B737 flight crew reported experiencing an abrupt pitch down event when engaging the autopilot system; resulting in an immediate return to the departure airport.
Synopsis
B737 flight crew reported experiencing an abrupt pitch down event when engaging the autopilot system; resulting in an immediate return to the departure airport.
Narrative
Today was an incredibly long day. I had just finished a two-day the previous day out of ZZZ; where we were delayed getting back into ZZZ by almost 5 hours due to weather. So I was hoping for a simple day trip to ZZZZ today. That simply was not the case. I arrived at the airport for our report time at XA:50. I was able to meet with the CA (Captain) and discuss the upcoming day and flight. We met the previous crew of our aircraft on the jet bridge who went into great detail about the current MEL items on that specific aircraft. After discussing the situation with the crew and Dispatch; the CA and I both felt that it was the best course to not take this aircraft to ZZZZ due to the MEL items on the aircraft. We then waited another 3 or so hours for the next aircraft to become available. The CA and I boarded the new aircraft to find that there was an inbound write-up on the aircraft in regards to the brake pressure accumulator not being able to hold pressure and had lost about 800 psi in about 30 minutes. Maintenance personnel then boarded that flight and performed a test on the aircraft to see what the issue was and if it could be deferred or if the issue was more complex. The test took roughly 45 minutes to see if the brake pressure would hold or not. Maintenance informed the CA and I that the aircraft did not pass the test and would need to be taken out of service. The time now is roughly XG:30. The CA and I have been at the airport roughly 6 hours and are now looking for our 3rd aircraft to take to ZZZZ. However duty period was now an issue. We later received a phone call from Scheduling who informed us that we will now be going to ZZZ1 because we would not be legal to operate the previous flight to ZZZZ and back to ZZZ2.The CA and I again boarded our new flight to ZZZ1. We got everything ready to only have the Ramp close for about 30 minutes due to the storms in the area. Once the storms cleared we were able to push back from gate. However; due to the construction behind the gate; the Tow Driver needed to tow us in a specific area to clear the construction. We informed the driver of where to go but he was very confused and it was clear that he may not have gotten the proper training for towing in this area with the construction. Due to this confusion we ended up breaking the shear pin on the push. So we had to get Maintenance to come out and verify everything was ok before continuing on with the pushback. Once given the ok to go from Maintenance; we were able to push back to the proper area and commence the flight.The time now is roughly XJ:15 and we were on our way for departure after 8.5 hours of delays and changes. There were still so many storms in the direction of which we were going but ZZZ2 was clear. We departed with no issues. We performed the after-takeoff check and continued our climb to 10;000 ft. It was around 8;000 ft. that the CA decided to put the autopilot on by pressing Command A. The autopilot engaged. I was the pilot monitoring and received a clearance to continue climb to 18;000 ft. At that moment within 30 - 60 seconds of the autopilot being on; there was a sudden violent jolt and pitch down of the aircraft. The autopilot remained engaged. The CA and I looked at each other trying to understand what just happened. The aircraft then did the same jolt and pitch down. The CA then clicked off the autopilot. We told ATC that we want to remain at 10;000 ft. and that we had a flight control issue. The CA and I then tried Command B autopilot. It appeared to be working normally. We then tried Command A autopilot and again within 30 - 60 seconds the jolt and nose down pitch occurred. We then saw the stab out of trim light appear. We ran through the checklist and decided that it was best to go back to ZZZ2 at that time. We advised ATC and got vectors back to ZZZ2. We notified Dispatch; the FAs (Flight Attendant); and customers. We did a great job using CRM to really divide and conquer at this time. We wouldmeet back and regroup with each other. We ran the overweight checklist and proceeded to land at ZZZ2 with no issues. ARFF (Airport Rescue and Firefighting) verified the aircraft was ok on the taxiway before proceeding to the gate.
