Hydraulic filter maintenance consists of filter element replacement only. You must be familiar with both replacement and general inspection procedures.

Replacement of hydraulic filter elements is normally a maintenance operation performed on a periodic basis, but need for prior replacement may be indicated during routine inspection. Hydraulic filter assemblies in some aircraft and SE are equipped with indicating devices (buttons or pins) that will extend when the differential pressure across the filter exceeds a predetermined value, indicating a loaded element. Upon appearance of this indicator, it becomes necessary to verify the condition of the filter element, and replace it if required. When checking or changing filter elements, also check the functioning of any pop-up mechanism.

Indications of a loaded filter must be verified to confirm that release of the button or pin is due to a loaded filter and not a result of system mechanical shock or cold start. Verification is accomplished by resetting the indicator (manually depressing it) and operating the system at full power. If the differential pressure indicator extends again during this test, the filter element should be replaced.

It is important that the applicable MIM be consulted for specific filter element replacement procedures. The following basic principles apply to most replacement operations:

1. Removal of the filter bowl is the first step in replacing the filter element. With most filter assemblies, this operation usually consists of removing a lockwire and unscrewing the bowl from the filter head. In most filter assemblies, an automatic shutoff valve in the head will prevent fluid loss from the system when the bowl is removed.

2. Once the bowl is removed, the fluid in it is discarded, and the bowl is cleaned of sediment by flushing with clean, unused hydraulic fluid or dry cleaning solvent, P-D-680. It is important that chlorinated solvents such as MIL-C-81302 or 1,1,1-trichloroethane are not used, as their residues may have harmful effect on the system.

3. The filter element is, in most instances, removed from the head by a gentle twisting and pulling motion. Once removed, the surface of the element should be visually inspected. An excessive amount of particulate on its surface, as determined from experience, may be indicative of upstream component failure and the need for investigation. Check the solid end of filter element for "Disposable" markings. If the filter element is disposable, it should be discarded. If the filter element is not disposable, it should be cleaned and handled carefully. 

4. The replacement filter element should not be removed from its protective packing until just prior to installation. Once removed from packing, the 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.

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.