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Licence By Post

The staplesin this book can catch

fiogen. Not lUitable for small
children. Care when handlin .
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 .

AUTHORITY

It is IMPORTANT to note that the information in this book is for

study / training purposes only.

When carrying out a procedure/work on aircraft/aircraft equipment you

MUST always refer to the relevant aircraft maintenance manual or equipment
manufacturer's handbook.

You should also follow the requirements of your national regulatory authority
--~-ltne-CM in the UK) and laid down companypoiicy as regards local
procedures, recording, report writing, documentation etc.

··~---------Fm:-health.and sa fety
in the.workplace you should foU0'-V tl!.e ~
regulations/ guidelines as specified by the equipment manufacturer, your
company, national safety authorities and national governments.

------
 ACKNO~EDGEMENTS

With thanks to:

AIRBUS INDUSTRIE
UK CIVIL AVIATION AUTHORITY

for their permission to reproduce drawings.

NOI E DrawIngs frofif-CiVil Ait PubhcatlOn (CAP) 562 may not be found in CUrrent CM
publications due to amendment action.

--- - ----------

----------- --- ---------- ----~-----~ - - ----------------~

 CONTENTS

PAGE PAGE

Principles 1 Components - cont

Fluids 6 Brake control valve 52
Basic hydraulic systems 8 Non return valve 53
Power supply circuits 11 Hydraulic fuse 54
A320 system 15 Restrictor valve 55
Concorde system 16 Selector valves 56
B757 system 18 Fluid jettison valve 60
Consumer circuits 20 Shuttle valve 60
Powered flying control 20 Sequence valves 62
Aligh ting gear 20 Modulator valve 62
Wheel brakes - small aircraft 23 Flow control valve 63
Flap - simple or plain type 24 Jacks/ actuators 64
Flap - Fowler type 27 Thermal relief valve 66
Flap & slat - A320 30 Throttling valve 66
Wheel brakes - large aircraft 31 Pressure relay valve 67
Components 33 Quick disconnect couplings 67
Reservoirs 33 Pressure relief valves 67
Heat exchanger 37 Drain cocks 68
Filters 38 Sampling valves 68
Hand pump 42 Fire shut-off valves 68
Accumulators 42 Hydraulic motors 69
Driven pumps 44 Emergency / standby systems 71
Pressure relief valve 48 RAT system 71
Automatic cut-out valve 49 RAT warming 72
Priority valve 51 RAT test ,.Z3_
Hydraulic seals 76
Power pack 80

lItE.'".·"
~.
~
...·•·.· ••..'..... •....; _ ..... , ,
--_._---~.

~._----_._---" . -- ,~-- -- --._- ~'------'

 HOW TO TACKLE THIS BOOK

Designed specifically for the category B 1 technician. For the category B

technician the level of knowledge required is level 3 (the level the book is
written at), for the category A line mechanic the level is level 1, so it should be
stressed that it is too deep for the knowledge required by JAR66 and should be
read with this in mind.

Because of the amount of material to be covered the book has been made up
into two parts. The first contains the 'lions share' of the work - principles,
I"" systems, and components. The second contains instrumentation and
maintenance procedures.

You should read both parts with the understanding that the CAA will expect
from you enough knowledge and ability to fault rectify a complex hydraulic
system. This means a thorough knowledge of the various systems, components
and associated systems - electrical- instrumentation - fuel etc.

It is an interesting subject and for some areas of the books a single read
through will suffice. For others a closer study with several reads may be
needed.

If you do not understand anything after the second or third read through then
contact your tutor - he will only be too pleased to help.

Other books published by LBP that should be read in conjunction with this
book include:

* Wheels, Tyres and Brakes - for brake and anti skid systems.
* Powered Flying Controls for - for PFCU, autopilot servo and yaw
damper operation.
* LandirrgGear - for power steering operation. ----~
* Airframe Instruments - for principles of operation of instruments
systems.
- -- - -------~
 HYDRAULIC SYSTEMS AND COMPONENTS

PRINCIPLES

The term "hydraulics" is used to describe methods of transmitting power

through pipes and control devices, using liquid as the operating medium. For
certain applications hydraulic systems are used in preference to mechanical or
electrical systems for a number of reasons, amongst which are ease of
application of force, ability to increase the applied force as necessary, ease of
routing of pipelines, and elimination of backlash between components. The
most important thing, however, is that hydraulic systems have a good
power jweight ratio.

Liquids are considered to be incompressible - at least up to pressures of 3,000

to 4,000 psi (21MPa to 27MPa) - not strictly true but we make this
,... assumption. The higher the pressure the more the fluid compresses and at
very high pressures (say 20,000 psi) then a fluid behaves very much like a gas
during compression. Many hydraulic systems have a working pressure up to
about 3,000 psi so the transmission of fluid power down a pipeline can be
achieved with very little power loss.

However, liquids will expand or contract as a result of temperature changes,

and a thermal relief valve is necessary to prevent damage from excessive
pressure in any closed circuit which may be subjected to large changes of
temperature.

Before we start looking at hydraulic systems we must have a knowledge of

some of the rules and laws which govern the behaviour of fluids under
pressure.

,- Pressure. This is defined as force per unit area or

P = F
A

Where P = Pressure
--- F = Force
A = Area

---------

Imperial Pressure Pounds per square inch (psi)

Force = Pounds force
Area = Square inches

Metric (SI) - Pressure The Pascal (1 Newton per square meter 1 Pal
Force = Newton
Area
Note. The Pa is the Pascal (named after Blaise Pascal 1623-62) - which is a
very small unit - nearly 7000 Pa to 1 psi.

Pascal's Law states that fluid held under pressure in a container exerts
pressure equally, instantly, and at right angles to all surfaces without loss.

Bramah's Press. Sometimes called a hydrostatic press, it is used to magnify a

force. It is a hydraulic press consisting of two cylinders - one larger in diameter
than the other. Sealed pistons are allowed to move within each cylinder.

In general a small force on the small piston will create a large force on the large
piston. But in pushing the larger piston up the smaller one will have to move
through a larger distance.

During operation the parameters that are common for both cylinders are:

(a) The volume of displaced fluid and,

(b) The pressure.

In other words the pressure is the same in both cylinders and the total volume
displaced from one cylinder is the same as that received by the other.

PISTONS

FLUID

Fig 1 BRAMAH'S PRESS

Example

Take the approximate sizes of an aircraft hydraulic lifting jack. The piston that
pumps the jack up being about 0.5 inches in diameter and the jack body
piston being about 3 inches in diameter.

__ -:.2_-=
 LARGE PISTON

Fig 2 HYDRAULIC LIFTING JACK

-,....
QUESTION: Given the diameters of the pistons can you work out their areas?
(5 mins)

ANSWER: Area of ----- small piston -------------- large piston

1td2
4

= 1t x 0.5 x 0.5 = 1tx3x3

4 4

= 0.2 in 2 = 7 in2

If a person can push down on the small piston with a 90 lb force (using most of
his/her own body weight and not using a handle (so no mechanical advantage
is obtained) then what weight will the jack be able to lift?

General. 1. Calculate pressure in the small cylinder.

2. Use this pressure in the largecytrrrderto calculate it's force.

Pressure equals force per unit area.

Pressure = force P =F
area A

so Force = Pressure x area F=PxA

~ -3-
 3" DIA

0.5" DIA

o
SHALL PISTON LARGE PISTON

Fig 3 PISTONS - RELATIVE SIZES

1. Pressure in small cylinder:

P = F = 90 = 450 psi
A 0.2

2. This is also the pressure in the large cylinder.

So the force on the large piston equals:

3. F = P x A = 450 x 7 = 31501bf.

The jack will lift 3150 lbs - nearly P/2 (imperial) tons (about Ph metric
tonnes) - the size of a large family saloon. With a lever on
the small piston (as with all lifting jacks) for the operator to push on, the
jack will lift ten times this load (The lever giving the operator a
mechanical advantage).

The same calculation using SI units.

Diameter of small piston = 13mm approx. (0.013m)

Diameter of large piston = 76mm approx. (0.076m)

Areas:
SMALL PISTON LARGE PISTON

1td2 1td2
4 4

1t x 0.013 x 0.013 = 1t x 0.076 x 0.076

4 4

0.0001 ms 0.004 m«

---- ._------

--- _._- ------ -_. - ------ ------ ------------~~

The pressure in the system with a 90 lb force on small piston is:
(90 lb force is approximately equal to 401 Newtons).

P = F = 401 = 4,010,000 Njm

A 0.0001

or 4,010,000 Pa

or 4 MPa pressure approx.

This pressure will act on the larger piston and the force produced = pressure x
area.

F = PxA

= 4,010,000 x 0.004

= 16040 Newtons

= 16.04 kN
All the above calculations assume that there are no losses in the system and
no friction to overcome in the seals.

QUESTION: Is the above statement true?

ANSWER: Not really - there are losses in all systems. But for general
calculations it 1S assumed that it IS true and the answer works out
quite well.

QUESTION: Can you work out how far the large piston will move up when the
small piston is pushed down?

If the answer is YES therr havea go assuming the small piston is

moved down 4". (Allow yourself about 10 minutes)

._ ._--~-ll-your answer.is NO then refer to the answer below,

ANSWER: Remember the basic rule here is - the volume displacement from
one cylinder is the same volume displaced to or from the other.

Volume moved out of small cylinder = nd 2 x h

4
where = height of piston movement

~~_._--~-_.
 So volume = 7t x 0.5 x 0.5 x 4
4

= 0.8in 3

This amount of fluid is displaced to the big cylinder.

