737-700 flight crew reported an un-commanded auto pilot pitch up during descent. Flight crew regained control and landed uneventfully.
Synopsis
737-700 flight crew reported an un-commanded auto pilot pitch up during descent. Flight crew regained control and landed uneventfully.
Narrative
On descent into ZZZ on the lateral path of ZZZZZ at 23000 ft. with autopilot (A/P) engaged and LNAV/VNAV; ATC cleared us the ZZZZZ arrival. We had been held up; which is not unusual; and immediately advised them we would not make the altitudes on the STAR. ATC then told us to disregard speeds and just make ZZZZZ at 12000 ft. We told them we would do our best but might be too high. The First Officer (FO); who was Pilot Flying; placed 6000 ft. in the altitude window and hit speed intervention-300 kias and brought the speed brakes out to make an attempt at it. They started to stow the speed brake from the flight detent to the 50%. We analyzed the problem; put 12000 ft. in the altitude window and went to level change as we continued the descent. Not finding anything new or any reason why it did this; we elected to try to put it back in VNAV. It took and then we put 6000 ft. in the altitude window. As the altitude at ZZZZZ looked like we were going to make it; the First Officer began to slowly bring the speed brakes to the up position. Within the last 2 inches of stowage; the aircraft abruptly went hard nose up and power began increasing. For about .5-2 (max) seconds; this happened until the autopilot kicked itself off. Two Flight Attendants were injured and one passenger. The seat belt sign was on during the event and the Flight Attendants were collecting trash for their final because I was sitting them early. Upon finding out that we had injuries onboard; we advised Dispatch and asked for Inflight Supervisors; as well as Customer Service Supervisor; to meet the aircraft. We asked Operations over the radio to have EMS to meet the aircraft at the gate. Suggestion: You know; I wrote that aircraft up and until it is resolved as to why this malfunction occurred; I would not put it back into service. We discussed being high; before ATC gave us the descent. I did not accept it; so really; I do not think that is a factor. This happened so quickly that I am not sure how to fix this problem.
Second reporter narrative
Before the event and descent. The seatbelt sign was on; and the Flight Attendants were given a five-minute heads up before some bumps going into ZZZ. After being left high due to traffic; we received a descend via clearance. The Captain told ATC we would be high at the altitudes. ATC said we could delete the speeds and just make the altitude at the ZZZZZ point. I started the descent using VNAV automation and using the speed brakes to increase our descent. As we were getting closer to being on PATH; I moved the speed break handle from flight detent to 50% position. At this point; the lateral navigation on the FMA changed from VNAV SPD to control pitch. At that point we were puzzled at this in the auto motion. We then selected level change. Once we got the aircraft back in the descent; we selected VNAV again. I saw we were still getting back to our PATH; I moved the speed breaks smoothly to the down position. Once the speed break handle went into down position; the autopilot immediately pulled the nose and the auto throttles advanced as it was starting a quick climb and then the autopilot disengaged. I disconnected the auto throttles and pushed the nose over smoothly and controlled the aircraft. After controlling the aircraft; we resumed our descent. We left the autopilot off until everything was controlled and smooth; then re-engaged the autopilot and auto throttles and continued the arrival without the use of any speed brake usage. The rest of the flight was uneventful with the automation being used. During the event though; two Flight Attendants were injured during their trash service and one passenger was injured with the seatbelt sign on.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.