Air carrier crew reported an altitude deviation while on initial approach resulting in descent below the published altitude. The First Officer recognized the low altitude and the Captain initiated a climb and continued to a safe landing.
Synopsis
Air carrier crew reported an altitude deviation while on initial approach resulting in descent below the published altitude. The First Officer recognized the low altitude and the Captain initiated a climb and continued to a safe landing.
Narrative
During descent into MMTL around 15;000 ft. MSL; cleared to 10;000 ft. MSL; the Captain (CA) Pilot Flying (PF) was concerned about landing weight; so they had the aircraft configured; slowed; with gear and speed brakes extended. Vert Speed was selected to shallow the descent to get configured. We were subsequently given direct OTUSO; cleared for the RNP Runway 12. The CA changed the altitude window to approach minimums (600) but did not reengage VNAV. I was distracted with the cabin PA; before landing checks; and frequency changes; and once I looked up from those tasks; I noticed out the window that we were lower than I mentally expected. I commented; we seem low" and looked inside at the PFD. I saw we were below the next fix altitude (OTUSO at 4;000; 15ish miles away) and descending. I said "we need to climb;" and the CA initiated a climb back to 4;000. We landed uneventfully and debriefed. Cause: Channelized attention on the overweight concern; and departure from normal habit patterns to cope with burning weight led to the excursion. Task saturation on my part led me to miss the altitude window change/non-VNAV and delayed noticing the excursion. Additionally; this was the first trip to MMTL for both of us; and communication was challenging; increasing perceived task load. Suggestions: Task prioritization and better crew communication when doing non-standard things."
Second reporter narrative
During cruise it became obvious that we were going to arrive above max landing weight. Well prior to descent I chose to turn on the APU to start reducing landing weight. During descent we were told to plan for the RNP to Runway 12. We were subsequently cleared the approach. I set up the LNAV/VNAV for the approach and set the MDA for the approach. Still concerned about the landing weight; I selected Vert Speed and shallowed the approach to burn more fuel. Concerned about the weight I allowed the aircraft to descend below the altitude restriction at OTUSO. The First Officer (FO) stated that we were low. I immediately added power and climbed back to 4000 feet and continued the approach and landed without incident. Cause: Distraction due to concern about fuel/landing weight.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.