Area is a measurement of a surface. In aircraft hydraulics the technician is concerned with the areas of piston heads. Knowing this area, the amount of force required to actuate a mechanism can be determined. Area is generally measured in square inches in the English system and square centimeters in the metric system.

Force is the amount of push, pull, or twist on an object. The force in a hydraulic system is derived from the pressure acting on the area of a piston head. In the English system, force is measured in pounds; in the metric system, it is measured in grams, kilograms, or newtons (N). To measure the force of hydraulics, we must be able to determine force per unit area. This is called pressure and is measured in pounds per square inch (psi) or kilopascals (kPa).

Stroke (length) is a measurement of distance expressed in inches or centimeters, and it represents the distance a piston moves in a cylinder.

Volume (displacement) is a measure of quantity, expressed in cubic inches or liters, which represents the amount of fluid contained in a reservoir or displaced by a pump or actuating cylinder.

Fluid is any substance that is liquid or gaseous in form. A liquid is a fluid whose particles form a definite volume. The term hydraulic fluid is used in this text as the common name for the fluid used in aircraft hydraulic systems and devices.

In general, fluids expand when they are heated and contract when they are cooled. If a fluid is confined so that it cannot escape when it is heated, pressure on the walls of the confining vessel will increase. Cooling of a confined fluid under pressure will cause a decrease in pressure.

In general, and for practical purposes, liquids are regarded as being incompressible. This means that the volume of a given quantity of a liquid will remain constant even though it is subjected to high pressure. Because of this characteristic, it is easy to determine the volume of hydraulic fluid required to move a piston through its operating range. For example, if a piston is 4 in [10.16 cm] in diameter and its stroke is 10 in [25.4 cm], the volume of liquid necessary to move the piston through its full stroke is 125.67 in³ [2.06 L]. This is determined as follows.

The volume of the cylinder through which the piston moves is equal to the area of the piston head multiplied by the length of the cylinder. The area of the piston head is determined by the formula A = πr²; therefore, for the piston in the example, A = 3.1416 × 2² = 12.567. Multiplying this value by 10, we obtain the volume 125.67 in³ [2.06 L]. We know then that it will require 125.67 in³ of hydraulic fluid to move the piston through its 10-in [25.4-cm] stroke.

Because of the relative incompressibility of a liquid, we know that a given output volume from a hydraulic pump will provide an equal volume of fluid at the operating unit. For example, if a hydraulic pump discharges 100 in³ [1.639 L] of fluid through a filled connecting line between the pump and an actuating cylinder, the piston in the cylinder will have to move through sufficient distance to provide a volume of 100 in³ [1.639 L] to accommodate the fluid.

Hydraulic fluids and other liquids expand as temperature increases; therefore, safeguards must be provided in hydraulic systems to allow for the expansion and contraction of fluid as temperature changes.

A confined liquid will seek its own level, as shown in Fig. 13-2. Here the liquid is in a container open at the top and is subjected only to the force of gravity. Assuming that the container shown in the illustration is level, the pressure is the same at all points on the bottom of the container. The liquid surfaces at A, B, C, and D are all equidistant from the bottom of the container.

A basic principle of hydraulics is expressed in Pascal’s law, formulated by Blaise Pascal in the seventeenth century. This law states that a confined hydraulic fluid exerts equal pressure at every point and in every direction in the fluid. The law holds under static conditions and when the force of gravity is not taken into consideration. In Fig. 13-3 if the piston has a face area of 1 in² [6.45 cm²] and a force of 10 lb [44.48 N] is applied to it, the fluid in the container will exert 10 psi [68.95 kPa] in all directions on all surfaces within the container. In actual practice the weight of the fluid would cause a small increase in the pressure on the bottom and lower sides of the container.

When liquids are in motion, certain dynamic characteristics must be taken into consideration. One of the principal factors in liquid motion is friction. Friction exists between the molecules of the liquid and between the liquid and the pipe through which it is flowing. The effects of friction increase as the velocity of liquid flow increases. The result of friction can be seen in the simple experiment illustrated in Fig. 13-4.

As the liquid flows from the container through a pipe that is open at the end, the pressure of the liquid decreases progressively until it becomes 0 psi at the open end of the pipe. If the pressure at A is 4 psi [27.58 kPa], then it will be 3 psi [20.69 kPa], 2 psi [13.79 kPa], 1 psi [6.895 kPa], and 0 psi at B, C, D, and E, respectively. As liquid velocity increases through a given length of pipe, the pressure differential between the ends of the pipe will increase. The same is true when a liquid is flowing through a restriction in a pipe. For this reason, the rate of fluid flow can be determined by measuring the pressure differential on opposite sides of a given restrictor. This is illustrated in Fig. 13-5. The differential pressure reading on the gauge will increase as liquid velocity through the restrictor increases, and the reading of the gauge can be converted to gallons per minute or some other rate measurement.

Friction in a moving liquid produces heat, and this heat represents a loss of energy in a hydraulic system. According to the law of conservation of energy, which states that energy can neither be created nor destroyed, energy converted to heat must be subtracted from the total energy of the moving liquid. Therefore, if a hydraulic pump is discharging hydraulic fluid at a rate and pressure equivalent to 3 hp [2.24 kW] and in the system the equivalent of 0.2 hp [0.149 kW] is converted to heat, the power available for useful work is reduced to 2.8 hp [2.09 kW).

Since energy cannot be created or destroyed, the multiplication of force is accomplished at the expense of distance. In the foregoing problem, the ratio of force multiplication is 25:1. The distance through which the pistons move must then be in an inverse ratio, or 1:25. That is, the large piston will move one twenty-fifth the distance the small piston moves. If the small piston is connected to a fluid input with check valves so it can act as a pump, it can be moved back and forth, and during each forward stroke it will move the large piston a short distance. This action can be observed in the hydraulic jacks used to raise airplanes.

In some aircraft hydraulic systems, fluid pressures of as much as 5000 psi [34 475 kPa] are employed. With a pressure of this level, a very small actuating cylinder can exert tremendous force. For example, a cylinder having a cross-sectional area of 2 in² [12.9 cm²] can exert a force of 10 000 lb [44 480 N].

A simple hydraulic system is shown in the diagram of Fig. 13-7. The pump (P) draws hydraulic fluid from the reservoir (R) and directs it under pressure to the four-way selector valve (V). When the selector valve is in the position shown, the fluid will flow into the left end of the actuating cylinder (A) and force the piston to the right. This moves the piston rod and any device to which it is connected.

As the piston moves to the right, the fluid to the right of the piston is displaced and flows out the port at the right end of the cylinder, through the tubing to the valve, and from the valve to the reservoir. When the valve is rotated one-quarter turn, the reverse action will take place.

It must be emphasized that the diagram shown is not intended to illustrate an actual system but only to illustrate the principle of a hydraulic system. In an actual system, a pressure regulator or relief valve is necessary between the pump and the valve in order to relieve the pressure when the cylinder reaches the end of its travel. Otherwise, the pump would be damaged or the tubing would burst.

Another simple system is illustrated in Fig. 13-8. This system is similar to a hydraulic brake system. Hydraulic fluid is stored in the cylinder and is directed through a valve into the master cylinder as it is needed. When the brake pedal is depressed, the fluid is directed to the brake cylinders; these cylinders push the shoes apart, thus causing them to bear against the brake drum and provide brake action. When the pedal is released, springs attached to the brake shoes cause the shoes to contract and push inward on the brake cylinders, thus causing some of the fluid to return to the master cylinder.

Vegetable-base fluids are usually mixtures containing castor oil and alcohol and are colored blue or blue-green or are almost clear. They are considered obsolete and are not generally found in any hydraulic-power systems but may still be found in some older brake systems.

Mineral-base fluids consist of a high-quality petroleum oil and are usually colored red. They are used in many systems, especially where the fire hazard is comparatively low. Small aircraft that have hydraulic power systems for operating wheel brakes, flaps, and landing gear usually use mineral-base fluids conforming to MIL-0-5606. Mineral-base fluids are less corrosive and less damaging to certain parts than other types of fluid.

A mineral-base, synthetic hydrocarbon fluid called Braco 882 has been developed by the Bray Oil Company and is used extensively by the military services in place of MIL-0-5606. The fluid is red in color and meets the specifications of MIL-0-83282. This fluid has the advantage of increased fire resistance, and it can be used in systems having the same types of seals, gaskets, hoses, etc. that are used with petroleum base MIL-0-5606. The seals required for mineral-base fluid may be synthetic rubber, leather, or metal composition.

Phosphate ester–base fluids utilized in most transport category aircraft are very fire resistant. Although phosphate ester fluids are extremely fire resistant, they are not fireproof. Under certain conditions phosphate ester fluids will burn.

The continual development of more advanced aircraft has resulted in modification to the formulation of phosphate ester–base fluids. The continual modification of the fluid specifications has resulted in the utilization of Type I, II, III, and now Type IV fluids. Typical examples of current Type IV fluids are Skydrol LD-4 and Skydrol 500B-4. These fluids are colored purple.

Two distinct classes of Type IV hydraulic fluid exist. The class definition is according to the airframe manufacturer’s hydraulic fluid specification. The classes are: Class 1, low density (Skydrol LD-4) and Class 2, high density (Skydrol 500B-4). Class 1 fluids are less dense and offer a weight savings, whereas Class 2 fluids possess handling characteristics that are beneficial in some aircraft hydraulic systems.

Seals, gaskets, and hoses used with the phosphate ester–base fluids are made of butyl synthetic rubber or Teflon fluorocarbon resin. Great care must be taken to see that the units installed in the hydraulic system are of the type designed for fire-resistant fluid. When gaskets, seals, and hoses are replaced, positive identification must be made to ensure that they are made of butyl rubber or an approved equivalent material, such as Teflon fluorocarbon resin.

Fire-resistant hydraulic fluid will soften or dissolve many types of paints, lacquers, and enamels. For this reason, areas that may be contaminated with this type of fluid must be finished with special coatings. When any of the fluid is spilled, it should immediately be removed and the area washed.

2. Never, under any circumstances, service an airplane system with a type of fluid different from that shown on the instruction plate.

3. Make certain that hydraulic fluids and fluid containers are protected from contamination of any kind. Dirt particles quickly cause many hydraulic units to become inoperative and may cause severe damage. If there is any question regarding the cleanliness of the fluid, do not use it. Containers for hydraulic fluid should never be left open to the air longer than necessary.

4. Never allow hydraulic fluids of different types to become mixed. Mixed fluid will render a hydraulic system useless.

5. Do not expose fluids to high heat or open flames. Vegetable-base and mineral-base fluids are highly flammable.

6. Avoid contact with the fluids. Although the vegetable- and mineral-based fluids do not cause any irritation for most people when in contact with skin, the phosphate ester fluids can cause severe skin irritation. If skin contact occurs, wash the fluid off with soap and water. Consult a physician if irritation persists. Take precautions to prevent any hydraulic fluid from getting in the eyes or from being inhaled as a mist or vapor. Phosphate ester fluids cause the greatest amount of irritation in these areas. For eye contact, flush well with water and, if a phosphate ester fluid is involved, apply an anesthetic eye solution. Reaction to inhaled vapors (coughing and sneezing) stops after the vapor or mist is eliminated. For all cases of eye contact and inhalation of hydraulic fluids, consult a physician.

7. Wear protective gloves and a face shield whenever handling phosphate ester fluids and whenever working around any hydraulic lines that are under pressure.

Hydraulic reservoirs vary in complexity from nothing but a can with a vent hose on top of the cap and an outlet at the bottom to sophisticated designs incorporating filters, quantity indicators, and pressure relief systems. Reservoirs can be broken down into two basic types, in-line and integral, and these can be further classified as pressurized and unpressurized.

