Air carrier flight crew reported descended below the published minimums during approach resulting in a low altitude alert from ATC. Flight crew corrected and continued to landing.
Synopsis
Air carrier flight crew reported descended below the published minimums during approach resulting in a low altitude alert from ATC. Flight crew corrected and continued to landing.
Narrative
Very busy approach with numerous threats. New CA training in left seat; Line Check Pilot (LCP) in right seat; F40 landing (lots of discussion about not forgetting to go to F40); close in traffic with TCAS TAs; multiple close in step downs from ATC resulting in constant manipulation of Mode Control Panel (MCP); long duty day; long trip with 4 reassignments; cumulative sleep deficit (numerous discussions by both pilots about how exhausted and 'brain fried' we were); shorter than planned and expected layovers due to delays and reassignments.At about 700' on visual approach to XXR; tower issued a low altitude alert for our flight. We both noticed that we were below our path; Trainee in left seat immediately turned off automation and adjusted flight path. We selected appropriate modes and completed the flight with a safe and successful approach and landing.During the thorough debrief; we found the reason that we ended up in that situation was the failure to remain in VNAV which was due to multiple close in step downs; leaving us high; and the subsequent failure of the PM to effectively monitor the FMA due to task saturation. Both pilots were affected by multiple long duty days with short layovers; delays; reassignments; poor sleep; and failed to acknowledge the cumulative fatigue and should have not operated.
Second reporter narrative
It had been a long 4 day trip with multiple reassignments and long days. We had an early 1.5 hour deadhead followed by a 3 hour sit; then the 5 hour 7 minute flight ZZZ1 to ZZZ. For the majority of the flight we had trained and discussed topics since it was IOE which was very beneficial and occasionally dealt with turbulence. We both had acknowledged our fatigue enroute from the long trip and multiple changes. However; we thoroughly briefed the entire approach; arrival and all threats. We were on the visual approach XXR; it was dusk; there was a 757 off our left to maintain visual separation; a strong right crosswind and multiple altitude changes from ATC. We were on the approach and forgot to arm VNAV and got a low altitude alert from ZZZ tower. We kicked off automation and hand flew; reengaged VNAV and successfully landed.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.