALSO ADVISED THAT DUE TO THIS ANOMALY, SOME AIRLINES HAVE ISSUED COMPANY BULLETINS NOT AUTHORIZING COUPLED APCHS TO RWY 4R.

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.