prevents loss of hydraulic fluid in the event of utility or normal system rupture. As a minimum requirement, filters are provided in each system pressure line, return line, and pump bypass or case drain line. Where hydraulic sequencing is critical, each  sequence  valve  is  protected  from  contamination  in each direction of flow by a screen-type filter. The filter is usually  included  as  a  part  of  the  sequence  valve.  The pressure line falters clean all fluids before they enter any major  equipment.  If  there  are  only  two  hydraulic  systems, the primary system is known as the No. 1 power control system (PC-1). The system supplying the other half of the flight control tandem actuating mechanisms and the utility hydraulic system is known as PC-2. The PC-2 system is also known as the combined hydraulic system. If there are three hydraulic power systems, they are generally identified as PC-1, PC-2, and utility system. Some manufacturers label the utility system PC-3. Each system has its own reservoir,  hydraulic  pump(s),  and  plumbing. Military  specifications,  MIL-H-5440  (series), provide  complete  design,  installation,  and  data requirements  for  aircraft  hydraulic  systems.  These specifications provide reference to all other specifications concerning aircraft hydraulic systems. Items such as hose assemblies,  hose  support  requirements,  minimum  bend radii, types of pumps, and types and classes of systems are found in the specifications. Many maintenance instruction manuals (MIMs) refer  to  aircraft  hydraulic  systems  as  being  open center  or  closed  center  systems.  The  following paragraphs  provide  a  discussion  of  these  systems. Open Center An open center system is one having fluid flow, but no pressure in the system when the actuating mechanisms are idle. The pump circulates the fluid from the reservoir, through the selector valves, and back to the reservoir. Figure 7-1 shows a basic open center system. The open center system may employ any number of subsystems, with a selector valve for each subsystem. Unlike the closed center system, the selector valves of the open center system are always connected in series with each other. In this arrangement, the system pressure line goes through each selector valve, Fluid is always allowed free passage through each selector valve and back to the reservoir until one of the selector valves is positioned to operate  a  mechanism. When one of the selector valves is positioned to operate an actuating device, fluid is directed from the pump through one of the working lines to the actuator. See view B of figure 7-1. With the selector valve in this position, the flow of fluid through the valve to the reservoir is blocked. The pressure builds up in the system  to  overcome  the  resistance  and  moves  the piston of the actuating cylinder, The fluid from the opposite end of the actuator returns to the selector valve and flows back to the reservoir. Operation of the  system  following  actuation  of  the  component depends  on  the  type  of  selector  valve  being  used. Several types of selector valves are used in conjunction with the open center system. One type is both manually engaged  and  manually  disengaged.  First  the  valve  is manually moved to an operating position. Then, the actuating mechanism reaches the end of its operating cycle, and the pump output continues until the system relief valve relieves the pressure. The relief valve unseats and allows the fluid to flow back to the reservoir. The system pressure remains at the relief valve set pressure until the selector valve is manually returned to the neutral position. This action reopens the open center flow and allows the system pressure to drop to line resistance pressure. The manually engaged and pressure disengaged type  of  selector  valve  is  similar  to  the  valve  pre- viously discussed. When the actuating mechanism reaches  the  end  of  its  cycle,  the  pressure  continues  to rise  to  a  predetermined  pressure.  The  valve  auto- matically returns to the neutral position and to open center  flow. Closed Center In  the  closed  center  system,  the  fluid  is  under pressure  whenever  the  power  pump  is  operating. Figure  7-2  shows  a  complex  closed  center  system. Figure 7-1.—Basic open center hydraulic system. 7-2