Hydraulic fluid reservoirs provide a compartment to store hydraulic fluid when it is not in use in a system, and they also provide sufficient fluid to make up for normal losses of fluid by seepage past seals. Reservoirs are not designed to be completely filled; they must allow for an air space above the fluid level to allow for expansion of the fluid when it is heated during system operation.

A reservoir will provide some means of checking the fluid level and of being replenished. The quantity-indicating method may be nothing more than a dipstick on the filler cap, or it may consist of a remote indicating system that displays the quantity on the aircraft flight deck. Replenishment is normally accomplished by adding fluid directly to the reservoir through a filler opening. When multiple reservoirs are used in an aircraft for independent or redundant hydraulic systems, the reservoirs may be filled from a common manifold.

Unpressurized reservoirs are normally used in aircraft flying at lower altitudes, such as below 15 000 ft [4583 m], or in aircraft whose hydraulic systems are limited to those associated with ground operations, such as brakes.

Pressurized reservoirs are commonly found in aircraft designed for high-altitude flight where atmospheric pressure is low. The most basic rule of hydraulics states that fluid cannot be pulled; it can only be pushed. At sea level the 14.7 psi of atmosphere provides the force to push the fluid from the reservoir to the pump. As altitude increases, atmospheric pressure decreases. With little or no pressure on the fluid, it tends to foam, causing air bubbles to form in the low part of the system. When an aircraft is operating at high altitudes, the pump will be starved for fluid unless some means of pressurization is used. Therefore, to provide a continuous supply of fluid to the pumps, the reservoir is pressurized. Along with providing a positive feed to the hydraulic pumps, a pressurized reservoir reduces or eliminates the foaming of the fluid when it returns to the reservoir. The reservoir may be pressurized by spring pressure, air pressure, or hydraulic pressure. The desired pressure to be maintained ranges from approximately 10 psi to 90 psi.

A simplified drawing of a spring-pressurized reservoir is shown in Fig. 13-9. The compression spring on top of the reservoir piston pressurizes the fluid in the reservoir and provides a positive pump inlet pressure for initial start-up of the pump. When the pump is running, system pressure (3000 psi [20 685 kPa] in this case) applied to the small area on top of the pressurization piston increases the pressure inside the reservoir to 75 psi [517.13 kPa). This assists the spring in providing a positive pressure to the pump inlet and prevents cavitation of the pump.

Turbine engine bleed air or a venturi-type aspirator can be used to pressurize a reservoir with air. Bleed air can be fed through a pressure regulator to establish the proper pressure and then into the top of the reservoir. When using a verturi-type aspirator, the low-pressure section of the venturi draws air into the reservoir and increases the pressure. The use of air pressure acting directly on the fluid eliminates the need for any elaborate chambering of the reservoir, as is required by the other methods; the reservoir is simply pressurized, with the air settling out to the top of the airtight reservoir.

Hydraulic pressure can be used to pressurize the reservoir in a manner similar to that used to assist the spring in pressurizing the reservoir. In Fig. 13-10 hydraulic pressure entering the pressure port flows through the depressurization valve and into the area just above the pressurization piston. The force on this piston causes the reservoir piston to try to move downward, which pressurizes the reservoir. The high pressure used is 3000 psi [20 685 kPa], which gives a reservoir pressure of 85 psi [586 kPa]. The depressurization valve is used to equalize the pressure in the two piston areas during servicing. An electric pump is used to pressurize the system before engine start so that the inlet lines of engine-driven pumps are under positive pressure, which provides a positive supply of fluid to the pump and reduces pump wear.

Another example of a hydraulic reservoir for a transport-category airplane is illustrated in Fig. 13-11. The reservoir shown is cylindrical in shape with a corrugated, perforated shield around the shell and has a fluid capacity of 2.5 U.S. gal [9.48 L]. A manifold, equipped with four main ports, a small pressure-line port, and a thermoswitch probe port, is welded to the bottom of the reservoir shell. The four main ports of the manifold are internally connected in tandem pairs, with both pairs opening into the supply fluid section of the reservoir. This configuration of ports makes possible either left or right installation of the reservoir by reversing the location of port connector unions and reducers. When connected in the system, one pair of ports delivers fluid to the suction ports of the engine-driven pump, the electrically driven auxiliary pump, and the ground service hand pump. One of the other pair of ports receives system return fluid, and the fourth port is equipped with a valve to drain the reservoir. A sight glass, located below the relief-and-bleed valve in the upper portion of the diaphragm-guide cylinder above the main portion of the reservoir, provides an indication of excessive accumulation of air in the reservoir. An instruction plate, mounted adjacent to the sight glass, and a pointer attached to the top of the relief-and-bleed valve above the sight glass, provide fluid-level instructions and direct fluid-level indications for system pressurized and depressurized conditions. A fluid-quantity transmitter, bracket, and actuating linkage, not shown in the illustration, is mounted on the inboard side of the reservoir cover. The lower end of the linkage is attached to the transmitter rotor, while the upper end is attached to the fluid-level pointer at the top of the relief-and-bleed valve. Fluid-level changes in the reservoir raise or lower the relief-and-bleed valve and pointer, thus extending or retracting the linkage and changing the position of the rotor in the fluid-quantity transmitter. The transmitter delivers a fluid-quantity signal to the fluid-quantity indicator in the flight compartment.

The relief-and-bleed valve, located at the top of the diaphragm guide and above the sight glass, is provided to relieve excessive reservoir pressure and to bleed off accumulated system air. The relief valve is set to relieve pressure above 47 psi [324.07 kPa]. A drain line is attached to the relief-and-bleed valve to conduct excess pressure and bleed air overboard. An air breather is provided in the reservoir cover forward of the diaphragm guide. It allows the upper, ambient-air section of the reservoir to breathe air in and out as the reservoir diaphragm lowers and raises with pressurization of the hydraulic system and operation of the various subsystem actuators. A filter screen is provided in the breather to remove atmospheric impurities from the air that enter the upper portion of the reservoir.

Internally, the reservoir is equipped with a piston and diaphragm assembly that utilizes system pressure from the small pressure-line port to maintain a pressure head on the supply fluid. This pressure is from 28 to 30 psi [193 to 207 kPa] and is reduced from the system pressure of 3000 psi [20 685 kPa] by means of the difference in area between the piston and diaphragm. This difference ratio is approximately 100:1.

The application of force to the diaphragm can be understood by a study of the drawing of Fig. 13-11. Observe the arrows that indicate fluid pressure to the inside of the piston shaft and out through holes at the top. The pressure is then exerted downward between the barrel and the piston to the surface at the lower end of the barrel. The 3000-psi [20 685-kPa] pressure on this small area is balanced by the 28- to 30-psi [193- to 207-kPa] pressure against the bottom of the diaphragm, which has about 100 times the area at the bottom of the barrel. As fluid enters the reservoir, the diaphragm is forced upward inside the reservoir shell, thus carrying the diaphragm guide and barrel upward.

A micronic filter contains a treated paper element to trap particles in the fluid as the fluid flows through the element. The micron filters can be designed to filter out particles as small as 3 μm (A micrometer is equal to 0.0000394 in, or 0.0001 mm.) A micron filter assembly is shown in cutaway in Fig. 13-14. Also, a magnetic-type element that will attract metal particles can be used in conjunction with the paper element to make a dual-element type filter.

Porous metal filters are composed of metal particles joined together by a sintering process. These filters can trap particles as small as 5 μm in size.

Most hydraulic filter assemblies are located in the pressure and return lines. In-line filters are generally constructed much like the one illustrated in Fig. 13-15. The filter illustrated consists of a head assembly, a bowl assembly, and a 15-μm paper filter element. The head assembly contains the fluid line connections and a shutoff valve to facilitate replacement of the filter element. A bypass valve in the filter prevents the system from becoming inoperative should the filter become clogged. Many filters incorporate a “pop-out” differential pressure indicator, such as the one shown in Fig. 13-15, to allow ready identification of a clogged filter. This pop-out indicator may also be electrically connected to the cockpit to provide notification of a clogged filter. The bowl assembly is mounted on the bottom of the head assembly and is sealed with an O ring. This is the housing that contains the filter element.

When filters are serviced, all pressure should be removed from the hydraulic system. When removing the bowl and element, care should be taken to prevent prolonged contact of the fluid with the aircraft, clothing, or skin—especially if a phosphate ester fluid is being used. If a micronic element is used, this is replaced with a new element; the old element can be opened to check for contamination. If the contamination indicator pin has popped out, then the fluid and filters downstream from the filter must be checked for contamination and the system flushed if required. If a porous metal element is used, this should be cleaned or replaced in accordance with the appropriate service manual.

Heat exchangers are often installed in the pumpcase drain return lines to cool the hydraulic fluid before it enters the reservoir. Cooling of the fluid may be provided by different means. The heat exchangers for some aircraft are installed in fuel cells and cool the hydraulic fluid by transferring the heat of the fluid to the fuel. Other aircraft utilize air to cool the fluid, with ram air used in flight and engine bleed air used when the airplane is on the ground.

The temperature-operated bypass valve in the hydraulic cooler fluid inlet controls the volume of return fluid circulating through the fluid cooler. As fluid temperatures rise, the bypass valve starts to close, porting return fluid through the hydraulic cooler. At high fluid temperatures, the bypass valve is fully closed, porting all return fluid through the cooler.

A double-acting piston-displacement type of hand pump is shown in the drawing of Fig. 13-18. The IN port from the reservoir is connected to the center of the cylinder, where there is a space between the two pistons and surrounding the shaft connecting the pistons. In each piston is a check valve (nos. 1 and 2) and a passage that allows fluid to flow from the center chamber to the spaces at each end of the dual-piston assembly. When the pump handle is moved to the right, the piston assembly moves to the left, forcing fluid out though check valve 3 into the system. The check valve in the left-hand piston is held closed by fluid and spring pressures. As the piston assembly moves to the left, a low-pressure area is created in the chamber in the right end of the cylinder, and this causes fluid to flow through check valve 1 into the chamber. Check valve 4 is held in the closed position by spring and fluid pressure. When the pump handle is moved to the left, the piston assembly moves to the right and the fluid is forced out of the right-hand chamber through check valve 4 into the system. Note that a fluid passage connects the outlet chambers at each end of the cylinder.

During operation, the gears turn clockwise. As the cavities move from the bottom of the gear to the top of the gear, they increase in volume, resulting in a vacuum being created in this area. As this cavity passes the inlet port, fluid is drawn into the cavity. As this cavity moves to the right side of the pump, the size of the cavity is decreased and fluid is forced out of the pump via the outlet port.

A cutaway illustration of a fixed-delivery pump manufactured by Sperry Vickers, Division of Sperry Rand Corp., is shown in Fig. 13-23. Fixed delivery means that the pump will normally deliver a fixed amount of fluid at a given number of revolutions per minute. A variable-delivery pump is shown in Fig. 13-24. This pump is designed so the alignment of the rotational axis of the cylinder block can be changed as desired to vary the volume of fluid being delivered at a given rpm. By changing the angle of the rotational axis, the stroke of the pistons is decreased or increased; therefore, the volume of fluid pumped during each stroke of the pistons is reduced or increased. If the axis of the cylinder block is parallel to the axis of the drive shaft, no fluid will be delivered, because there will be no movement of the pistons within their respective cylinders.

