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PRESSURE INDICATORS

Aviation Structural Mechanic (H&S) 3&2 - How airplanes are built and how to maintain them
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perform the same function—that of storing an extra volume of hydraulic fluid under pressure. Maintenance Accumulators should be visually examined for indications of external hydraulic fluid leaks. They should then be examined for external air leaks by brushing the exterior with soapy water, which will form bubbles where the air leaks occur. The air valve assembly should be loosened to examine the accumulator for internal leaks. If hydraulic fluid comes out of the air valve, the accumulator should be removed and replaced. The overhaul or repair of the accumulator is not a line maintenance function, but it is the responsibility of an intermediate-level activity. The air preload pressure should be checked after relieving the hydraulic system pressure by operating the wing flaps or other hydraulically actuated unit. The majority of the accumulators installed in naval aircraft are equipped with air pressure gauges for this purpose. When the accumulator is not equipped with a high-pressure air gauge, you may install one at the air preload fitting for this purpose. The required pressure can be found in the MIM for each aircraft. The preload pressure may be checked by another method in case the accumulator is not equipped with an air pressure gauge. With the system pressure (as indicated by the cockpit gauge) at the normal operating value, relieve system pressure by operating the wing flaps or another unit slowly. The pressure gauge reading must be watched carefully. The last reading before the indicator needle drops suddenly to zero is accepted as the accumulator preload air pressure. Before disassembly of any accumulator, ensure that the air preload has been completely exhausted. This may be accomplished by loosening the swivel nut on the air filler valve until all air is out; then remove the valve. Servicing The purpose of the hydraulic system accumulator is to store an extra volume of fluid under pressure. The energy stored in an accumulator is used for various purposes, such as the actuation of a unit in the event of normal hydraulic system failure. For example, sufficient energy can be stored in an accumulator for several applications of the wheel brakes. Most accumulators are installed with an air gauge and a high-pressure air valve mounted on a panel of the structure near the accumulator. Figure 7-35 shows the brake system accumulator installation used on one type of aircraft. The air valve used in the accumulator installations is usually the same type as that used on shock struts. Figure 7-35.—Accumulator air charge valve and gauge installation. 7-38







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