A method of increasing the control authority of redundant stability and control augmentation system (SCAS) actuators by utilizing feedback between systems such that one system may compensate for the position of a failed actuator of the other system. Each system uses an appropriate combination of reliable and unreliable inputs such that unreliable inputs cannot inappropriately utilize the increased authority. Each system may reconfigure itself when the other system actuator fails at certain positions so that the pilot or other upstream input maintains sufficient control authority of the aircraft.

An aerofoil has a main portion of aerofoil cross section and a trailing edge portion (12) movable with respect to the main portion by an actuating device (14), the trailing edge portion (12) is movable at or about an area/point of flexion A, the area/point of flexion A is located at or adjacent a pressure or suction surface of the aerofoil, the trailing edge portion (12) is connected to the main portion by first and second connection members (18, 20), the first connection member (18) extends inwardly away from the area/point of flexion A and has an end which is directly or indirectly connected to the main portion, the first and second connection members (18, 20) are flexible.

An improved actuator arrangement for operating a driven element with different actuators. The actuator arrangement comprises a first actuator that drives a driven element in a first direction via a first output member and a second actuator that drives the driven element in a second direction opposite to the first direction via a second output member. The first and second actuators are configured to work in a counter-acting manner to move the driven element in the first and second directions. An additional connecting and supporting device is connected between the first and second output members to support the output members to bear transversal forces transversal to the moving direction and/or to bear pre-load force for pre-loading the actuators.