An in-line variable delivery piston pump operates on the same principle as the multiple-piston pumps described in the foregoing section: however, the drive shaft is parallel to the pistons. The movement of the pistons necessary to create a pumping action is caused by a shoe-bearing plate in the yoke assembly. As the cylinder block rotates the piston shoe continues following the yoke-bearing surface and it pushes the pistons into the cylinders, causing them to eject the fluid into the out port. The pistons on the descending portion of the yoke can draw fluid from the in port of the pump. The angle of the yoke as established by the compensator valve determines the amount of fluid expelled during each stroke of the pistons, thus providing the variable delivery required. Each piston completes this cycle on each revolution of the drive shaft, providing a continuous, non-pulsating flow of fluid to the system. Check valves are located at the outlet of the each piston to prevent pressurized fluid from flowing back into the cylinders as the pistons move back in the cylinders. Internal leakage keeps the pump housing filled with fluid for lubrication of rotating parts and cooling. The leakage is returned to the return side of the hydraulic system through a case drain port. The case valve relief valve protects the pump against excessive case pressure, relieving it to the pump inlet.

The control mechanism for the variable-delivery pump is shown in Fig. 13-27. Remember that the output of this type of pump is determined by the angle of the shoe-bearing plate in the yoke assembly which produces a reciprocating action of the piston pumps. The yoke angle is changed by pressure on the yoke-actuating piston. Below system pressure of 2850 psi the compensator valve is in the up position and no high pressure can flow to the yoke actuator piston. In this position the pump delivers maximum flow. When the pressure increases above 2850 psi the compensator valve cracks open (valve moves down) and high pressure can act on the yoke actuator piston which reduces the yoke angle and as a result the output of the pump is decreased. When system pressure reaches 3025 psi the compensator valve is all the way open and high pressure acting on the yoke actuating piston will move the yoke to the zero position. The output of the pump decreases to zero and the unit pumps only its own internal leakage. The pump will provide a variable flow depending on the system pressure between 2850 and 3025 psi. The higher the pressure, the lower the flow.

The Bourdgn-tube type pressure switch illustrated in Fig. 13-29 is frequently used to control pressure within a hydraulic system. The flexible steel finger attached to the small end of the Bourdon tube moves outward as the tube (a) begins to uncoil. This finger presses against the toggle plate (b) until the desired system pressure is reached, at which time it will cause the toggle to pivot rapidly, thereby opening the contact points and breaking the electrical circuit.

A pressure regulator that is also an unloading valve is illustrated in Fig. 13-30. In this view, the unit is operating to supply fluid for charging an accumulator (hydraulic-pressure storage chamber) and to supply fluid pressure for operating units in the system. Fluid flows into port B and out of port A to the system. The check valve is off its seat because of the fluid pressure being exerted by the pressure pump. When the pressure in the accumulator builds up to the maximum level for the system, the same pressure is exerted in chamber F, Fig. 13-31. This pressure moves the plunger (piston) (3) upward to raise the pilot valve (2) against the pressure of the spring (1). In Fig. 13-30, observe that fluid pressure is applied to one of the pilot-valve chambers through a passage from the inlet line, around an annular groove surrounding the unloading valve [(4) in Fig. 13-31], and to the pilot valve. As the pilot valve is raised, this pressure is ported and directed through a passage to the left end of the directional spool (5). This spool moves to the right and causes hydraulic pressure to be directed against the right end of the unloading spool, thus moving it to the left. This permits the main flow of fluid to go from port B, through passage E, and out the return port C. Under this condition the power pump is unloaded because the fluid has free flow back to the reservoir. The regulator is said to be “kicked-out” in this position. The check valve has seated because of spring pressure and holds the pressure locked in the system. In the drawings, port D is the “bleed-off” port, which permits fluid from the chambers at the ends of the directional spool and unloading spool to escape and allow movement of the spools as required.

When a subsystem is operated and fluid pressure in the pressurized part of the system drops to a pre-determined level, the pilot valve and plunger will move back to the lower position, directing pressure against the right end of the directional spool, as shown in Fig. 13-32. The directional spool moves to the left and causes fluid pressure to be directed to the left end of the unloading spool, moving it to the right and blocking the return line. Pump pressure then builds up and opens the check valve (bullet valve) to allow fluid flow to the operating system through port A. This position is often referred to as “kicked-in.”

Another type of pressure regulator that serves a purpose similar to the one just described is the balanced type. A drawing of this regulator in the kicked-in position is shown in Fig. 13-33. In this drawing, the bypass valve E is held closed by spring pressure and system pressure. Fluid from the power pump enters at port F, and the pressure forces the check valve G off its seat and allows the fluid to flow out port H to the accumulator and the system. When the system-operating requirements are met, the pressure continues to build up from the pump and throughout the area in operation. This pressure bears against the poppet valve I and also increases above the piston N because of the sensing line O. The piston N has greater area than the poppet valve I; therefore, the force downward increases faster than the force upward. At a certain point the force downward becomes equal to the force upward and the valve is then said to be in the balanced condition. As pressure continues to increase, the downward force becomes greater than the upward force and the rod M moves downward against the force of the spring L. The hollow piston rod M moves downward and contacts the poppet valve, thus forcing it off its seat. Pressure fluid then can flow through the passage K to act against the directional valve C, which pushes the bypass valve E off its seat. When this occurs, pressure fluid entering port F can flow out through port D to the reservoir, and the pressure entering port F drops to the free-flow level. Check valve G is then immediately seated by spring pressure to trap the high pressure in the system. This is the kicked-out position, as illustrated in Fig. 13-34.

For proper operation, the regulator should remain in the kicked-out position until the pressure in the system has dropped to the lower operating level. This it will do, because the pressure for the initial kick-out was great enough to overcome the force of the spring L and the pressure against the poppet I. Since the poppet has been lifted from its seat, the only force necessary to keep the valve in the kicked-out position is that on the piston N acting against the spring L.

The kick-out pressure of the valve is adjusted by turning the adjusting screw A. This changes the effect length of the rod below the piston and changes the amount of force necessary to bring about the kicked-out condition.

A drawing to illustrate the construction of a relief valve is shown in Fig. 13-35. During normal operation, the valve is on its seat and fluid flows from the IN port to the OUT port without restriction. As the pressure on the line increases to a level above that for which the valve spring is adjusted, the valve lifts off its seat and the fluid then flows through the valve and out the return line. The pressure at which the relief valve lifts is called the cracking pressure. The design of the valve must be such that it will not rapidly open and close and cause chattering, since this would damage the system. Relief valves are used to control maximum system pressure and to control pressure in various parts of the subsystems. For example, a relief valve, called a wing-flap overload valve, is often placed in the DOWN line of the wing-flap subsystem to prevent lowering of the flaps at too high an airspeed. The pressure in the DOWN line will rise above a specified level because of air pressure against the wing flaps if the airspeed is too great. The wing-flap overload valve will then open and allow excess pressure to be relieved, thus causing the down movement of the flaps to stop.

When several relief valves are incorporated in a hydraulic system, they should be adjusted in a sequence that will permit each valve to reach its operating pressure. Thus, the highest-pressure valves should be adjusted first; the others are then adjusted in the order of descending pressure values.

The pressure-reducing valve illustrated in Fig. 13-37 has three ports. One is connected to system pressure, one is connected to the system return line, and the third is the reduced-pressure port. The pressure-reducing spring (A) is holding the reservoir return valve (B) and the poppet (D) to the left. Fluid under system pressure enters the system pressure port (C), where it passes around the unseated poppet and out the reduced-pressure port (E). As the fluid going out port (E) builds pressure, hydraulic force is transmitted back through the hollow poppet (D) and exerts a force on the reservoir return valve (B). When this force overcomes spring force, the reservoir return valve is pushed to the right, allowing the poppet spring (F) to seat the poppet. This prevents system fluid from going out the reduced-pressure port (E) to build up a further pressure. The inlet pressure (C) has no effect on the poppet itself because the areas on each end of the poppet exposed to the pressure are the same; therefore, the forces exerted on the poppet are balanced. The pressure exerts an unbalanced force only on the area of the reservoir return valve (B).

In actual operation, this pressure-reducing valve will close when the desired pressure is reached. When the actuator is in operation under reduced pressure, the valve will vary its opening to meter the fluid at the speed required to maintain the desired pressure.

Another type of pressure-reducing valve is a debooster valve used in an aircraft brake system to reduce system pressure; in addition to reducing pressure it will provide for a higher volume of fluid flow to the brakes for rapid application of braking forces.

A review of two basic formulas, F = P × A and V = A × L, will assist in providing a better understanding of deboosters. To determine the value of a force produced by pressure that acts on a given area, the formula F = P × A is used, where F is the total force, P is the value of the pressure expressed in pounds per square inch (psi), and A is the area of the surface exposed to the pressure. To find the volume of fluid required to move a piston within a cylinder or the amount exhausted, the formula V = A × L is used, where V equals the total volume delivered per stroke expressed in cubic units (usually cubic inches), A is equal to the surface area of the piston, expressed in square inches, and L is equal to the length of the stroke or the distance through which the piston moves as it discharges fluid.

A debooster valve operates by the differential area of two pistons. If a small-area piston is connected by a rod to a large-area piston, the two pistons will be capable of developing pressure in inverse proportion to their areas. Figure 13-38 illustrates the debooster principle. If the area of the small piston is 1 in² [6.45 cm²] and the area of the large piston is 4 in² [25.8 cm²], the large piston can transmit a pressure of only one-quarter that of the small piston. When 1000 psi [6895 kPa] is applied to the small piston, a force of 1000 lb [4448 N] will be exerted through the rod to the large piston. Since the large piston has a 4-in² [25.8-cm²] area, a force of 1000 lb [4448 N] will develop a pressure of only 250 psi [1723.8 kPa] in the large cylinder.

A debooster consists of two pistons of different sizes fastened together in a cylinder housing machined for each piston, as illustrated in Fig. 13-39. Movement of one piston causes the other also to move. The main feature of the debooster is the piston, which has two different areas. The upper area (N) and the lower area (H) will have a given ratio. The extra fluid that is discharged will cause rapid application of brakes. The outlet port (D) and line must be larger than the inlet port to accommodate the extra flow.

Accumulators are divided into three types according to the means used to separate the air and fluid chambers. The three types are diaphragm, bladder, and piston.

The diaphragm-type accumulator consists of a metal sphere separated by a synthetic-rubber diaphragm, as shown in Fig. 13-40. The sphere is constructed in two parts, which are joined by means of screw threads. At the bottom of the sphere is an air valve, such as a Schrader valve, and at the top is a fitting for the hydraulic line. A screen is placed at the fluid outlet inside the sphere to prevent the diaphragm from being pressed into the fluid outlet.

During operation of the accumulator, the air chamber is preloaded, or charged, with air pressure (approximately one-third maximum system pressure). As soon as a very small amount of fluid is forced into the fluid side of the accumulator, the system pressure gauge will show the pressure in the air chamber. This provides a means for checking the air charge (pressure) in the accumulator. If the system is inactive and the main pressure gauge shows zero pressure, a few strokes of the hand pump will cause the pressure gauge to rise suddenly to the charge pressure in the accumulator. The accumulator charge can also be checked by the reverse method. For example, if the system gauge shows a pressure of 1500 psi [10 342.5 kPa] when the system pump is not operating and the brakes are depressed and released a number of times, the pressure will decrease to the accumulator charge pressure and then will suddenly fall to zero.

It should be noted that some aircraft hydraulic systems monitor the hydraulic pressure by indicating the pressure on the air side of the accumulator. When the system has no hydraulic pressure, the gauge for the system indicates the accumulator air pressure. As soon as the hydraulic pressure is greater than the air charge, the air is compressed to the value of the hydraulic system pressure.

