Air carrier B737 flight crew reported they descended to an unsafe altitude; resulting in ATC issuing an alert.
Synopsis
Air carrier B737 flight crew reported they descended to an unsafe altitude; resulting in ATC issuing an alert.
Narrative
Set up for RNAV arrival into DEN Runway 17R. Briefed contingencies if given the visual approach. When visual approach was given; I extended JNKBX; as previously briefed. We were to treat established inbound on this extension as on the approach." As we were established on this segment; the PF (Pilot Flying) called to set touchdown elevation; which I did. I then went heads down and looked up the Tower frequency and entered the Tower frequency into the radio number 1 standby window. At this point; I looked outside and then cross checked the instruments and saw we were descending through 6;500 ft. I said; "Why are we so low?" and told the F/O (First Officer) (PF) to climb to 7;000 [ft]. At this point; the Approach Controller called with an altitude warning. I replied that were were climbing to 7;000 and we resumed visually the rest of the way to touchdown without incident. The PF had selected LEVEL CHANGE without announcing it (and followed the flight directors blindly) and left the protections of VNAV. A lengthy debrief followed on the proper sequence of events; the protections provided and discarded by his actions and the reasons for following the established SOP."
Second reporter narrative
Remained in LEVEL CHANGE during visual approach backed up by RNAV [Runway] 17R set in FMS. ATC cleared us for visual prior to JOULE (FAF) Runway 17R. FO (First Officer) remained in LEVEL CHANGE once approaching 7000 [ft.] prior to JOULE (1500 [ft.] AGL) and requested touchzone elevation set. Aircraft continued descent below 7000 due to being in LEVEL CHANGE where ATC advised of low altitude alert. FO disconnected autopilot climb to 7000 and acquired normal visual flight path for visual landing.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.