B737 Undifferentiated or Other Model
Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Troubleshooting; Workload
Aircraft Equipment Problem Less Severe
Flight Crew Landed As Precaution; Flight Crew Overcame Equipment Problem; Flight Crew FLC Overrode Automation; Flight Crew Returned To Departure Airport; Flight Crew Diverted
Aircraft
FL390 in clear smooth air enroute; [the Captain Pilot Flying (PF)]; autopilot had been on since 7;000 with no issues. Rudder trim as seen by yoke deflection seemed not normal. With yoke arrow pointing at centered; plane made a series of sudden 'kicks' to the left (gyro needle deflected right). Autopilot disconnect by PF and yoke showed steep deflection to the right. Trimmed out by rudder knob but when autopilot reengaged; same series of kicks occurred. Pilots decided to ZZZ before situation became worse and more severe. Autopilot seemed too dangerous; so hand flown entire trip back to [departure airport]. Dispatch notified; FAs coordinated well; PA announcements timely; and radio comms to [departure airport] worked fine. ZZZ decided more suitable than [a secondary diversion airport]. Descent; approach; and landing uneventful and smooth; with no recurrence of kicks/fluxuations. Trim held well.. Did notice drop of Hyd A fluid from 97 to 68 compared to departure amount; but recovered to 90 by block-in; had QRH out to 'Loss of Hyd System A' page; just in case.
737 Captain reported the flight returned to base due to a malfunctioning autopilot.