Date: 201808
Local time of day: 1201-1800
Location: MMMX.Airport, FO
Flight conditions: VMC
Light: Daylight
Aircraft: B737 Next Generation Undifferentiated
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Flight phase: Descent
ATC / advisory: TRACON MMMX
Aircraft component: GPS & Other Satellite Navigation
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Other / Unknown
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Detector: Person Flight Crew; Person Observer
When detected: In-flight
Result: Flight Crew Overcame Equipment Problem
Contributing factors: Chart Or Publication; Procedure
Primary problem: Ambiguous

Narrative: I was sent to MMMX as an observer in the jumpseat during this event. During the arrival; GPS-L INVALID and GPS-R INVALID enunciated repeatedly in the vicinity of MAVEK. The working crew advised this is a usual event and took no action. It seemed odd that no notice to the crew; no message; nothing in the company pages with regards to MMMX spoke of this issue. Once I arrived at the hotel for the layover; I investigated actions required in the event of an invalid GPS signal. There is nothing in the QRH; but there is a procedure in the ops manual. It's a pretty lengthy procedure that; in the end; requires monitoring your position using raw data or monitoring the POS REF page of the CDU (Control Display Panel) for accuracy. I recall back about six years ago the approach we used connected the enroute; arrival and approach across the SMO VOR. That approach; now the ILS DME 1 05R; changed from tracking the 160R from SMO to tracking the 164R from SMO. The first time we flew that approach; we were faced with a NAV failure. The fleet immediately issued a notice advising crews of this issue and reminded crews of the procedural requirement to tune; identify and track ground-based NAVAIDS during all phases of flight. With this in place; we weren't surprised by the failure nor unable to cope with the loss of NAV. There appears to be no notice in place to advise crews of this anomaly during the approach. Nor does there appear to be a procedure in place to accommodate this failure other than to return to the ops manual and work through the FMC Navigation Check - a lengthy process during a pretty critical phase of flight when distractions need to be kept to a minimum. Apparently; when the change in ILS procedure was made to place arriving aircraft close to a military installation that routinely jams GPS signals; GPS/NAV failures onboard aircraft arriving into MMMX became common. I would recommend filing the AVSAR 4A and requesting the ILS/DME1 05R so that crews have ground-based NAVAIDS that can be used to back-up their position. As well; I would suggest a message to crews advising them that during the arrival into MMMX; ground-based NAVAIDS must be tuned; identified; used and tracked so that when the GPS becomes invalid; the flight can continue safely in this mountainous area. The other option would be to advise crews of this issue during the arrival and require that one pilot monitor the POS REF page of the CDU. According to our procedures; if the two IRS positions are in agreement and the FMC position is not significantly different; the FMC position is probably reliable. Although this procedure is published in the ops manual; it's hard to find and might not be at the tip of your fingers; so to speak; during this critical phase. A notice indicating this loss should be expected and the procedure available to counteract the failure; would be most helpful.

Callback: Reporter has recommended to the Company Ops. Dept. that all flights into Rwy 5 at MMMX be filed for an ILS/DME approach and that crews be reminded of the requirement in the general operating manual to have all preparations completed to be able to transition to a non-GPS approach when flying an RNAV approach.



Synopsis: B737 observer pilot reported experiencing intermittent GPS reception issues while observing the flight crew on approach to MMMX.