V=Ah

Where V = volume, h = movement height of large piston &

A = piston area of large piston.

So the height of the big piston movement is:

h= V
A

= 0.8
7

= 0.1 in

It doesn't move far does it? - bearing in mind the movement of the
small piston is 4 in. When you jack an aircraft just note the
differences in the movements of the jack for each stroke of the
piston.

HYDRAULIC FLUIDS

Almost any type of fluid can be used in a hydraulic system, but the special
requirements of aircraft systems have resulted in the use of vegetable, mineral
and synthetic based oils. They must meet the requirements laid down by the
regulatory authorities (JAR25 for large aircraft for example), and an ideal
hydraulic fluid would have the following properties:

(a) Be a good lubricant.

(b) Have a low viscosity to minimise friction in pipelines and to
provide high-speed operation of motors and pumps.
(c) Be anti-corrosive.
(d) Have a wide operating temperature range.
(e) Be non-inflammable - not true of all fluids.
(f) Be user friendly (non-toxic etc) - not true of some fluids.
(g) Be inexpensive - some are costly.
etc.

- 6 --
 QUESTION: What does "viscosity" mean? (5 mins)

ANSWER: It is the resistance to flow of a fluid. The higher the resistance, the
higher the viscosity and the more energy the fluid requires to be
pumped around the system. Viscosity may change with
temperature (called "viscosity index") - usually the higher the
temperature the lower the viscosity.

Fluids are coloured which helps recognition, but in general fluids should only
be used if they are from an approved supplier; in sealed containers; to the
correct specification - as laid down in the AMM (Aircraft Maintenance Manual).
Fluids to different specifications must never be mixed. Fluids to the same
specification, but produced by different manufacturers, may be mixed when
permitted in the AMM.

,... Use of a fluid, which is not approved for a particular system, may result in
rapid deterioration of seals, hoses and other non-metallic parts. It may also
cause high wear rates and allow sludge to form.

Types of Fluids

LOCKHEED 22. (to MIL SPEC H-7644). Vegetable based and almost colourless
- but a slight brown/yellow hue. Pungent smell. Used with natural rubber
seals and hoses. Used in some braking systems but not often found in
hydraulic power systems.

DTD 585. (to MIL SPEC H-5606). Mineral based and coloured red. Uses
synthetic rubber seals and hoses. Used in hydraulic systems and landing gear
shock absorber struts. Excellent lubricacion and ant-corrosive qualities, but
flammable.

SKYDROL. (to MIL SPEC H-8446). Phosphate ester based (synthetic) and may
be green, purple or amber in colour. Used with butyl rubber, ethylene
propylene, or Teflon seals and hoses.

Widely used on modern aifcraftbecause of its fire resistance - though high-

pressure spray is combustible. Temperature operating range is between - 65°F
and + 225°F (- 54°C to +107°C). This fluid requires care in handling as it is an
irritant toskin-and-eye·~s.--~

Avoid contact with the body and avoid inhaling the fumes. Always use a barrier
cream and protective clothing such as fluid resistant gloves, goggles etc. It will
absorb atmospheric moisture and attack most plastics and paints (though not
epoxy and polyurethane based paints). May become acidic if overheated.

:.•".'. .•-.~_
~. ~~.=-
. . . . . . . •. •.•. . . . . . . .-..-'."',•..7 -
--_._._---
 HYDRAULIC SYSTEMS

Hydraulic systems may be used to operate such services as landing gear, wheel
brakes, powered flying controls, windscreen wipers, etc. Each of these services
has its own hydraulic circuit within the hydraulic system. These circuits are
usually connected to supply and return lines running to and from the supply
circuit. Thus the complete system is made up of a supply circuit connected to
various service circuits.

On some aircraft more than one hydraulic system is provided and these may
be interconnected. On most large aircraft three (or even four) independent
systems are used, each with it's own supply pumps, reservoirs, pressure and
return lines etc.

Each system supplies it's own services with the more important services
receiving supplies from more than one system. On most large aircraft the
Powered Flying Control Units (PFCUs) for example, receive three independent '-"
supplies. Emergency circuits may be provided for use in the event of hydraulic
system failure.

DIRECTION Of FLUID FLOW

--t>
RESERVOIR
PUMP

SELECTOR VALVE

JACK -:»

Fig 4 BASIC HYDRAULIC SYSTEM

Figure 4 shows a basic hydraulic system, which can be made to do useful

work. With the pump running, fluid is drawn from the reservoir and supplied
to the selector valve under pressure. When the selector valve is rotated one
side of the jack is connected to the supply whilst the other side is connected to
return. The jack will move. Returning fluid will go to the reservoir via the
selector valve.

--- ~ --~-~-~.~

._~.~---~--_._.~~- .~ .. --------- -~ -~~--------~~-~-----~~._--

 The selector valve may be manually or electrically operated. The drawing shows
a simple rotating type of valve, but some are operated by a slide type piston.

With the valve as shown the fluid causes the jack/ actuator to extend - with the
valve rotated clockwise by 90° the supply will be connected to the top of the
jack and the bottom will be connected to return. The jack will retract. This is
called a two-way selector valve - used for systems where only FULLY IN or
FULLY OUT selections are required.

Where intermediate selections are required - flaps for example - then a four-
way selector valve will be used. In the drawing it is similar to the one shown
but it can move 45° from the position shown so NOT aligning it's ports with
any of the connections - supply, return or the jack connections. In this
position it will form a hydraulic lock and the jack will not move.

QUESTION: What is a hydraulic lock? (5 mins)

ANSWER: It is trapped fluid within a pipeline system - normally between a

jack (or hydraulic motor or some other actuating device) and some
other component such as a selector valve. In the example above
the fluid is trapped between the jack (on both lines) and the
selector valve. Under these conditions the jack cannot move.

QUESTION: Can you see any problems with a hydraulic lock? (10 mins)

ANSWER: There may be several, but two you should be aware of.
1. The lock is not reliable. With all systems off, the fluid
pressure, over a period of time, will dissipate. (For example,
when jacking an aircraft using hydraulic jacks they are
reliably locked in the up position using a mechanical device
- usually a screw thread locking collar).
2. If the ambient temperature rises and fluid temperature
increases then the pressure will increase in the trapped
fluid. The pressures can get so great that structural failure
----of the pipes/ components will reault/Thermal relief valves are
fitted to prevent this - more on these valves later.

The pipeline leading from the reservoir to the pump is called the suction line,
with the line running from the pump to the selector valve the pressure line, and
the line returning to the reservoir the return line.

Most aircraft have systems which are considerably more complex than the one
shown above, so they are normally split up into sub systems, eg power circuits,
brake circuits, landing gear circuits etc.

- - - - - - - - _ .. _-- -9 -
There are two main types of system in use, the open-centre system and the
closed system. The former is most usually found on some light aircraft and is
not well known. The latter is common and found on many aircraft - large and
small.

Open-centre System

The main advantage of this system is its simplicity, and the main disadvantage
is that only one service can be operated at a time. When no services are being
operated the pressure in the system is at a low value, pump output passing
directly to the reservoir round the "open circuit" - with all valves in the OPEN
CENTRE position.

When a selection is made the appropriate jack moves. When the jack gets to
the end of its travel it makes contact with the selector valve lever and moves it --
to the "open centre" position (the positions shown in the drawing). This allows
the fluid from the pump to be pumped around the system under very little
pressure - thus saving energy consumption by the pump.

Should there be a delay between the jack getting to the end of its travel and the
de-selection of the valve, or should something fail, there is a pressure relief
valve to relieve excessive pressures.
RESERVOIR
PUMP

RELIEF VALVE
.JACK

<1:- _
Fig 5 OPEN CENTRE SYSTEM

Closed System

With this type of system, operating pressure is maintained in that part of the
system which leads to the selector valves, and some method is used to prevent
over-loading the pump. With this sort of system all consumer circuits have the
same pressure supply - but flow rates might be different.

_____ ~ LQ -
 In systems which employ a ftxed volume pump (constant delivery) an
automatic cut-out valve (ACOV) is fitted, to divert pump output to the reservoir
via the idling line when pressure has built up to normal operating pressure.

In other systems a variable volume pump (constant pressure pump) is used,

delivery being reduced as pressure increases, and an idling line (case drain)
allowing some fluid flow back to the reservoir to keep the pump cool and
lubricated.

In some simple light aircraft systems, operation of an electrically driven pump

is controlled by a pressure-operated switch, which may be part of a power-
pack assembly. The pump comes on when required and switches off when
system maximum pressure is reached and no component is selected.

Closed systems are used on most aircraft.

~_-ACCUMULATOR

--t>
RESERVOIR

IDLING CIRCUIT____ JAO

<l-
Fig 6 CLOSED SYSTEM

--rowER CIRCUITS

The power circuit supplies fluid to, and accommodates the fluid returned from,
the othercircuits. It also ma-y contain more than 0Ile p:ump. Pumps are ~~u~ly- - -
engine driven, but may be electrically driven. The power circuit may contain
one or more of the following components:

* Driven pumps - engine - electric - air - RAT - hydraulic (power

transfer units)
* Automatic cut-out valves (if constant volume pumps fitted)

- 11 -

-------_. ----- ---------~

------------ -------
 * Pressure relief valves
* Hand pumps
* Reservoirs
* Reservoir pressurisation system
* Reservoir pressure refill connection
* Oil coolers (in the fuel tanks)
* Accumulators
* Filters
* Priority valves
* Gauging - temperature - pressure - reservoir level - low pressure
warning
* Self sealing test couplings (ground test connections)
* Non-return valves (check valves)

Two Pump Power Circuit

In multi-engined aircraft it is usual to have each power circuit using two or

more pumps. They may both be engine driven (from different engines), or one
may be engine driven and the other driven by hydraulic pressure from another
system (power transfer unit), or electrically, or ram air operated.

