main system’s fluid supply become depleted, adequate fluid  is left for operation of the emergency system.  Figure 12-7 illustrates that the engine-driven pump (EDP) is not able  to draw fluid any more if the reservoir gets depleted below  the standpipe. The alternating current motor-driven pump  (ACMP) still has a supply of fluid for emergency operations. 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. Reservoirs  are either pressurized or nonpressurized.  Baffles and/or fins are incorporated in most reservoirs to keep  the fluid within the reservoir from having random movement,  such as vortexing (swirling) and surging. These conditions  can cause fluid to foam and air to enter the pump along with  the fluid. Many reservoirs incorporate strainers in the filler  neck to prevent the entry of foreign matter during servicing.  These strainers are made of fine mesh screening and are  usually referred to as finger strainers because of their shape.  Finger strainers should never be removed or punctured as a  means of speeding up the pouring of fluid into the reservoir.  Reservoirs could have an internal trap to make sure fluid goes  to the pumps during negative-G conditions. Nonpressurized reservoirs are used in aircraft that are not  designed for violent maneuvers, do not fly at high altitudes, or  in which the reservoir is located in the pressurized area of the  aircraft. 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 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 can be serviced by pouring  fluid directly into the reservoir through a filler strainer (finger  strainer) assembly incorporated within the filler well to strain  out impurities as the fluid enters the reservoir. Generally,  nonpressurized reservoirs use a visual gauge to indicate the  Most aircraft have emergency hydraulic systems that take  over if main systems fail. In many such systems, the pumps of  both systems obtain fluid from a single reservoir. Under such  circumstances, a supply of fluid for the emergency pump is  ensured by drawing the hydraulic fluid from the bottom of the  reservoir. The main system draws its fluid through a standpipe  located at a higher level. With this arrangement, should the  Figure 12-7.  Hydraulic reservoir standpipe for emergency operations. 12-9 PFCS Brakes (R SYS) T/R S Sample valve From RSVR servicing Heat exchanger ACMP filter module GND SVC RTN disk SYS pressurized XDCR Temp XDCR ACMP EDP Pressurized XDCR GND SVC pressurized disk RLF valve RSVR  pressurized  SW Return filter module RSVR pressurized module EDP filter module EDP supply shutoff valve RSVR pressurized shutoff valve Drain valve RSVR pressurized relief valve RSVR temp XDCR Pressurized air Supply Pressure Return Depress solenoid valve TEMP XDCR Pressurized XDCR Nonpressurized Reservoirs