ydraulic systems -...

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Hydraulic Systems

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6 HYDRAULIC SYSTEMS

Modern aircraft use up to three primary systems for operation of the landing gear,flaps, control surfaces and other devices: these are hydraulic, pneumatic and elec-trical systems. Each of these systems has its own characteristics, and the choiceof implementation is dependent upon availability and requirement. Some aero-planes use electrical systems to operate flaps and landing gear. Others use high-pressure air to operate this equipment. However, the majority of modern aero-planes use hydraulic power to operate such equipment. Many aeroplanes use sim-ple hydraulic systems to operate the brakes and to lower and raise the landinggear. Large aeroplanes use hydraulic systems to operate the primary and second-ary flight controls as well as many other functions such as nose wheel steering,engine reverse, doors, stairs and hatches etc.

The advantages of hydraulic systems are high power, light weight, easy installa-tion and minimum maintenance requirements. The hydraulic system provides anextremely accurate and efficient means of transmitting power over long distances.Since most advanced hydraulic systems operate with high pressure there is al-ways a possibility of hydraulic power loss due to fluid leak or failure of the hy-draulic pump. Another disadvantage of the hydraulic system is that the compo-nents have extremely tight tolerances, and filtering of the fluid is very important.

This section will discuss various hydraulic systems, but commences with a briefreview of the basic laws of physics regarding hydraulics.

6.1 Basic Principles of HydromechanicsHydraulic systems transmit power by moving incompressible fluid from one placeto another. A hydraulic system has a wide range of options for dealing with prob-lems of operating various systems in aeroplanes.

The international system of standards (SI) defines the units to be used to deter-mine physical standards and has been adopted for use in Europe. However, in theaviation industry, SI units have not yet been widely adopted and imperial systemof measurement that measures force in pounds, area in square inches, and pres-sure in pounds per square inch is still commonly used. This is especially true ofmaterial that originates in the USA where a quaint mixture of imperial and ‘USAhome grown’ units are the norm.

6.1.1 Relationship between force, pressure and areaThe amount of force a hydraulic system can provide is related to the amount ofpressure used and the area on which the pressure is acting. The relationshipbetween force, pressure and area may be expressed by various formulas, and inorder to determine the force produced from a given pressure and area, the follow-ing formula may be used.

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Force = Pressure X AreaThe product of the pressure and the area determines the force.The amount of pressure needed for a piston to produce a given force is expressedwith the formula.

Pressure = Force/ AreaThe area needed to produce a given force, with the hydraulic pressure available, isgiven by using the formula.

Area = Force/Pressure

6.1.2 Relationship between distance, volume and areaAnother relationship in hydraulics is the one between the distance travelled by thepiston, the volume of the fluid displaced and the area of the piston. The amount offluid needed to move a piston (with a given contact surface area), a given distanceis expressed in the formula below.

Volume = Area X DistanceThe distance a given amount of fluid will move a piston with a given surface con-tact area, is given by the formula.

Distance = Volume / AreaTo find the piston size needed for a given distance of movement when you know thevolume of the fluid, the formula becomes:

Area = Volume / Distance

6.1.3 Relationship between pressure and heightThe pressure produced by a column of liquid is directly proportional to the heightof the column and has nothing to do with the volume or the shape of the containerholding the liquid. Provided that the height of the liquid in a tube is exactly thesame as the height of the liquid in a tank, the pressure at the bottom of the tubeis the same as the pressure at the bottom of the tank.

Figure: AS 6.1

As illustrated in figure AS 6.1, the gauges in all the four containers will read thesame since the height of the liquid is the same, provided that all the vessels arefilled with liquid of the same density. Pressure can be varied by the density of thefluid and by the height of the fluid column. To find the pressure of a column offluid, the formula below is now.

Pressure = Density X Height

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6.1.4 Pascal’s lawThe law from Pascal’s theorem, is the basic law used regarding transmission ofpower by a hydraulic system. In simple terms, Pascal’s law says that the pressurein an enclosed vessel acts at right angles to the walls enclosing the fluid, and thatthe pressure is transmitted equally and undiminished to all parts of the vessel.

Figure: AS 6.2

In figure AS 6.2, it can be seen that the pressure in an open container increaseswith the depth of the tank. As illustrated by the gauges, the pressure is least atthe top and greatest at the bottom of the container. This is because the height ofthe column, and therefore the mass of fluid, is different above each gauge. It isleast at P1 and increases through P2 to P3. This pressure results from the accel-eration of gravity acting on the upper surface of the column – it is frequently re-ferred to as “Hydrostatic Pressure”.