If the accumulator must be charged, hydraulic pressure is removed from the system before the accumulator is charged. Nitrogen is the gas commonly used, but dry air may be used in some cases.

The installation of an accumulator is usually the reverse of removal. The air chamber of the accumulator is downward when the accumulator is installed. Sometimes the air charge is placed in the accumulator before installation. In any event, the manufacturer’s instructions should be followed. Particular care must be taken to see that all seals, valves, and fittings are of the proper type for the fluid being used in the system.

If hydraulic fluid is found in the air chamber of an accumulator, there is a leak between the two chambers. In such cases, the accumulator must be removed and repaired.

If the gear-control handle is placed in the neutral position, cam lobe 8 will open poppet valve 3, and cam lobes 6 and 9 will close poppet valves 1 and 4, assuming that the poppets are equipped with springs to keep them closed unless they are lifted by a cam lobe. When poppet valve 3 is open, fluid flow will pass from the pressure chamber PC to the neutral chamber NC and from there to the return manifold, thus permitting the fluid to flow freely and thereby reducing the load on the pressure pump. In the neutral position, valves 2 and 5 are closed because cam lobes 7 and 10 are not yet in position to open them.

When the gear-control handle is placed in the UP position, valves 2 and 5 will be open and the others will be closed. Fluid will therefore flow through poppet valve 2 and out the passage B to the left end of the actuating cylinder. The piston will move to the right and force fluid through passage A and poppet valve 5 to the return chamber and return line to the reservoir.

Some poppet valve assemblies are arranged with valves in a radial position, and they are opened and closed by means of a rotary cam unit. The results are the same in any case. Poppet valves are also manufactured with electric controls, and the individual valves are opened and closed by means of solenoids.

A simple piston-type selector valve is illustrated in the drawing of Fig. 13-46. In the first drawing, the valve is in the OFF position, and fluid flow is blocked because both port A and port B are blocked by the piston. In the second view, port A is open to allow fluid to flow out to an actuating cylinder; the return flow from the cylinder enters port B and flows out port B to the reservoir. The third view shows the reverse position where fluid flows out port B and back through port A. The center of the piston rod is provided with a drilled passage, which allows the return fluid to flow to the right and out through the return port R.

Like other forms of selector valves, the open-center selector valve provides a means of directing hydraulic fluid under pressure to one end of an actuating cylinder and of simultaneously directing fluid from the opposite end of the actuating cylinder to the return line. The advantage is that the valve automatically returns to neutral when the actuating cylinder reaches the end of its stroke. The fluid output of the power pump is directed through this valve to the reservoir when the valve is in neutral position.

The valve illustrated in Fig. 13-47 consists of a housing that has four ports, a piston, two metering pins, two check valves, and a spring-loaded roller and cam-arrangement that is attached to the end of the piston. The roller and cam arrangement is designed to hold the piston in either the operating position after it has been engaged or the neutral position.

In the illustration, the sliding piston is in one of the two operating positions. The sliding piston has been moved manually to the right and is held in position by the spring-loaded cam mechanism, Fluid under pressure flows from the inlet port A through port D to one side of the hydraulic actuating piston, moving the actuating piston to its fully extended or retracted position. Fluid returning from the opposite side of the piston enters the selector valve at B and discharges through the return port C to the reservoir.

Figure 13-48 shows the same valve in the neutral position, where it is held by the lever and cam arrangement. The position of the lever, which rotates about the shaft, is determined by the position of the roller, which rolls on the cam. In the neutral position, the inlet port A is connected directly to the return port C, thus allowing the fluid to flow freely through the valve.

The valve is automatically returned to the neutral position by action of fluid pressure, which opens the relief valve and admits fluid pressure to the end of the piston. This pressure forces the valve piston back to the neutral position. The action takes place for either position of the valve.

In Fig. 13-50 the right solenoid is energized and the right pilot valve will move to the left and will block fluid pressure from going to the right side of the main spool valve and will connect the right side of the main spool valve with the return line. The main spool valve will move to the right because the pressure on the left side of the main spool valve is now higher than on the right side. When the main spool valve moves to the right the pressure line will be connected with the left cylinder port. When the right solenoid is de-energized the pressure will again be allowed to go to the right side of the main spool valve and the hydraulic pressure on both sides will be equal and the main spool valve moves to the neutral position because of spring pressure.

The purpose of an orifice, or a variable restrictor, is to limit the rate of flow of the fluid in a hydraulic line. In limiting the rate of flow, the orifice causes the mechanism being operated by the system to move more slowly.

Figure 13-51 is a drawing of an orifice. This form of the device is merely a fitting that contains a small passage and has threaded ends. When fluid flows through the central passage, it will limit the rate of flow. An orifice of this construction may be placed in a hydraulic line between a selector valve and an actuating cylinder to slow the rate of movement of the actuating cylinder.

Figure 13-52 is a drawing of a variable restrictor. In this drawing, there are two horizontal ports and a vertical, adjustable needle valve. The size of the passage through which the hydraulic fluid must flow may be adjusted by screwing the needle valve in or out. The fact that the passage can be varied in size is the feature that distinguishes the variable restrictor from the simple fixed orifice.

During the installation of a check valve, the technician must observe the direction of flow indicated on the body of the valve. Usually there is an arrow on the body or case of the valve to show the direction of the free fluid flow.

An orifice check valve is also used for certain flap-control systems. Because of the air pressure on the flaps during flight, there is a continuous force tending to raise the flaps to a streamlined position. It is therefore advisable in some systems to restrict the UP movement by placing an orifice check valve in the DOWN line of the flap system.

The construction of an orifice check valve is illustrated in Fig. 13-54. When the valve is on its seat, fluid flow can occur only through the center orifice, but when the fluid flow is in the opposite direction, the valve moves off its seat and there is free flow of the hydraulic fluid through multiple openings.

The improper installation of an orifice check valve in a landing-gear system can cause serious problems. If the valve is installed in the reverse position, the movement of the landing gear is restricted as the gear is raised, but there is free flow as it is lowered. The hydraulic pressure in the system plus the force of gravity causes the gear to lower with excessive speed; when it reaches the end of its travel, the inertial forces are likely to damage the aircraft structure because of the sudden stop.

A drawing of a metering check valve is shown in Fig. 13-55. This unit has a housing, a metering pin, and a check-valve assembly. The pin is adjusted to hold the ball slightly off its seat. When fluid enters port B, it forces the ball away from its seat and then flows out through port A to the actuating cylinder. When the flow of fluid is reversed, the fluid entering from the actuating cylinder flows through the tiny opening between the ball and its seat, thus restricting the flow. By adjusting the metering pin in or out with a screwdriver, the rate at which the fluid can return from the actuating cylinder is controlled, because the position of the metering pin changes the width of the opening between the ball and its seat.

In reference to the figure, fluid enters the fuse through the passage at the right and flows between the outer case and the inner cylinder. It then passes through cutouts in the left end of the inner cylinder and out though the center opening. At the right end of the inner cylinder is a metering orifice, which permits a small amount of fluid to enter the cylinder behind the poppet valve.

During normal operation the pressure of this fluid is approximately the same as the pressure on the opposite side of this poppet; therefore, there is no movement of the poppet. As fluid flow increases, the pressure differential across the poppet also increases. If this differential becomes excessive, the poppet moves to the left and closes the exit port of the fuse, thus stopping the fluid flow. The fuse remains closed only as long as a substantial pressure differential exists. If the pressure differential decreases to a certain predetermined level, the spring unseats the poppet and permits normal flow to resume.

Figure 13-57 is a drawing of a sequence valve. It is essentially a bypass check valve that is automatically operated. There is a free flow of hydraulic fluid from port A to port B, but the flow from B to A is prevented unless the ball is unseated by depressing the plunger.

Figure 13-58 is a schematic diagram of a landing-gear system with sequence valves. During the retraction of the landing gear, the fluid flows under pump pressure from the selector valve to the landing-gear cylinder and to sequence valve A. In this position, sequence valve A is closed, thus preventing the fluid from entering the door cylinder. As the landing-gear actuating-cylinder piston approaches the end of its travel, either the piston rod or some other part of the landing-gear mechanism depresses the plunger of the sequence valve, thereby permitting the fluid to flow to the door-actuating cylinder and to close the doors. The fluid displaced at the other end of the landing-gear actuating cylinder by the motion of the piston passes to the DOWN line through sequence valve B. The flow is not restricted, because it is flowing directly from port A to port B in the sequence valve illustrated in Fig. 13-57.

During the extension of the landing gear, the fluid under system pressure flows through the DOWN line and enters the door-actuating cylinder, but it is prevented from moving the landing-gear actuating-cylinder piston because sequence valve B is closed. However, when the door actuating-cylinder piston reaches the limit of its travel, the plunger on sequence valve B is depressed, and a passage is opened for the fluid to enter the landing-gear actuating cylinder and extend the gear. The fluid displaced by the motion of the door’s actuating-cylinder piston flows to the UP line through sequence valve A without restriction.

Sequence valves are sometimes used in conjunction with hydraulically operated landing-gear UP locks and DOWN locks. In such instances, the sequence valve either blocks the fluid from the landing-gear actuating cylinder until the unlocking cylinder has released the lock or prevents the fluid from entering the locking cylinder until the landing gear has reached its fully retracted or extended position. Sometimes the sequence valve and the unlocking cylinder are manufactured as one unit.

To illustrate the operation of a sequence valve in a landing-gear system, the drawing of Fig. 13-59 is provided. This drawing shows a view of the sequence valve in the open position. If the valve is installed with port B connected to the landing gear UP line and port A connected to the gear door-actuating cylinder, fluid entering at port B cannot pass through to the actuating cylinder until the landing gear has been retracted and the valve plunger pressed in, as shown in the drawing. Therefore, the gear doors cannot be closed until the gear is retracted.

For the part of the system extending the landing gear, the sequence valve is installed with port B connected to the DOWN line of the landing-gear actuating cylinder. The plunger of the valve is not depressed until the gear doors are open; therefore, the gear cannot be extended until the door operation is nearly complete.

If, in the installation and adjustment of the landing gear system, the sequence valves are not set correctly, it is possible for the landing gear to strike the gear doors.

The operation of a shuttle valve is shown in the drawings of Fig. 13-60. Port 1 of the valve is the normal entrance for hydraulic fluid from the pressure system. Port 2 is the outlet leading to the DOWN line of the landing-gear actuating cylinder. In view A the valve is in the normal position with free passage of fluid from port 1 to port 2. If main system pressure fails and it is desired to lower the landing gear, the landing-gear selector valve is placed in the DOWN position and emergency pressure is applied to port 3. As explained previously, this pressure can come from an emergency pump or from a high-pressure air bottle. The emergency pressure forces the shuttle valve to the left, as in view B, and opens port 2 to port 3; thus fluid or air is permitted to flow through the valve to the DOWN line of the actuating cylinder. Return fluid from the cylinder will flow through the normal UP line, back through the main selector valve, and to the reservoir.

The installation of a shuttle valve in a landing-gear system is shown in the schematic drawing of Fig. 13-61. The DOWN line is blocked when emergency pressure is applied, and emergency fluid or air enters the cylinder, flowing from port 3 to port 2 and into the actuating cylinder.

The valve shown is a priority valve used for sequencing, but these valves are also used to give one component priority over another component in unrelated operations. For example, in some aircraft, a priority valve is used to give the flight-control actuators priority to system pressure over the landing gear and flap systems.

Figure 13-64 illustrates how the flow equalizer synchronizes the movement of the actuating cylinders in both directions. In the left-hand illustration, the equalizer is splitting the flow to extend to the pistons. The right-hand illustration shows the valve combining the two streams coming from the cylinders as the pistons retract. The two cylinders will move at the same rate and will always be synchronized even though unequal loads and pressure exist.

