A321 flight crew reported mountain wave action caused an uncontrolled excursion from their assigned altitude and airspeed fluctuations.
Synopsis
A321 flight crew reported mountain wave action caused an uncontrolled excursion from their assigned altitude and airspeed fluctuations.
Narrative
Our altitude excursion occurred while at FL320 with all automation on. While programming FMC for new arrival I felt aircraft react to mountain wave action. I looked at my Primary Flight Display (PFD) and noticed that we had gone from approximately 12 knots below MMO to just below MMO with an airspeed trend line at over 30 kts above MMO. We had quickly encountered the mountain wave that was smooth at first then was accompanied by moderate turbulence. I don't remember if the First Officer disconnected the Autopilot or it disconnected on its own but the aircraft started pitching down as it was climbing. We eventually reached 32;600 feet. The First Officer hand flew the aircraft back down to FL320 and reengaged the Autopilot. I reported the altitude excursion to ZZZ Center and reported back altitude. About this time I was starting to feel nauseated. (I would later find out was a bad crew meal I had eaten about 15 minutes prior to our mountain wave encounter. This left me very sick upon landing.) We flew the remainder of the arrival with no problems. Once lower and getting ready to start the ILS XXR; we were taken off the approach twice I think for some separation of traffic. On the second time being told to intercept the localizer we were ready for the approach. For some reason the aircraft did not intercept fully the localizer course. If I remember correctly we had about a 40 degree intercept heading to the inbound course. The aircraft rolled right to intercept but went through the course a bit to the left and re-intercepted from there. The remainder of the approach was flown without a problem. Of note here; either the final Approach Controller or the Tower asked us if we were intercepting the localizer and I replied in the affirmative. To the best of remembering; we had proper Flight Mode Annunciators (FMA's) for our approach. The controller made no more comments and we landed normally.
Second reporter narrative
Subject flight encountered moderate mountain wave turbulence at FL320 in the vicinity of ZZZZZ on the ZZZZZ1X arrival to ZZZ. The autoflight system was engaged but unable to compensate for the following tailwinds which caused some nose attitude deviations and pushed speed all the way up to Mmo (max mach) and bumped the trend vector to Mmo +30. Out of concern for overspeeding the aircraft; I disconnected Autopilot and initiated a slight (2-3 degrees) nose up attitude to arrest the speed trend below max mach. Slightly delayed (10-15 seconds afterwards) and due to the high altitude and rising Vls I slowly introduced 1/4 speed brake as well but not before the aircraft began to deviate above assigned altitude. Due to the acceleration and the lifting action of the mountain wave; the aircraft began to gain altitude rapidly despite opposite control inputs. I felt a little delayed in these inputs due to the startle effect of the circumstances but also did not want to introduce any form of pilot induced oscillation (PIO) or closing 'coffin corner' at altitude and elected to smoothly direct the aircraft back toward assigned altitude. We did not have any indication via TCAS of any aircraft in our vicinity and were back at FL320 in approximately 30 seconds. The max deviation was +650 feet (FL326). ATC did not initiate contact with respect to the altitude deviation but the pilot monitoring (PM) contacted ATC to alert them of the mountain wave encounter and that we had returned to assigned altitude without further incident. Autoflight was reconnected at that point.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.