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TEXTBOOK

Hydraulics

020 00 00 00 AIRCRAFT GENERAL KNOWLEDGE

021 03 00 00 HYDRAULICS


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Table of Contents:

Hydromechanics: basic principles ________________________________________ 3

Hydraulic systems ____________________________________________________ 20


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Hydromechanics: basic principles

Pascals law states that pressure applied to any part of a confined liquid is

transmitted with undiminished intensity to every other part.

Thus, if a number of passages exist in a system, pressure can be distributed

through all of them by means of the liquid.

That means in the ratio, force F1 to area A1 is equal to force F2 to area A2

and therefore it is constant like the pressure P, this will give us the formula

pressure P stays in relation to force F divided by area A.

We would name a fluid with this properties an ideal fluid.

Manufacturers of hydraulic devices usually specify the type of liquid best suited

for use with their equipment, in view of the working conditions, the service

required, temperatures expected inside and outside the systems, pressure the

liquid must with stand up to 3000 [ PSIG ], the possibilities of corrosion, and other

conditions that must be considered.

Hydraulic fluids must possess a number of properties, including the

incompressibility and fluidity, which is typical for the ideal fluid, as well as the

chemical stability, the viscosity, the flash and fire point.

F1

A1

F2

A2constant P

PF

Aideal fluid


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Hydromechanical aircraft systems assist the pilot to reduce his energy effort

during aircraft operation.

Hydraulic system liquids are used primarily to transmit and distribute forces to

various units to be actuated.

Liquids are able to do this because they are almost incompressible, meaning

almost no volume change under pressure.

The main components of a hydraulic system are:

- the hydraulic fluid ,

- the hydraulic power unit system, and

- the hydraulically driven system.


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There are three types of hydraulic fluids currently being used in civil aviation.

The vegetable base hydraulic fluid, is composed essentially of caster oil and

alcohol.

This fluid is used primarily in older type aircraft. Natural rubber seals can be

used. This type of liquid is flammable and not interchangeable with other

hydraulic fluids.

The mineral base hydraulic fluid, is processed from petroleum. It smells like oil

and is dyed red. Synthetic rubber seals are used.

This type of liquid is flammable and not interchangeable with other hydraulic

fluids. In practical use it is commonly known as the Aero Shell Fluid 4, and is

typically used for the landing gear shock struts.

The phosphate ester base hydraulic fluid a fire-resistant hydraulic fluid for use in

high performance piston engines and turboprop aircraft is the most used fluid in

Hydromechanical aircraft systems.

Special low weight version of this fluid is used on large and jumbo jet transport

aircraft where weight is a prime factor.

Manufactures names for this type of fluid includes Skydrol and Hyjet .

This type of fluid is not only fire resistant but it also has good low temperature

operating characteristics and low corrosive side effects.


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Regardless of its function and design, every hydraulic system has a minimum

number of basic components in addition to a means through which the fluid is

transmitted. Hydraulic lines or tubes connect components of a hydraulic system

together, to provide each hydraulic component or system with fluid pressure and

flow. The main pressure hydraulic lines are normally made from steel, the return

lines from aluminum alloy. Flexible tubes are used as pressure and return lines.

The core tube is made from steel mesh, covered with a rubber shielding or at hot

sections with a Teflon shielding. All hydraulic tubes are marked by a blue, yellow

and white tape with circulars.