Second reporter narrative
Our showtime for the original pairing was XA:50. We were assigned an aircraft that was removed from ETOPS operations due to an APU issue. I discussed this with Dispatch and requested either an airplane with an APU that was not degraded and certified to operate over water or in ETOPS or a route of flight that was mainly over land and closer to acceptable divert airports. Even though this flight was not ETOPS I do not operate our aircraft over water more than a distance that would provide for a safe and quick diversion in the event of system failures. The Dispatcher agreed to re-route our flight. Soon after discussing this aircraft and flight plan; we were assigned another aircraft. We were not told why this decision was made. The next aircraft was wrought with even more serious issues. In addition to having a cowl anti-ice valve wired in the open position; which required reference to flight manual procedures and information; it led to another MEL item that further complicated the flight crew duties including more referencing of the flight manual and MEL procedures. Bleed air and minimum power settings. as well as split throttle positions; on an approach are serious systems that require lots of attention. Had this flight been further complicated by other system failures not anticipated; we would have been up against an enormous amount of work just to get the aircraft on the ground safely. In addition to that; there were numerous areas of thunderstorm activity on and around our route of flight. We refused the aircraft. The next aircraft they gave us came in with a write-up of brake accumulator pressure bleeding down rapidly while parked. AMTs performed a required test on the brake system and it failed. This evolution took about an hour and further delayed our operation and the aircraft was eventually taken out of service. Following this evolution; the crew desk advised us that we would not be able to complete our one-day trip without going over our Part 117 limits of a 14-hour duty day. Thus; as is allowed by the [agreement] for irregular operations; we were reassigned to a two-day trip that was about to depart. That crew was reassigned to take our two-leg turn to ZZZZ. We spoke briefly with the swap crew on the jet bridge and began setting up the cockpit for our one-leg; first-day flight to ZZZ. The second day would have us flying a leg ZZZ - ZZZ1 followed by a deadhead back to ZZZ2; flying into our day off. This was the first time we had released the parking brake on what was a very long day. During the pushback; we received instructions to position the aircraft away from construction on the ramp with the jet facing north toward the concourse. The push crew was not familiar with these instructions and had not been trained on the pushback necessary for the construction being done south of the concourse on Taxiway 1. The Tug Driver removed the tow bar pin after we set the brake; but the Ramp Controller informed us that we were not positioned correctly. We then had the Tug Driver hook up the tow bar again to reposition us correctly. This led to the shear pin being broken when the tow bar was reattached and we released the parking brake for repositioning. Following SOP; we were informed that the nose gear had to be inspected. The ground crew then reattached another tow bar and repositioned the aircraft at the gate and station maintenance personnel performed a successful inspection. We pushed back successfully and taxied for takeoff on Runway XX at ZZZ2. After an uneventful departure; we were climbing through about 8;000 ft. to level off at our assigned altitude of 10;000 ft. I engaged autopilot A/CMD and we experienced an abrupt jolt of the control column followed by two more. I immediately disengaged the autopilot and we began to look for an indication of any system malfunction. There were no abnormal signs or indications; including hydraulic pressure or fluid. We discussed the possibility that we had flown through another aircraft's wake and I engaged the autopilot again; followed by the same abrupt jolts. I disengaged the autopilot again and hand-flew the jet; leveling off at 10;000 ft. This time I noticed a flash of the STAB OUT OF TRIM light on the forward instrument panel. We also noticed lots of autotrimming of the trim wheel. The trimming was not indicative of a runaway trim condition. Autopilot B was engaged and it operated normally. The First Officer immediately referenced the flight manual for the associated light. We reviewed the checklist and performed the steps called for. We discussed our options and decided to return to ZZZ2 as the safest and most prudent course of action. We also decided to advise ATC as an extra precaution and to obtain priority handling from ATC. Dispatch was notified; crew and passengers were briefed; and we made a slow approach back toward ZZZ2 about 30 minutes after takeoff. Landing performance data was requested and received from Dispatch. We reviewed the overweight landing checklist; briefed; and flew an uneventful approach and landing to Runway XY at ZZZ2. We were followed by numerous emergency vehicles on rollout. We spoke with the Fire Commander via VHF radio and he informed us that there were no external signs of damage or overheating of the brakes. We taxied back to the gate; shut down normally; performed all checklists; and debriefed thoroughly. We sent write-ups for the system malfunction as well as the overweight landing. Together we also debriefed the Chief Pilot. He called us again a short time later and we answered questions for a human factors review. He called one last time later and informed us that we were released from duty for the remainder of the day and night.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.