If a piston and a weight are placed on the top of the vessel, the situation changes.The force exerted by the piston will produce pressure inside the container, and thereadings, on each of the gauges, will increase to the same value. The pressure inthe container is dependent upon the force applied to the piston, and the area of thepiston. The pressure is the same on all the gauges, regardless of their position inthe vessel. Consequently, Pascal’s law states that the pressure in an enclosedvessel is transmitted evenly to all areas of the vessel. This theorem proves whyhydraulic brakes in automobiles have equal braking action. By depressing thebrake pedal, a pressure is generated which is transmitted equally to each of thewheels, regardless of the distance between the pressure source (the brake mastercylinder) and the wheel cylinder, given no pipe friction.

6.1.5 Implementation of these lawsThe application of these basic laws of physics provides mechanical possibilitiesfor a hydraulic system.



Figure: AS 6.3

In figure AS 6.3, the smaller piston has an area one tenth of the larger. A force often Newton (N) applied to the small piston results in a pressure of ten units beingbuilt up in the fluid. According to Pascal’s law, a pressure of ten units is the samethroughout the system, and this pressure acts on each square centimetre of thelarge piston. A pressure of ten N/cm2 applied to the larger piston will produce aforce of 100 N, (10 N/cm2 X 10 cm2)

If the system pressure is the same in the closed system the amount of pistontravel varies. If the smaller piston is moved one centimetre down, one cubic centi-metre of fluid is moved from the small cylinder to the larger cylinder. The cubiccentimetre of fluid spreads out over the larger piston and moves this only onemillimetre due to difference in area on the large and small pistons. The work doneby the small piston, however, is equivalent to the work done by the large one.

6.1.6 Hydraulic fluidsAny fluid can transmit a force, however, the fluid used in hydraulic systems musthave more extensive properties. The fluid must be as incompressible as practica-ble and at the same time have acceptable lubricating properties to avoid unneces-sary wear of the pump and system components. It is essential that the fluid has alow viscosity so it will flow through the system with a minimum of friction. It isimportant that the fluid has a high flash point and a high fire point. At the sametime it must be compatible with the elastic materiel used in gaskets, seals and themateriel used in the components of the hydraulic system. This to avoid internalmaterial corrosion.

In aircraft hydraulic systems there are three basic types of hydraulic fluids: veg-etable base, mineral base and synthetic base. All three fluids have individual char-acteristics and limitations, and it is important not to mix the various types be-cause they are not compatible.

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Vegetable-base, MIL-H-7644, Lockheed 22 (UK)Vegetable-base hydraulic fluid, MIL-H-7644 has a characteristic blue colour, andis primarily based upon castor oil and alcohol. This type of hydraulic fluid isbeing replaced with other, more sophisticated types of fluid. The vegetable-basefluid was used in systems that were far less complex. You are unlikely to find it inmodern hydraulic systems, due to their operational requirements.

Mineral-base, MIL-H-5606, DTD 585 (UK)Mineral-base hydraulic fluid, MIL-H-5606 is dyed red for identification and con-sists of a kerosene-type petroleum product. This hydraulic fluid is a good lubri-cant and additives prevent it from foaming and having a corrosive reaction withmetal. It is caustic to natural rubber, and must therefore be washed off if it gets incontact with the tyres, or the tyre should be changed. MIL-H-5606 is very stableand changes minimally in viscosity due to temperature changes. Unfortunately itis flammable. The fluid is used in light aircraft braking systems and landing gearoleo legs.

Synthetic-hydrocarbon-base, MIL-H-83282, MIL-H-83282This hydraulic fluid is dyed purple and has many of the characteristics of MIL-H-5606, but as opposed to the mineral-base, the MIL-H-83282 has a synthetic hy-drocarbon base. The synthetic-hydrocarbon-base replaces the mineral-base, mostlydue to it being fire resistant. For use in extremely low temperatures, the compat-ible MIL-H-81019 is preferred due to its superior low temperature characteris-tics. This fluid requires care in handling as it is irritant to skin and eyes andprotective equipment should be used to avoid skin contact.

Hydraulic fluid of different types should not be mixed. Seals for one type of fluidwill not be tolerant of any other type. Should an aeroplane system be serviced withthe wrong type of fluid it will be necessary to immediately drain and flush thesystem and check all the seals.

6.2 Basic aircraft hydraulic systemsHydraulic systems consists of various components and to better understand thefunction of the systems forthcoming section describes the intention and functionof some fundamental hydraulic components.



