Date: 201712
Local time of day: 1201-1800
Location: MMMX.Airport, FO
Light: Dusk
Aircraft: Large Transport
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
ATC / advisory: Center MMFR
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness
Second reporter function: Pilot Not Flying; First Officer
Anomaly: ATC Issue All Types; Inflight Event / Encounter CFTT / CFIT
Detector: Automation Aircraft Terrain Warning; Person Flight Crew
When detected: In-flight
Result: Flight Crew Became Reoriented; Flight Crew Took Evasive Action
Contributing factors: Human Factors; Procedure
Primary problem: Human Factors

Narrative: On approach to Mexico City; received a terrain warning as we maneuvered for ILS 23L. We had planned an ILS 05R and after 3 off course vectors to intercept various radials inbound to SLM; we were then given a vector to intercept 053 radial to MEX with a 120 degree intercept angle. We were then cleared to intercept 053 radial MEX with a 90 degree intercept as we came close to radial and descend to 12000 ft. We were within 30 DME of MEX; assigned 200 kts. We were then cleared to 'complete the approach'. I elected to start descent to 10200 ft for a crossing alt at 10 DME. As I maneuvered slight right of course to the north to remain clear of the terrain; we received a terrain warning. We landed uneventful on 23L. We were VMC and I saw the terrain. I disconnected the autopilot and stopped descent until we were past terrain. I did not note high terrain on [the] plate for an approach to 23L. I thought I was inside the terrain area based on the approach plate for ILS 23L 2 DME.I should have delayed the start of my descent until I was past the terrain.


Second narrative: Planned for and set up for ILS 05R MEX. While on arrival they switched to RWY 23L and from that point we received three separate vectors to intercept radials to the MEX for and track inbound. We were finally told to intercept the 053 Radial MEX and track inbound for runway 23L @ 120 degree intercept angle. Subsequent vectors alternated between intercepting the 053 VOR radial and the 23L localizer. Once queried; they gave us a 90 degree intercept heading to capture the inbound course at 12;000 ft. I asked if we were cleared for the approach and we were told to 'complete the approach'. At that point the Captain put 10200 ft on the MCP as the altitude for the next 10 mile fix on the ILS 23L. VMC prevailed throughout the approach. Sometime after leaving 12000 ft we received a terrain caution. We could see terrain ahead. PF began to maneuver aircraft north and parallel to course to avoid visible terrain when we received warning. PF stopped descent; began climb adjacent to visible terrain; climbed back up and continued approach. We did not note high terrain on the plate due to multiple confusing nonstandard vectors. We were using the information on the ILS 23L 2 DME plate.Delay start of descent in accordance with all necessary data available and to clarify ATC instructions that appear nonstandard even for international operations.


Synopsis: Air Carrier flight crew reported receiving non-standard vectors and a GPWS terrain warning on an approach to MMMX.