Narrative:

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 first officer while I managed to emergency with ATC; company; crew; and pilot monitoring duties. The first officer 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.

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Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.