piston downward, breaking the magnetic attachment between the indicator button and the magnetic piston. This allows the red indicator to pop out, signifying that the element must be cleaned. The button or pin, once extended, remains in that position until manually reset and provides a permanent (until reset) warning of a loaded element. This feature is particularly useful where it is impossible for an operator to continuously monitor the visual indicator, such as in an aircraft. Some button indicators have a thermal lockout device incorporated in their design that prevents operation of the indicator below a certain temperature. The lockout prevents the higher differential pressure generated at cold temperatures by high fluid viscosity from causing a false indication of a loaded filter element. Differential pressure indicators are a component part of the filter assembly in which they are installed, and, as such, are normally tested and overhauled as part of the complete assembly. With some model filter assemblies, however, it is possible to replace the indicator itself, without removal of the filter assembly, if it is suspected of being inoperative or out of calibration. It is important that the external surfaces of button-type indicators be kept free of dirt or paint to ensure free movement of the button. Indications of excessive differential pressure, regardless of the type of indicator employed, should never be disregarded. All such indications must be verified and action taken, as required, to replace the loaded filter element. Failure to replace a loaded element can result in system starvation, filter element collapse, or the loss of filtration where bypass assemblies are used. Verification of loaded filter indications is particularly important with button-type indicators, as they may have been falsely triggered by mechanical shock, vibration, or cold start of the system. Verification is usually obtained by manually resetting the indicator and operating the system to create a maximum flow demand, ensuring that the fluid is at near normal operating temperatures. Maintenance 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 7-35