Air carrier flight crew reported a GPWS warning while on approach due to improper verification of the automation mode. The crew leveled the aircraft; captured the glide slope; then landed safely.
Synopsis
Air carrier flight crew reported a GPWS warning while on approach due to improper verification of the automation mode. The crew leveled the aircraft; captured the glide slope; then landed safely.
Narrative
While on a westward right base to final to a visual backed up with ILS XXR VMC (planned on using LNAV AND VNAV to intercept final before switching to VOR/LOC and Approach Mode) approach into ZZZ; I Pilot Flying (PF) failed to ensure VNAV mode was engaged resulting in decent below FAF altitude as the aircraft was inadvertently in level change. The resulting decent below 2;900 ft. MSL outside of ZZZZZ FAF resulted in a terrain caution (Towers beyond - and west - of the final course) as the aircraft was about to turn to final. Upon receiving the caution; the PF canceled all automation and hand flew the aircraft from approximately 1;300 ft. AGL to 1;500 ft. AGL and continued a right turn to the final course to intercept the glideslope. ATC did not issue a caution; the aircraft terrain alerting system was the only alert during this event and due to my decent and base leg flight path it alerted correctly. I am not sure what caused me to fail to confirm 'LNAV/VNAV PATH setting zeros' as I normally do. I thought I was in VNAV and did not confirm on the flight mode annunciator as I normally do when I set the mode control panel to field elevation. The Pilot Monitoring (PM) mentioned afterward that he should have queried me (PF) when I had LNAV selected without VNAV; and field elevation set in the mode control panel. We debriefed and discussed our errors; however as the Captain and PF I take full responsibility for this deviation from normal and will use it as a lesson learned. Suggestions: This event is a lesson learned and will ensure I am confirming my flight mode annunciation modes match my intended flight requirements.
Second reporter narrative
I was the Pilot fling (PF); Captain was the Pilot monitorng PM. Descending into ZZZ for the visual approach. We were high coming in and configured early in level change. When cleared for the visual the Captain went direct to the FAF; with confirmation selected LNAV. While on base I was task saturated with communicating with Tower; adding flaps; and monitoring altitude and airspeed. Through that theme the PF had selected a lower altitude of field elevation. I went to check the approach plate on the altitudes of the FAF and airport elevation. I briefly mentioned 'did you want that altitude?' Remembering we were in level change we descended below the FAF altitude and around 1;500 ft. AGL received a GPWS caution oral 'obstacle'. The Captain immediately disconnected the autopilot and leveled around 1;100 ft. -1;200 ft. AGL. The caution went away; we configured; and held altitude until glidepath intercept and continued the approach and landing without incident. Suggestions: I wish I would have been more direct with my language about the altitude instead of a 'suggestive question' during a critical phase of flight. Was also task saturated and trying to multitask instead of aviate; navigate; then communicate. And will be more diligent in my verification of the active lateral and vertical modes active.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.