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.