2025-09 · NASA ASRS report 2282644
A320 flight crew reported they descended below the assigned altitude at the FAF and received a Low Altitude Warning from ATC.
During our descent/approach into ZZZ; I was pilot monitoring. We received the following clearance; 'Cross ZZZZZ at or above 14 thousand cleared RNAV GPS X runway XX ZZZ'. I set 14;000 into the altitude window of the FCP and pointed while my Captain (PF) verbally confirmed and pointed to the assigned altitude. Since we were cleared approach he selected approach mode. I verified app nav on the FMA (Flight Mode Annunciator) and saw the brick; sticks and the proper FMAs. Shortly after the FMA changed to 'Final App'. We set the missed approach altitude which coincidentally was also 14;000 feet. In my head; I expected the airplane to maintain 14;000 feet until reaching ZZZZZ then capture the path. I became distracted verifying the next frequency for tower on the approach plate and tuning it in. As I was doing this we received a new clearance to cross ZZZZZ at or above 13;000 cleared RNAV GPS X runway XX approach. This confused me as we had already been cleared for the approach at 14;000 so I started to say that to the Captain. At the same time I set 13;000 into the altitude select. At this point ATC issued a low altitude alert and stated we needed to be at 13;000 I looked up and saw we were approximately 12;800. I was surprised because my mental model was thinking that we would stay level at 14;000 until the IAF. My Captain disconnected the auto pilot and I pulled for open climb to reach the assigned 13;000 I believe the lowest altitude I saw during the recovery was approximately 12;400. ATC stated that the lowest IFR altitude for the area was 12;500 and that we needed to be at 13;000 ft. She then re-cleared us to cross ZZZZZ at or above 13;000 cleared RNAV GPS X runway XX. We continued the approach to a normal landing; although the 13;000 foot restriction made it challenging to capture Final App as the charted altitude there is 11;700. I continued to think about what happened as we continued on the approach. I realized that the correct sequence should have been to set 14;000 in the altitude and then come down to the FMC and hard code the altitude for ZZZZZ at 14;000. This would have honored the 14;000 foot restriction. I believe a combination of 3 legs and a relatively long duty day resulted in some fatigue on my part and lead to the error of thinking the automation would honor the restriction.
We flew a segment from ZZZ1 to ZZZ. The landing runway was XX. We were cleared to cross ZZZZZ (IAF) at 14;000 and cleared the RNAV (GPS) X RWY XX. We set 14;000 in the altitude selection window and started to descend to 14;000 feet from a higher altitude. Since we were cleared the approach; I armed the approach and saw APP NAV FMA (Flight Mode Annunciator); but we were not in FINAL APP mode. As we approached ZZZZZ; ATC cleared us to cross ZZZZZ at 13;000 and cleared the RNAV (GPS) X RWY XX. After the FO set the altitude; I noticed we were descending below 13;000 and we were in FINAL APP mode on the FMAs. I initially pushed the Vertical Speed/Flight Path Angle Selector to try to level off; but that wasn't reacting fast enough; so I disengaged the auto pilot and climbed back to 13;000 feet. I think I saw that we had descended to approximately 12;300 feet. We were subsequently recleared the approach from ZZZZZ at 13;000 feet and executed the approach to a landing.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.
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