Air carrier captain experiences loss of GPS signal on three separate occasions during approach to MM...

Narrative:

This event has happened to me on my last 3 trips to MMMX in almost exactly the same location while executing the ILS DME runway 5R with the smo transition. The FMS went into dr mode about half way from smo to starting the turn to the northeast to intercept the localizer. The map still shows on the proper track; and there is little time to set up raw data to track the smo 160 radial and set up the lead in radial. After a short time in dr the FMA shows heading hld and the map shifts to show the aircraft right of course. An immediate left turn is required to not overshoot the localizer. This is especially dangerous due to the fact that an overshoot of the localizer put the aircraft heading directly towards very high terrain. Undetected this navigation failure and failure to intervene with heading select and rearming the ILS mode could lead to a serious GPWS/terrain event. No prior dr mode annunciations were present during any of these flights until this point. The flights where this event occurred were (all different tail numbers) all in april.

Original NASA ASRS Text

Title: Air Carrier Captain experiences loss of GPS signal on three separate occasions during approach to MMMX between SMO and the turn to intercept the Runway 5R localizer.

Narrative: This event has happened to me on my last 3 trips to MMMX in almost exactly the same location while executing the ILS DME Runway 5R with the SMO transition. The FMS went into DR mode about half way from SMO to starting the turn to the northeast to intercept the localizer. The MAP still shows on the proper track; and there is little time to set up raw data to track the SMO 160 radial and set up the lead in radial. After a short time in DR the FMA shows heading HLD and the map shifts to show the aircraft right of course. An immediate left turn is required to not overshoot the localizer. This is especially dangerous due to the fact that an overshoot of the localizer put the aircraft heading directly towards very high terrain. Undetected this NAV failure and failure to intervene with heading select and rearming the ILS mode could lead to a serious GPWS/TERRAIN event. No prior DR mode annunciations were present during any of these flights until this point. The flights where this event occurred were (all different tail numbers) all in April.

Data retrieved from NASA's ASRS site 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.

Attributes
ACN	1172651
Time
Date	201405
Local Time Of Day	1801-2400
Place
Locale Reference	MMMX.Airport
State Reference	FO
Aircraft 1
Make Model Name	Medium Large Transport
Operating Under FAR Part	Part 121
Flight Phase	Initial Approach
Flight Plan	IFR
Component
Aircraft Component	GPS & Other Satellite Navigation
Person 1
Function	Pilot Flying Captain
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Procedural Published Material / Policy Deviation - Track / Heading All Types

Air carrier captain experiences loss of GPS signal on three separate occasions during approach to MMMX between smo and the turn to intercept the runway 5R localizer.