According to the military specifications, which establish the requirements for aircraft hydraulic systems, all hydraulically operated systems considered essential to flight safety or landing must have provisions for emergency actuation. The specifications further state that these emergency systems may use hydraulic fluid, compressed gas, directed mechanical linkage, or gravity for their actuation.

Some aircraft use mechanical linkage or gravity in conjunction with pneumatic pressure for emergency actuation of landing gear and other actuating systems where limited actuation is required. Most other essential hydraulically operated systems have emergency power systems that are powered by a hand pump, electric motor-driven pump, ram-air turbine-driven pump, or a pneumatic compressor. On some aircraft the hand pump is a part of the auxiliary hydraulic system and is not considered as part of the emergency power systems. The hand pump is used for ground operation of the canopy, extensible electronics platform, nose radome opening, and to recharge the brake accumulator. These systems may be operated by aircraft system pressure or, if the aircraft is shutdown, they may be powered by the auxiliary electric motor-driven hydraulic pump or the hand pump.

Learning Objective: Identify the various hydraulic system components and recognize the procedures required for their maintenance.

Various types of hydraulic components makeup a power system. The components discussed in this chapter are representative of those with which you will most likely be working. Values such as pressure, temperature, and instructional tolerances have been given to provide detail in the coverage.

When actually performing the maintenance procedures discussed, the exact location and make up of the various hydraulic and pneumatic components will vary with the design of the hydraulic system. You should consult the current applicable technical publication for the latest information on items such as location, pressure, temperature, and tolerances.

The reservoir is a tank in which an adequate supply of fluid for the system is stored. Fluid flows from the reservoir to the pump, where it is forced through the system and eventually returned to the reservoir.

The reservoir not only supplies the operating needs of the system, but it also replenishes fluid lost through leakage. Furthermore, the reservoir serves as an overflow basin for excess fluid forced out of the system by thermal expansion (the increase of fluid volume caused by temperature changes), the accumulators, and by piston and rod displacement. The reservoir also furnishes a place for the fluid to purge itself of air bubbles that may enter the system. Foreign matter picked up in the system may also be separated from the fluid in the reservoir, or as it flows through line filters.

Figure 7-3.Hydraulic reservoir instruction plate.

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:

7. System pressure (accumulator charged or discharged)

Additional information may be added, when required, such as the following:

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.