Directional control systems provide a means of controlling and/or stabilizing the aircraft about its vertical axis. Most Navy aircraft use conventional-type rudder control systems for this purpose.

The rudder control system, shown in figure 9-15, is operated by the rudder pedals in the cockpit. The system is powered hydraulically through the rudder actuator. In the event of hydraulic power failure, the hydraulic portion of the power system is bypassed. The system is then powered mechanically through control cables and linkage. An aerodynamic irreversible hydraulic system is employed in the rudder system. To accomplish trim, the complete rudder surface is repositioned.

The actuation of the rudder pedals causes the control cables to move a cable sector assembly. The cable sector, through a push-pull tube and linkage, actuates the power mechanism. The rudder actuator deflects the rudder to the left or to the right. A load-feel bungee is connected to the push-pull tube, and is compressed when the push-pull tube is actuated. When the pedals are released, the compressed bungee returns the system to the neutral position. In the event of hydraulic failure, a slip link allows movement of the control valve linkage to port hydraulic fluid from the actuating cylinder. Then the cylinder can be mechanically driven by pilot input during manual operation. In manual operation, surface travels are reduced by the lost-motion effect of the slip link.

The load-feel bungee is also the connecting link from the rudder trim actuator to the power mechanism. When the trim actuator is operated, the bungee repositions the power mechanism. The power mechanisms deflect the rudder for nose-left and nose-right trim. Figure 9-15 is a functional schematic of the operation of the rudder control system. The rudder power mechanism is actuated when movement from the cable sector assembly is transmitted through the push-pull tube to the primary control crank. The crank is connected to the load-feel bungee, a slip link to the secondary crank, a link and spring to the pedal position transmitter, and a link to the control valve of the actuator assembly. The actuator assembly consists of an electromechanical dual input control valve, a rudder surface position transmitter, and a power cylinder. When the mechanism linkage is actuated, the control valve directs hydraulic pressure from both the utility hydraulic system and the surface control hydraulic system to the power cylinder. The valve directs the hydraulic pressure to two separate chambers in the cylinder. Each chamber has a separate piston that is mounted on as common shaft. The shaft is connected to a push-pull tube that moves the rudder. The actuator assembly normally operates from both hydraulic systems. If one system should fail, the other supplies sufficient pressure to operate the rudder with some lost hinge movement. In the event both hydraulic systems fail, the slip link will allow movement of the control valve linkage to port hydraulic fluid from the actuating cylinder.

When the automatic flight control system is engaged, the actuator initiates the movement of the rudder system through the electrical impulse received by the control valve from the surface control amplifier. The pedal position transmitter and the rudder surface transmitter function only when the automatic flight control system is engaged. Rudder pedal movement transfers mechanically to the left and right rudder servo cylinders through the rudder feel assembly, the yaw summing network, and the reversing network. These servo cylinders, normally powered by the flight and combined hydraulic power systems, move the rudders. If both hydraulic systems fail, the rudder servo cylinders automatically receive hydraulic power from the backup hydraulic system (flight control backup module). The rudder trim switch on the EXT ENVIRONMENT/THROTTLE control panel enables trimming of the aircraft in yaw. Setting the switch to L or R provides a trim-left or trim-right input, respectively, to the rudder trim actuator. The actuator provides rudder movement through the rudder-feel assembly, the yaw summing network, the reversing network, and the rudder servo cylinders.