B737 captain reports descending below the arrival path to MMMX ILS 5R after being vectored by ATC an...

Narrative:

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.

Original NASA ASRS Text

Title: 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.

Narrative: 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.

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	1180820
Time
Date	201406
Local Time Of Day	0601-1200
Place
Locale Reference	MMMX.Airport
State Reference	FO
Environment
Flight Conditions	VMC
Aircraft 1
Make Model Name	B737 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Final Approach
Component
Aircraft Component	GPS & Other Satellite Navigation
Person 1
Function	Captain Pilot Flying
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance Deviation - Procedural Published Material / Policy

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.