Cylindrical accumulators consist of a cylinder and piston assembly. End caps are attached to both ends of the cylinder. The internal piston separates the fluid and air/nitrogen chambers. Both the end caps and piston are sealed with gaskets and packings to prevent external leakage around the end caps and internal leakage between the chambers. In one end cap, a hydraulic fitting is used to attach the fluid chamber to the hydraulic system. In the other end cap, an air filler valve is installed to perform the same function as the filler valve installed in the spherical accumulator.

In operation, the compressed-air chamber is charged to a predetermined pressure, which is somewhat lower than the system operating pressure. This initial charge is referred to as the accumulator preload.

As an example of accumulator operation, let us assume that the cylindrical accumulator in figure 7-34 is designed for a preload of 1,300 psi in a 3,000 psi system. When the initial charge of 1,300 psi is introduced into the unit, hydraulic system pressure is zero. As air pressure is applied through the air pressure port, it moves the piston toward the opposite end until it bottoms. If the air behind the piston has a pressure of 1,300 psi, the hydraulic system pump will have to create a pressure within the system greater than 1,300 psi before the hydraulic fluid can actuate the piston. Thus, at 1,301 psi the piston will start to move within the cylinder, compressing the air as it moves. At 2,000 psi it will have backed up several inches. At 3,000 psi the piston will have backed up to its normal operating position, compressing the air until it occupies a space less than one-half the length of the cylinder.

When actuation of hydraulic units lowers the system pressure, the compressed air will expand against the piston, forcing fluid from the accumulator. This supplies an instantaneous supply of fluid to the hydraulic system.

Many aircraft have several accumulators in the hydraulic system. There may be a main system accumulator and an emergency system accumulator. There may also be auxiliary accumulators located in various unit systems. Regardless of the number and their location within the system, all accumulators perform the same function-that of storing an extra volume of hydraulic fluid under pressure.

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.

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.

To service an accumulator, the hydraulic pressure that is trapped in the accumulator must be relieved. This is accomplished by actuating the units involved. For example, the hydraulic pressure in a brake accumulator may be relieved by applying the emergency brake several times. When the hydraulic pressure is relieved, the accumulator gauge should indicate the air or nitrogen pressure specified for the particular accumulator installation. If the pressure indicated is below the specified pressure, the accumulator must be recharged with dry compressed air or nitrogen.