A319
Flight Crew Air Transport Pilot (ATP)
Confusion; Workload; Situational Awareness; Communication Breakdown
ATC Issue All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT; Inflight Event / Encounter Unstabilized Approach
Flight Crew Became Reoriented
Human Factors; Procedure
On ILS approach to Mexico City we were given headings to fly for a minute; then a turn to intercept the course. We asked for an altitude 3 times before we were given one; the first couple times just getting headings and airspeeds while being vectored on and off the approach. When I selected the altitude for about the third time while the first officer was calling the Tower; I accidentally put in 8000 ft instead of 8800 ft. The FO told me I was 300 ft high. I looked out the window as we were rolling onto the localizer and noticed the PAPI had 4 red lights. Since I was VFR and had the runway in sight; I turned off the autopilot and the autothrottles and stopped the descent and climbed to the proper PAPI glidepath.  We got a warning at about the same time as I joined the proper glidepath; and I continued the approach visually to landing. With a good visual on the runway and on the glidepath and stable I determined we were in a safe position to land. We briefed the arrival and approach and reviewed the airport info for Mexico City several times and were well prepared for the arrival and approach. We were configured and at speed and on profile early and all was going well when ATC gave us a heading to fly for a minute then a turn to intercept without an altitude. Since we were being vectored off the course on the chart we asked for an altitude which took several times to get from ATC. If we were left on the arrival and approach as cleared I'm sure it would have gone as planned. The non-standard surprise clearances we received took us out of the green and into the yellow. Since the FO was on the radio rather than calling for the altitude to be set by him I set it myself.  I set it wrong and neither of us caught it with all the changes we kept getting at a critical time of flight. Both the FO and I have learned from this event.  In the future I will make sure that I get the entire clearance including altitude before turning to a heading while descending on an approach. I also will call for altitudes at critical times and verify them with my FO before communicating with ATC. Aviate; navigate; communicate.
A319 Captain reported several ATC clearance changes on approach to MMMX led to descending below the glideslope after incorrectly setting the altitude on the autopilot.