B737 Undifferentiated or Other Model
Distraction; Human-Machine Interface; Situational Awareness
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Aircraft; Human Factors; Procedure
While on [Runway] 5R ILS approach to Mexico City; approximately 10 miles prior to SMO we were given a heading for spacing. [We were] then re-cleared; direct SMO and [Runway] 5R approach. Approaching SMO we dialed 8;800 in the MCP window. Just after crossing the VOR we got a GPS invalid message. I checked ANP [Actual Navigation Performance] not to exceed [Required Navigation Performance] and continued the approach. We also were able to maintain VMC conditions to the airport. I noticed we were below VNAV path. I disengaged the autopilot and climbed back to the VNAV path. I believe our deviation to be no more than a couple hundred feet. Our altitude deviation was caused by not verifying the re engagement of VNAV after our vector from ATC. The GPS invalid message only served to compound the problem by being another distraction.
B737 Captain reports descending below the arrival path to MMMX ILS 5R after being vectored by ATC and forgetting to reengage VNAV. A GPS invalid message caused a distraction at a bad time.