A321 flight crew reported auto flight anomalies after encountering wake turbulence on approach to DFW airport in trail of a B737-800.
Synopsis
A321 flight crew reported auto flight anomalies after encountering wake turbulence on approach to DFW airport in trail of a B737-800.
Narrative
Approaching BRDGE on the SEEVR 4 arrival; FMC in APPROACH phase; speed pulled at 250; relatively smooth conditions; we encountered apparent wake turbulence from the 737 ahead as evidenced by two fairly fast uncommanded roll maneuvers to the left; followed by the A/P (Autopilot) aggressively correcting to the right. Expecting the A/P might respond too aggressively; I disconnected A/P; but NOT the auto thrust. A few seconds later we experienced a few more milder left/right roll disturbances which I easily managed hand-flying. Shortly after I noticed the speed bleeding from 250 kt. to about 235 kt. (we previously selected SPD); FMA (Flight Mode Annunciator) showed SPEED; 250 was in the window; but the light in the A/THR push button was not lit. I pushed the PB; and thrust increased to limit and speed shot thru 250 kt. I pulled speed again; to no avail; and increased to 270 before I got A/T (autothrottle) off and pulled thrust back to idle. We gave up on A/T and controlled thrust manually for the rest of the approach. We asked for and received heading and altitude from ATC for the remainder of the flight. When A/P was re-engaged; it appeared not to be following commands from the FGU; first we selected 3;000 ft. and pulled open descent; but the aircraft would not descend from 5;000 ft. I disconnected AP and we completed the approach and landing in full manual flight.There were no warnings for the initial failure of A/T. Most of the FMA indications and displays showed normal response to our commands; but the automated pitch and thrust systems either did not respond to input; or responded incorrectly; with several attempts to establish normal engagement. I don't recall every FMA mode change because just trying to manage the automation or lack of it took all my attention. The startle factor was definitely in play; as well. The overall lack of warnings; and then the lack of consistency and aircraft correlation with the FMA indications was unsettling. My only supposition is that something about the wake turbulence encounter caused partial incapacitation of the autopilot and auto thrust. We did not attempt to engage A/P 2. The environment was quite confusing until we settled the airplane down in manual flight. F/O (First Officer) handled PM (Pilot Monitoring) duties well and helped with his observations and coordination with ATC. In addition; the poor readability of EFIS and LCD screens in bright sunlight is always a factor.This is the third such major event I've experienced with the A320; where the automation responds erratically; unpredictably; or seems to just freeze up. I hope these odd and unexpected events are analyzed for possible software/firmware anomalies; or recommendations for better ways for us as pilots to mitigate automation failures. I am constantly reminded how easy it is for a pilot to be 'out of the loop' when things start to go wrong in this highly automated airplane. The lack of thrust lever movement in the governing range and any other real obvious clues to off-schedule thrust continues to be a big human factor; in my view.
Second reporter narrative
On the SEEVR4 we encountered some intense wake turbulence from aircraft in front of us. Fast left and right rolling. Captain announced disconnecting the autopilot. We received a few more rolls and bumps. Captain asked me to find out what we were following. ATC said B737-800 12 or 13 miles. ATC said we could fly a parallel course or a heading I believe. During that time we had slowed below the 250 set in the speed window and speed selected. Captain announced the speed decreasing and we looked for the cause. He said the auto throttle had disconnected. Neither of us had done that. He reengaged the auto throttle and the aircraft went to a high thrust setting. So he took control of the thrust manually. Captain stabilized the aircraft and we continued. Further down the arrival the Captain reengaged the autopilot and everything seemed normal. But as we received a descent from 5 or 6 thousand ft. to 3 thousand the aircraft would not go into open decent. Captain tried multiple times to no avail. Captain disconnected auto flight and flew a manual approach.We had nice clear weather. No atmospheric turbulence. Just some heavy rolling due to wake turbulence. Crew that rode in the cabin did report that the wake turbulence was strong in the back. Captain handled the event very calm and professionally.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.