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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 are provided at the reservoir to remove this air. Because free air resulting from maintenance actions or other causes may enter the system at a point remote from the system reservoir, the system should be extensively cycled with full power to transfer air to the reservoir, where it can be bled off. Air bleed valves are sometimes found at high points in the aircraft circulatory system, filter assemblies, and remote system components such as actuators. These valves make the removal of free air easier. Refer to the applicable MIMs for the location and use of additional bleed points. In systems not equipped with additional bleed points, you may have to loosen line connections temporarily at strategic points in the system, which permits removal of entrapped air from remote or dead-end points. When you bleed a system in this manner, be careful to avoid excessive loss of hydraulic fluid, and prevent the induction of air or contaminants into the system. In many cases, air inspection procedures are inadequate. SE specifically designed to detect and measure air is not presently available in the fleet. You should use indirect methods to determine the amount of air present in a system. Operating the air bleed valve on the reservoir reveals whether or not there is air present in the reservoir. Large amounts of air might be present somewhere else in the system and go undetected. An effective means for measuring the air in your system is known as the reservoir sink check. down period, in which case dissolved air has had lots of time to come out of solution. All air bleed operations must be followed by a check of the system hydraulic fluid level. Fluid replenishment may be required, depending upon the amount of air and fluid purged from the system. |
 
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