Fluid-Pressurized Reservoirs Reservoir Servicing Some aircraft hydraulic system reservoirs are pressurized  by hydraulic system pressure. Regulated hydraulic pump  output pressure is applied to a movable piston inside the  cylindrical reservoir. This small piston is attached to and  moves a larger piston against the reservoir fluid. The reduced  force of the small piston when applied  by the larger piston is  adequate to provide head pressure for high altitude operation.  The small piston protrudes out of the  body of the reservoir.  The amount exposed is used as a reservoir fluid quantity  indicator.  Figure 12-13 illustrates the concept behind the  fluid-pressurized hydraulic reservoir. the reservoir until air-free fluid flows through the bleed  drain port. The reservoir fluid level is indicated by the markings on  the part of the pressurizing cylinder that moves through  the reservoir dust cover assembly. There are three fluid  level markings indicated on the cover: full at zero system  pressure (FULL ZERO PRESS), full when system is  pressurized (FULL SYS PRESS), and REFILL. When the  system is unpressurized and the pointer on the reservoir lies  between the two full marks, a marginal reservoir fluid level  is indicated. When the system is pressurized, and the pointer  lies between REFILL and FULL SYS PRESS, a marginal  reservoir fluid level is also indicated. The reservoir has five ports: pump suction, return,  pressurizing, overboard drain, and bleed port. Fluid is  supplied to the pump through the  pump suction port. Fluid  returns to the reservoir from the  system through the return  port. Pressure from the pump enters the  pressurizing cylinder  in the top of the reservoir through the  pressurizing port. The  overboard drain port drains the  reservoir, when necessary,  while performing maintenance. The bleed port is used as  an aid in servicing the reservoir. When servicing a system  equipped with this type of reservoir,  place a container under  the bleed drain port. The fluid should then be pumped into  Nonpressurized 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. Many  reservoirs also have a quick disconnect service port at  the bottom of the reservoir. A hydraulic filler unit can be  connected to the service port to add fluid to the reservoir. This  method reduces the chances of contamination of the reservoir.  Aircraft that use pressurized reservoirs often have a central  filling station in the ground service bay to service all  reservoirs from a single point.  [Figure 12-14] A built-in hand pump is available to draw fluid from  a container through a suction line and pump it into the  reservoirs. Additionally, a pressure fill port is available for  attachment of a hydraulic mule or serving cart, which uses an  external pump to push fluid into the aircraft hydraulic system.  A check valve keeps the hand pump output from exiting the  pressure fill port. A single filter is located downstream of  both the pressure fill port and the hand pump to prevent the  introduction of contaminants during fluid servicing. It is very important to follow the maintenance instructions  when servicing the reservoir. To get the correct results when  the hydraulic fluid quantities are checked, or the reservoirs are  to be filled, the airplane should be in the correct configuration.  Failure to do so could result in overservicing of the reservoir.  This configuration could be different for each aircraft. The  following service instructions are an example of a large  transport-type aircraft.  Before servicing always make sure that the: •  Spoilers are retracted, •  Landing gear is down, •  Landing gear doors are closed, 12-12 Figure 12-13.  Operating principle behind a fluid-pressurized  hydraulic reservoir.  Hydraulic system pressure (3,000 PSI) 30–50 psi Hydraulic fluid Fluid pressurized  reservoir Pressurized fluid to pump inlet Amount piston protrudes indicates  reservoir fluid quantity Large piston Vent Small piston