TEMPSENSORQTY

SENSOR

EICAS

EXTERNALPOWER

RETURNSHUTOFF

VALVE

ENGINEDRIVEN

PUMP

PRESSURERELIEF

VALVEFILTER

RESERVOIR

ELECTRICPUMP

EICAS

PRESSURETRANSDUCER

MECHANICALREVERSION

MECHANICALREVERSION

RUDDER

LEFT AILERON

LEFT THRUSTREVERSER

INBOARDSPOILER

OUTBOARDBRAKES

PRIORITYVALVE

ACCUMULATOR

LANDING GEAR

MAIN DOOR

STEERING

EMERGENCYBRAKES

ACCUMULATOR

INBOARDBRAKES

FREEFALL

OUTBOARD SPOILER/

SPEED BRAKES

RIGHT THRUSTREVERSER

RIGHTAILERON

MECHANICALREVERSION

NO.2 HYDRAULICSYSTEM

(IDENTICAL TONO.1 SYSTEM)

HYDRAULICPANEL

FIREEXTINGUISHING

HANDLE

EICAS

PRESSURETRANSMITTER


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The reservoir stores the supply of hydraulic fluid for operation of the system. It

replenishes the system fluid, provides room for thermal expansion, and in some

systems provides a means for bleeding air from the system.

Reservoirs are either vented to the atmosphere, where the atmospheric pressure

and gravity are the forces which cause fluid to flow from the reservoir to the pump

intake, or if the atmospheric pressure becomes too low to supply the pump with

hydraulic fluid, the reservoir must be pressurized.

One method of pressurization is pressurizing with hydraulic fluid. In this case

pressure from the hydraulic pressure system is used to force a small piston,

which in turn moves a large piston and thus produces a pressure of

approximately 40 [PSIG] in the reservoir which provides supply of hydraulic fluid

to the pump. This type of reservoir must be completely filled with hydraulic fluid

and have all the air bled from it.


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The hydraulic pump produces the hydraulic pressure and flow, which is used to

provide a hydromechanics driven system.

The hydraulic pump can be driven by a power source like:

- A/C electric power or

- D/C electric power,

- by a hydraulic motor,

- piston engine or

- turbine engine.

We call them the power driven pumps.

There are different types of pumps which are used to provide aircraft hydraulic

systems.

The types are: The Gear type pump, where the driving gear is driven by a power

unit, and the driven gear is meshed into the driving gear. When the gear turns the

hydraulic fluid is trapped between the gear teeth and the housing, and is then

carried around the housing to the outlet port.


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An other type of pump is the vane type pump, where a rotor is positioned off

center within the sleeve in a housing.

The vanes, which are mounted in the slots in the rotor, together with the rotor,

divide the bore of the sleeve into four sections. When the rotor turns the four

chambers changes their volume during the rotation, and if they pass the inlet or

outlet port through a slot in the sleeve, the fluid is drawn into or displaced out of

the chambers.

A commonly used kind of pump is the piston type mechanism pump. The basic

pumping mechanism of piston pumps consists of a multiple bore cylinder block,

a piston for each bore, and a valving arrangement also for each bore. The

purpose of the valving arrangement is to let fluid into and out of the bores as the

pump operates.


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A typical type of a piston pump is the angular type pump. The angular housing

of the pump causes a corresponding angle to exist between the cylinder block

and the drive shaft plate to which the pistons are attached. It is this angular

configuration of the pump that causes the pistons to stroke as the pump shaft is

turned.

Another type is the cam type pump. With this pump a cam causes stroking of the

pistons. There are two variations of cam type pumps. One in which the cam is

driven and the cylinder block is stationary, which causes the stroke of the pistons

and the other in which the cam is stationary and the cylinder block rotates which

causes the pistons to move. Both kinds of piston pumps produce a practically non

pulsating discharge of hydraulic fluid.

stationary cylinder block stationary camstationary cylinder block


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The hydraulic motor is driven by the hydraulic pressure and flow and transforms

the hydraulic energy into mechanical force.

There are mainly two types of hydraulic motors in use. The Gear type motor with

gears driven by the hydraulic fluid pressure and flow. The rotation of the gears

are transmitted by an output drive shaft to activate a mechanical mechanism. A

commonly used kind of motor is the piston type mechanism.

The basic mechanism of piston motors consists of a multiple bore cylinderblock, a piston for each bore, and a valving arrangement for each bore. The

purpose of the valving arrangement is to let fluid into and out of the bores as the

motor operates.