On small aircraft there may only be one pump and there may only be one
supply system.

GAS OiARGING
CONNECTION

NRV PRESSURE
PRES
LOW GAUGE
PRES INO INO
TEMP PRES
WARN

FWIO TEMP ::::::::::::

TO NON ESSENTIAL
SERVICES

DRIVEN PUMP

TO ESSENTIAL
SERVICES

COMMON RETURN •••••••.•••••••••••••• : ELECTRICAL CONNECTIONS

- - - - : PIPELINES

Fig 7 TWO PUMP POWER SUPPLY CIRCUIT USING SELF IDLING PUMPS
Figure 7 shows a circuit fitted with two self-idling (constant pressure) pumps,
which, should one fail, will still provide fluid flow but at half the normal rate -
so all systems will work at half the normal speed.

The self-idling nature of the pumps means that they will automatically adjust
the flow rate to suit the demand, and completely shut-off when demand is zero
- although they run continuously. The idling line (case drain) will allow fluid
flow to the reservoir when in idling mode to keep the pump cool and lubricated.

The pipelines to the pumps are called suction lines; from the pumps are called
pressure lines and idling lines (case drain).

The purpose of the accumulators in this circuit is to give speedier operation of

components and provide a source of hydraulic power when the engine-driven
pumps are not working - for a very limited time period.

As an additional safety factor, some aircraft are fitted with a high-pressure

relief valve in the power circuit - in case the pump fails to off-load.

The gauging (from the pump) includes a pressure gauging system (moving coil,
de or ac ratiometer, or synchro systems) a low pressure warning system
(Bourdon tube or bellows operated micro-switch) and an oil temperature
indicating system (thermister - moving coil or de ratiometer systems).

Gauging at the reservoir includes: oil level; temperature, and pressure - if the
reservoir is of the pressurised type.

Indication that the power circuit is functioning correctly is provided by low

pressure warning lamps, pressure gauges, and temperature gauges situated in
the cockpit. Filter by-pass warnings may also be available. If for any reason,
such as a defective pump, defective valves, low fluid level or leaks, the pressure
falls below working pressure, the pressure switch in the power circuit will
operate a warning lamp - the gauge will also show a low reading - of course.

Over-temperature warning may indicate a failing pump; lack of fluid supply or

reduced cooling effect to the returning fluid. In the event of failure of the
engine-driven pump, thersecorrd" pump will maintain the supply (but--at half
flow rate); the hand pump, if situated in the cockpit, could be used in an
emergency.
--~~------

The hand pump has its own filter, pressure relief valve and non-return valve.

Both hand and engine driven pumps are provided with non-return valves to
prevent the flow of fluid from either pump by-passing back to the reservoir
through the pump not in use.
When the aircraft is on the ground, the hand pump can be used for
maintenance purposes - operation of cargo doors, bleeding etc.

Some circuits are of vital importance to the safe operation of the aircraft, whilst
others are of lesser importance. In general powered flying controls (PFCs),
wheel brakes and other essential services must have priority over non-essential
circuits such as powered nose wheel steering and landing gear.

Should the power circuit supply pressure fall below a pre-determined figure, a
priority valve shuts-off the flow of fluid to the non-essential services and
maintains the fluid pressure for the essential services.

The priority valve may be situated at the power supply circuit (as shown) with a
pipeline going to all essential services and another pipeline going to all non-
essential services. Alternatively, the valve may be fitted at the supply point to
each non-essential service to shut it off in the event of reduced supply -/
pressure.

GAS CH.A.RGING
CONNECTION

NRV PRESSURE
LOW PRES GAUGE
PRESIND PRES IND
WARN ACCUMUlJ\TOR
FlUID LEVEL zzzzz: GROUND TEST
CONNECTIONS
FlUID TEMP =::::=••••
TO NON ESSENTIAl
SERVICES

DRIVEN PUMP

V///1.
PRIORITY
VALVE
PRESSURE
RELIEF VALVE
'-----'--'~ t--------T"I TO ESSENTIAL
SERVICES

COMMON RETURN - - - - = PIPELINES •••••••••••••••••••••• = ELECTRICAL CONNECTIONS

Fig 8 TWO PUMP CIRCUIT USING CONSTANT VOLUME PUMPS

 QUESTION: It is usual to consider the landing gear retraction and nose wheel
steering circuits as non-essential - why? (5 mins)

ANSWER: The landing gear has it's own emergency down systems - to
include "freefall", gas operated, separate pumps etc.

As far as steering the aircraft on the ground is concerned there is

always differential braking if the nose wheel steering fails.

During servicing of the aircraft it will be necessary to test the hydraulic system,
but as the engine cannot always be used to run the engine-driven pump (when
landing gear retraction testing for example), ground test connections are
provided which connect the power circuit to a ground engine driven hydraulic
"... servicing trolley (cart).

The ground test connections are of the self-sealing type - either screwthread or
bayonet type.

Of course, the hand pump may be used for testing purposes, but the rate at
which it can deliver fluid is very slow and it will not reproduce actual operating
conditions.

The constant volume pump shown in figure 8 (which pumps fluid all the time it
is running) will need some form of "pressure relief" when services do not need a
fluid supply. If an ordinary pressure relief valve was fitted in the system it
would do the job - but at a cost. The pump would have to keep pumping at it's
normal working pressure to keep the pressure relief valve open - using energy
all the time (using fuel and costing money). So a special relief valve is fitted
called an Automatic Cut Out Valve (ACOV).

When operated it allows the pump to pump fluid at almost zero pressure via
the idling line or case drain line back to the reservoir - keeping the pump
lubricated and cool. More of this later.

Note. The two circuits shown above have two driven pumps each but may have
only one pump on some aircraft, and may have three on others.

A320 System

Figure 9 shows an overview of the A320 aircraft hydraulic supply system.

It has three systems- blue, green and yellow. All have accumulators with
priority valves in front of the non-essential services.

------- --~-
15
 The pumps include a hand pump (for cargo door operation), ram air driven
turbine (RAT) - for emergencies, engine driven pumps and electrically driven
pumps.

A power transfer unit is provided between the green and the yellow system.

Note that some services have three supplies - PFCUs for example, whilst others
have only one - landing gear and steering.

ENG 2
RAT PUMP HAND
70l/mn 140l/mn PUMI
20.'Uaq/mn 37 Ulq/mn .:»

La R.AIlERON
l.a R.SPOIlER 3 SPOILER 2. 4

R.ElEVATOR

TAIL TRIM

RUDDER RUDDER RUDDER

YAWDAMPER YAW~AMPER

REVERSEENG. 1 REVERSE ENG. 2

CARGO DOORS

WHEELS BRAKES
FlAPS FLAPS
STBY rue GEN SLATS SLATS

Fig 9 AIRBUS A320 HYDRAULIC SYSTEM ARCHITECTURE

Concorde System

Figure 10 shows the layout of the Concorde hydraulic system.

As you can see it also has three separate systems - blue, green and yellow.

~ - ~~----~~-

..
~. -----~_.~ ~~~ ------_._--~~-~
Each system is served with two engine driven pumps as well as electrically
operated pumps.

Yellow and green system have the use of a ram air turbine (RAT) which falls
into the airflow automatically if fluid pressure gets too low.

The visor (raised for high-speed flight, lowered for landing) circuit is not shown.

An accumulator is fitted to the yellow system (wheel brake circuit) to allow

aircraft movement (towing etc) without engines running.

TANK 11

AIR
INTAKES
3&L

FllCHT
CONTROl

NOSE- AIR
WHEEL INTAKES
STEER- 1&2
ING

Fig 10 CONCORDE HYDRAULIC SYSTEM

QUESTION: Study the drawing above and note which services use which
system. Which service has the use of all three systems? (2 mins)

ANSWER: The PFCUts are supplied from all three services - as, I'm sure,
you would expect.

- 17 -:,
 The B757 System

Figure 11 shows the power supply circuits of the Boeing 757. Note the
following:

* Three systems (left, centre & right) using seven pumps.

* Pumps:

2 engine driven (EDP) 3000psi @ nearly 40 galls per minute.

4 electrically driven (ACMP). Pressure supply similar to

above but flow rates very low (about 7 galls per minute)

1 RAT - for emergencies - manual or automatic deployment

based on both engines low rpm - also possibly connected
to airspeed and weight switch inputs. Driven by a VP
propeller and supplies about 2000 psi at about V4 of the --./
normal pump flow rate.

1 air driven (additional pump on the 767 fitted to the

centre system).

* Single point reservoir filling.

* Fire shut-off valves (SOV).
* Pressurised reservoirs.
* Fuel cooled heat exchanges.
* Power transfer unit - comes on automatically when left engine rpm
or left EDP pressure is lost. Is a bent axis hydraulic motor driving
a hydraulic pump. About 2,000 psi at about half the normal pump
flow rate.

