B747 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Communication Breakdown; Situational Awareness
ATC Issue All Types; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued New Clearance; Flight Crew Returned To Clearance
Human Factors
Approach ILS RWY 05R MEX Flight conditions night. Approaching position MAVEK aircraft was configured at flaps 20 degrees and crossing MAVEK configured gear down and flaps 25 (planning for 30 degrees) for descent from 12;000 to the next altitude of 8;800 feet for the interception of final approach to runway 05R. Upon crossing MAVEK we thought we were given a heading of 250 to fly due to closeness behind traffic ahead. I selected HDG Select and made the turn thinking this was a small widening vector for a subsequent turn to intercept final; but was concerned for terrain. Controller promptly asked what we were doing and we clarified what we thought was our assigned heading. He then gave us a right turn to 360 and a climb to 14;000 feet and then 15;000 feet. We cleaned up the aircraft to flaps 10 and re-inserted MAVEK into the FMC for a subsequent descent to 12;000 and the arrival for runway 05R. The arrival was subsequently flown with no further event.  Listen and confirm all instructions; Controller was very busy.
[Report narrative contained no additional information]
A B747 flight crew reported turning toward terrain on a night approach to MMMX when they apparently misunderstood an ATC clearance.