Dash 8-200
Flight Crew Air Transport Pilot (ATP)
Other / Unknown
Aircraft Equipment Problem Less Severe
Flight Crew Landed in Emergency Condition; Flight Crew Returned To Departure Airport
Aircraft
During climbout the FO experienced a gradual left wing down tendency. He attempted to counter this with RWD trim. Upon clicking RWD on the aileron trim the trim went to full LWD. Attempts to move it to the RWD position were not successful and the EA checklist for Runaway Aileron Trim was accomplished. We notified ATC immediately that we would like to stop the climb at 5;000 to run checklist and may return to the field. The EA checklist was completed however the situation remained. The FO was able to with right control force hold wings level with some effort. We adjusted the speed through a range of speeds to ensure the plane would be controllable at low level low speed and determined that the condition was not made worse with a reduction of airspeed. We however felt that it would be unsafe to continue to fly for 40 minutes to burn enough gas to land below max landing weight. The flight attendant was notified that though we would be arriving in an emergency state; we were anticipating a normal landing and an evacuation was not foreseeable. We informed the passengers of the same thing.  During the landing roll; the FO set the plane down smoothly and brakes were lightly used as the plane was slowing safely without them.
Dash 8 Captain reported experiencing a runaway aileron trim malfunction. The flight returned to the departure airport after unsuccessful troubleshooting.