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The double action hydraulic hand pump is used in some aircraft hydraulic

systems as a backup unit.

For example to provide the alternate landing gear extension system. This kind of

hand-pump produces fluid flow and pressure on each stroke of the handle. Two

spring loaded check valves, one located in the pump housing and the other one

in the piston, are responsible for letting fluid into and out of the cylinder bore

when the pump is operated.

The filter or a filter package.

A filter is a screening or straining device used to clean the fluid, thus preventing

foreign particles and contaminating substances from remaining in the system.

If such objectionable material is

not removed, it may cause the

entire hydraulic system of the

aircraft to fail through the

breakdown or malfunctioning of a

single unit of the system.


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Most filters used are the inline type, consisting of three basic units:

The difference between a simple filter assembly and the filter package is the filter

package includes not only the main pressure filter but also the return manifold

with bypass valve, the system relief and check valve, and the pressure

transmitter.

From the pressure manifold the hydraulic line will run to the pressure relief valve,

which is in most cases included in the filter package. The relief valve is used to

limit the amount of pressure being exerted on a confined fluid, to prevent failure

of components or rupture of hydraulic lines under accesive pressure.

The head assembly which includes a bypass valve which routes

the hydraulic fluid directly to the outlet port in case of a clogged

filter element to prevent blockage of the whole hydraulic system.

The filter bowl which is the housing which holds the filter element

to the head assembly. And the filter element itself, which may be

either a micronic, porous metal, or magnetic type. The micronic

element is made of a special paper and is normally thrown away

when removed.

The porous metal and magnetic filter elements are designed tobe cleaned by various methods and replaced in the system.


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The accumulator, a steel sphere divided into two chambers by a synthetic rubber

diaphragm or piston assembly, will dampen pressure surges in the hydraulic

system.

One chamber is filled by the hydraulic fluid and the other chamber is pre-charged

with air or nitrogen, with a pressure of 1000 [ PSIG ]. This acts as a spring, for the

further functions of the accumulator.

Aid or supply the power pump.

Store power for the limited operation of a hydraulic unit when the pump is not

operating, like an alternate brake system for example.

The check valve allows free flow of fluid in one direction, but not in the opposite

direction, to control hydraulic systems or components to operate as intended.

A simplified hydraulic power driven system is represented here by a selector

valve and an actuator. The selector valve is used to control the direction of

movement of an actuating unit.

And supply fluid under pressure tocompensate for small internal or external

undesirable leaks, which would cause

the system to cycle continuously by

action of the pressure switches on off.


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Different types of selector valves are used, differentiated by their valve bodies

such as the rotor type or spool type.

Or they are different by their inlet and outlet ports for example like a

three way valve or the four way valve.

But the purpose of all selector valves is the same, to provide a pathway for the

simultaneous flow of hydraulic fluid into and out of a connected actuating unit.

For example an actuating cylinder, shown here.

RETURN

PRESSURE

DN UP

MLG ACTUATOR

LDG SELECTOR

VALVE

DOWN LOCK

RELEASE

ACTUATOR

UPLOCK ACTUATOR

TO NLG

TO LH

MLG

PRESSURE

REGULATOR

CUT OFF

VALVE

CHECK

VALVE


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The relief valve is used to limit the amount of pressure being exerted on a

confined fluid, to prevent failure of components or rupture of hydraulic lines under

excessive pressure.

The pressure regulator is installed between hydraulic lines, if the system pressurehas to be adjusted to a constant pressure. Normally this pressure is lower than

the normal system pressure.

The regulator works much like a relief valve, but it isnt. A regulating piston moves

toward right or left, to open or close orifice holes. This causes too high a fluid

pressure to be dumped back into the reservoir, and if the pressure in the system

is too low the drain holes are closed, so that the pressure will increase.

The cut off valve is normally used as a safety device. It has only an open orclosed position, so that if the system is switched off, no fluid can be transported

out of the system.