Note the power supplies to the following consumer circuits:

* Wheel brakes - 2
* Powered steering - 1
* Landing geaJC T~g_action - 1
* Flaps and Slats - 1
* Yaw damper - 2
* Tail plane trim - 2
Auto pilot sel vas 3-
* Aileron, rudder and elevator PFCU's - 3
* Spoilers - 3
* Thrust reversers - 2

-"
~~._-_._-----

18 -
"~rn.IF'···
:llll!-i
i i: j

I I ii
li )'1 )1
I'

L SYSTEM SYSTEM R {} R SYSTEM

RESERVOIR
PReSSURIZATION c:> -+-~ C SYSTE..
RETURN t::l
;::o::::::n=o:, \2 0 0 0 0
~QRESERV01R
0 ~
.R!.Zl
i _ -
PRESSURIIATION
• ••• 1-
rflLILr;:n:::a:::r; • •• • • •
..
.. . .
• •
• • • •
• • ) () n 0 L}' ()
,.....1L
() n
~~
Cl
• • •• ··
n
.---J-.L-

: jl
I
• • • •
• • •
n if
SYSTEM HEAT
EXCHANGER
·• • • 11 •
I· • •

~ [ ___-=rf ~
C

1
L.....n- RETURN
HEAT
,
TO PTU ~ EXCHANGER
•
~
lli
fW
RESERVOIR
FILL SEL
VALVE n

~
HAND PUMP
RAT
"'=" ACTUATOR
SPOILERS
L-

SOV SOV
lLzz
4 AILERONS A
~ -8 ELEVATORS ~
A
-'Bl'

L THRUST LEGEND
REV~~SERS
RIzm • • • • SUPPLY
! YA 0
It,IlZIIlIl? PRESSURE
r$j0V
o 0 d 0 RETURN

ALTERNATE BRAKES ggSkkis, AIR

EDP - ENGINE DRIVEN PUMP
NORMAL BRAKES ACMP - ALTERNATING CURRENT MOTOR PUMP
LEAD ING EDGE c:;g,:neX>OQS RAI - RAM AIR TURBINE
SLATS A ,-_. PTU - POI/ER TRANSfER UNIT
L RESV SOV - SHUT Off VALVE

~AI/';~~~~~--
:::cc:,::::
j
- --
,I ~
"" "",---~
SlEERING
ooccoccoc

Fig 11 B757 HYDRAULIC SYSTEM

CONSUMER CIRCUITS

Power Flying Control Circuit

The power flying control circuit incorporates components such as a selector

valve, filters, accumulators etc. The selector valve is of the "two way" type and
is used to switch hydraulic power on or of! On many aircraft the selector valve
is not fitted so when the hydraulics are on the PFCUs are on automatically.

Control of the hydraulic fluid is achieved by the servo valve in the PFCU -
depending on pilot's flying control inputs. The filter is of the micronic type - see
Filters in this book. For operation of the PFCU see the book on in this series
entitled Powered Flying Control Systems.

SUPPLY

ACCUMULATOR

THROTTLING VALVE
, ~CRONIC FILTER
SELECTOR
VALVE ", 1--------.

SERVO VALVE

RETURN

P F tu
Fig 12 PFCU CIRCUIT

Alighting Gear Circuit

The circuit illustrated shows the layout and components required to control
the raising and lowering of one landing gear unit with-the other units having a
similar arrangement - with the selector valve being common for them all. Fluid
is supplied from the power circuit to the control valve via a non-return valve.

This valve ensures that the alighting gear circuit is isolated from the rest of the
aircraft hydraulic system by providing a hydraulic lock. The control valve,
which may be manually or electrically selected, directs fluid to the desired end
of the jacks/ actuators and at the same time connects the other line to the
reservoir. The lines to/from the actuators are known as the up and down
lines.
 The purpose of the remaining components in the circuit are as follows:

Thermal Relief Valve (TRY). Due to thermal expansion of the fluid (eg, hot
climates, operating temperature rise etc) in a closed circuit, there is a risk of
burst pipelines and damaged components. To prevent this a thermal relief
valve is fitted in both the up and down lines. The valves will relieve expanding
fluid to the circuit return line. They are pressure operated.

SUPPLY

NRV
JETTISON
VALVE SHUTTLE VALVE
SELECTOR VALVE

UP LINE DOWN LINE

TO OTHER
T R V T R V UNDERCARRIAGES

ONE-WAY
I RESTRICTOR DOOR
JACK MANUAL
EMERGENCY
SELECTOR
"Up"
SEQUENCE
'l'O OTHER VALVE
UNDERCARRIAGES r -..... _-.::
lTl---------t----

GAS
BOTTLE
"DOWN"
SEQUENCE
VALVE

LEG RETURN
JACK
'1' RV. THERMAL RELIEF VALVE

Fig-1:3--L-MfDING GEAR RETRACTION CIRCUIT-- SINGLE SEQUENCE

One-Way RestrictQr Valve. Wh~n alighting gear down is selected the free fall
(static drop) of the alighting gear could damage the undercarriage unit
attachment points on the airframe and cause cavitation in the down line (the
fluid being drawn by the falling undercarriage quicker than the pump can
supply), therefore, to slow-down the rate of fall, a one-way restrictor valve is
provided in the up line.

- 21 -
This valve, which restricts the flow of fluid in one direction, but permits full
flow in the opposite direction, offers no restriction to the flow of fluid when
alighting gear up is selected. It prevents fluid getting away from the jacks too
quickly when down is selected - thus slowing them down.

Shuttle and Fluid Jettison Valves. Separate the Alighting Gear circuit from the
Emergency Down gas operated circuit - more of their operation later.

Mechanical Sequence Valves (Single Sequence Circuit). To avoid collision

between the undercarriage leg and it's fairing door during operation, the
undercarriage components must move in the correct order (or sequence).
For example the undercarriage leg must fully retract before it's fairing door
starts to close. Typical sequencing operation would be:

UP SELECTION

1. Pressure fluid to leg jack and leg retracts.

2. Once retracted, 'UP' sequence valve operated mechanically by leg jack.
3. Fluid flows through sequence valve to door jack.
4. Door closes.

DOWN SELECTION

1. Pressure fluid to door jack and door opens.

2. At fully open position 'DOWN' sequence valve operated mechanically by
door jack contact.
3. Fluid flows through sequence valve to leg jack.
4. Leg lowers.

Landing gear without fairing doors do not require this (rather complex) system
and some (small) aircraft with fairing doors have the doors mechanically
connected to the main undercarriage leg and, therefore, employ only one
operating jack.

The sequence valves may be operated mechanically (as above), or hydraulically,

or electrically.

Many of the larger aircraft have double sequence systems, which go something
like:
--~~._--

DOWN SELECTION Door opens

Leg comes down
Door closes

UP SELECTION Door opens

Leg retracts
Door closes

-- ---_.~-_._-
 Additional sequence valves are fitted to cope with a double sequence system.

Emergency Operation. Many large aircraft have a free-fall emergency facility

and some have a separate de motor driven pump and hydraulic system to
lower the landing gear should all else fail. The one shown has a gas operated
system. Some have a manually operated wind-down system.

The gas operated system has nitrogen stored under pressure in the gas bottle.
Should an emergency arise and the undercarriage fail to lower by the hydraulic
system then the pilot can operate a manually operated emergency selector.

This allows nitrogen under pressure to the shuttle and jettison valves.

The shuttle valve is caused to move across and gas under pressure enters the
undercarriage down line. The door jack, sequence valve, and leg jack operate
,.... as normal.

Returning fluid from the circuit has to be allowed to be jettisoned (the

emergency could be a jammed selector valve - or loss of electrical power to the
valve, if electrically operated). So the compressed gas operates the jettison
valve which allows returning fluid to be jettisoned overboard.

Note. When servicing the circuit after emergency operation it is important to:

(a) Select emergency selector to OFF.

(b) Release gas pressure carefully from hydraulic lines.
(c) Ensure jettison and shuttle valves are reset.
(d) Rectify original fault.
(e) Bleed hydraulic system.
(f) Recharge gas bottle - check for leaks.
(g) Functional test hydraulic system.

Wheel Brake Circuits - Small Aircraft

On some small aircraft the brakes may be operated by a lever and cable system
- operating a drum, or brake calliper and disc-assembly.

On many light aircraft a small hydraulic system is provided. Figure 14 shows

~----~sil-Jul4"c'-Ah-asystem.

When the toes are pushed down (on the rudder pedals) on the foot motor,
pressure in the hydraulic fluid causes the brake cylinders to operate. When the
toes of both feet are pushed down in-line braking is achieved, when one is
pushed down, one brake will operate and the aircraft will turn.

-~~- - 23 ---

--~- ---------- -------------- ---------~

With the hand brake applied, fluid pressure causes the pistons in the shuttle
valves to move over and the brake cylinders will operate. The provision of the
shuttle valves means there is an alternative brake system.

HANDBRAKE
LEFT
FOOT
MOTOR

STBD
SHUTTLE SHUTTLE WHEEL
VALVE VALVE BRAKE

Fig 14 TYPICAL WHEEL BRAKE SYSTEM - SMALL AIRCRAFT

Flap Circuit - Simple, Plane or Split Flap (Figure 15)

The flap circuit will allow the flaps to be raised and lowered and set to any
intermediate position. This may be done by using an electrically operated
hydraulic four-way selector valve wired into a Wheatstone bridge or drum
circuit; or the valve may be manually operated. The non-return valve prevents
other "fluid hungry" circuits from drawing fluid from the flap circuit and hence
inadvertently operating the flaps.

