Air carrier flight crew reported receiving a low altitude alert from ATC on approach to EINN.
Synopsis
Air carrier flight crew reported receiving a low altitude alert from ATC on approach to EINN.
Narrative
I was the Captain and flying pilot on [a flight to] EINN. This event occurred during the approach phase of flight in VMC and light winds. After coast in off the NATs we were advised of radar contact and subsequently cleared direct to ELPOM; the IF for ILS or LOC Runway 06. We were cleared for several descents with a final clearance to descend to 3000 ft. on 1031 hectopascals. At 3000 ft. outside ELPOM I began to slow for the approach and we were cleared for the ILS Runway 06. With the autopilot still engaged I armed the approach and checked the FMA to see that LOC and GS were armed. Moments later the LOC and GS captured at exactly the same time and our ASA switched to NO AUTOLAND while our raw data GS indicator began acting erratic. As we were in VMC conditions with the FMA confirming LOC and GS capture and I had the runway visually I elected to continue the approach despite the anomalies. At about 2600 ft. with reference to outside cues I lost confidence in the Glide Slope indication I was following and confirmed my concerns with my First Officer. I disengaged the autopilot and began a climb back to 3000 ft. to reestablish a visual approach and try and recapture a true Glide Slope. At that time we received a low altitude alert from Shannon Control. We advised Control we understood and confirmed we were returning to 3000 ft. I continued to fly the approach visually as the First Officer turned both FDs off then on in an attempt to reestablish some instrument guidance for the approach. At the final approach fix of ASGON with the approach reprogrammed by the First Officer all indicators began to settle down and the GS indication on the FD seemed to be giving me guidance I could trust. I continued the approach to an uneventful landing and taxied to gate; after completion of the parking check list; the First Officer and I debriefed the event.
Second reporter narrative
I was the First Officer as the pilot monitoring and the Captain the pilot flying. After crossing the Atlantic in EINN approach sector. We were cleared to the initial fix ELOPM for ILS 06 at 3000 ft.; the aircraft was level with 3000 ft. selected in MCP altitude window with LNAV to ELOPM and approach mode armed; at about 19 miles to runway we began to slow the aircraft for approach with the autopilot engaged. And was trying to visually acquire the airfield. The autopilot captured the localizer; simultaneously the autopilot also captured a false glide slope; the ASA indicated 'no auto land'; the raw data glide-slope was erratic and the aircraft descended to approximately 2650 ft. The Captain and I recognized the aircraft descending and the Captain disconnected the autopilot and climbed aircraft back to 3000 ft. ATC advised of an altitude alert. After recovering to 3000 ft. the Captain continued to hand fly and upon reaching final approach fix raw data and flight director indicated appropriate data and landing was accomplished without further incident.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.