the major part of the electrical system components from the hydraulic system. Hydraulic System The hydraulic system has two components—a reservoir, which supplies fluid through a helical, screw-type boost pump and a filter to a variable volume, pressure-compensated, axial piston, high-pressure pump. Also, the hydraulic system has three circuits-the dynamic test circuit, the static test circuit, and the pump test circuit. DYNAMIC TEST CIRCUIT.—The dynamic test circuit is used to test double-acting hydraulic cylinders and other components requiring combined pressure and flow. STATIC TEST CIRCUIT.—The static test circuit is included in the hydraulic system. It is essentially a compressed-air operated, low-displace- ment, high-pressure pump that supplies fluid for static pressure tests. This circuit may be operated independently of the other two test circuits. A safety interlock prevents operation of this circuit when the door of the test chamber is open. PUMP TEST CIRCUIT.—The pump test circuit supplies controlled pressure and flow to a variable-displacement, reversible-rotation, hydraulic motor that, in turn, supplies power for driving hydraulic pumps during tests, Pneumatic System The pneumatic system is composed of two circuits. One circuit provides control, indication, and filtration of externally supplied compressed air for the operation of the hydraulic fluid temperature control system, the hydraulic static pressure pump, and the pneumatic static pressure booster. The second circuit consists of a portable, compressed nitrogen cylinder that supplies gas to a supply port through a manually adjusted pressure regulator for static pneumatic testing. A safety interlock prevents operation of this circuit when the door of the test chamber is open. Electrical System Externally supplied electrical power is controlled by a system located on the right-hand control panel. The test stand START switch, pump ON/OFF switches, and a test stand STOP switch are located along the lower portion of this panel. There is also a test stand STOP switch on the top left side on the front of the test stand. AIR BLEEDING Air bleeding is a service operation. In this operation, entrapped air is allowed to escape from a closed hydraulic system. For specific air bleed procedures for each model aircraft, you should refer to the applicable MIM. Excessive amounts of free or entrained air in an operating hydraulic system results in degraded performance, chemical deterioration of fluid, and premature failure of components. Therefore, when a component is replaced or a hydraulic system is opened for repairs, the hydraulic system must be bled of air to the maximum extent possible upon repair completion. Hydraulic fluid can hold large amounts of air in solution. Fluid, as received, may contain dissolved air or gasses equivalent to 6.5 percent by volume, which may rise to as high as 10 percent after pumping. Dissolved air generates no problem in hydraulic systems so long as it stays dissolved, but when it comes out of solution (as extremely minute bubbles), it becomes entrained or free air. Free air could enter a system during component installation, filter element installation, or opening the system during repairs. Free air is harmful to hydraulic system performance. The compressibility of air acts as a soft spring in series with the stiff spring of the oil column in actuators or tubing, resulting in degraded response. Also, because free air can enter fluid at a very high rate, the rapid collapse of bubbles may generate extremely high local fluid velocities that can be converted into impact pressures. This is the phenomenon known as cavitation. Cavitation causes pump pistons and slide valve metering lands to wear rapidly, commonly causing component failure. Any maintenance operation that involves breaking into the hydraulic system introduces air into the system. The amount of such air can be minimized by prebilling replacement components with new, filtered hydraulic fluid. Because some residual air may still be introduced, all maintenance of this type is followed by a thorough air bleed of the system. Most hydraulic systems in high-performance aircraft are of the closed, airless type; they are designed to self-scavenge free air back to the system reservoir. Air bleed valves remove this air. are provided at the reservoir to Because free air resulting from 4-37