Commercial Fixed Wing
Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Flight Crew Last 90 Days 199; Flight Crew Total 3444; Flight Crew Type 772
Human-Machine Interface; Workload
Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Flight Crew Overcame Equipment Problem; Flight Crew Returned To Clearance
Human Factors
While approximately 2 hours from our destination; we discovered that ZZZZ1 weather would not allow us to land at destination. We made a decision with fuel and time to hold and then subsequently divert to ZZZZ2. The communication barrier was difficult with the controllers and while that took place we were also using the radio for Dispatch. I was the PM (Pilot Monitoring) and was coordinating the divert with Dispatch and the handling in ZZZZ2. Task saturation after a long flight was certainly a challenge. I also was having freezing issues with Jepp FD pro. After being handed to ATC; we had the approach and plan in place for the approach Runway XX. On the decent the controllers left us very high even after multiple requests for lower. The PF (Pilot Flying) was using FLCH and max speed brakes most of the time. We were able to get to an appropriate altitude safely. I had a brief moment of thought about still being in FLCH on decent and through saturation; I failed to mention it. Inside the IAF I set the TDZE as normal and confirmed with crew; as we were cleared the approach. We had still been descending constantly from events before; we never really stopped descending since just after the divert was made. As we were in the arc; we were completely configured for landing and I noticed that we had descended below the published altitude on the ARC. I quickly realized we had been still in FLCH and immediately said to stop decent. We were approximately 400-500 ft. Low. And after arresting settled at about 600 below. The PF and crew were in VMC conditions. CAVOK. We were approximately 1/2 to 3/4 of the way through the arc once I noticed. We had the airport in sight and the PF elected to stay at the approximate pattern altitude as we were over water in VMC and had the airport in sight and about to turn final. The approach continued visually and the PF made a normal landing and taxi to hard stand.
Air carrier First Officer reported an altitude deviation during a weather diversion uneventful landing during a task saturated weather diversion which included an altitude excursion.