B737 captain reported an abnormal amount of rudder trim was required to maintain level flight. After...

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.

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.

Attributes
ACN	1349279
Time
Date	201604
Local Time Of Day	1801-2400
Place
Locale Reference	ZZZ.Airport
State Reference	US
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Make Model Name	B737 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Climb
Flight Plan	IFR
Component
Aircraft Component	Rudder Trim System
Person 1
Function	Captain Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 100 Flight Crew Total 12500 Flight Crew Type 5000
Events
Anomaly	Aircraft Equipment Problem Less Severe Inflight Event / Encounter Other / Unknown

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.