Air carrier flight crew reported receiving a low altitude alert from ATC during approach when they descended below a fix crossing altitude. Flight crew continued approach and landed.
Synopsis
Air carrier flight crew reported receiving a low altitude alert from ATC during approach when they descended below a fix crossing altitude. Flight crew continued approach and landed.
Narrative
Descending on the QUERVO1 arrival shortly after passing 10;000 ft. we were cleared direct to QUESO at 4;000 ft. and cleared for the approach. Before arriving at QUESO; our approach clearance was cancelled due to traffic not responding to Approach Control. We were cleared direct QUESO and to descend and maintain 4;100 ft. The Captain identified the clearance cancellation and altitude restriction as a threat and verbalized that we were likely going to be left high and get slam dunked onto the approach as the controlled tried to reach the conflict traffic. Shortly before QUESO at 4;100 ft. we were cleared for the approach again. I re-armed approach mode and the final fix altitude (2;200 ft.) was set in the altitude window. At this point I believe due to confirmation bias; the Captain suggested use of Level Change to get down. I failed to verify our vertical position relative to the published approach altitudes and glide slope indication prior to initiating the descent in LVL CHG. As we turned onto the final approach course; the aircraft descended below the charted altitude of 3;200 ft. at NUGIT and Tower notified us of an altitude alert. At that point we were in VMC conditions approaching NUGIT. By the time we recognized the error we had passed NUGIT and captured the glide slope from below at 2;200 ft. and continued to land.
Second reporter narrative
Got cleared direct Queso for the ILS 13R approach into SAT. Once established toward Queso got descend 4;000 ft. cleared ILS 13R approach. At this time the airplane was descending in LNAV/VNAV with VNAV Path captured. Approximately a couple miles before Queso descending through approximately 4;700 ft. Approach Control cancelled approach clearance and said to maintain 4;100 ft. 4;100 ft. was set in the Mode Control Panel (MCP) and than Level Change was selected for vertical guidance. Approach re cleared us for the ILS 13R at about Queso. The FAP altitude of 2;200 ft. for ALAMO was set. From than on we descended in Level Change with Approach mode armed. Just after NUGIT while level at 2;200 ft. we realized our mistake. Due to descending in Level Change and not VNAV Path we realized we had busted the intermediate fix altitude at NUGIT of 3200ft. From there the approach continued uneventfully and flight completed with a thorough debrief of what had happened.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.