B737-500 aircraft descending on STAR and FMC set for ILS intercept to runway 5R (MMMX), apparent map shift from VOR or localizer resulted in aircraft turning inbound on the ILS north of actual on course. Flcs must manually correct for the approach.

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Attributes
ACN	384361
Time
Date	199710
Day	Fri
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : mex
State Reference	FO
Altitude	agl bound lower : 1500 agl bound upper : 1500
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Operator	common carrier : air carrier
Make Model Name	B737-500
Operating Under FAR Part	Part 121
Navigation In Use	Other Other
Flight Phase	descent : approach descent other
Route In Use	arrival other arrival star : star
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 240 flight time total : 21000 flight time type : 10000
ASRS Report	384361
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial
Events
Anomaly	aircraft equipment problem : less severe other anomaly other other spatial deviation
Independent Detector	other flight crewa
Resolutory Action	flight crew : overcame equipment problem flight crew : returned to intended course or assigned course
Consequence	Other
Supplementary
Primary Problem	Navigational Facility
Air Traffic Incident	other
Situations
Navigational Aid	Unspecified

Narrative:

On numerous occasions flying the smo arrival to the ILS runway 5R at mexico city (MMMX) over the past 1-2 months, the flight guidance/autoplt is capturing the localizer approximately 6 mi from the runway end and turning to/or commanding a course of about 052 degrees (runway course) heading even though the actual aircraft position is approximately 1/4 mi north (and paralleling) the final approach course. It usually follows this path for approximately 1 mi or more and sometimes starts on its own to correct by turning to intercept the proper inbound course. (It might have corrected more often had I not intervened.) in all cases it would not have been on the proper final approach course until after passing the LOM (mw). In addition, rarely does the LOM (mw) point properly after station passage. I am constantly amazed that there has not been a major problem with this facility to date. Numerous other pilots have seen this phenomenon as well.

Original NASA ASRS Text

Title: B737-500 ACFT DSNDING ON STAR AND FMC SET FOR ILS INTERCEPT TO RWY 5R (MMMX), APPARENT MAP SHIFT FROM VOR OR LOC RESULTED IN ACFT TURNING INBOUND ON THE ILS N OF ACTUAL ON COURSE. FLCS MUST MANUALLY CORRECT FOR THE APCH.

Narrative: ON NUMEROUS OCCASIONS FLYING THE SMO ARR TO THE ILS RWY 5R AT MEXICO CITY (MMMX) OVER THE PAST 1-2 MONTHS, THE FLT GUIDANCE/AUTOPLT IS CAPTURING THE LOC APPROX 6 MI FROM THE RWY END AND TURNING TO/OR COMMANDING A COURSE OF ABOUT 052 DEGS (RWY COURSE) HDG EVEN THOUGH THE ACTUAL ACFT POS IS APPROX 1/4 MI N (AND PARALLELING) THE FINAL APCH COURSE. IT USUALLY FOLLOWS THIS PATH FOR APPROX 1 MI OR MORE AND SOMETIMES STARTS ON ITS OWN TO CORRECT BY TURNING TO INTERCEPT THE PROPER INBOUND COURSE. (IT MIGHT HAVE CORRECTED MORE OFTEN HAD I NOT INTERVENED.) IN ALL CASES IT WOULD NOT HAVE BEEN ON THE PROPER FINAL APCH COURSE UNTIL AFTER PASSING THE LOM (MW). IN ADDITION, RARELY DOES THE LOM (MW) POINT PROPERLY AFTER STATION PASSAGE. I AM CONSTANTLY AMAZED THAT THERE HAS NOT BEEN A MAJOR PROB WITH THIS FACILITY TO DATE. NUMEROUS OTHER PLTS HAVE SEEN THIS PHENOMENON AS WELL.

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.