6.2.1 Hydraulic reservoirs

Figure: AS 6.4

Hydraulic components have different tasks. The purpose of a hydraulic systemvaries and to safeguard its operation, the system has to include som basic compo-nents. A hydraulic system requires fluid to transform the force, a pump to pres-surise and move the fluid, a reservoir is the component that stores the fluid andserves as an expansion chamber to provide a space for the fluid when its volumeincreases because of temperature. During the operation of the hydraulic systemthe fluid becomes contaminated with air and the reservoir serves as a place wherethe air can purge itself of any air it has accumulated. A reservoir has to fulfilcertain requirements depending on system complexity and there are two mainconstructional differences; non-pressurised and pressurised reservoirs as shownin figure AS 6.4.

A non-pressurised reservoir are usually used on aeroplanes that fly at relativelylow altitudes. Aeroplanes that operate at high altitudes require pressurised reser-voirs because the outside pressure is low at high altitudes. There is not enoughair pressure to force the hydraulic fluid from the reservoir to the pump. Pressuri-sation assures a positive flow of fluid both in the outlet and the return lines athigh altitudes where low atmospheric pressure appears. There are different waysto pressurise the reservoir, either by air or fluid from the system pressure andfigure AS 6.4 show a reservoir that is pressurised by system pressure.

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6.2.2 Hydraulic accumulators

Figure: AS 6.5

An hydraulic accumulator is a device charged with hydraulic fluid that shall sup-port the system with backup pressure in case of temporarily or permanent loss ofhydraulic pressure. It also servers as a cushion or shock absorber by maintainingan even pressure in the system. Besides these two-fold purposes, the accumulatoralso supplements the pump output under peak loads by storing energy as pressu-rised fluid. Accumulators are being used in most hydraulic systems but they areespecially important as emergency sources in the landing gear and the brake sys-tem.

There are basically three types of accumulators;· the piston type· the bladder type· the diaphragm type.

The piston type as shown in figure AS 6.5 is as a robust cylinder, while both thebladder and the diaphragm type are in the shape of a steel sphere. The piston typeaccumulator consists of either a steel or aluminium alloy cylinder and a floating



piston. In one end of the cylinder compressed air or nitrogen is supplied and inthe other end, the hydraulic fluid is stored. Accumulators are charged with com-pressed air or nitrogen to a pressure of approximately one third of the systempressure. During normal system operation hydraulic fluid is forced into the accu-mulator by the pump and as the gas is further compressed it put forth a force onthe hydraulic fluid and holding it under pressure.

6.2.3 Hydraulic pumpsFluid power is produced when fluid is moved under pressure. The pumps used ina hydraulic system are simply fluid movers rather than pressure generators. Pres-sure is produced only when the flow of fluid from the pump is restricted. The twobasic types of hydraulic pumps are those operated by hand, and those driven byan external source such as an aircraft engine, electrical- or pneumatic motor, calledpower pumps.

Hand pumps. A hand operated pump moves fluid during the stroke of the pumphandle and are often used in connection with back up- or emergency systems. Thetwo most common types of hand pumps are;Single acting. This is a hand operated fluid pump that moves fluid only duringone stroke of the pump handle. One stroke pulls the fluid into the pump and theother forces the fluid out.

Double acting. A hand operated fluid pump that moves fluid during both strokesof the pump handle.

Power pumps. Hydraulic system used in aviation industry widely use power pumpsclassified as either constant- displacement or variable displacement pumps.

Constant displacement pump. This type of pump moves a specific amount of fluideach time it rotates. A pump of this type must have some sort of relief valve orunloading device to prevent its building up so much pressure that it will rupturea line or perhaps damage itself.

Variable displacement pump. A hydraulic pump whose output is controlled bythe demands of the system. These pumps normally have a built-in system pres-sure regulator. When the demands of the system are low, the pump moves verylittle fluid, but when the demands are high, the pump moves a lot of fluid.

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Figure: AS 6.6

The gear type pump illustrated in figure AS 6.6 is a power driven constant- dis-placement fluid pump that contains two meshing large tooth spur gears. One ofthe gears is driven e.g. by an engine accessory drive, and this gear drives the otherone. As the gears rotate in the direction shown by the arrows, the space betweenthe teeth on the inlet side of the pump becomes larger. Fluid is drawn into thepump as the teeth separate and is carried around the inside of the housing withthe teeth and is forced from the pump when the teeth come together. Here the teethof the two gears come into mesh and decrease the volume. As the volume is de-creased, fluid is forced from the pump outlet. A small amount of fluid leaks pastthe gears and around the shaft to help lubricate, cool and seal the pump. Geartype pumps move a medium volume of fluid under a pressure of between 300 and1500 psi.



