The four fixed body servo actuators are equal and interchangeable. The servo actuators can operate in three modes. - active mode - damping mode - centering mode In active mode the jacks are electrically controlled. In damping mode the jacks will follow the surface movement, and in centering mode the jack is hydrauli-cally maintained in neutral position. In the event of high load-factor demand that would cause one servo actuator to stall, the second actuator in damping mode automatically becomes active, both actuators will thus be active. Active mode means solenoid valves ( 1 ) de-energized and pressure line clo-sure valve ( 2 ) open, return line closing valve ( 3 ) also open, connecting the actuator to pressure and return. The High pressure flow will thus activate the mode selector valve ( 5 ).The mode selector LVDT ( 11 ) will provide mode feed back to the ELAC and SEC's. Steering input from the side stick or the FMGC is routed via the EFCS com-puter in command to the servo valve ( 4 ), controlling the actuator piston. Servo loop feed back to the EFCS is provided by the position RVDT ( 10 ). The se-cond RVDT is back-up, used in case of failure. The servo valve transducer ( 12 ) is used for monitoring. Damping mode means solenoid valves ( 1 ) are energized, powered from the standing-by ELAC and SEC. The mode selector valve is displace due to the spring, this causes the intercon-nection of the two actuator chambers through the damping orifice ( 6 ). The mode selector LVDT identifies the mode change to the EFCS. A certain amount of reserve fluid is maintained in the actuator by the reserve reservoir ( 9 ) and the check valve ( 7 ). - to hold the volume of fluid if there is a leakage or if the hydraulic fluid temperature changes. When the Elevator surface is moved by the active actuator, the actuator in damping mode will follow the movement and provide a certain resistance to the movement.
Centering mode becomes active in case of loss of power to both controlling computers and hydraulic pressure are still present. The centering device ( 13 ) mechanically keeps the actuator in the center posi-tion, preventing movement of the surface. The deflection of the remaining surface is in this case limited in order to pre-vent excessive asymmetrical load on the tailplane and the rear fuselage.Numbers in text refer to the diagram in the previous post.