The type of flow equalizer shown in Fig. 13-65 is used to divide the pump’s fluid output in the two different systems. Fluid enters the inlet port (C), where it pushes down the plug (D) and uncovers the two fixed orifices in the sleeve (B). The fluid then splits and tends to flow equally down the two side passages (A and E). It then flows through the two splitting check valves (G and P) and metering grooves (K and O) in the valve body. It then flows out the exit ports (J and N) to the actuating cylinders. Any difference in the rate of flow between the two passages results in a pressure differential between these passages. Then the free-floating metering piston (L) shifts to equalize the internal pressures, and the flow equalizes. If the pressures are different in the exit ports (J and N), there will also be a difference in the pressure drops across the metering orifices (K and O), with the greatest drop in the orifice leading to the line with least pressure in it.

To illustrate this equalizing action, assume that the actuating cylinder attached to the left-hand exit port (J) moves easier than the one attached to the right-hand exit port (N), The fluid would then try to flow faster out the left-hand exit port, but in doing so it must also flow faster through the left-hand fixed orifice, creating a greater pressure drop.

A double-acting actuating cylinder is designed so hydraulic pressure can be applied to both sides of the piston. Thus, the cylinder can provide force in either direction. A double-acting cylinder is shown in Fig. 13-66(b). This type of cylinder is used for the operation of retractable landing gear, wing flaps, spoilers, etc.

Many variations of the basic actuator cylinder have been designed, usually for special applications. The technician will find information on these in the applicable manufacturer’s maintenance manuals.

To unlock and extend the piston (C), fluid under pressure enters the extend port (F) and is directed through passageways to the head of both the piston (C) and the lock rod piston (J). Sine the piston (C) cannot move until the actuator is unlocked, the lock-rod piston (J) must first be moved to the left. Although the pressure is equal on both sides of the head of the lock-rod piston (J), the difference in working areas allows it to be moved to the left. This action compresses the lock spring (G) and allows the toggle (L) to collapse, so that the main actuating piston (C) can force the locking segment (M) inward as the piston begins to extend to the right. The piston (C) movement then allows the retainer spring (K) (which was compressed by the piston on the retract cycle) to move the segment retainer (E) to the right, as shown in the bottom portion of Fig. 13-67. This position holds the locking segment (M) in the retainer (D) while the piston (C) is in the extended position. This prevents the locking segment (M) from interfering with the piston (C) on the retract cycle.

To retract and lock the piston, fluid under pressure enters the retract port (A) and is directed to the piston head. When the piston (C) has retracted a specified distance, the piston head contacts the segment retainer (E) to move it to the left. As the segment retainer moves to the left, it compresses the retainer spring (K). After the piston (C) moves sufficiently to the left (fully retracted position), the lock spring (G) positions the lock rod piston (J) to the right, thus extending the toggle (L). This, in turn, positions the locking segment (M) behind the lip of the concave piston head to lock the actuator in the locked position.

The lock bleed port (H), which is connected to the return line, ensures the positive movement of the lock rod piston (J) by preventing the small amount of operational leakage around the shaft from building up pressure and creating a fluid lock. The hollow portion of the lock and piston shaft can be used for the attachment of a cable operated control so that the actuator can be unlocked manually during an emergency or ground operation.

This type of actuator can be used in many hydraulically operated subsystems and is ideal for hydraulically operated doors.

A servo actuator is designed to provide hydraulic power to aid the pilot in the movement of various aircraft controls. Such actuators usually include an actuating cylinder, a multiport flow-control valve, check valves, and relief valves together with connecting linkages. Figures 13-68 and 13-69 show the components of a servo actuator. When the pilot moves a cockpit control in a particular direction, force is applied to the flow control valve in the actuator, causing it to direct fluid pressure to the actuating cylinder. The piston in the actuating cylinder moves in a direction to assist the pilot in moving the flight control. As the piston moves to the position called for by the pilot’s control, the linkage between the piston and the flow-control valve moves the flow-control valve back to a neutral position and stops the flow of hydraulic fluid and, therefore, the movement of the piston. The reverse action takes place when the pilot moves the cockpit control in the opposite direction.

A hydraulic motor has the advantage over an electric motor of being able to operate through a wide range of speeds from 0 rpm to the maximum for the particular motor. Variable-speed electric motors can provide some flexibility in the rate of actuation; however, they lose efficiency as speed decreases.

Seals used in hydraulic and pneumatic systems and components are of two general classes, packings and gaskets. Packing rings are used to provide a seal between two parts of a unit that move in relation to each other. The gasket is used as a seal between two stationary parts. This classification will apply in most cases; however, deviations will be found in some instances.

The handling, installation, and inspection of rings, seals, and gaskets in an aircraft hydraulic system are extremely important because a scratched or nicked packing ring can cause the failure of a unit in the system at a critical time.

Typical shapes for hydraulic packing rings, seals, and gaskets are shown in Fig. 13-71. These drawings show the standard O-ring seal, chevron seal, universal gasket, and crush washer.

The O-ring seal is probably the most common type used for sealing pistons and rods because it is effective in both directions. A color code on the O-ring seal identifies the type of system in which it is used and, generally, the manufacturer. O-rings have either a series of colored dots arranged clockwise or a colored stripe around the outer circumference. The first dot, reading in a clockwise direction or the stripe indicates the system in which the O-ring is used. The following color codes are used: Red indicates use in the fuel systems only; blue indicates use in hydraulic and pneumatic systems; and yellow indicates use in oil systems only. Note: A white dot appearing first indicates a nonstandard seal. The second dot indicates the system. A white dash with a yellow dot 90° from the dash indicates a system using Skydrol.

Generally, the O-ring seal requires no adjustment when it is installed. Figure 13-72 shows a typical O-ring installation. However, a few precautions must be observed at installation, or early failure will result. Always install new O-rings with each disassembly. Do not use old rings that have been in service. The seal must be the correct size and installed without defects such as cuts, nicks, or other flaws. The seal and O-ring groove must be lubricated freely with the same type of hydraulic fluid that is used in the system unless otherwise specified in the repair manual. Care must be exercised when installing O-rings to prevent scratching or cutting the seal on threads or sharp comers. Also, make certain that the O-ring is not installed in a twisted condition; otherwise it will not function correctly.

The O-ring seal should not be used alone in a system where the pressure is greater than 1500 psi [10 342.5 kPa]. If the hydraulic pressure is too great, the ring can become pinched between the moving part and stationary part of the unit, as is shown in Fig. 13-73, thus damaging the ring and destroying the seal. When the O-ring is used in higher-pressure systems, backup rings are installed. Backup rings are used in conjunction with seals subjected to high pressure. Their purpose is to prevent the high pressures from pinching the seal between the moving and stationary parts, such as between a piston and cylinder wall. They give additional strength to the seal. Backup rings are usually made of Teflon.

If pressure is exerted on the seal in one direction only, the backup ring should be placed on the side away from the pressure. Where the pressure is exerted alternately in each direction, these backup rings are installed on both sides of the O-ring. Proper installation of backup rings is shown in Fig. 13-74.

The V-shaped (chevron) seal and the U-shaped seal must be installed with the same care as that used for other types of seals. The size of the seal must be correct and there must be no nicks, scratches, or other damage on the seals. When high pressure is exerted in only one direction, one set of seals is sufficient, but when the pressure is alternately applied, first in one direction and then in the other, two sets of seals are required because this type of seal works effectively in only one direction. The installation of a dual set of chevron seals is shown in Fig. 13-75.

The general procedure to follow in installing V-shaped or U-shaped seals is as follows:

2. Use shim stock (very thin metal sheet) to protect the packing rings if the packing crosses sharp edges or threads. The shim stock should be from 0.003 to 0.010 in [0.076 to 0.254 mm] thick. It is rolled and then placed over the threads or sharp edges. After the packing is installed, the shim stock is removed.

3. If the unit in which the packing is being installed does not have an adjustable packing gland nut, insert metal shims of graduated thickness behind the adapters to hold the packing securely in place.

4. If the unit in which the packing is being installed has an adjustable packing gland, adjust the gland nut until the V-ring stack is held together firmly but not squeezed. The gland nut is then loosened to the first lock point.

5. Whenever possible, the technician should check the unit by hand for free operation after installation before the hydraulic pressure is applied.

6. In all cases, the technician should consult the manufacturer’s instructions regarding the installation of seals.

The power section may either be an open or a closed system using an engine-driven pump or a pump driven by an electric motor. Pressure developed by the pump in an open system is controlled by one of the following three valves: an open-center valve, a power-control valve, or a pump-control valve. An open system means there is a free path for fluid flow back to the reservoir until one of these units is actuated and directs fluid flow from the selector valve. Fluid flow is then directed to the actuators (cylinders, motors, etc.), and pressures will build sufficiently to move the actuating units.

All three types of open system valves are automatically shifted by hydraulic pressure to the open position after the units have been actuated. All three type valves employ time-lag or resistance features to prevent them from shifting to “bypass” too soon, which would result in a loss of fluid flow and pressure to the units before the hydraulic cycle has been completed. When the power valve, pump-control valve, or the open-center valve is in the bypass position, the pump is said to be idling under no load and with little power-input requirement.

In a closed system the pressure developed by the engine-driven or electric motor-driven power pump may be regulated by a pressure regulator (unloading valve), by the power pump (which would then have an integral control valve), or by a pressure switch (which would shut off the pump driven by an electric motor when the desired system pressure is reached). The closed system will always have fluid stored under pressure whenever the power pump is operating. However, after system pressure is built up to a predetermined value, the load is automatically removed from the pump by an unloading valve called the pressure regulator or the integral control valve of the pump (called the compensator valve, which controls the pump output). In this way the pump is allowed to idle when no units are in operation and until there is further demand upon the system.

A typical open system is shown in Fig. 13-77. This system is a combination closed and open system, with both parts of the system receiving their fluid supply from the same engine-driven constant-volume power pump. The pump supply is divided equally to the two systems by a flow equalizer. Other units included in the closed section of the system are discussed at a later time.

Open systems develop no pressure except when a mechanism is being operated; the pressure is then metered by the selector valve and limited by a relief valve. Each subsystem has an individual relief valve that limits the maximum pressure of the system.

One of the advantages of the open-center system is that it does not require expensive and complicated pressure regulators; the power pump can be a simple gear pump, although a fixed-displacement piston pump may be used.

A disadvantage of the open-center system is that the operation of only one subsystem at a time is possible without interference from other systems. For example, if the flap subsystem precedes the landing-gear subsystem and the two systems are operated at the same time, the speed of gear operation will be limited by the amount of fluid returning from the flap-actuating cylinder. As soon as the flap operation is complete and the flap-selector valve kicks out, the landing-gear operation proceeds at its normal rate.

Open systems are generally found only on light, general aviation aircraft. Transport-category aircraft require more complex systems, which may have several units operating at the same time.

When a selector valve is positioned to operate a mechanism, the system pressure drops until it reaches the kick-in settings of the regulator, at which time the pump again directs the flow into the system to increase the system pressure until the maximum setting of the regulator is reached again. The accumulator used in the system stores fluid under pressure, stabilizes system pressure, and ensures smooth operation of the regulator. A system relief valve safeguards the system if the regulator fails.

Multiple power pumps are used in many hydraulic closed systems. Normally they are used in multiengine aircraft, where they can be driven by separate engines. This ensures hydraulic system operation in the event of an engine failure or the failure of one of the pumps. Figure 13-78 shows how dual power pumps are connected into the system. Notice that both pumps supply the same system by combining their volume output into a common pressure manifold. The pumps may be either the constant-volume type or the variable-volume type.