OPEN


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An actuating cylinder transforms energy in the form of fluid pressure into

mechanical force. It is used to impart powered linear motion to some movable

object or mechanism. There are two basic types of actuators. The single action

actuating cylinder, where a piston is driven by the hydraulic fluid pressure against

a spring load, which will bring back the actuating piston after pressure loss.

And the double action actuating cylinder, where the fluid pressure activates the

piston in both directions, controlled by a four way valve.

For example the double action actuator can also have two exposed piston rod

ends. Or two pistons in one housing, provided by three fluid ports.

Hydraulic systems have many advantages as a power source for operating

various aircraft systems or units. Hydraulic systems combine the advantages of

light weight, ease of installation, simplification of inspection, and minimum

maintenance requirements. Hydraulic operations are also almost 100% efficient,

with only a negligible loss due to fluid friction.


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The typical basic simplified hydraulic power unit system consists of:

- the hydraulic reservoir,

- the cut off valve,

- the power pump or engine driven pump,

- a check valve,

- pressure filter with by-pass valve,

- pressure transmitter,

- system pressure relief valve,

- the accumulator,

- the hydraulic ports to and from the hydraulic power driven systems,

- the return filter and

- the hydraulic fluid coming back to the reservoir.

Normally there is also a back-up system connected to the power system, in case

of a power system failure.

This consists of:

- a standby power unit, normally an electric driven pump,

- a pressure transmitter, and

- a check valve.

During pump operation, some of the hydraulic fluid is leaked from the pumping

section to provide pressure lubrication to the pump moving parts, and to assist

cooling.

The leakage flow is directed through a case drain filter and back to the system

reservoir.

Identify the components on the schema at the next page


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Main users of hydraulic systems are:

- Landing gear,

- wing flaps,

- speed and wheel brakes, or thrust reversers,

- flight control surfaces,

- nose wheel steering,

- and in some cases, especially for bigger aircraft movement of air-stair- or

baggage door.

TEMPSENSORQTY

SENSOR

EICAS

EXTERNALPOWER

RETURNSHUTOFF

VALVE

ENGINEDRIVEN

PUMP

PRESSURERELIEF

VALVEFILTER

RESERVOIR

ELECTRICPUMP

EICAS

PRESSURETRANSDUCER

MECHANICALREVERSION

MECHANICALREVERSION

RUDDER

LEFT AILERON

LEFT THRUSTREVERSER

INBOARD

SPOILER

OUTBOARDBRAKES

PRIORITYVALVE

ACCUMULATOR

LANDING GEAR

MAIN DOOR

STEERING

EMERGENCYBRAKES

ACCUMULATOR

INBOARDBRAKES

FREEFALL

OUTBOARD SPOILER/

SPEED BRAKES

RIGHT THRUSTREVERSER

RIGHTAILERON

MECHANICALREVERSION

NO.2 HYDRAULICSYSTEM

(IDENTICAL TONO.1 SYSTEM)

HYDRAULICPANEL

FIRE

EXTINGUISHINGHANDLE

EICAS

PRESSURETRANSMITTER


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Hydraulic systems

Hydraulic systems are not only used for big or modern aircraft's. They are also

used for small aircraft's, especially to provide the brake system.

Large and new aircraft types are using mainly hydraulic driven systems, to

operate systems of a high power demand. The main reason, hydromechanical

controlled systems are almost 100 % efficient, ease of installation and a

construction of light weight.

Big aircraft's are not using only one hydraulic system. There are always a

minimum of two main (#1/#2) and two alternate systems (PTU/RAT) installed.

The pressure and flow which is necessary to operate the systems are produced

by a power pump system.


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The normal system pressure which is used are the result of an economical and

technical calculation. For the practical use, today are 3000 [PSI] the normally

used system pressure.

