Air carrier flight crew reported receiving a low altitude alert from ATC during approach. Flight crew climbed back and continued approach.
Synopsis
Air carrier flight crew reported receiving a low altitude alert from ATC during approach. Flight crew climbed back and continued approach.
Narrative
We had multiple changes and adjustments to our schedule throughout the day. We were delayed more than two hours in ZZZ due to the inbound aircraft diverting due to thunderstorms. Then we delayed more than two hours in ZZZ1 waiting for a weather delayed airplane from ZZZ2. The flight to RAP took 43 minutes. When we took off out of ZZZ1; we were given a heading to deviate west around weather. We then turned towards RAP and had to deviate east to avoid more thunderstorms. While still deviating east; we were given a descent at pilots discretion to 17;000. Hindsight we should have started the descent at that point. We waited until we were closer in and started the descent. They then cleared us direct to HELTA for the ILS. Due to the need for engine anti ice; we were not descending quickly. The FO slowed and we lowered the gear to descend more quickly. We were descending in level change because we were on a heading when we started the descent. We were still going to be above the altitude needed at HELTA; so we requested a right 360. We were told we could fly the procedure turn at HELTA; so I loaded it and we entered the turn. Because we were on a portion of the approach; we set the FAF altitude of 5000'. The altitude prior to HELTA is 5500'. We did not engage VNAV and the plane descended to 5000'. ATC let us know they got a low altitude alert and asked if we were established on the approach which we were. The FO disengaged the autopilot and climbed up to 5500. He intercepted the GS from there. We never received any alerts in the airplane other than the call from ATC. We flew the approach to an uneventful landing. During the debrief; we discussed how all the changes had led to some symptoms of fatigue by the end of the flight from ZZZ1-RAP.
Second reporter narrative
Shortly after our departure from ZZZ2 we were given a heading to deviate around weather in route to RAP. Once cleared of that weather; we proceeded direct to RAP; however; we had to deviate once again for weather. During the second deviation; ATC cleared us to descend to 17;000 feet at pilot's discretion. I had put range rings in the box around the airport for altitude guidance due to the fact there was no arrival into the field. I felt we had some time before we needed to descend so I stayed at flight level 270 for a bit longer. Looking back; I should have began my descent immediately as that was the root cause of the incident. Shortly after I did begin the descent ATC cleared us direct to HELTA for the ILS runway 32. At that point; I was in level change and knew I was going to be high so I slowed the aircraft and dropped the gear to assist with the descent. Seeing that we were still going to be high at HELTA; the captain requested a right 360. ATC gave us the procedure turn at HELTA. Once established in holding the captain set the FAF altitude of 5000 feet. I was still descending in level change so the plane leveled at 5000 feet; the altitude prior to crossing HELTA inbound is 5500 feet. ATC called us saying they got a low altitude alert at which point I disengage the auto pilot and climbed up to 5500 feet. I intercepted the glideslope from that altitude and continued the approach. We landed and taxied to the gate with no further issues.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.