QUESTION: How is it possible for another circuit to draw fluid from this
circuit? (5 mins)

ANSWER: If another circuit is selected it might demand more fluid than the
pump can supply so it will draw fluid from where-ever it can. A
good example is when thelanding--gea:r-is-~electead own--EEl,.\\~Tean-l-----~~­
though there is a one-way restrictor fitted to the landing gear
circuit to try to prevent it falling too quickly, it may still want to
move quicker than the pump can supply the fluid. If there were
not a NRV (one-way valve, or check valve) in front of the circuit the
landing gear circuit would "pinch" fluid from it and cause it to
move - even though it was not selected.

- _._-_._- - . ._ - - - - - - - - - -
 The blow-back valve allows the flaps to be blown back if lowered at an
excessive air speed (or left down after take-off) which would otherwise cause
them to be damaged by the air-flow. This provision only applies to flap circuits
that operate using jacks - as the aerodynamic loading can be felt back through
the jack and hence back through the fluid.

If the flaps are operated using hydraulic motors - as with many large aircraft -
the flaps will have a separate load relief system. This consists of a Pitot
operated pressure capsule connected to a motor connected to the flap selecting
linkage. (With flaps down, as the airspeed increases so the Pitot capsule will
make an electrical contact to select the motor to move the flap selector linkage
to select the flaps to a higher position).

The thermal relief valve relieves excessive pressure built up by temperature

,... increase in a closed circuit.

The throttling valve ensures a constant rate of flow of fluid irrespective of

supply rate and is always fitted in the flap down line. It works in both
directions.

QUESTION: Why could the flap circuit (or any other circuit for that matter)
have a varying flow rate supply? Surely the pump - particularly a
constant delivery pump - will supply the fluid at a constant rate.
(5 mins)

ANSWER: If a second circuit is selected, then fluid supply from the pump / s
would be shared (on a closed hydraulic system), and hence speed
of operation, of the first circuit will be reduced.

QUESTION: So why is it important that the flaps always move at the same
speed each time? (I'm not talking asynchronous here). (5 mins)

ANSWER: Operation of the flaps causes a trim change of the aircraft. This
must be the same orieveryflight so that the pilot knows how
to react. So the throttling valve sees to it that the flaps always
move at the same speed each time they are selected.
-- ~ - - - - - ---------~--.

QUESTION: Not another one!!!!

What other circuits might use a throttling valve?

ANSWER: Possibly PFCU circuits the power steering circuit.

--_._---~-
When a selection is made fluid flows to one side of the operating jacks.
Returning fluid from the other side of the jacks goes via the selector valve to
the reservoir through the return line.

Irrespective of the selection the fluid flows through the throttling valve which
ensures a constant rate of flow at all times - IN & OUT.

SUPPLY
ONE·WAY
/VALVE
PORT THROTTLING STARBOARD
OPERATING VALVE SELECTOR OPERATING
JACK
\ VALVE JACK

...
BLOWBACK THERMAL
VALVE RELIEF
VALVE

RETURN
...
PORT FlAP
TORQUE SHAFT

Fig 15 FLAP CIRCUIT - PLAIN OR SIMPLE FLAP

This system operates a simple plain or split flap.

If any pipeline full of fluid is trapped at any time - in this case, the flap up-line
- a thermal relief valve must be fitted - by regulation. This works by pressure
which has been created by an increase in fluid temperature.

Flap Synchronisation
----_. --- -- - -~---_ .. ----
Port and starboard flaps must go up and down together, if they do not for any
reason, a roll would be induced. If this happened at low altitude (and it has)
the role would be severe enough to cause the aircraft to crash.

Flap synchronisation may be achieved by connecting the flaps together

mechanically - in that way one flap cannot move without the other. For most
aircraft this is the solution.

_. -~_. .~-------'- --_._. __ ._.. - ._._._._--------

 For small aircraft an operating rod/torque tube connecting both port and
starboard flaps may be connected to a pilot-operated handle. For many large
aircraft the flaps are moved using hydraulically/electrically operated motors
that rotate common drive shafts to operate screw jacks in the wings to port and
starboard flaps.

Figure 16 shows a hydraulic method of synchronisation - which is not

common, but interesting. It uses 2 constant volume jacks, one connected to
the port flap and the other to the starboard flap.

SYNCHRONISING JACKS

OPERATING JACKS

,...--
.i .: .,''''
! i
.
L.-J :
:
rr1
:
:

!I! : : :
: i':
l Ii i i j
i: i
i
PORT FLAP STARBOARD FLAP

Fig 16 HYDRAULIC METHOD OF FLAP SYNCHRONISATION

They are cross connected in such a way that if one flap is moved the other will
be forced to move in unison because of the transfer of fluid from one side of the
,... first jack to the other side of the second jack.

Fluid make-up valves and thermal relief valves are fitted to the cross feed lines
(not shown in the drawing).

Fowler Type Flap Circuit (Figure 17)

On most large aircraft the flaps are lowered by being pushed back and down
orrtracksystems (tho ughsome large aircraft - tfieBG 10 still have a simple.c,_ _-
hinge arrangement}.

The flaps are pushed back on tracks using a screwjack and drive-nut
assembly. The screwjack is rotated by the drive shafts and the drive-nut
(attached to the flap) is caused to move forward or back.

- 27-
The screwjacks are rotated using drive shafts and gearboxes. The drive shafts
are rotated using a duplicate system of hydraulic motors.

Flap asymmetric operation is prevented, in the first instance, by the drive

system being common to both sides, but if it does happen it is detected by
detectors at the end of each drive shaft. The detectors pick up the drive shaft
rotation and send the signal to the asymmetric drive unit.

This unit compares the signals of both sides and if one shaft rotates quicker or
slower than the other then the fluid shut-off valve is caused to operate.

This type of system is also used to operate the leading edge slats / Krueger
flaps.

.ASSYMETRIC DRIVE UNIT

r--~-----------~ r---------------j
~ r------U------l :
I J I I
I FLUID SHUT-orF v I 1 I
I ~ I SEIlVO VALVES J I
I I
I HYD. SYSTEM -,,- HYD. SYSTEM -8- I
I
TANDEM HYDRO-
I
I GEARBOX MECHANICAL I)RIVE I
UNIT

ASSYMETRIC
D!:TECTOR UNIT

1
'"
DRIVE "UTS
SELECTOR INPUT

/PORT FLAP
SCREWTHREADS
STARbOARD FLAP

Fig 17 FOWLER TYPE FLAp· CIRCUIT

-Airspeedsensor~ win operate to select the flaps up if,. for any. reason thevare__
left down in an accelerating airflow (after take-off for example). If the flaps are
selected down at high speed the air speed sensor will prevent the selection
being made to the hydraulic motors.

-- ... _------_ ..... ._-_. - - .- - - • . .-

_._ .. _ . _ - - - - - ---_._-----
 )1., ):

PowerControl Unit (PCU)

1. Power control unit
E<j;AM Control lever ~ Command 2. Instrumentation
position pick off unit
indication • sensorumt 3. Feedback position
pick off unit
I. I ' I
t=
4. Pressureoff brake
+ 5. Valve block and
r-- +:I'-+--I Slat-~ Flal Slat/Flap :
hydraulic motor
Control ....--......:::=:;.-Ir-;::=====~
Control
r-----;'-~ Computer 2 I
(SFCC 2)
II ..
,
Computer 1
(SFCC 1)
--, 11 i
Hydraulics'
i

t J 1
I I t
I B Blue system

I . 41.
I-L
..c§5 ~~~§;~t~~t1 . .A
• ; .

l\l:'\"
l
_."~
LGCIU

~~~~.
-t-
11+4- - - - 7
G
Y
Green system
Yellow system

LGCIU: landing Gear

Control Interface
Unit

FWC: Flight Warning

Computer

~\\~
Rotary
actuator

Drive gearbox

Aap attachment Asymmetry position

switch pick up unlt
.,&......-_-- Rotary actuator
~~~~~~:;;---Surface support track

_ ~~~ ~ : :~ il ~!1j~i1j1i~i~ j~ ~ 1i~j; : :Jj~j!~ ~1· ~IIII

i................................................. :
~ ~ I Wl11~!~1!1!!!1!1~!111ji1iil:::
R.n' ~~
.. )~II
.:.:-:-:.:.:.:.:.:.:.:.:.:-:....
..........................:.:.:
J:.:1!!...
;.:w:.:.:
::;:::;:;:::::::::::::::::::::::::::::
....
::::::::: : :=::::::::::::::: :::::::~::: : ::::=::::'

Fig 18 A320 FLAP & SLAT SYSTEMS

The A320 Flap System (Figure 18)

This is similar in principle to the system shown in figure 17.

Each Power Control Unit (PCU) is supplied by two hydraulic systems and these
operate drive shafts to operate the flap drive gearboxes.

Asymmetry detectors are fitted at the end of each drive shaft to monitor their
rotation and to inform the computer if the flaps move asymmetrically. If
detected the computer will cause the flaps to stop moving and a warning to be
sent to the flight-deck.

When the pilot makes a selection via the computer the hydraulic valve on the
hydraulic motor selects and the motor causes the drive shafts to rotate. As the
shafts rotate so a pick-off on the motor power control unit sends the positional
feedback information to the computer. J

When the flaps get to the selected position the computer will operate the
control valve to the mid-off position to stop the motor - the flaps being held in
their new position by a hydraulicjmechanicallock.

QUESTION: What sort of feedback is used in this system? (1 min)

ANSWER: Negative feedback

QUESTION: Can you define negative feedback? (2 mins)

ANSWER: It can be defined as a system where the output of the system tries
to cancel the input.

