Air carrier flight crew reported they were on a visual approach and descended below terrain minimums. They received a low altitude alert from the Tower.
Synopsis
Air carrier flight crew reported they were on a visual approach and descended below terrain minimums. They received a low altitude alert from the Tower.
Narrative
I was the Pilot flying and this was my first time into ZZZ. We were cleared direct to ZZZZZ for the GPS XXC approach; but then when on with Tower we were cleared for the visual. Weather was VFR and wind was down the runway. We were to cross ZZZZZ1 at 3000 ft. and once captured I selected 2300 ft. We descended down to 2300 ft. by ZZZZZ1. I rolled the altitude up to 2700 ft. I saw our VNAV vertical guidance 'snow flake' start to go below us and selected VS and started a descent down. Then noticed we were not even to ZZZZZ2 yet and outside we were low. I disconnected the autopilot and shared my mental model that I had made a mistake and we were low and started to adjust the altitude back up to 2300 ft. to intercept a glide path. The tower gave us a Low Altitude Alert right at this time. We were at flaps 20 and about 180 knots; was slowing to VREF straight in speed to continue to get configured. The snow flake adjusted around this time and bounced back up above us. Cause: Poor airmanship. We were on a visual approach; but I was using guidance from the approach to help with my situational awareness. Acted without verifying the position of the aircraft and in managing the correct flight path. The 4.6 miles at 2300 ft. between two fixes is also not something you see very often. The snow flake was moving below us though and it was a muscle memory V/S and role the wheel down. Suggestions: Cross check the situation; as soon as I hit V/S; I knew it was wrong; but hesitated because of what I was seeing with the snow flake. Needed to not have pushed the V/S; unit verifying our position. I also immediately went to setting the altitude for a missed approach which cost me a few moments of time as well.
Second reporter narrative
On approach to Runway XXC in VMC conditions we were at 2;300 ft; Pilot flying set 2500 ft. as the go around altitude. Crossing ZZZZZ1 the VNAV snowflake indicated we were high and since we were on a visual started to descend to follow it. When we got to 1900 ft the gear horn sounded and the autopilot was disconnected and we initiated a climb. ATC then advised us to check altitude at which point we returned to 2300 ft. The rest of the approach and landing was uneventful. There were several factors that contributed to the confusion of when we could descend. the first being the indication from the VNAV snowflake saying we were high. The second was a lack of ability to see the PAPI; the PAPI was very faint and we could not see it properly until a short final. The third factor was fatigue on my part; having woke up at 4am; fatigue contributed to my ability to catch we were getting too low. Even though the approach was conducted in VMC conditions; we still should follow the approach profile and altitudes; this will ensure that the aircraft is at a proper altitude for the distance away it is. Also briefing threats such as fatigue can help make both pilots aware of each others cognitive states. Briefing the approach profile can also be beneficial to make ourselves aware of different altitudes and distances from the runway
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.