Figure: AS 6.7

A power driven variable displacement pump is far more complicated. The pumpillustrated in figure AS 6.7 consists of a coupling shaft, drive cam, pistons withrelief holes and sleeves, a spider and a compensator spring and compensatorstem. It uses nine axially oriented cylinders and pistons which are driven up anddown inside the cylinders by a wedge shaped drive cam, and the pistons pressagainst the cam with ball joint slippers. When the slipper is against the thick partof the cam, the piston is at the top of its stroke, and, as the cam rotates, thepiston moves down in the cylinder until the slipper is riding on the thin part of thecam. When the slipper is in this position, the piston is at the bottom of its stroke.The physical stroke of the piston is the same regardless of the amount of fluiddemanded by the system, but the effective length of the stroke controls the amountof fluid moved by the pump. A balance between the fixed compensator spring forceand the variable force caused by pump output pressure acting on an enlargedportion of the compensator stem moves the sleeves up or down over the outside ofthe pistons. This varies the position of the piston when the pressure is relieved,and thus varies the effective stroke of the piston. Aircraft systems that require arelatively large volume of fluid under a pressure of 3000 psi use this type of pumps.

On large transport aircraft hydraulic power is normally provided by two hydrauli-cally separate, closed circuit hydraulic systems. Each system may be equippedwith an engine driven pump of the variable displacement type which normallyprovides system pressure. In addition an electrically driven auxiliary pump of

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same type may be installed as a backup. But as redundancy the two systems maybe mechanically interconnected by a hydraulic transfer pump.

Figure: AS 6.8

Transfer pump. This is rather a pump assembly than one separate pump andconsists of two coupled hydraulic units so constructed that either unit can oper-ate as a motor while the other operates as a pump. The transfer pump thus servesas a fluid pressurisation backup for both systems as one side of the assemblymay operate as a pump, driven by the other side acting as a motor.

Figure AS 6.8 shows a situation where the right engine-driven pump has failed,while the left systems is working normally.

Switching ON the the transfer pump will make both transfer pump shut-off valvesopen and high pressure fluid enters the left side of the assembly. Thus left pump/motor will act as motor and drive the right pump through the mechanical inter-connection. Right pump gets fluid from right hydraulic system reservoir and sup-plies fluid to the right system as long as right system reservoir contains fluid.During normal system operation left system will apply fluid with a pressure of3000 psi to the motor. Due to energy loss across the assembly, the right pump willdeliver a pressure which is somewhat lower.

6.2.4 Hydraulic actuatorsThe ultimate function of a hydraulic system is to convert the pressure in the fluidinto work. In order to do this, there must be some form of movement, and thismovement takes place in the actuator. The two main types of hydraulic actuatorsare linear actuators and rotary actuators(hydraulic motors).

TRANSF

1

HYD PUMPSENGHI

LOW

OFF

ON

OFF

2

LEFTCONSUMERS

DUALCONSUMERS

L EngineDrivenPump

Motor

TRANSFER PUMP

PumpRIGHT

CONSUMERS

L Hydr Fluid Tank

R Hydr Fluid Tank

R EngineDrivenPump



Figure: AS 6.9

Linear actuators. Uses a piston inside a cylinder to change pressure into linear,or straight- line motion. The cylinder is usually attached to the aircraft structure,and the piston is connected to the component that is being moved. The rate ofmovement of the piston is controlled by restricting the fluid flowing into or out ofthe cylinder. Linear actuators have features that adapt them to operate e.g. thelanding gear, landing gear doors and speedbrakes.

Figure shows three basic types of linear actuators; single acting, double actingunbalanced and double acting balanced actuators.Single acting actuator. This type has a piston that is moved in one direction byhydraulic fluid, and is returned by a spring.

Double acting unbalanced actuator. Uses hydraulic fluid to move the piston inboth directions. It has a piston rod extending from only one side of the piston andtherefor more area on the opposite of the piston rod because much of this area istake up by the piston rod. When using this type of actuator for lowering and risingthe landing gear the full piston area side will normally be used to raise the landinggear. Not as much force is needed to lower the landing gear, so this sequencedirects fluid into the end of the actuator that has the piston rod.

Double acting balanced actuator. An actuator that has a shaft on both sides ofthe piston, so the area is the same on each side, and the same amount of force isdeveloped in each direction. Balanced actuators are often used for hydraulic servosused in connection with the autopilot.

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Figure: AS 6.10

Hydraulic motors. A hydraulic motor is a hydraulic actuator that converts fluidpressure into rotary motion. They are similar to hydraulic pumps expect for cer-tain design detail differences and is used to maintain continued rotation. Hydrau-lic motor’s advantage over electrical motors are its ability to instantaneously re-verse the direction of rotation and its lack of fire hazard in the event of a stalledmotor.

