Air carrier flight crew reported they received a low altitude alert from Tower while descending for a TCAS/RA for parallel runway traffic.
Synopsis
Air carrier flight crew reported they received a low altitude alert from Tower while descending for a TCAS/RA for parallel runway traffic.
Narrative
Once established on the XXR LOC and maintaining 8;000; we were cleared ILS XXR approach. At this time; the TCAS system alerted and a Resolution Alert commanding a descent. The Captain disengaged the autopilot and auto throttles and flew to the save fly zone and the FO declared the RA and descending to Tower. The traffic continued to approach our position so the Captain continued to descend. The Tower notified the crew that we descended below the FAF altitude which was 7;000 and to say intentions. The crew agreed and FO notified Tower that we were still responding to an RA and intended to continue. The Traffic continued to descend and closed the distance. At this time; the crew acknowledged the vertical separation was -400 feet and not correcting course. At this point it was apparent to both pilots that the other aircraft was either not responding to the TCAS/RA or that it was not alerting them of the decrease in vertical separation. The Captain increased the rate of descent. The FO called out the radio altimeter altitude of 1;000 feet. Soon after the Tower notified our flight that we were low altitude alert. At that moment the RA advisory completed and the Captain increased pitch and power to climb back to the appropriate glide path. Resulting in a stabilized approach to a normal landing.
Second reporter narrative
While on the localizer (we had not intercepted the glide slope yet) to Runway XXR in ZZZ; we received a DESCEND" Resolution Advisory while in IMC. Another aircraft was descending into us while turning left (coming from the west) to intercept the Runway XXL localizer. I immediately punched off the autopilot and autothrottles and started a manual descent. The First Officer announced to ZZZ Tower that we were reacting to a Resolution Advisory. Tower asked if we wanted to continue on the approach; and I told the FO that we did--which he relayed to the Tower. We had already been slowed to 170 knots by Approach Control--so we had the flaps at 10 degrees. During the RA descent; I watched the other aircraft on the radar display continue its descent as it was intercepting the XXL localizer. I am not certain of the lateral separation; but the vertical separation at one point got as low as 400 feet. I continued the descent per the RA-- which took me below the glideslope for the ILS. The FO announced 1;000 feet on the radar altimeter and I saw us descend to approximately 6700-6800 feet MSL a mile or two prior to ZZZZZ (which is an at or above altitude of 7000 feet MSL). At that point; the DESCEND RA was still active--and I was thinking about only going down as far as 6500 feet MSL before turning right to get away from the traffic. Also at this point; the senior controller came on the radio with a low altitude alert--reprimanding us that the minimum altitude at ZZZZZ is 7000 feet. The FO had to reiterate that we were following a Resolution Advisory; and she did not say anything else. Fortunately at that point; the DESCEND Resolution Advisory stopped (again; we were approximately 6700 feet MSL). I am confident it stopped not because of altitude deconfliction; but because the aircraft headed to Runway XXL rolled out on its course. I manually climbed back up to 7000 feet; intercepted the glideslope and manually flew the aircraft down to near minimums (250 feet AGL before breaking out of the weather.)"
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.