B747 flight crew reported descending below a charted altitude on initial approach. The flight crew immediately climbed back up to the correct altitude and continued the approach.
Synopsis
B747 flight crew reported descending below a charted altitude on initial approach. The flight crew immediately climbed back up to the correct altitude and continued the approach.
Narrative
On arrival into ZZZZ; ATC cleared us to 3000 ft. and then direct to ZZZZZ and then shortly thereafter cleared us for the ILS Y XXR approach. The First Officer was pilot flying and was utilizing FLCH (Flight Level Change) in an effort to get down as ATC had held us up high until fairly late as they took us off the STAR and put us on radar vectors. As we were simultaneously approaching ZZZZZ and 3000 ft. I noticed what I thought was the FO (First Officer) reaching up to re-engage VNAV and I looked away to check the chart to try to verify the speed control limits for the upcoming segments. When I looked back at the instruments I realized that we were still descending in FLCH and had descended approximately 1000 ft. below the charted 3000 ft. altitude. I immediately told the pilot flying to stop descent and climb back to 3000 ft.; which he did with very little delay. The rest of the approach and landing was uneventful. We were over water at the time of event so terrain was no factor and no other traffic was conflicted.As pilot monitoring I allowed myself to assume the pilot flying was re-engaging VNAV without taking the time to positively verify it as I turned my attention to the approach plate to check charted speed limits. The pilot flying mistakenly remained in FLCH while also setting the next lower altitude of 1600 ft. for the FAF / glideslope intercept - which took away the 3000-ft.altitude protection. The other pilot monitoring in the Observer's seat also did not catch the mistake. As Captain and pilot monitoring I should have instructed the pilot flying to return to VNAV earlier as we were nearing our VNAV flight path profile and prior to reaching 3000 ft. I also should not have assumed the pilot flying was re-engaging VNAV button without taking the time to positively verify it prior to looking away to check the chart. Reminding the pilot flying about being cautious about use of FLCH as we neared ZZZZZ probably would have kept this from happening. Possibly delaying setting the final charted altitude until safely confirming being at the proper altitude and back into VNAV to help guard against this sort of FLCH trap - loss of altitude segment protection.
Second reporter narrative
On the arrival into ZZZZ we were on radar vectors with me flying. The controller cut our arrival short and we were told to delete altitude restrictions. Soon after we were told to expedite descent to 3000 ft. and given clearance direct to ZZZZZ. I selected level change and 3000 ft. Shortly after the aircraft ahead of us was told to descend to 1600 ft. and cleared for the approach. Shortly after that we were cleared for the approach ourselves without an altitude given. I selected the final approach fix altitude of 1600 ft. but did not select VNAV. The Captain verified 1600 ft. was selected and we in turn descended through the 3000 ft. altitude restriction at ZZZZZ. After noticing I selected 3000 again and selected level change; we climbed back up to the altitude restriction. The controller never mentioned it and we continued the approach without event.Don't get complacent when switching out of VNAV; and don't let yourself expect the same clearance that another aircraft ahead of you got.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.