Figure 13-79 is a schematic drawing of a direct pressure system that has its pressure regulated by an unloading valve (pressure regulator) operating in conjunction with an accumulator. This system is also termed a closed-center system.

When the actuating equipment is not in use, pressure is relieved from the continuously operating engine-driven pump by means of the unloading valve after the accumulator is charged to the correct level. As soon as a subsystem is operated, pressure first comes from the accumulator for operation; when accumulator pressure has dropped to a predetermined level, the unloading valve kicks in and directs pump flow to the operating system. A study of the diagram shows that the pump output flows directly through the unloading valve and back to the reservoir when no pressure is required. When a subsystem (brakes, landing gear, or flaps) is operated, pump pressure flows through the subsystem and back to the reservoir through the subsystem return line.

The main relief valve in the system is located between the unloading valve and the return line. If the unloading valve should become stuck in the kicked-in position, the excess pressure would be bypassed through the relief valve to the reservoir. The pressure gauge would show higher than normal pressure, and the system would probably be making noise because of the operation of the relief valve. The relief valve is usually set at least 100 psi [689.5 kPa] above the normal operating pressure.

The Powerpak assembly is a modular unit that includes the reservoir, relief valve, hand pump, landing-gear selector valve, wing-flap selector valve, filters, and numerous other small parts essential to the operation. When both selector valves are in the neutral position, the system acts as an open-center system, in that the fluid flows first through one selector valve and then through the other before returning to the reservoir. During this time the fluid flows freely at a reduced pressure. Since the fluid supply line runs first through the landing-gear selector valve, the flaps cannot be operated while the landing-gear subsystem is in operation. Each selector valve has a separate return line to allow fluid from the actuating cylinder or cylinders to flow back to the reservoir.

The diagram of Fig. 13-81 shows the fluid flow when the landing-gear selector is placed in the UP position. When the selector valve is placed in either operating position, it is held in the position by a detent consisting of a ball, O-ring, plunger, and spring. The ball snaps into a groove in the spool, which operates the poppet valves and prevents the spool from moving linearly until the operation is complete. When the actuating cylinder reaches the end of its movement, pressure builds up to approximately 1150 psi [7829 kPa]. This pressure acts against the plunger of the detent mechanism and relieves the pressure of the spring, thus allowing the ball to pop out of the groove. A spring then causes the spool and selector lever to return to the neutral position.

Figure 13-82 shows the fluid flow in the system when the wing-flap selector valve is placed in the DOWN position. The action is generally the same as that described in the foregoing paragraph. Note that the fluid to the flap-selector valve has passed through the landing-gear selector valve first and that the landing-gear valve is in the neutral position.

The fluid used in the just-described system is petroleum type, MIL-0-5606. The operating pressure is 1150 psi [7829 kPa], and the main relief valve “cracking” pressure is 1250 to 1300 psi [8618.8 to 8963.5 kPa]. Thermal relief valves are set to open at 2000 to 2050 psi [13 790 to 14 134.8 kPa] or 1850 to 1900 psi [12 755.8 to 13 100.5 kPa], depending upon the particular model of Powerpak. The main reservoir capacity is approximately 4.5 p [2.13 L], with an emergency supply of 0.95 p [0.45 L].

Another example of a comparatively simple hydraulic system is illustrated in Fig. 13-83. Since this system serves one function only, raising and lowering the landing gear, it requires no selector valves. Direction of fluid flow is controlled by the reversible, electrically driven pump.

The reservoir, gear pump, relief valves, reversible motor, filter, and shuttle valve are all integrated into one assembly. A pressure switch is installed on a cross fitting connected to the pump-mount assembly. This switch opens the electrical circuit to the pump solenoid when the gear fully retracts and the pressure in the system increases to approximately 1800 psi [12 400 kPa]. The switch continues to hold the circuit open until the system pressure drops to approximately 1500 psi [10 340 kPa], when it again operates to build up pressure as long as the gear-selector handle is in the UP position. The DOWN position of the selector does not affect the pressure switch.

The pump is a gear-type unit driven by a 14-V reversible motor designed to operate in a pressure range of 2000 to 2500 psi [13 790 to 17 237 kPa]. A thermal relief valve is connected to the gear-up pressure line to prevent excessive pressure due to thermal expansion of the fluid. This valve maintains pressure in the system up to 2350 + 50 psi [16 230 + 345 kPa]. An additional relief valve is incorporated in the system to return hydraulic fluid to the reservoir if pressure should exceed 4000 psi [27 436 kPa]. The shuttle valve, located in the base of the pump, allows fluid displaced by the actuating cylinder pistons to return to the reservoir without back pressure.

The system includes a free-fall valve that allows the landing gear to drop in the event of a system malfunction. The valve is controlled manually or by a gear backup extension device that is operated by a pressure-sensing chamber. This chamber moves the free-fall valve to lower the gear, regardless of gear-selector handle position, depending upon engine power (propeller slipstream) and airspeed. Landing-gear extension occurs even if the selector is in the UP position at airspeeds below approximately 118 mph [190 km/h] with engine power off. The device also prevents the gear from retracting at airspeeds below approximately 93 mph [150 km/h] at sea level with full power, even though the selector switch may be in the UP position. The sensing device operation is controlled by a differential air pressure across a flexible diaphragm, which is mechanically linked to the hydraulic free-fall valve and an electric switch that control the pump motor. A high-pressure and static air source for actuating the diaphragm is provided in a mast mounted on the left side of the fuselage above the wing. Manual override of the device is provided by an emergency gear lever. The emergency free-fall lever must be held in the UP position to retract the gear and in the DOWN position to extend the gear.

Modular filter units are used in each system to combine a number of the smaller system components in one case to simplify maintenance. Each pump has its own filter module. For instance, the EDPs have a filter module that contains a temperature transducer, pressure transducer, ground servicing connection, filters, pressure relieve valve, and check valves for the direction of flow. Both the pump pressure output and the pump case drain line are routed through the filter module. The case drain line fluid is returned to the reservoir after being cooled in the heat exchanger. The ACMP has a filter module that contains filters, check valves, temperature transducer, and pressure transducer. The hydraulic fluid used for the Boeing 777 is Skydrol 5 (type V), a fire-resistant, phosphate ester-base, synthetic fluid. Servicing of any airplane’s hydraulic system must be accomplished according to the specific instructions issued by the manufacturer for the particular airplane. On the Boeing 777, the servicing station is located in the aft right-wing fairing area and is accessible through an access door. The filing equipment consists of a manually operated hand pump, hydraulic reservoir (system) selector valve, reservoir fill filter, and reservoir fill quantity gauge. The arrangement of the equipment in the filling area is shown in Fig. 13-85.

The engine-driven pump (EDP) draws fluid through a standpipe. The electrical pump (ACMP) draws fluid from the bottom of the reservoir. If the fluid level in the reservoir gets below the standpipe, the EDP cannot draw any fluid any longer and the ACMP is the only source of hydraulic power. The reservoir can be serviced through a center servicing point in the fuselage of the aircraft. The reservoir has a sample valve for contamination testing purposes, a temperature transmitter for temperature indication on the flight deck, a pressure transducer for reservoir pressure, and a drain valve for reservoir draining.

The left system return filter module removes particles from the hydraulic fluid before the fluid returns to the reservoirs. Each module has these components: replaceable filter, automatic filter bowl removal shutoff valve, differential pressure indicator (red pop-up indicator), two check valves, bypass relief valve with indicator, and ground service disconnect. Return fluid from airplane hydraulic systems and the heat exchangers goes through the return filter modules before it goes into the hydraulic reservoirs again. The left and right return filter modules have a negative pressure loop. The negative pressure loop and the check valves permit backward flow of fluid from the reservoir to the system without back-flush of the filter. Bypass relief valves open to permit continued hydraulic flow if the filter clogs. The relief valve starts to open at approximately 165 psi.

The purpose of the relief valve is to protect the system against excessive pressure.

The hydraulic supply shutoff valves are provided to stop the flow of hydraulic fluid to the engine area. These valves are electrically operated and are automatically shut off when the engine fire switch is operated.

The reservoir pressure shutoff valve is a manual shutoff valve placed in-line before the pressurization module.

The reservoir drain valve is a manual valve that is used to drain the fluid out of the reservoir.

The reservoir sample valve is a manual valve that is used to take a sample of hydraulic fluid for fluid analysis. Never use the drain valve for collecting a sample because the bottom of the reservoir could contain more contaminants.

The reserve and nose gear isolation valves are normally open. Both valves close if the quantity in the center system reservoir is low (less than 0.40) and the airspeed is more than 60 knots for more than 1 s. When CHIS is active, this divides the center hydraulic system into different parts. The NLG actuation and steering and the leading edge slat hydraulic lines are isolated from center system pressure. The output of ACMP C1 goes only to the alternate brake system.

The output of the other center hydraulic system pumps goes to the trailing edge flaps, the MLG actuation and steering, and the flight controls. If there is a leak in the NLG actuation and steering or LE slat lines, there is no further loss of hydraulic fluid. The alternate brakes, the trailing edge flaps, the MLG actuation and steering, and the PFCS continue to operate normally.

If there is a leak in the trailing edge flaps, the MLG actuation and steering, or the flight control lines, the reservoir loses fluid down to the standpipe level (0.00 indication). This causes a loss of these systems. But, the alternate brake system continues to get hydraulic power from ACMP C1. If there is a leak in the lines between ACMP C1 and the alternate brake system, all center hydraulic system fluid is lost.

The reserve and nose gear isolation valves open again automatically when the center system quantity is more than 0.70 and airspeed is less than 60 knots for 5 s. Both valves also reset when the center system quantity is more than 0.70 and both engines and both engine-driven pumps operate normally for 30 s. Figure 13-94 shows the center hydraulic isolation system.

In this aircraft the electro motor pumps (EMP) will be used for ground purpose only and are located by couple in the inner pylon. Electrical systems can power the local electrohydraulic power packs. The flight control systems can be operated by electro-hydrostatic actuators (EHAs) and electrical backup hydraulic actuators (EBHAs). The brake and nose wheel steering system can be operated by local electrohydraulic generation systems (LEHGS). Figure 13-95 shows a hydraulic block diagram of the A380 hydraulic system.

The green and yellow hydraulic circuits are fully independent. Four engine-driven pumps (EDPs) per circuit pressurize the hydraulic fluid at 5000 psi in normal operation. Each circuit has a single hydraulic reservoir (RSVR). There are two EDPs per engine and each engine has one fire shutoff valve (FSOV) to supply hydraulic fluid to both EDPs on that engine. The green and yellow circuits generate hydraulic power to operate: the primary flight controls (F/CTL), the secondary flight controls, the landing gear, and the braking and steering system.

Two hydraulic system monitoring units (HSMUs), one for each hydraulic circuit, control and monitor the green and yellow hydraulic circuits, respectively. They also give indication on the control and display system (CDS).

The hydraulic power for each system is generated by its own variable-displacement hydraulic pump. The PC1 pump is powered by the helicopter transmission, and the PC2 and utility pumps are powered by the combining gearbox. The pumps pressurize their system to 3000 psi [20 685 kPa], and they also assist the reservoir spring in pressurizing the reservoir. Each system is equipped with temperature and pressure sensors which warn the pilot of malfunctions. Protection from excessive pressure is provided by a pressure-relief valve in each system. The utility system is not discussed further, since it is very similar to the flight-control systems.

