An ATR72 rudder trim system failed to move in flight. The flight crew returned to the departure aircraft with the aircraft in trim after coordinating with Dispatch and Maintenance Control.

Narrative

At FL250; I noticed that the rudder trip was not moving. I noticed that the indicator was stuck at about 0. I tried moving the rudder trim to the right and the left and neither the indicator or the slip/skid ball would move. I sent an ACARS to Dispatch advising them of the problem. The Dispatch advised me to return to the departure airport. I contacted Maintenance Control to ensure that there was nothing that could be done in-flight. I also asked if there was any operational issues that I should consider when operating the aircraft with an inoperative rudder trim system. Maintenance Control said that since there was no MEL for the problem; that they could not give me any operational guidance on how to operate the aircraft. I made the decision that since the trim failed at 180 KTS IAS and that the slip/skid indicator was centered; that I would fly the remainder of the trip at 180 KTS IAS. I tested the rudder trim switch on the ground in accordance with the origination checklist and it worked properly. Additionally; the trim switch worked when we tested it on the ramp upon returning to our departure airport. The ATR Technical Representatives queried me on the decision to return to departure airport rather than continuing to destination. I explained that Maintenance Control had requested that we return and that additionally; if the aircraft experienced an engine failure in-flight; that the aircraft would be difficult to control; so I needed to reduce the time or exposure. I noticed that the slip/trim indicator was not moving in response to my input on the rudder trim. There was no guidance in the flight manual on how to handle this problem; nor was Maintenance Control able to offer any guidance.