Citation X (C750)
Flight Crew Air Transport Pilot (ATP)
Confusion; Fatigue
Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Flight Crew Overcame Equipment Problem
Human Factors; Procedure; Aircraft
During the last leg of a very long day; we ran into an issue where status of the autopilot caused us to climb when we should have been descending. It also led a situation where the airplane was greatly out of trim for the phase of flight which we should have been in; descending. We were on downwind for an expected visual approach. Controllers had left us high; during the downwind. The autopilot was on during the downwind with Flaps 5. We were cleared for the visual approach. On the wide base leg turn; probably a 4 mile base; I extended speed brakes to get us to final configuration speed. On the base leg; I believed I had pressed the autopilot disconnect button. I retracted speed brakes and pushed down on the control wheel. This is the point the autopilot was truly disconnected and it was discovered that the trim was not neutral but was greatly pitched for nose up to hold altitude with the decaying speed from having the speed brakes out. In the confusion of it all; I thought the control wheel was jammed as it was very difficult to get the nose down and asked the other pilot to check his. That is when he discovered that it was not jammed but greatly out of trim. During the course of the re-trimming for nose down; the airplane did climb temporarily when we should have been descending. The airplane was re-trimmed and hand flown from the base leg to the final leg and subsequently a stable approach and landing.I should have done a better job ensuring the autopilot was off by checking the MCP and getting a better feel for the control wheel; in this case it was left on and then truly shut off in an out-of-trim condition. Normally I would have caught this issue sooner but I believe fatigue at the end of the day led to the brief confusion. The pilot monitoring did a great job quickly catching my mistake and pointing out the out of trim condition.
As I was looking outside on my side I heard the PF say something that caught my attention. When I looked over I saw the PF pushing with both arms against the controls with a desperate look on the face and saying something to the extend of 'I can't go down; it won't work.' The speed was dropping quickly towards about 190 kts; the speed brakes were now retracted again; the thrust was still set at idle; the controls were pushed full forward; and the trim indicated full Nose Up. I immediately took control of the thrust and started to add when I noticed I was able to control the speed (now steady at 170 kts) and arrest the pitching up moment. I stated a few times that 'I have the controls' and we did have a positive transfer when I took the control column; pressed and held the AP disconnect; and pushed forward. I re-trimmed the aircraft; arrested the now climb (about +300ft); and returned to a gradual descent on base.
CE750 flight crew reported that the autopilot was disconnected while the speedbrakes were engaged resulting in an excessive nose up trim.