Flap (and slat) positions are indicated on the flight deck by ECAM (Electronic
ge~!~Cl1 Aircraft Monitor). The signal will go via a Symbol Generator Unit (SGU)
to be displayed on a CRT (Cathode Ray Tube)=-in-co!our.

The slat operating and asymmetric detection system operate in a similar way to
the flapoperattng-ftftd-cletectiofl-Systems.-------------
 Brake Circuits - Large Aircraft (Figure 19)

Two supplies are provided - one normal and one emergency. Both are identical
from the non-return valve (NRV) to the pressure-reducing valve. The
accumulators hold a quantity of fluid under pressure to be used when there is
no supply (towing for example). The NRV ensures that this fluid is not used by
other circuits when the pumps are off.

SUPPLY 2
SUPPLY 1

LOW PRESSURE
WARNING LAMP
-:
"t-O-----
LOW '+
~
L- _ -t'
----~.

PRESSURE
SWITCH
FOOT MO'roRS

--0-----
-t-/
/I -----0.;
PRESSURE ./
GAUGE
TRANSDUCER

EMERGENCY
BRAltE
~<I----1

/
PRESSURE
REDUCING
VALVE
SLAVE
UNITS -l
~<I---t
~-----1
ANTI SKID VALVE
\

Fig 19 BRAKE CIRCUIT - LARGE AIRCRAFT

- 31 -

-----------~~- -- --------------
Notes:

1. Pressure transducers (to flight deck gauges) are fitted to the brake lines
between the brake control valve and the anti skid units, and between
the emergency brake control valve and shuttle valves - but these are not
shown for clarity.
2. The foot motors are similar to those shown in figure 42, with each having
its own reservoir. On some aircraft - the Tristar for example - the brake
control valve is operated by cables from the rudder bar so there is no
separate foot motor hydraulic system for each foot on each rudder pedal.

The pressure operated switch provides a warning (light) on the flight deck
should pressure drop to some low value.

The pressure transducer and gauge provides pressure indication on the flight
deck. The gauge could be a moving coil, de or ac ratiometer, or a synchro
system. (See the book in this series entitled Airframe Instruments).

The pressure reducing valve reduces pressure from say 3000 psi to say 600
psi. For supply 1 this pressure goes to the brake control valve.

Lines to the brakes are often fitted with hydraulic fuses so if lines become
ruptured (due to runway debris damage etc) the system fluid will not be lost.
(not shown in the drawing - but would be fitted upstream close to the most
vulnerable section of each line).

The brake control valve is operated by slave units which in turn are operated
by master cylinders on the rudder pedals (see Brake Control Valve in this
book).

When a master cylinder is operated this causes the slave unit to move - which
in turn causes the brake control valve to allow fluid pressure from
supply 1 to the brakes. (For Brakes refer to the book in this series Wheels,
Tyres and Brakes).

This supply then goes via the anti-skid unit also described in the book Wheels,
-Tyres,"and Brakes - to the brakes. The operation of both master cylinders will
achieve in -line braking.

The aircraft can be steered, however.by.using either the left or the right foot
motor separately. (For normal operation large aircraft are power steered
through the nose wheel).

In an emergency, system 2 can be operated directly using the emergency brake

lever.

-- - - - - - - - - - ---- -------
 In the system shown the emergency system by-passes the anti-skid valve, but
with some aircraft the standby system is an alternate system with a complete
set of duplicate components to the main system, but powered from another
supply circuit.

System 2 also stops the wheels rotating during retraction, so they are not
spinning in the wheel bay. This is automatic in operation. When "up"
selection of the undercarriage is made fluid pressure causes the spring return
jack to operate and the brakes to come on.

After a short interval the pressure in that part of the "up" line is released - the
jack spring returns the control valve to the off position and the brakes are
released.

,... Fluid pressure from supply 2 goes to the brakes via shuttle valves.

HYDRAULIC COMPONENTS

This part of the book describes various aircraft components of a general

nature. Several manufacturers produce components and there are many
different types in use.

However, all components designed for one task rely upon the same basic
principles although their appearance and name may differ.

Reservoirs

Some Functions of a reservoir are:

(a) Supply the pumps - with a head of pressure.

(b) Accept return fluid from the system.
(c) Hold a reserve of fluid to allow for small leaks.
(d) Act as a heat sink.
re)·· ·~AITow for jack ram displacement.
(1) Provide a filling point.

The aGt-ual size-and-shape willvary from aircraft to aircTCiftdepending on

capacity, location etc. A simple unpressurised reservoir is shown in figure 20.
Figure 21 shows a pressurised type with a separator piston - note the contents
gauge at the top. (Not a very common type) .

. ~ 33 ~

---~.. _---~
 VEtff VALVE

FILLER CAP

FILLER

Fig 20 UNPRESSURISED RESERVOIR

Fig 21 PRESSURISED RESERVOIR - WITH SEPARATOR PISTON

 ANTlllllULSlOH
DlVltl

.-.~

OUVIL IlUIIIVI

~~f~~~~tSi--- • LtTlAS
~ OF .
-
""-....... ".,

fO'ACC 'J8:f' "O.ACIe.

t""&fAGI

- --
auTlUCfOIl

tt~----------»~---------_

.. oo,l"'LOII AI'ooO '''l~'

"iLllf "4&,"1 .....

Fig 22 A300 PRESSURISED RESERVOIR & PRESSURISING SYSTEM

- 35 -
-~------.~ - ------
----- --.~._---~-

-:-~
~.~

'~
Figure 22 shows a pressurised type as fitted to Airbus aircraft. Study the
pressurisation system and make sure you know how it works. Note the
indications - reservoir quantity, oil temperature and pressurisation pressure.
Note the negative "G" trap so that fluid is always available to the suction line,
even under negative "G" flight conditions.

Figure 23 shows the location of a reservoir.

MAIN RETURN UNE

LOCATl
eecr

PACKING

Fig 23 RESERVOIR LOCATION - EXAMPLE

Since it is not always possible-t~mountthe reservoir above the pump.iand.tc-.

ensure a positive supply of fluid to the pump, many reservoirs are pressurised.
The pressure (relatively low - about 30 psi - but check your AMM) in the
reservoir also helps to reducefluid~Jrothi!1K'l,Vhicl:J: could affect t!te op~ration of
the system.

The method of pressurising varies, but may include the use of compressed gas
acting against a piston or diaphragm in the reservoir, or air from a compressor
stage of the jet engine.

-36 -

-~--~~-- -~ ~~~~~----~----
Heat Exchangers

Heat exchangers are used to cool the hydraulic fluid from the driven pumps -
engine driven - electrically driven etc. This extends the services life of the fluid
and the hydraulic pumps. Not fitted to all aircraft.

They are positioned in the fuel tanks so providing a heat sink from the
hydraulic fluid to the fuel.

They are normally situated in the idling line (case drain) between the pump
and the reservoir - so cooling the fluid returning to the reservoir.

Where more than one supply circuit is used each will have a separate heat
exchanger positioned in a separate fuel tank.

Il IN

HYDRAULIC FLUID

" OUT

FUEL

Fig 24 HEAT EXCHANGER

They are positioned low in the-tank-Ifigure 25) so should always be in fuel.e.but.L,

on large aircraft as much as 1000 US gallons (3790 litres) may have to be in
the tank for proper cooling.

The hydraulic pumps can be operatedi(ihe fuel goes~belowTheselevels, but

the pumps should not be operated if the oil temperature indication getsmore
than that laid down in the AMM {eg 100 0 e (212°F)} or after the pump fault light
comes on. If the pumps are operated above these values, the hydraulic fluid
can become too hot.
 --iJ--'.,---M- - - -

CENTRE SYSTEM
HEAT EXCHANGER

RIGHT SYSTEM
HEAT EXCHANGER it
@r:::::> INBD I !i§~~1~-===i1
FWD
Fig 25 HEAT EXCHANGER - FUEL TANK LOCATION

Filters

The CAA lays down the requirement that the hydraulic system should be
adequately filtered. This means that 'dirty' systems would have more filters
fitted that 'clean' systems.

In general filters may be fitted:

* After the reservoir in the pump supply line (low pressure filter).
* After the pump in the pressure line (high pressure filter).
* In the return line to the reservoir (low pressure filter).
* In front of some circuits that require special protection - eg where
valves are fitted that rely on metal-to-metal contact for fluid
sealing (some PFCUs).

The working parts of hydraulic components have very small clearances and
working limits and it is, therefore, most important that the hydraulic fluid in
the system is scrupulously clean otherwise excessive wear, damage or-even
blockage could occur.

BeforeenteringJbesystemtheJluid passes throughafine.Iilter on the ... ..~_. ._.

dispensing equipment and then through another filter at the filling point (on
some reservoirs). The filter in the dispensing equipment is a micronic type, it's
filtration level being in microns; one micron = O.00004in; a five micron filter
therefore would allow particles smaller than O.0002in to pass unhindered.

The type of filter element varies depending upon its position in the system and
the manufacturer.

- 38 -
 HEAD

r-.-O-RING

- - FILTER ELEMENT

O-RING

Fig 26 THE FILTER OF THE A300

The basic construction of all filters is similar with those in the pressure lines
being of more robust construction that those in the return lines. Elements are
made from:

(a) Felt
.. (b) Paper
"""'" (c) Wire gauge or cloth
(d) Wire wound spool
(e) Sintered metal
(f) Aluminium foil
(g) Magnetic plug

QUESTION: What does" sintered" mean? (5 mins)

ANSWER: It is a process of manufacturing small metal parts using fine

powder.