The hydraulic pumps are located side by side on the cabin roof in the area shown in Fig. 13-101. A schematic diagram of the PC1 system is shown in Fig. 13-102. Fluid from the reservoirs is pressurized by the pumps and passes through a check valve and a pulsation dampener, which removes surges from the pump output. The fluid next flows to the hydraulic module, first passing through a filter assembly. If the filter becomes clogged so that a pressure differential of approximately 70 psi [483 kPa] exists across the filter, a red button extends from the assembly. The filter is not designed to bypass when it becomes clogged.

From the filter, fluid flows to a shutoff valve and a solenoid pilot valve. The pilot can shut down either system by opening the solenoid valve with a cockpit toggle switch. When the solenoid is energized, pressure is directed to the shutoff valve and system pressure is routed to the return line, depressurizing the system downstream of these control valves. If power to the solenoid is interrupted, the shutoff valve is closed and full system pressure is restored. Electric interlocks prevent both PC1 and PC2 from being shut down in this manner at the same time.

Excess pressure in the system is released by the system relief valve, and the relieved fluid returns to the reservoir. The relief valve begins to open at 3500 psi [24 132 kPa] and is fully open at 3850 psi [26 345.8 kPa]. The valve reseats when pressure drops to 3250 psi [22 408.8 kPa].

Downstream of the relief valve, the pressure is applied to a pressure switch, a pressure transmitter, and the reservoir pressurization line. The pressure switch operates a cockpit annunciator when pressure is lost and disables the solenoid switch for the opposite system. The pressure transmitter drives a pressure gauge in the cockpit. The reservoir pressure is maintained at 75 psi [517 kPa] by the system pressurization piston.

After passing the reservoir pressurization line, the fluid exits the module and flows to the flight-control actuators. Return fluid from the actuators is routed back to the module, through the return filter in the module, and into the reservoir. The return filter has a red button to indicate when the filter is clogged, but unlike the pressure filter, the return filter can be bypassed.

From the reservoirs, the fluid is fed back to the pumps. A temperature switch and a temperature transmitter in the line between the reservoirs and the pumps send signals to cockpit displays. The transmitter allows the pilot to monitor the fluid temperature, and the switch activates an annunciator light to warn of excessive heat in the fluid.

The system can be operated by a ground cart through the ground test connection. The quantity of fluid in the reservoirs is indicated at the sight gauge on the module, as shown in Fig. 13-101.

The air in a pneumatic system must be kept clean by means of filters and also be kept free from moisture and oil droplets or vapor. For this reason, liquid separators and chemical air driers are incorporated in the systems. Moisture in a pneumatic system may freeze in the low temperatures encountered at high altitudes, resulting in serious system malfunctions.

Another important feature of a pneumatic system is that there is no need for return lines. After the compressed air has served its purpose, it can be dumped overboard, which saves tubing, fittings, and valves.

The power section of the primary pneumatic system is that portion located in each engine nacelle shown in Fig. 13-103. It consists of a gearbox-driven compressor, bleed valve, unloading valve, moisture separator, chemical drier, back-pressure valve (right nacelle only), and a filter. In addition, each nacelle contains a shuttle valve, disk-type relief valve, and a ground charging connection to aid in ground maintenance or initial filling. Each power section independently supplies compressed air to the primary and emergency systems. The air in the primary system is stored in two storage bottles, and the system delivers the air for normal operation components, as required by directional valves. A schematic diagram of the pneumatic power system is shown in Fig. 13-104.

A 100-in³ [1.64-L] bottle is used for the main-wheel brakes, and a 750-in³ [12.3-L] bottle is used for gear operation, nose-wheel centering, propeller brakes, and passenger-door retraction. A pressure-relief valve is installed to protect the system from excessive pressure buildup. In the air-supply tube from the large primary bottle, an air filter is installed to filter the air for the primary system. An isolation valve, which is in the tube between the large primary bottle and the smaller brake bottle, permits maintenance to be performed downstream of the valve without discharging the large bottle. On the downstream side of the isolation valve, a pressure-reducing valve is used to reduce system pressure from 3300 to 1000 psi [22 753 to 6895 kPa]. All components except the pressure gauges and the power-section components are located in the fuselage pneumatic compartment.

The arrangement of the crankshaft and cam in the four-stage compressor is such that when the no. 1 cylinder is on the compression stroke, the no. 2 cylinder is on the intake stroke and the compressed air enters the no. 2 cylinder. When the no. 2 cylinder compresses, the air enters the no. 3 cylinder, which is on the intake stroke. When the no. 3 cylinder compresses, the air is delivered to the no. 4 cylinder, which is on the intake stroke. The no. 4 cylinder then compresses, and the air is delivered to the system. Thus the air has been moved from the no. 1 cylinder through the four stages to be further compressed in each stage. The interstage lines are finned to provide cooling and reduce the heat of compression.

Ducted ram air is provided for cooling of the finned cylinders and the finned interstage lines. Oil pressure from the gearbox provides lubrication for the compressor through a drilled passage in the mounting flange. Air is compressed by stages 1 through 4 of the compressor, whereas overpressure protection is provided by means of relief valves between stages 1 and 2 and 2 and 3. Compressed air from stage 4 is then routed by an intercooler line to the bleed valve mounted on the compressor.

Although the version of the F-27 being discussed makes use of an engine-driven air compressor to power the pneumatic system, other aircraft commonly make use of other power sources. Air for the pneumatic operation of controls can be provided by the engine compressor bleed air. Some aircraft use this medium-pressure air (100 to 150 psi [689 to 1034 kPa]) for operation after passing it through a pressure regulator and conditioners such as filters and driers. Other aircraft use this air to operate air turbine motors, which allows an increase in operation pressure on the pump side of the air turbine motor. Still other aircraft make use of a hydraulic motor or an electric motor to operate a high-pressure pneumatic pump. This high-pressure air is then regulated, conditioned, and allowed to flow on to the systems to be operated.

Air-storage bottles are used for emergency operations when hydraulic or pneumatic pressure sources have been lost. The bottles are normally pressurized with nitrogen, but some aircraft require the use of carbon dioxide. These bottles will power a system, normally a landing-gear extension system or a brake system, for only a brief period of time. The bottles must be recharged on the ground.

Aircraft that use the pneumatic system only for emergency purposes have the storage bottles equipped with a charging valve for ground servicing and a control valve to release pressure into the system being controlled. The control valve may be an on-off type valve, where all of the bottle’s contents are released at one time, or it may be such that the pilot can control the amount of pressure being applied to a system. The on-off control is found on landing “blow-down” systems, whereas the controllable valve is typically used for brake systems.

In each compressor pressure line, a mechanical moisture separator, shown in the drawing of Fig. 13-106 is installed to remove approximately 98 percent of any moisture and/or oil that may pass from the compressor. The separator is an aluminum tubular chamber, mounted vertically on the outboard side of the right nacelle and on the inboard side of the left nacelle.

Two valves are installed in the bottom of the separator. An inlet air pressure of 750 psi [5171 kPa] maximum closes the drain valve, whereas the inlet valve remains closed, thus preventing air from entering the separator. When pressure reaches 900 + 150 psi [6205.5 + 1034 kPa], the inlet valve opens and allows air to flow. The inlet valve stays open as long as inlet pressure is above 1050 psi [7240 kPa]. The air entering the inlet port flows through the inlet valve seat, guide, and tube assembly and through the orifices in the retainer assembly onto the baffle. The moisture droplets are deflected by the baffle, separated from the air stream, and caused to settle at the bottom of the air chamber against the closed drain valve.

As long as the inlet air pressure remains above 400 + 150 psi [2758 + 1034 kPa], the flow of air is through the inlet port, guide, tube assembly, and outlet port. When inlet air is reduced to 0 psi, the residual air flows through the ball check of the retainer assembly and out the open drain valve, pushing the collected moisture ahead of it. The separator is equipped with a safety disk that will burst at 5800 to 6200 psi [39 991 to 42 749 kPa] as a safety measure against over-pressurization.

In the unit base is a small thermostatically controlled heater that prevents freezing of any moisture in the bottom of the air chamber. The heater is automatically operated by a hermetically sealed thermostat, located in the moisture-separator body. The thermostat closes the electrical circuit at 35°F [1.67°C] to provide heating and opens the contacts at 85°F [29.44°C] maximum. To function properly, the moisture separator must be mounted vertically with the overboard drain at the bottom.

A tubular, chemical-drier housing is installed in each nacelle downstream of the mechanical moisture separator. Each drier has an inlet and outlet port and contains a desiccant cartridge. Air is directed through this replaceable cartridge, and any moisture that the mechanical moisture separator has failed to remove will be absorbed by the dehydrating agent in the cartridge. The cartridge with dehydrating agent specification MIL-D-37 16 incorporates two bronze filters, one at each end, which allow a 0.001-in [0.03-mm] maximum-size particle to pass through.

Other types of filters commonly found in pneumatic systems include micronic and wire-screen filters. The micronic filter has a replaceable cartridge, whereas the wire-screen filter can be cleaned and reused. The basic construction of these filters is the same as the micronic filter units used in hydraulic systems.

Pressure output from the bleed valve is routed to a relief valve in the unloading valve. The relief valve protects all components of the compressor circuit from excessive pressure buildup in the event any component downstream of the compressor malfunctions. The relief valve is set to open at 3800 psi [26 201 kPa].

The unloading valve contains a sensing valve and a directional control valve, and it directs compressor output through dehydration equipment to the system or vents the output overboard. The directional control valve, controlled by the sensing valve, opens and vents overboard when system pressure reaches 3300 psi [22 753.5 kPa] and closes when system pressure is less than 2900 psi [19 955.5 kPa].

A back-pressure valve, illustrated in Fig. 13-107, is installed in the pressure tube of the right nacelle only, to ensure that the engine-driven air-compressor output is kept at a predetermined value of 1700 + 150 psi [11 721.5 + 1034 kPa]. This is to provide and maintain a fixed back pressure of approximately 1700 psi upstream of the air-drying equipment. The back-pressure valve is similar in construction and operation to a check valve and consists of a valve seat, piston, and spring. It is installed in the outlet port of the chemical drier and is directly connected to the inlet port of a check valve. The right-engine compressor or a ground charging unit must be used for normal charging of the pneumatic system.

Poppet-type, spring-loaded, line-check valves are installed in the primary system, and each is identified by part number according to tube size. One valve is located in the inlet tube just before the point where the tube enters the primary bottle inlet port.

Another check valve is installed in the supply tube leading to the brake storage and is located on the left-center edge of the fuselage pneumatic panel. Two spring-loaded, poppet-type, line-check valves are installed in each pneumatic power system in each nacelle. One valve is installed in the outlet end of the back pressure valve; the other valve is installed in the upper outlet port of the “T” fitting.

A spring-loaded, piston relief valve is installed in the primary system. The valve is preset, safetied, and tagged by the manufacturer for a cracking pressure of 3800 + 50 psi [26 201 + 345 kPa]. It protects the primary system from excessive pressure buildup in the event of thermal expansion or compressor power system malfunction.

This relief valve is designed to relieve pressure as a metering device instead of an instantaneous on-off valve. Its rate of relief automatically increases in direct proportion to the amount of overpressure. It reseats when the pressure decreases to approximately 3400 psi [23 400 kPa].

As mentioned previously, the air pressure employed for the operation of the landing-gear, propeller-braking, nose-wheel-steering, and passenger-door-retraction subsystems is reduced from 3300 [22 753.5] to 1000 psi [6895 kPa]. This is accomplished by means of the pressure-reducing valve installed in the primary pressure supply line. This valve is mounted on the pneumatic power panel just below the isolation valve, as shown in the drawing of Fig. 13-108. The pressure-reducing valve contains dual springs and poppets, which not only reduce pressure but also provide for pressure relief of the reduced pressure should the valve fail. A cross section of the valve is shown in Fig. 13-109.