Hydraulic pump's for transport aircraft's can be split up roughly into two differenttypes of pumping mechanism: the rotor type and the piston type pumps. A typical

rotor type pump is the gear type pump, the gerotor type pump and the vane type

pump.

Representative piston type pump's are: the radial type pump, the axial type pump

which is represented by a axial cam type, an angular type pump and the hand

pump.

NO 1RESERVOIR

NO 2RESERVOIR

WHEEL BRAKESLANDING GEARINBOARD ROLL SPOILERRUDDER

ANTI SKID

WHEEL BRAKESFLAPSOUTBOARD ROLL SPOILERRUDDERNOSEWHEEL STEERING

POWERPUMP

1

POWERPUMP

2

POWER TRANFER UNIT

RAM AIR TURBINE

STANDBY PUMP 1

STANDBY PUMP 2

0

1

23

4

5

PSI


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An important safety device for the hydraulic system are the alternate power units,

used to provide the hydraulic system in case of a failure of the normal used

power unit system for example engine driven pump.

You can see here the Ram air turbine (RAT).

The RAT consists of the propeller or turbine which will be driven by the air-flow

and these will drive the pump section or electric generator section which

produces the hydraulic pressure and flow or electric volt and ampere to provide

an alternate driven hydraulic or electric system.

The PTU = power transfer unit is an assembly of a hydraulic power driven motor,

which drives a hydraulic pump. This system is incorporated in one hydraulic

power system to provide hydraulic power for the other system in the event of a

power pump failure.


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Electrical driven standby pumps are also used to provide a hydraulic system in

the event of a power unit failure, these electrical driven pump can also be used

on ground without running engines to provide the hydraulic system to operate for

example the landing gear door, air-stair or baggage door.

Hand pumps are mainly used for alternate hydraulic systems, where the hydraulic

fluid flow does not to be constant such as in alternate landing gear extension.

Hydraulic driving units are used to transform energy in the form of fluid pressure

and flow into mechanical force. The main systems which are driven by such units

are:

- the landing gear system including the nose wheel steering and brake system

and

- the flight control system consisting of the wing flap system, the roll spoiler,elevator and rudder controls.


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Other mechanisms which may be driven by hydraulic driving units are:

Windshield wipers, air-stair-doors and baggage doors for example. The driving

unit itself can be an actuating cylinder, for example to extend or retract the

landing gear, but it can also be a hydraulic motor, for example a cam type piston

motor which turns the wing flap drive, to extend or retract the flaps.

Typical types of actuating cylinders include:

The single action actuating cylinders split up into:

- the Plunger with or without spring;

- single action actuating cylinders with or without spring,

- telescope type single acting piston.

The double action actuating cylinders are split up into the:

- differential cylinder,

- double action cylinder,

- three port actuating cylinder,

- rotation action cylinder.

Plungerwithout spring

Plungerwith spring

single actionactuating cylinder

without spring

single actionactuating cylinder

with spring

telescope type

differential

cylinder

double action

cylinder

three port

actuating cylinder

rotation

action cylinder


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See an example for a normal switching unit of a double action cylinder in the

CBT. If the valve is switched to the No # 1 position, hydraulic fluid will go into

chamber No #1 and the piston will be moved out.

At position No # 2 the piston doesn't move = neutral valve or off position,

hydraulic fluid will be returned and at position No # 3 hydraulic fluid enters

chamber No # 2 and the piston will be moved in.

Hydraulic power unit system indication is provided in the flight compartment for

the hydraulic fluid quantity the main pump system pressure, standby pump

pressure, low pressure warning light, and the warning light for the hydraulic fluid

over-temperature.

For every hydraulic power system a separate fluid quantity indicating system is

provided.

Each system consists of a mechanical or visual sight gauge at the reservoir and

an electrical indicator in the cockpit provided by a synchro -transmitter.

You will get a wrong quantity indication for pressurized reservoirs if air is in the

hydraulic fluid, due to the fact - air reacts like a spring and will be compressed

under pressure but expands at no or low pressure, so you will get without a

pressurized system a too high quantity indication.