The powder is measured out to a precise quantity and placed into

accurately machined dies. Here heat and pressure is applied to
form small parts such as gear wheels, valve bodies and filters.

- 39 -

--~_._~--------~
 The item leaves the die without any further machining necessary.
If the applied pressure is not too great then the fine particles bond
together in such a way as to allow small passages between them -
thus forming a filter element.

Filter elements are normally made in discs about 2" to 4" in

diameter (50 to 100mm) and stacked one upon the other inside
the filter body.

Sintered metal filter elements are cleaned using ultra-sonics and a counter
flow of fluid in a special cleaning rig.

Many filters are fitted with a clogging indicator. When the filter element
becomes blocked (or nearly so) a pressure differential is created (about 30 psi -
AP) in the filter body to cause an indicator button to be pushed out. This is
usually coloured red and gives a visual indication that the filter/filter element ~
needs changing.

The indicator is kept out by a spring and may be pushed back (after the
removal of the transparent cover) once the filter/filter element has been
changed - or as directed by the AMM.

Provision is made that it will not operate until the fluid has attained it's correct
working temperature.

TUHSPAafHT
PlASTIC CAP
MAGNETS

Fig 27 FILTER CLOGGED INDICATOR

 " -
""";

..
MANIFDLD

Fig 28 FILTER LOCATION - EXAMPLE AIRBUS

All filters must, by regulation, have provision for by-pass should there be any
chance that fluid starvation could occur if the elements became blocked,

If the filter element becomes blocked then a differential pressure (~P of about
60 psi) will cause the by-pass valve to open. This allows fluid through
unfiltered - better this than no fluid at all. Of course, the clogged indicator will
show.

Wherichariging the filter element always changeanyseals at the same time,

and always make sure that the replacement element is the same as the one
removed - or an acceptable alternative as per the IPC.

With some filters, as the bowl is lowered, internal valves close off the inlet and
outlet to the filter body thereby reducing the amount of fluid lose. Remember to
bleed the system after a filter change - as per the AMM.

---
- 41 -
---~-----

- -------~~~
----- ------------ -~~~------'
Hand Pumps

A hand pump is included in some aircraft installations, for emergency use and
for ground servicing operations. Figure 29 illustrates a double-acting hand
pump (ie a pump which delivers fluid on each stroke).

+ INLET
Drawing from CAP 562
Fig 29 DOVBLE ACTING SINGLE CYLINDER HAND PUMP

As the piston moves upward in the cylinder, fluid is drawn in through a non-
return valve (NRV) at the inlet connection into the cylinder; at the same time
fluid above the piston is discharged through a non-return valve in the outlet
connection.

As the piston moves downwards, the inlet NRV closes and the transfer NRV
opens, allowing fluid to flow throughthepiston. Since the volume below the
piston is larger than the volume above the piston, some of the fluid (about half)
is discharged through the outlet port. When pressure in the outlet line exceeds
the relief valve setting, discbarged.fluid.is by-passed back to thepump inlet.~~~_~_

Accumulators

QUESTION: Can you list 2 or 3 reasons why an accumulator is fitted to a

system? (2 mins)

--------

----------- -------------~-~
 ANSWER: My list goes something like:

* Holds a reserve of fluid under pressure for when pumps are

off.
* Allows instantaneous operation of systems.
* Assists the ACOV in operation.

It may also be used to damp out pulsations from the pump and thus cushion
the shock loads which the circuit might otherwise have to withstand. However,
with modem pumps this function is not always necessary.

GASCHARGING CONNECTION

PRESSURE GAUGE
(BOURDON TUBE 1YPE)

SEPARATOR PISTON

PISTONlYPE
DIAPHRAGM TYPE

Fig 30 TYPES OF ACCUMULATORS

Accumulators may also be fitted to essential services, such as powered flying

control circuits and wheel brakes, mainly to provide a reserve of pressure in
the event of supply non-availability.

Accumulators are normally charged with compressed nitrogen, but might, on

some small aircraft, be spring-loaded or charged with air.

-_. May consist of a cylinder and floating piston or a spherical container and
flexible diaphragm.

-- ~~- ---
QUESTION: Why charged with nitrogen and not air? After-all nitrogen is
less expensive than air. (5 mins)

ANSWER: Nitrogen does not support combustion - air does. Should

a fine spray of hydraulic fluid occur in the gas chamber a
condition similar to a diesel engine is set up if the gas is
air - and combustion can occur.

-----
-- -----~

---.------- ~.
=-.
.....• - .
~3
 PRESSURE - CHARGING VALVE
GAGE CONNECTlON..-.::::::~

T.COUPLlNG ---I
/

Fig 31 AIRBUS ACCUMULATOR

Driven Pumps

Most aircraft are fitted with multi-piston type hydraulic pumps, driven from
the engines. Other types of pumps, such as gear or vane positive displacement
pumps may be found in some installations, but these normally do not provide
sufficient pressure - though flow rates are high.

Pumps may be powered by:

(a) Another hydraulic circuit (hydraulic motor - power transfer unit).

(b) An electric motor - ac normally but dc on some emergency systems.
(c) A ram air turbine - RAT.
(d) The aircraft engine - widely used (EDP).
{e)uu Theairflew in the.by-passsectionof a turbo fan jet.enginec.L ~~_
(f) The APU.

The driven pumps may be classified as either:

(a) A constant volume or non self-idling pump

(b) A constant pressure or self-idling pump.

- 44_"C
 A constant volume pump (figure 32) has 2 hydraulic connections - suction and
pressure - and has to have an Automatic Cut Out Valve (ACOV) fitted in the
power circuit with an accumulator.

A constant pressure pump (figure 33) has 3 connections - suction, pressure

and idling (called case drain on many aircraft).

ACCUMULATOR

SUCTION LINE PRESSURE LINE

IDLING L'INE

Fig 32 CONSTANT VOLUME PUMP - GENERAL ARRANGEMENT

SUCTION LINE SUPPLY LINE

IDLING LINE

Fig 33 CONSTANT PRESSURE PUMP - GENERAL ARRANGEMANT

Constant Volume or Non Self-Idling Pump

These pumps deliver a fixed quantity of fluid to the system at a particular rpm,
regardless of system requirements, and means must be provided for diverting
pump output when it is not required by the system.

One type of constant volume multi-piston pump is illustrated in figure 34. The
cylinder block and drive shaft rotate together, and because of the angle
----- betweenthe.cylinder.block and shaft axes, each piston_moves into and outof
it's cylinder once per revolution.

The stationary valve block has two circumferential slots leading to the cylinder
block, which are connected to the fluid inlet and outlet ports, and are arranged
so that the pistons draw fluid into the cylinders on the outward stroke, and
push out fluid to the system on the inward stroke.

- 45 -

. - .... _ - - - ~
 DlllYl
IMA"

Drawing from CAP 562

Fig 34 CONSTANT VOLUME AXIAL PISTON PUMP

Another type of constant volume pump is illustrated in figure 35. In this pump
the cylinders are arranged radially around a crankshaft, so that when the
crankshaft is rotated, each piston moves in and out within it's cylinder once
per revolution. Fluid is drawn into the pump body and enters each cylinder
through ducting in the cylinder block, whenever the associated piston moves to
the bottom of its stroke. As it moves outwards into its cylinder, it covers the
inlet port, and forces fluid out of the top of the cylinder, past a delivery valve,
to the pump outlet connection. The drawing shows one piston for clarity but
pumps have many pistons with some having a dozen or more.

"".UVoll
OUtUT !'OlIT

Drawing from CAP 562

Fig 35 CONSTANT VOLUME RADIAL PISTON PUMP

- 46-
Constant Pressure Self-Idling Pump (Figure 36)

This type of pump is similar in construction to the fixed volume pump but the
cylinder block and drive shaft are co-axial and piston travel can be varied. The
pistons are attached to shoes which rotate against a stationary yoke, and the
angle between the yoke and cylinder block is varied to increase or decrease
pump stroke to suit system requirements.

CUT OFF SOLENOID

'\
INLET

FIXED VALVE PLATE-

ROTATING CYLI"DEJl _ _~1

BLOCK

DRIVE
DllIVE SRAn'
SRAn'

Fig 36 CONSTANT PRESSURE PUMP

Figure 36 shows the operation of the pump. When pressure in the system is
low as would be the case following selection of a service, spring pressure
causes the-yoke-te move to its maximum angle, and the pistons are at full
stroke, delivering maximum output to the system.

When the selected system has completed its operation, pressure builds up in
the supply line and under the control piston. This moves the yoke to the
minimum stroke position. In this position a small flow through the pump is
maintained (from inlet to case drain) to lubricate the working parts, overcome
internal leakage and dissipate heat.

47

---~------------------'
On some pumps a solenoid-operated depressurising valve is used to block
delivery to the system, and to off-load the pump.

Note: Pipeline connections often vary in size but are usually:

1. Suction line - largest.

2. Pressure line - medium.
3. Idling or case drain - smallest.

PRESSURE
PORT

PUMP
BODY

Fig 37 ENGINE DRIVEN PUMP LOCATION - EXAMPLE B767

Pressure Relief Valves (Figure 38)

Pressure relief valves are fitted between pressure lines and return lines. They
are designed to relieve pressure should it build up above normal working
pressure. They are not designed to operate continuously as this would keep
the pump "on load". Pressure relief valves are usually a spring-loaded ball or a
spring-loaded plate - often adjustable - by the manufacturer only (of course).

