2025-06 · NASA ASRS report 2256206
A321 flight crew reported an Autopilot malfunction which resulted in excessive pitch commands and descent below glide slope. The flight returned to the glide slope and continued the approach.
Second day of trip. Second leg of day. Visual backed up by ZZZ RNAV Z XXL. From a northern arrival; we were given a descent from 4000ft to 2000 and a dog leg to final Runway XXR. Pilot flying (PF) used open descent to get aircraft down to altitude which coincided with that of the FAP. We were level; 180kts and below glideslope at Flaps 2 upon reaching ZZZZZ. Final Approach captured but was slow to pitch down for G/S. When it did; it was high and descending steeply in an attempt to capture it; exceeding 2000FPM. Both pilots made mention of the need to intervene; but before that was possible; the aircraft began pitching back up to reacquire the G/S which it had flown through.Due to the initial pitch down; it was necessary to wait to extend further flaps; which resulted in the aircraft being Vapp and 20-25 at 1100ft. Fortunately; the aircraft had reduced its VS to such a degree that it was possible to slow to stabilized criteria by the time the aircraft reached 1000ft.The remainder of the approach was uneventful.I am not certain why the Autopilot lacked sufficient authority to pitch the nose under what seemed normal" flying parameters. However; I would not say that it is uncommon for the 320 series to fly a sloppy approach. I also believe that we as a community do not do a very good job of writing up such performance; which I did in this case.Were I to be in a similar situation again; I would have likely intervened somewhat earlier as I trusted the Autopilot a little too much in this case."
After cleared for visual approach XXR which was backed up by RNAV Z XXR; before final approach fix while Autopilot engaged airplane encountered Autopilot errors by diving the aircraft at a rate of 1200-1400 feet per minute before the final approach fix which was +1100 . Although Autopilot recovered before reaching final approach fix it was an unusual behavior of the aircraft automation. Flight was stable for the rest of the approach and the landing was uneventful Cause: Autopilot error. Autopilot was written up by the Captain
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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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