Abstract: An aircraft flight control surface has an actuator that employs a magneto-rheological (MR) fluid for dynamically adjusting the responsiveness of the control surface. The MR fluid may be used as a primary or secondary control in the event that a primary control actuator fails. In the event of system failures associated with the primary control, an alternate level of performance may be provided by the secondary control and communicated to the overall flight control system. This permits the control surface actuator to reactively and proactively respond to changes associated with the flight control system. If the design fails, it also permits a safe mode of operation should the ability to dynamically adjust the viscosity of the MR fluid be negatively impacted. The safe mode of operation may involve a reduced level of performance.