The power pump may be one used with a separate pressure regulator control. The power pump may be used with an integral pressure control valve that eliminates the need for a pressure regulator. This system differs from the open center system in that the selector or directional control valves are arranged in parallel and not in series. The means of controlling pump pressure will vary in the closed center system. If a constant delivery pump is used, the system pressure will be regulated by a pressure regulator. A relief valve acts as a backup safety device in case the regulator fails. If a variable displacement pump is used, system pressure is controlled by the pump’s integral pressure mechanism compensator. The compensator automatically varies the volume output. When pressure approaches normal system pressure, the compensator begins to reduce the flow output of the pump. The pump is fully compensated (near zero flow) when normal system pressure is attained. When the pump is in this fully compensated condition, its internal bypass mechanism provides fluid circulation through the pump for cooling and lubrication. A relief valve is installed in the system as a safety backup. An advantage of the open center system over the closed center system is that the continuous pressurization of the system is eliminated. Since the pressure is built up gradually after the selector valve is moved to an operating position, there is very little shock from pressure surges. This action provides a smoother operation of the actuating mechanisms. The operation is slower than the closed center system, in which the pressure is available the moment the selector valve is positioned. Since most aircraft applications require instantaneous operation, closed center systems are the most widely used. Power systems are designed to produce and maintain a given pressure. The pressure output of most of the Navy’s high-performance aircraft is 3,000 psi. The hydraulic system, shown in figure 7-2, is an example of a representative 3,000 psi hydraulic power system. The aircraft has three independent hydraulic power systems. The two primary systems are the flight hydraulic power system and the combined hydraulic power system. These systems are pressurized by two independent engine-driven hydraulic pumps on each engine. The auxiliary power system also operates on 3,000 psi pressure. It is pressurized by the hydraulic hand pump and/or the electric motor-driven hydraulic pump. The auxiliary power system is similar to the combined hydraulic power system. The primary difference is that the combined system supplies hydraulic pressure to utility hydraulic circuits and the flight controls. The hydraulic control valves and actuators that operate the primary flight controls are of the tandem construction type. This design permits operation from either or both of the two power systems. With this arrangement, either engine can fail or be shut down without complete loss of hydraulic power to either system. The flight system reservoir supplies fluid to the two engine-driven flight system pumps. The combined system reservoir supplies fluid to the two engine-driven combined system pumps and to the auxiliary hydraulic power system. Both reservoirs are of the pressurized piston type. They are pressurized by engine bleed air during engine operations and by an external air (nitrogen) source during maintenance operations. Hydraulic system pressure is indicated on the integrated hydraulic pressure indicator. This indicator displays the output pressure of the flight and combined hydraulic power systems. The flight hydraulic power system provides power for the operation of the rudder, stabilizer, and flaperons. It also provides power for operation of the automatic flight control system actuators, which are an integral part of the rudder and stabilizer control surface actuators. The flight hydraulic system also controls the automatic operation of the isolation valve. This valve is a part of the combined hydraulic system. The combined hydraulic power system consists of two parallel circuits—one to power the primary systems and the other to power the secondary systems. The primary system consists of spin recovery, rudder, stabilizer flaperon, speed brakes, and electric ram air turbine systems. The secondary system consists of wing slats, wing flaps, wing fold, landing gear, arresting gear, wheel brakes, nosewheel steering, and the nose strut locking systems. The isolation valve shuts off flow to the secondary systems during flight and limits the combined system’s pressure requirements to operation of the primary circuit. Operation of the isolation valve is both automatic and manual. The reservoir pressurization system provides the reservoir with a differential pressure of 40 psi to prevent engine-driven pump cavitation. The pressure is maintained at 40 psi by the air regulator. In the event of regulator failure, the relief valve installed between the regulator and the reservoir prevents overpressurization. The relief valve opens at 50 psi. 7-4