Narrative:

I would like to bring to your attention a very serious safety concern that I have regarding flying into mexico city and a subsequent erroneous ILS signal for runway 5R. Since this is not the first time that I have noted peculiarities flying into mexico city on this same approach, I find it necessary to report the following incident. On sep/xx/95 I was the first officer operating and the PF, from houston to mexico city. The WX was reported clear, with 6 mi visibility. The aircraft was a B737-500. Upon arrival into the mexico city area, our flight was cleared for the mateo 5 arrival and ILS approach for runway 5R. Upon reaching the mateo VOR I switched from LNAV to manual VOR and tracked south of mateo on the 160 degree radial. Upon reaching the 9.2 DME from mateo, I switched over to the imwx localizer frequency 109.1 and 052 degree course inbound. I immediately had an indication of intercepting the localizer and was within 1 DOT of showing on centerline with the runway. I completed the turn to the inbound course and started to descend on the GS. The captain noticed that the airport was approximately 1-2 mi off to the right of the aircraft. I pointed out the ILS indication that was erroneously showing that the aircraft was on course. I then corrected the course after visually seeing the runway and made a normal landing. At the point I selected and tuned the ILS frequency (9.2 DME south of mateo) I should have had a full scale deflection of the localizer, not an erroneous signal showing the aircraft to being close to centerline on the approach. There is no terrain between that point and the runway that would explain a possible weak or inaccurate signal. It is my opinion that the problem with the localizer signal, is ground based and has nothing to do with any of our aircraft. Someone needs to investigate and insure the reliability of this signal to avoid the very real possibility of a serious incident. Please feel free to contact me for any other information. Callback conversation with reporter revealed the following information: the reporter said he was flying the B737-500 with a complete glass cockpit, 2 IRS's, 2 FMC's, and 2 CDU's. (The older B737-300 and 400 with round dials had 1 FMC ported to the 2 CDU's). The reporter believes the problem may be a false localizer (like at toronto and brussels). Why he got the false localizer and others didn't he agreed was puzzling. He did say on a subsequent trip the controller told him he would call the turn on to the ILS, so, others may be having a problem. Being CAT ii ILS equipped, this airline B737 equipment may have an increased sensitivity to false localizer signals that older CAT I equipment does not have. He wishes an FAA ILS standards check were flown on this ILS at mexico city. He said he has flown the localizer benito juarez runway 05R ILS 30-40 times during his career. Possibly 5-8 of the recent flts have had unusual ILS indications. For example, one time he was approaching the ILS localizer coupled to the autoplt. The autoplt captured the localizer but there was no symbol generated localizer and no ILS aural identify until on centerline. The absence of the symbol generated localizer is the same as a localizer invalid flag. He said it is true if both navigation receivers are manually tuned in raw data for more than 12 mins the FMC will flash a warning 'IRS navigation only' but this did not happen. When questioned he responded there were no passenger electronic devices being operated in the cabin as the airline flight attendants are especially conscientious about that on this airline. The analyst suggested next time he might turn on his standby ILS which would be the only ILS localizer indication independent of the symbol generator other than the autoplt ILS coupler. He thought that would be a good idea.

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Original NASA ASRS Text

Title: FLC VISUALLY DETERMINES ILS GUIDANCE OF CTRLINE 1-2 MI.