Additional protection is afforded the reducer relief valve by a restrictor elbow upstream of the isolation valve. The restrictor prevents excessive flow to the reducer and reducer relief valve.

The pneumatic power system is provided with an isolation valve so that the air-pressure storage section of the system can be isolated from the subsystems; thus work on the subsystems can be performed without discharging the entire system. The isolation valve is a two-position (open-close) type installed on the top right side of the pneumatic panel. The valve contains a spring-loaded, poppet-type plunger, which is cam- and lever-operated. The location of the valve in the system is shown in Fig. 13-104. Connected to the valve lever is a push-pull rod-and-latch assembly, which prevents closing the pneumatic panel door while the valve is in the closed position. This arrangement eliminates the possibility of operating the airplane while the air pressure is shut off.

Selector valves for pneumatic systems are similar in construction and operation to those used for hydraulic systems.

The charging valve also acts as a protective device against an excessive rate of inlet airflow. The valve serves as a restrictor by providing a fixed orifice through which all ground-supplied compressed air must pass.

When servicing a hydraulic reservoir, the technician must make certain to use the correct type of fluid. Hydraulic fluid type can be identified by color and smell; however, it is good practice to take fluid from an original marked container and then to check the fluid by color and smell for verification. Fluid containers should always be closed except when fluid is being removed. This practice reduces the possibility of contamination. Funnels and containers employed in filling the hydraulic reservoir must be clean and free of dust and lint.

When a system has lost a substantial amount of fluid and air has entered the system, it is necessary to fill the reservoir, purge the system of air, then add fluid to the FULL mark on the reservoir. Air is purged from the system by operating all subsystems through several cycles until all sounds of air in the system are eliminated. Air in a system causes a variety of sounds, including banging, squealing, and chattering. The reservoir should be filled before the air is purged.

When cleaning filters or replacing filter elements, be sure there is no pressure on the filter. When changing hydraulic filters, wear the appropriate protective clothing and use a face shield as necessary to prevent fluid from contacting the eye. After filters have been cleaned or replaced, the system may have to be pressure-checked to locate any leaks in the filter assembly. Hydraulic fluid leaks are readily apparent because of the appearance of fluid. Large pneumatic leaks are indicated by the sound of escaping air and the feel of an airflow. Small pneumatic leaks should be located by the use of a soap-and-water solution.

2. See that the fluid in the test unit is the same type as that used in the aircraft.

3. Pump clean, filtered fluid through the system and operate all subsystems until no further signs of contamination are found upon inspection of the filters. The airplane must be raised on jacks so the landing gear can be operated.

4. Disconnect the test stand and cap the ports.

5. See that the reservoir is filled to the FULL line.

Leakage from any stationary connection in a system is not permitted, and if any such leak is found, it must be repaired. A small amount of fluid seepage may be permitted on actuator piston rods and rotating shafts. In a hydraulic system a thin film of fluid in these areas indicates that the seals are being properly lubricated. When a limited amount of leakage is allowed at any point, it is usually specified in the appropriate manual.

Tubing must not be nicked, cut, dented, collapsed, or twisted beyond approved limits as explained previously. The identification markings or lines on flexible hose will show whether the hose has been twisted.

All connections and fittings associated with moving units should be examined for play evidencing wear. Such units should be in an unloaded condition when they are checked for wear.

Accumulators should be checked for leakage, air or gas preload, and position. If the accumulator is equipped with a pressure gauge, the preload can be read directly. Otherwise, the system pressure gauge can be used as explained previously. The accumulators should be mounted with the air chamber upward.

An operational check of the system can be performed using the engine-driven pump, an electrically operated auxiliary pump if such a pump is included in the system, or a ground test unit. The entire system and each subsystem should be checked for smooth operation, unusual noises, and speed of operation for each unit. The pressure section of the system should be checked with no subsystems to see that pressure holds for the required time without the pump supplying the system. System pressure should be observed during operation of each subsystem to see that the engine-driven pump will maintain required pressure.

Lack of pressure in a system can be caused by a sheared pump shaft, defective relief valve, pressure regulator or unloading valve stuck in the “kicked-out” position, lack of fluid in a hydraulic system, check valve installed backward, or any condition that permits free flow back to the reservoir or overboard. If a system operates satisfactorily with a ground test unit but not with the system pump, the pump should be examined.

If a system fails to hold pressure in the pressure section, the likely cause is the pressure regulator or unloading valve, a leaking relief valve, or a leaking check valve.

If the pump fails to keep pressure up during operation of the subsystem, the pump may be worn or one of the pressure-control units may be leaking.

High pressure in a system may be caused by a defective or improperly adjusted pressure regulator or unloading valve or by an obstruction in a line or control unit.

Unusual noise in a hydraulic system, such as banging and chattering, may be caused by air or contamination in the system. Such noises can also be caused by a faulty pressure regulator, another pressure-control unit, or a lack of proper accumulator action.

Maintenance of hydraulic and pneumatic systems involves a number of standard practices together with specialized procedures set forth by manufacturers. The technician may be called upon to replace valves, actuators, and other units, including tubing and hoses. In every case, the technician must exercise care to prevent system contamination; avoid damage to seals, packings, and other parts; and apply proper torque in connecting fittings.

Flexible hose, particularly the type with a Teflon fluorocarbon resin tubing, tends to take a set after a period of time. During the process of removing and reinstalling such hose sections, the technician should make no effort to straighten or change the shape of the hose. It is recommended that the hose be tied with a piece of cord or safety wire to hold its shape. When flexible hose is installed, the metal sleeve of the hose fitting should be held with a wrench while the nut is tightened to the fitting. Otherwise, the hose may be twisted and damaged. Twisted hose can be detected by observing the woven wire braiding or the painted lines on the outside of the hose.

Overhaul of hydraulic and pneumatic units is usually accomplished in approved repair facilities; however, replacement of seals and packings may be done from time to time by technicians in the field. When a unit is disassembled, all O-ring and chevron seals should be removed and replaced with new seals. The new seals must be of the same material as the original and should carry the correct manufacturer’s part number. No seal should be installed unless it is positively identified as the correct part and its shelf life has not expired.

When installing seals, the technician must exercise care to ensure that the seal is not scratched, cut, or otherwise damaged. When it is necessary to install a seal over sharp edges, the edges should be covered with shim stock, plastic sheet, or masking tape. Sharp-edged metal tools should not be used to stretch the seals and force them into place.

When it is necessary to replace metal tubing, the old section of tubing can be used as a template or pattern for forming the new part. In many cases, the tubing section can be procured from stock or from a parts dealer by specifying the correct part number.

The replacement of hydraulic units and tubing always involves the spillage of some hydraulic fluid. The technician should take care to see that the spillage of fluid is kept to a minimum by closing valves, if available, and by plugging lines immediately after they are disconnected. All openings in hydraulic and pneumatic systems should be capped or plugged to prevent contamination of the system. It is particularly important to see that fire-resistant hydraulic fluid is not allowed to get on painted surfaces or other material that would be damaged. Fire-resistant fluids (phosphate ester type) have a powerful solvent characteristic and should be immediately removed from any surface that may be damaged, including the skin.

The importance of the proper torque applied to all nuts and fittings in a system cannot be overemphasized. Too much torque will damage metal and seals, and too little torque will result in leaks and loose parts. In every case, the technician should procure torque wrenches with the proper range before assembling units of a system.

Additional information regarding aircraft plumbing, including plumbing for hydraulic and pneumatic systems, is provided in the text Aircraft Basic Science.

  1. What is the principal difference between a hydraulic power system and a pneumatic power system?

  2. Explain how force is developed through the use of hydraulic fluids.

  3. Explain the multiplication of force by means of hydraulics.

  4. Explain the relationship between volume, area, and the length of stroke.

  5. Name three types of hydraulic fluids and describe their basic characteristics.

  6. What precaution must be taken in regard to spills on a painted surface with a phosphate ester fluid such as Skydrol?

  7. Describe a hydraulic reservoir.

  8. What is the purpose of a standpipe in a reservoir?

  9. Why is it necessary to pressurize some reservoirs?

10. Give three methods by which a hydraulic reservoir can be pressurized.

11. What is the purpose of a hydraulic filter?

12. What are different filter elements that may be found in hydraulic systems?

13. What is the purpose of the “pop-out” indicator pin found on many filters?

14. Describe the purpose of a heat exchanger and how it functions.

15. Why is a gear-type pump described as a positive displacement pump?

16. In an axial multiple-piston pump, what causes the reciprocating motion of the pistons?

17. Describe two types of variable-delivery pumps and explain the operation of each.

18. Explain the purpose of a ram air turbine.

19. Describe the purpose of a pressure regulator.

20. How does a pressure regulator or unloading valve serve to prolong the life of the system pump?

21. List the different types of selector valves.

22. Explain how poppet valves control fluid flow.

23. Explain the operation of spool-type valves.

24. How are the selector valves in an open-center system connected with respect to one another?

25. Describe an orifice-check valve. For what purpose would such a valve be used?

26. Explain sequence valve operation and purpose.

27. What can occur if a sequence valve is not properly adjusted?

28. Under what conditions is a shuttle valve required?

29. Explain the operation and use of a hydraulic fuse.

30. Explain the operation and use of a priority valve.

31. Describe the operation of a relief valve.

32. What determines the cracking pressure of a relief valve?

33. Why is a wing-flap overload valve installed in the down line of a flap-actuating system?

34. Why are thermal-relief valves needed in certain sections of a hydraulic system?

35. Explain the principle of a debooster valve.

36. Explain the purpose of a flow equalizer.

37. What are the two functions of an accumulator?

38. Describe a piston-type accumulator.

39. What precautions should be taken before removing an accumulator from a hydraulic system?

40. What is a hydraulic actuator?

41. Describe a double-acting hydraulic actuating cylinder.

42. How does a servo actuator differ from an actuating cylinder?

43. Compare a hydraulic motor with a piston-type pump.

44. Discuss the purpose of colored markings on O-ring seals.

45. When O-ring seals are used in high-pressure systems, what devices are often necessary?

46. Compare a closed-pressure hydraulic system with an open-center system.

47. List the principal hydraulic systems in a Boeing 777 aircraft.

48. What is the function of a power-transfer unit on the 777 aircraft?

49. List the principal hydraulic systems on an A380 aircraft.

50. Name the three principal hydraulic systems for the Boeing 777 airplane.

51. What types of pumps are employed in the hydraulic power systems for the Boeing 777 airplane?

52. What systems are served by system A in the Boeing 777?

53. What is the purpose of the ground interconnect valve?

54. List the advantages of a pneumatic power system.

55. What is the difference in the principles of operation between a pneumatic system and a hydraulic system?

56. What are uses for pneumatic bottles in a hydraulic system?

57. For what functions is the pneumatic power system on the Fairchild F-27 airplane used?

58. How is the high-pressure air stored in the Fairchild F-27 pneumatic system?

59. Describe the operation of the four-stage compressor.

60. What is the purpose of the isolation valve in the F-27 pneumatic system?

61. Explain the need for dry air in a pneumatic system.

62. Describe the operation of the moisture separator.

63. What is a desiccant?

64. What is the function of the back-pressure valve?

65. How is lowered pressure obtained from the high-pressure air source?

66. Explain the use of the ground-charging valve.

67. List the principal consideration necessary when servicing a hydraulic reservoir.

68. What conditions can cause banging or chattering in a hydraulic system?

69. What conditions may cause lack of pressure in a hydraulic system?

70. What may cause the pressurized section of a hydraulic system to lose pressure?