element must be carefully handled to protect it from contamination and mechanical damage. 5. The replacement element is installed in reverse order of its removal. In most instances the element is inserted up into the head, employing a gentle twisting motion.    O-ring seals located in the head, or sometimes in the element itself, prevent fluid from flowing around the element. It is important that these seals be inspected and replaced, if required, in accordance with the applicable MIM. 6. Prior to installation of the cleaned filter bowl, the bowl is first filled with new filtered hydraulic fluid to minimize the introduction of air into the hydraulic system.    It is important that the fluid used for this operation be obtained only from an authorized hydraulic fill service unit. 7. Once filled, the filter bowl is carefully and slowly slid up over the installed element and screwed into the head. A quantity of fluid from the bowl will normally be displaced by the element and spilled. Provisions must be made to collect or absorb it. 8. The installed filter bowl should be torqued to the value specified in the applicable MIM. The bowl is then lockwired, using standard tools and the lockwire provisions in the filter assembly. 9. All filter element installations should be followed by test and inspection of the system to ensure proper operation. This is generally accomplished by operating the system at its normal pressure and flow rates and inspecting for external leakage at the filter assembly and for indications of excessive differential pressure. Any external leakage is unacceptable, and requires that the system be shut down and the problem corrected. 10. Should the filter assembly differential pressure indicator continue to extend after a new element has been installed, the indicator itself is probably defective. Consult the maintenance instructions to determine what corrective action is to be taken. Inspect the filter element as follows: 1. Visually inspect the element for dents, broken wires, holes, creases, and sharp corners of pleats. Permissible damage is to be confined to small dents that will not impede the required flow, or increase the filter pressure drop beyond tolerance, or fail to pass the required bubble test point. Deeper dents, broken wires, holes, creases, and sharp corners of pleats are cause for rejection of elements. 2. Remove the O-ring from the filter element and visually inspect the O-ring groove, including chamfers, for nicks, dents, visible roughness, out-of-roundness, and pitting. Blend out nicks and/or scratches that are deeper than 0.002 inch with crocus cloth P-C-458. 3. Visually inspect mating surfaces, including chamfers, or other parts that mate with the O-ring grooves. Make sure that all surfaces (grooves and mating surfaces) are smooth and capable of sealing with the O-ring installed. 4. Dispose of unacceptable filter elements according to existing instructions. ACCUMULATORS The purpose of the accumulator in a hydraulic system is to store a volume of fluid under pressure. There are several reasons why it is advantageous to store a volume of fluid under pressure. Some of these are listed below: 1. An accumulator acts as a cushion against pressure surges that may be caused by the pulsating fluid delivery from the pump or from system operations. 2. The accumulator supplements the pump’s output when the pump is under a peak load by storing energy in the form of fluid under pressure. 3. The energy stored in the accumulator may be used to actuate a unit in the event of normal hydraulic system failure. For example, sufficient energy can be stored in the accumulator for several applications of the wheel brakes. There are two general types of accumulators in use on naval aircraft. They are the spherical type and the cylindrical type. Until a few years ago, the spherical type was the more commonly used accumulator; however, the cylindrical type has proved more satisfactory for high-pressure hydraulic systems, and is now more commonly used than the spherical type. Examples of both types are shown in figure 7-34. Spherical Type The spherical type accumulator is constructed in two halves that are screwed together. A synthetic rubber diaphragm is installed between both halves, making two chambers. Two threaded openings exist 7-36