2024-05 · NASA ASRS report 2123322
B737 flight crew reported descending well below a fix crossing altitude during approach. Flight crew climbed and leveled off until glide slope intercept and continued descent.
I was pilot flying and we were on the ZZZZZ X arrival ; and set up for the ILS XXL approach into ZZZ. We were on the approach frequency at the time of the occurrence (frequency XXX.XX I think); just passing through ZZZZZ1 level at 3000 ft. After passing through ZZZZZ1 at 3000 ft.; the controller said '4Aircraft X; you're cleared for ILS Runway XXL'. Once we were cleared for the approach; I asked my First Officer to dial in final approach fix altitude (1500 ft.); he then put in 1500 ft. and we both acknowledged the altitude change by saying '1500 feet set'. I then hit Level Change and we descended to 1500 ft.; prior to reaching ZZZZZ2 (3000 ft. restriction) on the approach for Runway XXL. We were also setup for the CAT III approach for this approach because of the ceilings were lower than CAT I mins (OVC002; VIS 1 3/4 mile). The workload at the moment was a bit more than normal; and it was at night. I realized my mistake once the controller came back on the frequency and asked why we were descending below altitude restrictions for that approach. Once he told us of my mistake; we apologized to the controller for it and I realized what I had done wrong immediately; he then suggested that we climb 500 ft.; and I asked my FO to dial the altitude up to 2000 ft.; and we both responded '2000 feet set'; and we climbed to 2000 ft. immediately. Being that the workload was already high; and we were now established on the approach; I decided to level off at 2000 ft. instead of climbing back up to the 3000 ft. restriction that we should have been at initially for that portion of the approach. Although the ceilings were calling for OVC002 over the airport; at that moment in time; from the time we were cleared for the approach; we were in visual conditions and were clear of any terrain/obstacles. We stayed on the approach at 2000 ft. as we intercepted the LOC and until we intercepted the GS. Once we were established on the LOC and GS; we commenced the approach for a CAT III ILS for Runway XXL into ZZZ. Other than my mistake of breaking an altitude restriction on the approach; everything else was safe and conducted as planned and briefed. We then landed safely on Runway XXL and taxied to our gate. I never once felt we were in any harm or I would have chosen to go-around and retry the approach again.
On approach into ZZZ; at approximately 20 miles; Approach Control cleared the flight for the ILS to Runway XXL. Weather was 200 ft. overcast with 1 3/4 miles visibility so the Captain; who was pilot flying; briefed a cat III approach. The IMC layer was between 200 and approximately 700 ft. When cleared for the approach; the Captain verbalized that he was going to hold off on arming the approach until the aircraft was closer to the field. At approximately 16 miles he verbalized and armed the approach. I acknowledged and armed Autopilot B as the second autopilot which is required for the cat III approach. He then asked me to set final approach altitude and I set 1500 ft. for the final approach fix altitude at ZZZZZ3 when we were between ZZZZZ1 and ZZZZZ2. The Captain verified the altitude. I failed to notice the flight was in level change mode and assumed we were protected in VNAV. At that point my attention shifted to reviewing the autoland procedure in the flight manual to ensure switches were correct for the cat III autoland. The plane started to descend immediately and I did not register the early descent. At 1500 ft. in the vicinity of ZZZZZ2; Air Traffic Control (ATC) asked what altitude we were at; to which I responded '1500 feet' immediately recognizing the altitude restriction of 3000A at ZZZZZ2. ATC recommended we immediately climb at least 500 ft. which the Captain did immediately. Glideslope was intercepted at 2000 ft. at ZZZZZ4 and the rest of the approach was flown without incident.
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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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