Figure AS 6.10 illustrates a balanced vane type hydraulic motor that has pressuredirected to both sides of the rotor. Rectangular vanes are free to float and fitted toslots in the rotor. A elliptical shaped steel housing held the vanes against the wallduring rotation. As fluid under pressure are supplied to the inlet ports the fluidwill act on the vanes. Due to the elliptical shape of the housing the vane areadiffers and are greater left of the inlet ports, resulting in a greater force producedon the left side. This again causing a clockwise rotation of the rotor. When the vanereaches the return port the two vane area holding the fluid volume are much thesame and no resultant force are dominant. The fluid is led back to through returnlines

6.2.5 Hydraulic valvesHydraulic valves are used to control the flow and the direction of the fluid flow.They may be controlled manually, electrically, mechanically, hydraulically or com-binations of these methods. In hydraulic systems there are two types of valves;those that control flow and those that control pressure.



Figure: AS 6.11

Check valves. In hydraulic systems there are situations that require devices toobstruct the fluid from flowing in both directions. Figure AS 6.11 shows a widelyused ball type check valve. When fluid is supplied from the left side the fluidpressure will force the spring to compress and the ball moves off its seat. Thisallows fluid to flow through the valve from left to right. In the opposite directionthe fluid is obstructed because the spring held the ball tightly against its seat.Besides if the fluid tries to flow from the right the fluid pressure will act on theball forcing it even tighter toward its set.

Figure: AS 6.12

Relief and by-pass valves. Relief and by-pass valves are mainly used as a backupdevice to prevent high pressure from damaging the system. These valves may havea pressure adjustment mechanism which allows us to fit the relief pressure of therelief valve to the system requirements. In normal operation the spring act on theball which holds the relief valve closed and no fluid is directed through the valve.The pressure port is connected to a hydraulic pressure line and when the pres-sure exceeds the value relatively to the adjustment of the spring force, the linepressure will force the spring to compress and unseat the ball. This allows fluidto flow trough to the return port and doing so the valve relief or by-passes thepressure.

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Figure: AS 6.13

Restrictor valves. This is a valve that controls the rate of e.g. actuator movementby restricting the flow of fluid into or out of the actuator. The valve illustrated infigure AS 6.13 allows full flow of fluid in one direction but a restricted flow in theopposite direction. A restrictor valve may be used in a landing gear system to slowthe extension of the gear and yet allow it to retract as quickly as possible.

Figure: AS 6.14

Selector valves. The selector valve controls the direction of flow of the fluid usedto actuate hydraulic components. They direct fluid under pressure to one side ofthe hydraulic component and vent the opposite side to the return line. Usuallyselector valves are connected in parallel between the pressure manifold and thereturn manifold. Figure AS 6.14 illustrates a simplified diagram containing anactuating cylinder and a selector valve. When the valve is positioned so that thepressure port is connected to the right side of the cylinder the piston is forced intothe cylinder. At the same time the other side is connected to the return line. Toreverse the movement, the selector valve is to be rotated 90 degrees which con-nects the left side of the actuator to the pressure side, resulting in a rightwardmovement of the piston. When the selector valve is in neutral position, in whichthe actuator is isolated from both the pressure and the return manifold, the flowof fluid is stopped and the actuator remains its position.



Figure: AS 6.15

Shuttle valves. This is an automatic selector valve mounted on critical compo-nents such as landing gear actuation cylinders and brake cylinders. In normaloperation the spring drives the piston onto the piston movement blocks. Systemfluid flows into the hydraulic component attached to the shuttle valve. If normalpressure is lost emergency system pressure forces the shuttle over and emergencyfluid flows into the hydraulic component attached. The valve illustrated in figureAS 6.15 has an emergency air source but hydraulic oil e.g. from an accumulatormight also act as emergency source.

Fire shutoff valves. In normal system operation the valve is open but in case offire it shuts off hydraulic fluid to the pumps. Normally they are located in thesupply line between the reservoir and the main pumps. They may be mechanicallyor electrically operated through a fire handle in cockpit. When activated they willblock fluid to the pumps. Fire shut-off valves look much the same as selectorvalves except that its choice of position is either open or closed.

Hydraulic Fuses. Modern aircraft depend upon their hydraulic systems not onlyfor raising and lowering of the landing gear and flaps, but for brakes, and manyauxiliary systems. For this reason, most aircraft use more than one independenthydraulic system as seen previously. If a serious leak should occur, it is essentialthat the leaking system be isolated, and this is done by using a hydraulic fuse.There are two basic types of hydraulic fuse in common use. One of these typesoperate in such a way that it will shut off the fluid flow if there is a sufficientpressure drop across the fuse. Another type operates on a different principle andwill be shut off the flow after a given amount of fluid has passed through the line.Hydraulic fuses are used to protect the wheel brakes.

