Actuators are used in conjunction with power-operated flight control systems. Their function is to assist the pilot in handling the aircraft, in the same way as power steering aids in handling an automobile. In a power-operated flight control system, all the force necessary for deflecting the control surface is supplied by hydraulic pressure. Each movable surface is operated by a hydraulic actuator incorporated in the control linkage. Some aircraft manufacturers refer to these units as power control cylinders; however, all flight control system actuators and power control cylinders perform the same function, and are similar in principle of operation.

Figure 8-4.Typical finger-lock actuating cylinder.

A typical flight control surface actuator is shown in figure 8-5. This is a tandem-type hydraulic unit, which means, in this case, that two control valves are incorporated within a common housing. One of the control valves is connected to the aircrafts primary flight control hydraulic system, while the other is connected to a separate hydraulic system. This is a typical arrangement since Navy specifi-cations require two independent hydraulic systems for operation of the primary flight control systems on all high-performance aircraft.

Although the two control valves in the actuator are interconnected mechanically by a synchronizing rod, they are not interconnected hydraulically. The purpose of the synchronizing rod is to equalize the flow of fluid into the actuator piston chambers. Because the two control valves operate independently of each other as far as hydraulic pressure is concerned, failure of either hydraulic system does not render the actuator inoperative. Failure of one system does reduce the output force by one-half; however, this force is sufficient to permit handling of the aircraft at certain airspeeds (always well above that required for a safe landing). 

This complete actuator consists of the two isolated piston chambers, a shaft assembly with two pistons, two end cap assemblies, the two control valves, and the previously mentioned synchronizing red.

In this particular installation, the piston shaft end is attached to the aircraft structure and remains stationary. The cylinder body is attached to the control surface, and provides control surface deflection by its movement. Two adjustable stops are provided as a means of adjusting actuator movement, thereby limiting the travel of the control surface. When these steps are used in an aileron or elevator control system, one stop limits the UP travel, and the other limits the DOWN travel. In a rudder system, one stop limits the travel to the right, and the other to the left.