An actuator system for controlling a movable surface has a main shaft connected between a power drive unit and the movable surface to transmit a command from the power drive unit to move the movable surface. If a failure in the main shaft is detected a failure control device connects a secondary shaft from the movable surface to the power drive unit e.g. by releasing a solenoid which releases gears so as to permit differential rotations between an outer ring and a sun gear of a planetary gear system, within prescribed limits. When these limits are exceeded, the solenoid brake is de-energised braking the planetary gear outer ring and tying the gears through an epicycle ratio corresponding to the ratio of gears connecting the main and secondary shafts. The gear system acts as a low mass asymmetry brake that allows post-failure operation of the surface control.