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Figure: AS 6.16

Priority valves. A priority valve is a valve in a hydraulic system that requires acertain action to be completed before another action can begin. The priority valveis opened by the hydraulic pressure rather than by mechanical contact. E.g. prior-ity valves are used to assure that the hydraulically actuated wheel well doors arecompletely open before pressure can be directed to the landing gear to lower it.Figure AS 6.16 illustrates a priority valve in a landing gear actuation system in theposition it is in before the wheel well doors are fully open. As soon as the doorsare fully open, pressure builds up in the gear down line and moves a poppet insidethe priority valve which allow fluid into the main landing gear actuator.

6.3 Advanced aeroplane hydraulic systemsA hydraulic system is much like an electrical system. It must have a source ofpower, a means of transmitting this power, and some type of mechanism to usethis power.

Passive hydraulic systems. The most basic form of a hydraulic system is thatused by hydroelectric power plants. Large dams block streams of water to formlakes that store billions of litres of water. This stored water represents the poten-tial energy in the system and it is converted to kinetic energy as the water flowsdownward through pipes to a turbine. The kinetic energy of the flowing water isconverted to mechanical energy as it turns the turbine. This mechanical energy isused to drive a generator and we have a open hydraulic system also called a pas-sive hydraulic system.

Active hydraulic systems. Passive hydraulic systems work well to operate a mill,or for the production of electrical energy, but has no practical application to hy-draulic systems used in the aviation industry. To apply hydraulic power to aircraft



systems we must enclose the fluid, move it through a system of rigid lines andhouses, and put its energy to use in various types of actuators and hydraulicmotors. These systems are called closed hydraulic systems or active hydraulicsystems.

Hydraulic components have different tasks. The purpose of a hydraulic systemvaries and to safeguard its operation, the system has to include some basic com-ponents. A hydraulic system requires fluid to transform the force, a pump tomove the fluid, a reservoir to store the fluid, lines to carry the fluid, actuators totransform the flow of fluid into work and valves to control the system sequence.Components, such as accumulators, filters, pressure regulators, hydraulic fuses,etc. provide the operational efficiency of the system. Hydraulic systems on modernaeroplanes can be quite complex and to better understand the functional princi-ples of such systems, this section gives a brief introduction to the function of twobasic hydraulic systems.

6.3.1 Hydraulic landing gear system, manually operatedWhen parasite drag became a problem, the solution was retractable landing gear.The hydraulic system in figure AS 6.17 illustrates a retractable landing gear sys-tem with a double-acting actuator. This type of system is unusual on today’saeroplanes but its simplicity provides a good start to an understanding of themore complex hydraulic systems.

Figure: AS 6.17

This system includes a reservoir, a hand pump, a selector valve and an actuatingcylinder. The landing gear down sequence is as follows: The landing gear selectorvalve is to be set in GEAR-DOWN position. This directs fluid to the upper side ofthe actuator’s piston. When the hand pump is operated, fluid is fed from thereservoir and enters the top cylinder. The landing gear causes resistance to thepiston and fluid pressure builds until it overcomes the resistance from the land-ing gear mechanism and the landing gear is forced down. The fluid on the lowerside of the piston returns to the reservoir through the selector valve. The loweringsequence requires less power from the hydraulic system because the weight of thelanding gear will assist it to fall under gravity.

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To raise the landing gear the selector valve is to be placed in GEAR-UP position,which directs fluid to the lower side of the piston. When the hand pump is oper-ated, pressurised fluid on the top of the cylinder is returned to the reservoir throughthe selector valve.

Such a system has no pump redundancy and if the hand pump should fail, thepilot might not be able to operate the landing gear and certainly not raise it. Thissystem is normally designed to allow the landing gear to lower itself from theweight and the aerodynamic drag from the airflow. The drag assistance is, of course,only available if the landing gear is hinged to the aeroplane structure, behind thelanding gear’s centre of gravity.Hand pump systems are often used as emergency systems or back up to powerpump systems in the event of a failure of or loss of fluid from the power pump. Itis then designed to lower the gear and flap but not raise them.

6.3.2 Power pump system with pressure regulatorParagraph 6.3.1 explained the principle of a very simple hydraulically operatedsystem. This paragraph looks at a more sophisticated hydraulic system, sup-ported by a power pump, a pressure regulator called an unloading valve or auto-matic cut-out valve, and various other valves and components.