PSI x 1000

HYD PRESS

0

1

2

3

4

0

1

2

3

4

1 2

PSI x 1000

HYD PRESS

0

1

2

3

4

0

1

2

3

4

1 2

STANDBY MAIN

NORM

STD BY HYD PRESS

1 2

H

Y

D

PW

R

0

1

2

3

0

1

2

3

4

5

1 2

U.S. QTS

HYD QTY

PSI x 1000

HYD PRESS

0

1

2

3

4

P

A

R

K

B

RA

K

E


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A separate pressure indicating system is installed in each main system to provide

pressure indication. An additional indicating system is installed with each standby

power unit. A low pressure warning switch on each main pressure system closes

on a decreasing pressure is 2000 [PSI] or below to operate the appropriate low

pressure warning light in the flight compartment.

The indication of the fluid over-temperature is provided by a warning light located

in the flight compartment. The warning light is controlled by a over-temperature

switch which is installed at each hydraulic power system (Pump return line). The

over-temperature switch operate at a fluid temperature of 109 degree Celsius,

and will remain actuated until the fluid temperature falls to 95 degree Celsius.

The windings of the electrical motor of an A/C powered hydraulic pump is also

monitored by an over-temperature switch to protect overheating and damage of

the electrical motor by illuminating of a warning light.

Hydraulic pumps are permanently driven by an accessory gear of a power plant,

we call them the engine driven hydraulic pumps. Engine driven pumps are

permanent running as long as the power plant is running.

NO 1RESERVOIR

NO 2RESERVOIR

WHEEL BRAKESLANDING GEARINBOARD ROLL SPOILERRUDDER

ANTI SKID

WHEEL BRAKESFLAPSOUTBOARD ROLL SPOILERRUDDERNOSEWHEEL STEERING

POWER TRANFER UNIT

RAM AIR TURBINE

STANDBY PUMP 1

STANDBY PUMP 2

In case of an engine fail we need a

backup system to provide hydraulic

power for the hydraulic power

driven system. This is

accomplished either by an

electrical driven or -hydraulic motor

driven pump.

These can be switched on

manually such as the electrical

driven pump here or automatically

as you can see by the power

transfer unit.


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The hydraulic systems used in modern transport aircraft are one of the most

important components used to operate transport aircraft in a save and

economical manner.

The lifetime of any of moving part or component including the hydraulic system is

restricted by the wear caused by his movement. Here are some of the reasons

why hydraulic systems can fail:

Air in the hydraulic system can cause serious problems in the hydraulic driven or

driving systems. For example air might enter into the hydraulic park brake system

via a worn piston seal of a park brake accumulator from the nitrogen chambre

into the hydraulic chambre, and results in a reduced, or worse, insufficient

parking brake action.

For the driving unit, for example a piston power pump air in the system can cause

power drop of the pump and if air enters the pump it might cause heavy wear of

the pistons due to lack of lubrication and afterwards the failure of the pump.


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Hydraulic leaks especially at engine mounted components are resulting from a

simple hydraulic leak up to a worse engine oil leak if the required maintenance

action hasn't been carried out on time.

The reason, hydraulic phosphate ester and synthetic base fluids are

decomposing the engine oil seals. An other kind of defect is, but it's not really one

during the operation of the normal brake system followed by setting of the park

brake, a little amount of hydraulic fluid will swap from one system into the other

caused by the shuttle valve switching. Fluid pressure moves the shuttle valve to

close the port where lower pressure comes from so that before the low pressure

port is closed hydraulic fluid will be transferred. This fluid transfer is easy to

correct for the flight crew by opposite operation of the system.

Maintenance of hydraulic filters and replenishing the fluid is relatively easy but

also very efficient. Because the regular replacement of filters and fluid on time

are reducing the probability of contamination of the hydraulic fluid and the

probability of a system or component fail.

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