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 aircraft’s 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. MAINTENANCE OF ACTUATING CYLINDERS During preventive maintenance inspections, you inspect actuating cylinders in accordance with the applicable maintenance requirements cards (MRCs) for the specific aircraft. Actuating cylinders are inspected for leakage and binding. You should clean the exposed portion of the piston shaft with a dry-cleaning solvent, and then wipe it with a clean cloth moistened with hydraulic fluid. All mounting fittings are lubricated with specified grease only. NOTE: All lubrication fittings and lubri- cation areas must be cleaned prior to lubrication, and all excess lubricants must be removed at its completion. External leakage is the most common trouble encountered with actuating cylinders. This can be caused by static or dynamic seals. Static seal leakage around end caps or fittings may be stopped by tightening the affected components or replacing the leaking seal. Dynamic seal leakage around an actuator shaft will require seal replacement. Refer to the appropriate MIM or 03 manual for specific maintenance instructions. WARNING Applying too much torque while tightening fittings or other components under pressure may cause catastrophic failure. Such failures can result in injury to personnel or damage to the aircraft. Internal leakage is harder to detect. This leakage is usually caused by failure of piston seals, and will require repair. Internal leakage is usually indicated by weak, sluggish, or slow movement of the actuator. Refer to the appropriate MIM or 03 manual for repair instructions. This problem is usually resolved by replacement of the actuator. After the repairs are made, you must test the actuator to verify its performance. HYDRAULIC MOTORS Hydraulic motors are used to convert hydraulic pressure into rotary mechanical motion. The type of hydraulic motor used in naval aircraft is similar in general design and construction to the piston-type pumps. The difference in the operation of a hydraulic motor and a hydraulic pump is as follows: In the operation of a pump, when the drive shaft is rotated, fluid is drawn into one port and forced out the other under pressure. This procedure is reversed in a hydraulic motor. By directing fluid already under pressure into one of the ports, pressure will force the shaft to rotate. Fluid will then pass out the other port, 8-6