The internal piston separates the fluid and air/nitrogen  chambers. The end caps and piston are sealed with gaskets  and packings to prevent external leakage around the end  caps and internal leakage between the chambers. In one end  cap, a hydraulic fitting is used to attach the fluid chamber  to the hydraulic system. In the other end cap, a filler valve  is installed to perform the same function as the filler valve  installed in the spherical accumulator.  [Figure 12-50] Maintenance consists of inspections, minor repairs,  replacement of component parts, and testing. There is an  element of danger in maintaining accumulators. Therefore,  proper precautions must be strictly observed to prevent injury  and damage. Before disassembling any accumulator, ensure that all  preload air (or nitrogen) pressure has been discharged. Failure  to release the preload could result in serious injury to the  technician. Before making this check, be certain you know  the type of high-pressure air valve used. When you know  that all air pressure has been removed, you can take the unit  apart. Be sure to follow manufacturer’s instructions for the  specific unit you have. In operation, the compressed-air chamber is charged to a  predetermined pressure that is somewhat lower than the  system operating pressure. This initial charge is referred to  as the accumulator preload. As an example of accumulator  operation, let us assume that the cylindrical accumulator is designed for a preload of 1,300 psi in a 3,000-psi system.  When the initial charge of 1,300 psi is introduced into the  unit, hydraulic system pressure is zero. As air pressure is  applied through a gas servicing valve, it moves the piston  toward the opposite end until it bottoms. If the air behind  the piston has a pressure of 1,300 psi, the hydraulic system  pump has to create a pressure within the system greater than  1,300 psi before the hydraulic fluid can actuate the piston.  At 1,301 psi the piston starts to move within the cylinder,  compressing the air as it moves. At 2,000 psi, it has backed  up several inches. At 3,000 psi, the piston has backed up to  its normal operating position, compressing the air until it  occupies a space less than one-half the length of the cylinder.  When actuation of hydraulic units lowers the system pressure,  the compressed air expands against the piston, forcing fluid  from the accumulator. This supplies an instantaneous supply  of fluid to the hydraulic system component. The charged  accumulator may also supply fluid pressure to actuate a  component(s) briefly in case of pump failure. Transport-type aircraft use heat exchangers in their hydraulic  power supply system to cool the hydraulic fluid from the  hydraulic pumps. This extends the service life of the fluid and the  hydraulic pumps. They are located in the fuel tanks of the  aircraft. The heat exchangers use aluminum finned tubes  to transfer heat from the fluid to the fuel. The fuel in the  tanks that contain the heat exchangers must be maintained  at a specific level to ensure adequate cooling of the fluid.  [Figure 12-51] 12-32 Figure 12-51.  Hydraulic heat exchanger. Figure 12-50.  Cylindrical accumulator. Maintenance of Accumulators Heat Exchangers Operation Right system heat exchanger Center system heat exchanger FWD INBD