Figure: AS 6.18

An engine driven pump receives its fluid from the reservoir, which is usually pres-surised. Note that the line protrudes into the bottom of the reservoir. If there is aleakage before the pressure manifold, the engine driven pump will not be able toempty all the fluid from the reservoir. The remaining fluid may be used in anemergency situation where the hand pump supports the system with fluid.Alternatively, the reservoir might be divided into two cells, one for the engine drivenpump and one to support hand pump operation.

Under normal operation, the engine driven pump moves the fluid through theunloading valve and into the pressure manifold. Depending on the position of theselector valves, the actuator pistons will move in or out of their cylinders. If the



system is unloaded, the accumulator maintains the system pressure whilst thefluid output from the pump is directed back to the reservoir through the unload-ing valve.

When the system pressure reaches a predetermined value, indicating that the ac-tuating cylinders have sufficient fluid supply, the unloading valve becomes opera-tive and directs the pump outlet into the return manifold instead of the pressuremanifold. In this situation, the pump directs the fluid through the unloading valveand filter, back into the reservoir as long as the system pressure is above thepredetermined value set on the unloading valve. Very limited amounts of enginepower are used to circulate the fluid in this configuration because there is almostno opposition.

If some components in the system are operated, the system pressure will dropbelow the pressure set on the unloading valve to circulate the fluid. The unloadingvalve will shift and direct the pump output into the system pressure manifold.Now the pump supplies all the fluid needed for operation and the pressure risesabove the circulation value and the unloading valve will once again circulate fluidthrough the filter to the reservoir. The system pressure relief valve will unload thesystem pressure if this exceeds the predetermined pressure for fluid circulationthrough the unloading valve. One reason for the relief valve to operate is in thecase of heat expansion when the system is inoperative.

6.4 Advanced aeroplane hydraulic systemEarly aeroplanes flew at speeds where drag was no great concern. Therefore, re-tractable landing gear was not needed. Landing speeds were so slow that therewas no need for lift generating devices, such a flaps and slats, nor high perform-ance brakes. Today’s aeroplanes are bigger, faster and much more sophisticatedand there is a requirement for systems to provide power to operate these devices.Hydraulic power is so important in modern aeroplanes, that more than one mainhydraulic system is installed. Some aeroplanes may include two, even three over-lapping main hydraulic systems and, in addition, emergency systems in case ofmain system failure.

The next section uses the hydraulic system of the Boeing 737 as an example.

6.4.1 General system descriptionThe Boeing 737 has a complex hydraulic system that supplies power to operatethe landing gear, flaps, spoilers, primary flight controls and auxiliary functions,such as the thrust reversers. Hydraulic power is provided by three independentsources: system A, system B and a standby system. An engine driven pump oneach of the aircraft’s engines provides pressure to system A. System B is pressu-rised by electric pumps. The standby system is used if there is loss of power insystem A or system B. Pressure to the standby system is provided through oneelectric motor-driven pump. The nominal system pressure delivered by the pumpsis 3000 psi (pounds per square inch) and each hydraulic system has a fluid res-ervoir located in the main wheel-well area. To prevent fluid cavitation, the reser-voirs are pressurised with engine bleed air. This provides a constant pressure

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that ensures a positive supply to all hydraulic pumps and controls the fluid levelin the reservoirs. System A, system B and the standby system all have definedsystems to which they deliver pressurised fluid but an interconnect valve allowssystem B to pressurise system A. This is only a system check when the aircraft ison the ground and the parking brake is set.

It should be noted that system pressures vary with aeroplane type. Some smallturbo-prop aeroplanes may have a 2000 psi system while a large transport aero-plane could have a system pressure of 4000 psi.

6.4.2 Hydraulic system A

Figure: AS 6.19



The fluid from system A supplies the inboard brakes, inboard flight spoilers,ground spoilers, ailerons, elevators, rudder, trailing edge flaps, leading edge de-vices, landing gear, nose wheel steering and thrust reversers. This section dis-cusses the options the crew can take to control the hydraulic power throughswitches in cockpit and what indications there are.

The fluid quantity is displayed on indicators at the reservoir and in the instru-ment panel in the cockpit. The engines drive the pumps but the pump outputpressure is controlled by the ENG pump ON/OFF switch in the cockpit. If thisswitch is positioned to OFF, a solenoid valve in the pump is energised and shutsoff the fluid flow to the systems. For normal operation, the switch is placed in ONposition and fluid flows from the reservoir through a shutoff valve placed neareach of the pumps. A FIRE WARNING SWITCH, not directly related to the hydrau-lic system, controls these valves.

