Date: 201706
Local time of day: 1801-2400
Location: MMMX.Airport, FO
Flight conditions: Mixed
Weather elements / visibility: Thunderstorm; Rain
Light: Night
Aircraft: A319
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
ATC / advisory: Center MMFR
Aircraft component: FMS/FMC
Component problem: Improperly Operated
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Workload; Situational Awareness
Anomaly: Deviation - Speed All Types; Inflight Event / Encounter Weather / Turbulence
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Flight Crew Returned To Clearance; Flight Crew Became Reoriented
Contributing factors: Weather; Procedure; Human Factors
Primary problem: Ambiguous

Narrative: On arrival to MMMX with the FO as Pilot Flying; I as CA was Pilot Monitoring. Night conditions with significant weather in the form of rain and thunderstorms scattered along our route and near the field. Arriving from the south we were given the MEX5A Arrival. PF had selected the ILS DME 1 Rwy 05R in the MCDU. CA had previously briefed the FO on the company pages and the likelihood that we would be cleared to MAVEK and the RNAV (GNSS) Or ILS DME 2 Rwy 05R procedure. While on the arrival we were given multiple altitude; heading; and speed changes; interspersed with directions to proceed direct to MAVEK; and clearance for the ILS DME 2 procedure. Both pilots became task saturated with these changes; weather avoidance; and procedures required to reprogram the MCDU and verify those changes. Shortly before the final approach turn PF allowed speed to reduce below the 160 KIAS given by ATC; to approximately 140 KIAS. PM did not notice because of task saturation. ATC inquired about the speed and PM acknowledged we had inadvertently slowed and would briefly speed back up. Task saturation; especially in my view since the PF had elected to disconnect the autopilot and hand fly the approach. PF had the wrong approach selected and waited too long to correct the MCDU. On making the change he neglected to select the 'No Star' to prevent the discontinuity between MAVEK and the next waypoint. Upon checking the points I noticed the discontinuity and reselected the approach with the No Star to correct this. Unfortunately that caused the 'approach' button to disarm; which initially went unnoticed by both pilots. It was at this point that the PF elected to disconnect the autopilot; thinking he could better control the situation. I feel he should have left it engaged and used the automation to relieve the workload. Nearly descending through an altitude; almost missing a turn on course; erratic control of final approach course; and the inadvertent 20 KIAS speed loss during this event bear that out; in my opinion. We briefed this procedure very early; covering all applicable pages and eventualities. However; although we had not received the eventual clearance I should have insisted the PF set up the likely procedure much earlier. This would have prevented the scramble to catch up in an extremely task saturated environment.




Synopsis: A319 Captain reported difficulties managing the automation during an arrival and approach to MMMX due to ATC changes and workload.