Narrative: I WOULD LIKE TO BRING TO YOUR ATTN A VERY SERIOUS SAFETY CONCERN THAT I HAVE REGARDING FLYING INTO MEXICO CITY AND A SUBSEQUENT ERRONEOUS ILS SIGNAL FOR RWY 5R. SINCE THIS IS NOT THE FIRST TIME THAT I HAVE NOTED PECULIARITIES FLYING INTO MEXICO CITY ON THIS SAME APCH, I FIND IT NECESSARY TO RPT THE FOLLOWING INCIDENT. ON SEP/XX/95 I WAS THE FO OPERATING AND THE PF, FROM HOUSTON TO MEXICO CITY. THE WX WAS RPTED CLR, WITH 6 MI VISIBILITY. THE ACFT WAS A B737-500. UPON ARR INTO THE MEXICO CITY AREA, OUR FLT WAS CLRED FOR THE MATEO 5 ARR AND ILS APCH FOR RWY 5R. UPON REACHING THE MATEO VOR I SWITCHED FROM LNAV TO MANUAL VOR AND TRACKED S OF MATEO ON THE 160 DEG RADIAL. UPON REACHING THE 9.2 DME FROM MATEO, I SWITCHED OVER TO THE IMWX LOC FREQ 109.1 AND 052 DEG COURSE INBOUND. I IMMEDIATELY HAD AN INDICATION OF INTERCEPTING THE LOC AND WAS WITHIN 1 DOT OF SHOWING ON CTRLINE WITH THE RWY. I COMPLETED THE TURN TO THE INBOUND COURSE AND STARTED TO DSND ON THE GS. THE CAPT NOTICED THAT THE ARPT WAS APPROX 1-2 MI OFF TO THE R OF THE ACFT. I POINTED OUT THE ILS INDICATION THAT WAS ERRONEOUSLY SHOWING THAT THE ACFT WAS ON COURSE. I THEN CORRECTED THE COURSE AFTER VISUALLY SEEING THE RWY AND MADE A NORMAL LNDG. AT THE POINT I SELECTED AND TUNED THE ILS FREQ (9.2 DME S OF MATEO) I SHOULD HAVE HAD A FULL SCALE DEFLECTION OF THE LOC, NOT AN ERRONEOUS SIGNAL SHOWING THE ACFT TO BEING CLOSE TO CTRLINE ON THE APCH. THERE IS NO TERRAIN BTWN THAT POINT AND THE RWY THAT WOULD EXPLAIN A POSSIBLE WEAK OR INACCURATE SIGNAL. IT IS MY OPINION THAT THE PROB WITH THE LOC SIGNAL, IS GND BASED AND HAS NOTHING TO DO WITH ANY OF OUR ACFT. SOMEONE NEEDS TO INVESTIGATE AND INSURE THE RELIABILITY OF THIS SIGNAL TO AVOID THE VERY REAL POSSIBILITY OF A SERIOUS INCIDENT. PLEASE FEEL FREE TO CONTACT ME FOR ANY OTHER INFO. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR SAID HE WAS FLYING THE B737-500 WITH A COMPLETE GLASS COCKPIT, 2 IRS'S, 2 FMC'S, AND 2 CDU'S. (THE OLDER B737-300 AND 400 WITH ROUND DIALS HAD 1 FMC PORTED TO THE 2 CDU'S). THE RPTR BELIEVES THE PROB MAY BE A FALSE LOC (LIKE AT TORONTO AND BRUSSELS). WHY HE GOT THE FALSE LOC AND OTHERS DIDN'T HE AGREED WAS PUZZLING. HE DID SAY ON A SUBSEQUENT TRIP THE CTLR TOLD HIM HE WOULD CALL THE TURN ON TO THE ILS, SO, OTHERS MAY BE HAVING A PROB. BEING CAT II ILS EQUIPPED, THIS AIRLINE B737 EQUIP MAY HAVE AN INCREASED SENSITIVITY TO FALSE LOC SIGNALS THAT OLDER CAT I EQUIP DOES NOT HAVE. HE WISHES AN FAA ILS STANDARDS CHK WERE FLOWN ON THIS ILS AT MEXICO CITY. HE SAID HE HAS FLOWN THE LOC BENITO JUAREZ RWY 05R ILS 30-40 TIMES DURING HIS CAREER. POSSIBLY 5-8 OF THE RECENT FLTS HAVE HAD UNUSUAL ILS INDICATIONS. FOR EXAMPLE, ONE TIME HE WAS APCHING THE ILS LOC COUPLED TO THE AUTOPLT. THE AUTOPLT CAPTURED THE LOC BUT THERE WAS NO SYMBOL GENERATED LOC AND NO ILS AURAL IDENT UNTIL ON CTRLINE. THE ABSENCE OF THE SYMBOL GENERATED LOC IS THE SAME AS A LOC INVALID FLAG. HE SAID IT IS TRUE IF BOTH NAV RECEIVERS ARE MANUALLY TUNED IN RAW DATA FOR MORE THAN 12 MINS THE FMC WILL FLASH A WARNING 'IRS NAV ONLY' BUT THIS DID NOT HAPPEN. WHEN QUESTIONED HE RESPONDED THERE WERE NO PAX ELECTRONIC DEVICES BEING OPERATED IN THE CABIN AS THE AIRLINE FLT ATTENDANTS ARE ESPECIALLY CONSCIENTIOUS ABOUT THAT ON THIS AIRLINE. THE ANALYST SUGGESTED NEXT TIME HE MIGHT TURN ON HIS STANDBY ILS WHICH WOULD BE THE ONLY ILS LOC INDICATION INDEPENDENT OF THE SYMBOL GENERATOR OTHER THAN THE AUTOPLT ILS COUPLER. HE THOUGHT THAT WOULD BE A GOOD IDEA.

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.