Most nonpressurized reservoirs contain filters to maintain the hydraulic fluid in a clean state, free from foreign matter. They are usually located in filler necks and internally within the reservoir. The mesh-type filter (finger strainer), usually installed in the filler neck, removes foreign particles from fluid that is added to the reservoir. Internally installed filters clean the fluid as it returns to the reservoir from the system. This type of installation may have a bypass valve incorporated to allow fluid to bypass the filter if it becomes clogged. Some modern aircraft hydraulic reservoirs do not incorporate this feature. All reservoirs containing filters are designed to permit easy removal of the filter element for cleaning or replacement. A reservoir instruction plate is usually attached to the reservoir, or it may be attached to the aircraft structure adjacent to the filler opening. Navy specifications designate the minimum information that must be contained on this plate. Figure 7-3 shows the reservoir instruction plate. Information on an instruction plate must include the following: 1. 2. 3. 4. 5. 6. 7. 8. Simple and complete instructions for tilling Reservoir fluid capacity at full level Full level indication Refill level indication Specification number and color of fluid Position of operating cylinders during filling System pressure (accumulator charged or discharged) Instructions regarding air bleeding Additional information may be added, when required, such as the following: 1. 2. 3. 4. Additional full and refill levels under various conditions of system pressure Safety precautions Filter element servicing information Total fluid capacity of the system There are two classes of hydraulic reservoirs— class I and class II. Class I reservoirs are constructed in such a manner that the air and hydraulic fluid are not separated. Class II reservoirs are constructed in such a manner that the pressurizing agent and fluid chambers are separated. This is accomplished by installing a piston between the chambers. Nonpressurized reservoirs are vented to the atmosphere so the reservoir can “breathe.” This is done to prevent a vacuum from being formed as the fluid level in the reservoir is lowered. The vent also makes it possible for air that has entered the system to find a means of escape. The reservoir on aircraft designed for high- altitude flying is usually pressurized. Pressurizing assures a positive flow of fluid to the pump at high altitudes when low atmospheric pressures are encountered. On some aircraft, the reservoir is pressurized by bleed air taken from the compressor section of the engine. On others, the reservoir may be pressurized by hydraulic system pressure. Nonpressurized Reservoirs Nonpressurized reservoirs are used in several transport, patrol, and utility aircraft. These aircraft are not designed for violent maneuvers; in some cases, they do not fly at high altitudes. Those aircraft that incorporate nonpressurized reservoirs and fly at high altitudes have the reservoirs installed within a pressurized area. High altitude in this situation means an altitude where atmospheric pressure is inadequate to maintain sufficient flow of fluid to the hydraulic pumps. Most nonpressurized reservoirs are constructed in a cylindrical shape. The outer housing is manufactured from a strong corrosion-resistant metal. Filter elements are normally installed internality within the reservoir to clean returning system hydraulic fluid. In some of the older aircraft, a filter bypass valve is incorporated to allow fluid to bypass the filter in the event the filter becomes clogged. Reservoirs serviced by pouring fluid directly into the reservoir have a filler strainer (finger strainer) assembly incorporated within the filler well to strain out impurities as the fluid enters the reservoir. Generally, reservoirs described in the above paragraph use a visual gauge to indicate the fluid quantity. Gauges incorporated on or in the reservoir may be either a glass tube, a direct reading gauge, or a float-type rod, which is visible through a transparent dome. In some cases, the fluid quantity may also be read in the cockpit through the use of quantity transmitters. 7-7