Power-boosted flight control systems are used on high-speed jet aircraft. Aircraft traveling at or near supersonic speeds have such high airloads imposed upon the primary control surfaces that it is impossible for a pilot to control the aircraft without power-operated or power-boosted flight control systems. In the power-boosted system, a hydraulic actuating cylinder is built into the control linkage to assist the pilot in moving the control surface. The power-boost cylinder is still used in the rudder control system of some high-performance aircraft; however, the other primary control surfaces use the full power-operated system. In the full power-operated system, the force necessary to operate the control surface is supplied by hydraulic pressure. Each movable surface is operated by a hydraulic actuator

Figure 9-2.Hydraulically powered elevator control system.

(or power control cylinder) built into the control linkage.

Learning Objective: Recognize the functions of the three primary flight control systems (longitudinal, lateral, and directional) and the maintenance associated with each system.

Different aircraft manufacturers call units of the primary flight control system by a variety of names. The types and complexity of control mechanisms used depend on the size, speed, and mission of the aircraft. A small or low-speed aircraft may have cockpit controls connected directly to the control surface by cables or pushrods. Some aircraft have both cable and a pushrod system. See figure 9-1. The force exerted by the pilot is transferred through them to the control surfaces. On large or high-performance aircraft, the control surfaces have high pressure exerted on them by the airflow. It is difficult for the pilot to move the controls manually. As a result, hydraulic actuators are used within the linkage to aid the pilot in moving the control surface. Figure 9-2 shows a mechanically controlled, hydraulically assisted system. Because these systems reduce pilot fatigue and improve system performance, they are now commonly used. Such systems include automatic pilot, automatic landing systems, and stability augmentation systems.

Navy specifications require two separate hydraulic systems for operating the primary flight control surfaces. Current specifications call for an independent hydraulic power source for emergency operation of the primary flight control surfaces. Some manufacturers provide an emergency system powered by a motor-driven hydraulic pump. Others use a ram-air-driven turbine for operating the emergency system pump.

LONGITUDINAL CONTROL SYSTEMS 

Longitudinal control systems control pitch about the lateral axis of the aircraft. Many aircraft use a conventional elevator system for this purpose. Aircraft that operate in the higher speed ranges usually have a movable horizontal stabilizer.