B737 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 100; Flight Crew Total 12500; Flight Crew Type 5000
Distraction; Troubleshooting
Aircraft Equipment Problem Less Severe; Inflight Event / Encounter Other / Unknown
Air Traffic Control Provided Assistance; Flight Crew Landed in Emergency Condition; Flight Crew Returned To Departure Airport; Flight Crew Requested ATC Assistance / Clarification; General Maintenance Action
Aircraft
During climb; it took an unusual amount of rudder trim-initially 3 units-to maintain straight flight. While continuing to climb; I moved the rudder trim back to a more normal setting-around 1.5 to 2 units to see how the aircraft would respond. The control wheel continued to indicate a need for more left rudder trim. As TAS increased; the need for additional left rudder trim increased up to 5 units to maintain straight flight. After level off in cruise; I contacted maintenance to know if 5 units was within tolerance. They replied yes as long as we can maintain straight flight. Soon after; it took additional left rudder trim; now up to 6 to 7 units of left rudder trim to maintain straight flight. Then without any pilot input; we noticed the rudder trim gauge was increasing on its own; including some minor fluctuations in the gauge. At 10 units on the gauge; I decided to return to ZZZ and [advise ATC]. The rudder trim gauge eventually went to its maximum limit although actual rudder input wasn't at maximum. I gave the flying duties to the FO while I managed to emergency with ATC; company; crew; and pilot monitoring duties. The FO confirmed that there was still sufficient rudder input available though it still was about 7 to 8 units of rudder trim to maintain straight flight; both with and without the autopilot.
B737 Captain reported an abnormal amount of rudder trim was required to maintain level flight. After consulting with Maintenance; a decision was made to return to the departure airport where an uneventful landing was accomplished.