Figure 4-22 shows a basic system with the addition of a power-driven pump and other essential components. These components are the filter, pressure regulator,   accumulator,   pressure   gauge,   relief   valve, and two check valves. The function of these components is described below. The   filter   (fig.   4-22)   removes   foreign   particles from the fluid, preventing moisture, dust, grit, and other undesirable matter from entering the system. The    pressure   regulator    (fig.   4-22)   unloads   or relieves   the   power-driven   pump   when   the   desired pressure in the system is reached. Therefore, it is often referred  to  as  an   unloading  valve.  With  none  of  the actuating   units   operating,   the   pressure   in   the   line between the pump and selector valve builds up to the desired    point.    A    valve    in    the    pressure    regulator automatically opens and fluid is bypassed back to the reservoir.  (The  bypass  line  is  shown  in  figure  4-22, leading from the pressure regulator to the return line.) NOTE: Many  aircraft  hydraulic  systems  do  not use a pressure regulator. These systems use a pump that automatically  adjusts  to  supply  the  proper  volume  of fluid as needed. The  accumulator  serves a twofold purpose. 1. It  serves  as  a  cushion  or  shock  absorber  by maintaining an even pressure in the system. 2. It stores enough fluid under pressure to provide for  emergency  operation  of  certain  actuating units. The  accumulator  is  designed  with  a  compressed-air chamber    separated    from    the    fluid    by    a    flexible diaphragm, or a removable piston. The pressure    gauge indicates    the    amount    of pressure in the system. The  relief  valve  is  a  safety  valve  installed  in  the system. When fluid is bypassed through the valve to the return   line,   it   returns   to   the   reservoir.   This   action prevents excessive pressure in the system. Check   valves allow   the   flow   of   fluid   in   one direction    only.    There    are    numerous    check    valves installed  at  various  points  in  the  lines  of  all  aircraft hydraulic systems. A careful study of figure 4-22 shows why the two check valves are necessary in this system. One check valve prevents power pump pressure from entering the hand-pump line. The other valve prevents hand-pump    pressure    from    being    directed    to    the accumulator. HYDRAULIC CONTAMINATION Hydraulic    contamination    is    defined    as foreign material in the hydraulic system of an aircraft. Foreign material might be grit, sand, dirt, dust, rust, water, or any other substance that is not soluble in the hydraulic fluid. There    are    two    basic    ways    to    contaminate    a hydraulic  system.  One  is  to  inject  particles,  and  the other is to intermix fluids, including water. Particle     contamination     in     a     system     may     be self-generated through normal wear of system components.  It  is  the  injection  of  contaminants  from outside that usually causes the most trouble. Regardless of its origin, any form of contamination in the hydraulic system   will   slow   performance.   In   extreme   cases,   it seriously affects safety. A  single  grain  of  sand  or  grit  can  cause  internal failure of a hydraulic component. Usually, this type of contamination comes from poor servicing and fluid-handling procedures. For this reason, the highest 4-20 ANf0422 1.   Reservoir 2.   Power pump 3.   Filter 4.   Pressure regulator 5.   Accumulator 6.   Check valves 7.   Hand pump 8.   Pressure gauge 9.   Relief valve 10.   Selector valve 11.   Actuating unit Figure 4-22.—Basic hydraulic system with addition of power pump.