The filter assembly is composed of three basic units. The units are a head assembly, a bowl, and a filter element. See figure 7-32.

Figure 7-31.Typical filter arrangement in hydraulic system.

HEAD ASSEMBLY. The head assembly is secured to the aircraft structure and connecting lines. The head assembly of some filters have a pressure-operated bypass valve, which will route the hydraulic fluid directly from the inlet to the outlet port if the filter element becomes loaded with foreign matter.

BOWL. The bowl is the housing that holds the element to the filter head, and it is removed when element replacement is required.

FILTER ELEMENT. The filter element may be of the 5-micron noncleanable, woven mesh, micronic, porous metal, or magnetic type. The micronic and 5-micron noncleanable elements have nonmetallic filter media, and are discarded when removed. Porous metal, woven mesh, and magnetic filter elements are usually designed to be cleaned and reused. However, some metallic filters are considered noncleanable and are normally discarded. Noncleanable filter elements rated at 5-microns (absolute) represent the current state of the art in hydraulic filtration. Elements of this type afford significantly improved filtration and have greater dirt-holding capacities than other types of elements of the same physical size. They are particularly effective in controlling particles in the 1- to 10-micron size range, which are normally passed by other types of elements, and they are capable of maintaining a hydraulic system at much cleaner levels than could previously be achieved. The use of 5-micron (absolute) filters is presently specified for all new design aircraft, and they are being retrofitted to existing fleet aircraft where practicable. The most common 5-micron filter medium is composed of organic and inorganic fibers integrally bonded by epoxy resin and faced with a metallic mesh upstream and downstream for protection and added mechanical strength. Filters of this type are not to be cleaned under any circumstances, and will be marked DISPOSABLE or NONCLEANABLE, usually on the bottom end cap.

Five-micron, noncleanable, hydraulic filter elements should be replaced with new elements during specified maintenance inspection intervals in accordance with the applicable procedures. Refer to the applicable MIM or maintenance requirement cards (MRC) for replacement intervals and procedures.

Another 5-micron filter medium of recent design employs layers of very fine stainless steel fibers drawn into a random but controlled matrix. The matrix is then processed by an exclusive procedure, which in successive steps compresses and sinters (bonds all wires at their crossing points) the material into a thin layer with controlled filtration charac-teristics. Filter elements of this material may be cleanable or noncleanable, depending upon their construction, and are marked accordingly.

To ensure delivery of contaminant-free hydraulic fluid, all SE must be provided with 3-micron (absolute) non-bypass filtration in their fluid discharge or output pressure lines. With many test stands, the filter used for this application, in addition to having a low micron rating, must be capable of withstanding high-collapse pressures and holding large amounts of dirt.

Unlike most filter elements, 3-micron, high-- pressure SE filters are not normally replaced on a prescribed periodic basis. Because of their large dirt-holding capacity and nature of service, it is more effective to replace such elements only when indicated as being loaded by their associated differential pressure indicators. Element replacement procedures vary with the particular type, and applicable maintenance instructions should be consulted for specific procedures.