In case of an engine fire, the pilot can shut off the flow of fluid to the respectivehydraulic pump to isolate the hydraulic system from the fire.

In a hydraulic system, the fluid is also used for cooling and lubrication of thepumps. This is the reason why some hydraulic systems include a heat exchanger.In this system, the heat exchanger is placed in a bleed line between the pumps andthe reservoir. To obtain sufficient cooling, the heat exchanger is placed in the fueltank and uses fuel as coolant.

In the output line of the engine driven pumps, pressure transmitters are installed,sending electric signals to illuminate the LOW PRESSURE light if the pump pres-sure output is below limits. The system A pressure is measured at a pressuretransmitter in the line to the system which send signals to a pressure gauge whichshows the output pressure of the two pumps.

When a hydraulic reservoir is initially serviced (filled with fluid) the pumps arenot running and the accumulators are not pressurised except by dry air or nitro-gen. When the reservoir is full, the indications in the cockpit or reservoir sightglass are misleading because, when the pumps are started, fluid is moved fromthe reservoir to the accumulators and the content level will drop. The aircraftflight manual will state which accumulators must be charged and which relievedbefore a fluid contents check is made.

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6.4.3 Hydraulic system B

Figure: AS 6.20

The user systems that are fed from system B are outboard brakes, outboard flightspoilers, ailerons, elevators, rudder, yaw damper, autopilot and outboard brakes.The system B reservoir is connected to the system A reservoir and the standbyreservoir through balance lines for the purpose of single point pressurisation andservicing. The pumps in system B are driven by electrical motors, which are con-trolled by the HYD PUMP switches in the cockpit. If the electrical power to eitherof the pumps should fail, an automatic load shedding feature will deactivate therespective hydraulic system B pump. In the cockpit, such a situation can be de-tected through the illumination of the LOW PRESSURE light. When the electrical



power is recovered, the pump is once again activated and the light extinguishes. Inaddition, there are pressure sensors located in the pump output lines that sendsignals to illuminate the LOW PRESSURE light if the output pressure is belowlimits. A check valve isolates the two pumps’ bleed line and a temperature sensorsends signals to illuminate the OVERHEAT light if the fluid becomes overheated.

The HYD SYS PRESS indicator has needles both for system A and system B, andthe system pressure transmitter sends electrical signals from the pressure of thepumps to the B needle in the indicator.

As for system A, some of the fluid is used for cooling and lubrication of the pumps.The heat exchanger is placed in a bleed line between the pumps and the reservoir.System A has cockpit indication from the reservoir quantity, but since the reser-voirs are interconnected, the reservoir in system B is only equipped with a LOWQUANTITY warning light that illuminates when the reservoir fluid is low.

6.4.4 Hydraulic standby system

Figure: AS 6.21

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If system A and system B fail, the standby system can be used as backup for therudder, leading edge flap and thrust reverses. The Standby system reservoir isconnected to the system B reservoir through a balance line for the purpose ofpressurisation and servicing.

The electrical motor driven pump is activated by placing either FLT CONTROLswitch to its STDBY RUD position, or by placing the ALTERNATE FLAPS switchto ARM. Positioning either FLT CONTROL switch to STDBY RUD, a flight controlshut off valve will shut off the corresponding hydraulic system pressure to ailer-ons, elevators and rudders. If there are a major leakage in system A, the shut offvalve prevents the Standby system from being emptied by this leakage.

Since the Standby system is normally inoperative, the low pressure light is onlyarmed when standby pump operation has been selected and will illuminate if thepump output is below limiits.

6.4.5 Ram Air TurbineMost modern aircraft are equipped with an APU that provide additional electricalpower in flight in case of main system(s) failure. In the event that all hydraulicpower is lost, some airplanes can extend a ram air turbine(RAT) into the air-stream outside the aircraft. The air flowing through the turbine blades drives ahydraulic pump to produce enough hydraulic power to actuate the systems neededto get the aircraft safely on the ground.

Figure: AS 6.22



In normal operation the RAT unit is stowed in the body fairing. In flight the RATautomatically deploys into the airstream if main hydraulic sources should fail.Manual control of extending the RAT may be provided by a RAT switch. A hydrau-lic pump is interconnected to the RAT and provides adequate power for flightcontrol operation, and in some cases lowering of flaps and landing gear, throughoil supply from one of the main hydraulic reservoirs. To produce sufficient powerthe speed of the aircraft has to exceed a limiting speed of e.g. 130 knots. Hydraulicsupply produced by the RAT is isolated from the aircraft’s main hydraulic sys-tems. Therefor RAT system pressure has no indication in cockpit. Once extended,the RAT can only be retracted on the ground.

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