Air carrier flight crew reported they descended below glidepath on approach and received a Low Altitude Alert from ATC.
Synopsis
Air carrier flight crew reported they descended below glidepath on approach and received a Low Altitude Alert from ATC.
Narrative
Upon commencing the approach to ZZZ; we were given vectors to intercept the final approach course. We were at 3000msl and instructed to cross ZZZZZ at 3000msl; cleared for the approach. After VOR/LOC capture and FMA (Flight Mode Annunciator) confirmation of being on the lateral course; I asked the CA/PF if he wanted the FAF altitude. He did and I entered 1700 into the MCP (Mode Control Panel); we were in ALT HOLD at this time as the CA had previously used V/S to descend to 3000. I confirmed/cross referenced that the box had large (protected) altitudes for ZZZZZ1 and ZZZZZ2; it did have the correct altitudes. I did not notice what vertical mode was selected to start the descent; I had looked down at my IPAD to locate the field elevation and missed approach altitudes as I knew they would be the next MCP altitude inputs. I then confirmed the ground frequency and looked at the radio stack to get that preloaded as we were now on with tower frequency. I looked out at the runway as we were told landing clearance would be short final as there were departing aircraft. It was then I noticed 4 red PAPI lights. I looked down at our altitude and it was 1700; I looked at the CDU (Control Display Panel) and it said the next fix; ZZZZZ1; was at 1800A. I begin to state we were low when tower issued a low altitude alert. I immediately responded with 'correcting' as I watched the CA/PF apply power; disconnect the AP (Autopilot); and pitch up. We began a climb to 1800 when G/S annunciated on the MCP. I watched as the command bars began a descent then a slow rise until both lateral and vertical bars disappeared from the display altogether. I immediately reached up and turned off both FD (Flight Director) switches; I then turned on the CAs and then my FD switches in sequence. FD annunciated on both PFDs (Primary Flight Display); but no command bars present. CA /PF then commanded I cycle the switches again; I did with the same result. He stated he would fly the approach using raw data and visual conditions. This sequence of events caused us to be slightly behind in configuring the aircraft for landing; barely making the 500ft stability gate criteria. We landed and exited the runway; parking without further event at the gate.
Second reporter narrative
We were cleared direct ZZZZZ and the ILS RWY XX in visual conditions. We set 1700 feet as this was the final approach fix altitude. Approach was armed and V/S selected. Localizer was captured; and as we descended through 1800 feet we received a low altitude alert from the air traffic controller. ZZZZZ1 has a crossing restriction of 1800 just outside of the FAF which has an altitude of 1700. We missed that when descending. We intercepted the glide slope and began the final approach. The flight director then disconnected abruptly and after a few seconds of startle; I executed a raw data hand flown visual approach. The PM attempted to re-engage the flight director a couple of times without success. We landed without incident.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.