BALL--a:L RESERVOIR

Drawing from CAP 562

Fig 38 PRESSURE RELIEF VALVE
 QUESTION: These valves often all look alike but are set by the manufacturer to
operate at different pressures depending on the system they are
designed for. How would you know which is the correct one for
your system and location? (1 min)

ANSWER: By reference to the description and part number, (on the

component), the AMM and the IPC. Too easy eh?

Automatic Cut-Out Valve (ACOV)

A cut-out valve is fitted to a system employing a constant volume pump, to

provide the pump with an idling circuit when no services have been selected.
An accumulator is essential when an ACOV is fitted, since any slight leakage
I"- internally or externally would result in continuous operation of the ACOV
(hammering) .

QUESTION: Why wouldn't an ordinary PRY (Pressure Relief Valve) do? It would
be a lot cheaper. (2 mins)

ANSWER: If the pump operated a PRY during idling it would have to work all
the time against the spring - thus consuming the same energy as
when the pump is operating the services normally. Obviously not a
good idea. When the pump is off-loaded by an ACOV it does little
or no work during it's idling cycle. As an example one pump made
by Dowty used 14HP (10,444 Watts) when on-load and when off-
load used only 2HP (1490 Watts) - quite a saving in energy and
fuel.

QUESTION: Why would an ACOV 'hammer' if an accumulator was not fitted?

(10 mins)

ANSWER: Because the fluid is (more or less) incompressible, pressure build

up to normal maximum would be almost instantaneous without-~--­
any service selected so the ACOV would cut-out almost
instantaneously.

There are always some internal (allowable) leaks in a system. The

quantity (volume) of fluid to be lost to drop the pressure from (say)
3000 psi normal working pressure to (say) 2500 psi cut-in
pressure of the ACOV would be very small. This could be 'lost' in
seconds - causing the ACOV to 'cut-in'.
 When cut-in and on-load, the pump would take a fraction of a
second to build up the pressure from 2500psi to 3000psi (as the
volume required is so small). Thus the ACOV would cut back out
again - in a fraction of a second. This cycle would be repeated very
quickly causing ACOV hammering.

The accumulator gives the system some 'resilience' it takes time to

build the pressure up - and time to loose it so the ACOV may
cycle, say once every 30 minutes or so instead of every second or
so.

Figure 39 shows the operation of an ACOV. When a service has been selected
and the pump is delivering fluid to the system, the NRV is open and equal
pressure is applied to the poppet valve and piston. The force of the spring
combined with the pressure on the poppet valve is greater than the force on
the piston, so the valve is closed and the return line to the reservoir is shut.

When the service selected has completed it's travel and fluid is no longer
-
required for the systems, the pressure applied to the piston is sufficient to lift
the poppet valve off its seat. This results in a sudden drop in pressure on the
pump side of the valve which snaps the poppet valve open and the NRV closed.

Pressure in the idling line drops to a low value and the load on the pump is
removed.

POPPET VALVE

PUMP SUPPLY --~--(~~~~=- TO SYSTEM

~\\~~~

Pressure in the system is maintained by the accumulator until further

selection is made - when pressure drops, and the pressure on the cut-out
piston becomes less than the spring force, the poppet valve closes and pump
output is again directed to the system through the NRV.

-5o_- _

- - --------------~----------'"
Priority Valve (Figure 40)

A priority valve is basically a pressure relief valve which is kept open if system
supply pressure is normal. Should this drop to some pre-designed value then
the valve will close - shutting off the supply to the secondary services. In this
way, if the supply is failing, what is available will be for the primary services
only.

The valve will open to allow fluid to the secondary services if normal supply is
resumed.

SECONDARY
-SERVICES

Drawing from CAP 562

Fig 40 PRIORITY VALVE

A priority valve is generally used to safeguard operation of important services

such as flying controls and wheel brakes. Figure 40 shows the valve in the
open position, pressure being sufficient to move the piston against spring
pressure and connect the main supply to both the primary services and the
secondary services.

A priority valve may be fitted immediately after the power circuit. One
pressure line supplies all the secondary services and another pressure line
supplies all the primary services.

On some aircraft-the priority valve is fitted in front of each secondary circuit.

Each secondary circuit will be connected to the secondary services port and
the primary services port will be blanked off.

Pressure Reducing Valve (Figure 41)

A pressure-reducing valve is used to reduce main system pressure to a value

suitable for operation of services such as wheel brakes. Figure 41 illustrates a
pressure-reducing valve, which also acts as a relief valve for the services
operating at the reduced pressure.

-- 51 -
----------

------~----------~
-----~--~
 Fluid enters the inlet port, and flows through the valve to the low pressure
sub-system. When the fluid pressure exceeds the spring-loading on the valve,
the valve is lifted and gradually covers the inlet port until sub-system pressure
reaches the specified value - when the supply is shut-off. If sub-system
pressure increases for any reason, the valve is lifted further and uncovers the
return port to relieve excess pressure.

Drawing from CAP 562

Fig 41 PRESSURE REDUCING VALVE

Brake Control Valve

This valve is quite "a box of tricks" so you will need to take it slowly. If you are
not too sure after the second read through, contact your tutor.

If the brake control on your aircraft is different and you understand it then
fine - don't bother with this one. But either way the CM will expect you to be
able to describe how the brakes are operated on a civil aircraft - large or small.

This brake control valve is essentially a variable pressure valve, which controls
pressure in the brake system according to the position of the pilot's brake "'"
pedals. The valve usually contains four elements, one pair for the brakes on
each side of the aircraft, to provide duplicated control. Figure 42 illustrates a
single element, in this case operated by a slave servo from the brake pedal
master cylinder.

When either pilot's brake pedal master cylinder on the appropriate side is
~-~~-----------depressed.oLthe~d brake is operated, thJ~.JootPrC\k~servo _~plies a _f~rce to _
the linkage on the control valve, which, via the lever assembly and plunger,
presses down the exhaust valve cap. This action initially closes the gap
between the exhaust valve cap and the exhaust valve seat, then moves the
cradle down to open the inlet valve and direct fluid to the brakes.
 + HANDBRAKE
LEVER ASSEMBLY
fiXED
KNIFE EDGE

EXHAUST _:;;::::::

EXHAUST
VALVE CAP

EXHAUST lit PILOrS

VALVE SEAT PEDAL

2nd PILOrS
PEDAL

INLET PORT
CRADLE

BRAKES _~~II[] INLET VALVE

Drawing from CAP 562

Fig 42 BRAKE CONTROL VALVE AND SLAVE UNIT

Pressure builds up in the brakes and under the valve until it is sufficient,
assisted by the spring, to overcome the inlet pressure and the force exerted by
the plunger force. This pushes the whole assembly upwards to close the inlet
valve (the increase in pressure to the brakes stops).

An increase in the load applied to the va've plunger will be balanced by

increased delivery pressure, and a decrease in the load applied will be balanced
by relief of delivery pressure past the exhaust valve cap to exhaust. In this way
the pressure applied to the brakes is proportional to brake pedal pressure.

When the brake pedals are released, the cradle moves to close off the inlet port,
the exhaust valve cap lifts, and exhausts the pressure from the brakes to the
_ . _ - ~ - .

reservoir.

-- -~------------ - - ---

A common device used to control the flow of fluid is the non-return valve (NRV)
or check valve or one-way valve. It permits full flow in one direction, but blocks
flow in the opposite direction. Simple ball-type non-return valves are common
but design may vary. When a non-return valve is used as a separate
component, the direction of flow is indicated by an arrow moulded on the
casing, in order to prevent incorrect installation.

------~-----_a5ct_3~__cc

------------------------~
Hydraulic Fuse

This valve allows normal fluid flow through, but should it become excessive
due, say, to a massive leak, then it will shut and prevent further fluid flow. It
is sometimes known on American aircraft as a Waterman fuse. It operates
using the principle of differential pressure across the valve.

The valve will permit normal flow, but if the flow rate rises above a pre-
determined level the valve will close its outlet line preventing further flow.

Often fitted to wheel brake lines due to the high probability of damage (and
leaks) from flying runway or tyre debris. On some aircraft, fuses are fitted at
many locations throughout the hydraulic system.

Figure 43 shows the operation of the valve. The fluid flows into the fuse and
enters the upper chamber via a small metered orifice in the spring loaded
piston. The fluid also flows through the valve to the brakes.

BLEED
SCREW <,

TO
BRAKE
-PlSTO''---------'-

OPEN RESET

Fig 43 BRAKE LINE FUSE

------~~- ~ ~ ~
During normal operation the pressure differential that exists either side of the
spring-loaded piston is minimal therefore the piston remains at the full-flow
position and an uninterrupted supply goes to the brakes. If an excessive flow
rate occurs through the fuse (due to a large leak), the pressure difference on
top of the piston through the metered orifice will be sufficient to push the
piston down and block the exit side of the fuse.

It will remain closed even if the brake pedals are released - as reservoir
pressure will hold it closed.

This fuse will need to be bleed to be reset - with the brakes off.

IMPORTANT. Always carry out a full brake/system test after fuse

operation/bleeding to check for correct re-setting.

Restrictor Valves

A restrictor valve may be two-way or one-way. A two-way restrictor restricts

flow in both directions; a one-way restrictor restricts flow in one direction -
with full flow in the other.

The restriction is usually of fixed size, as shown in figure 44. A restrictor valve
is used in a number of locations, in order to limit the speed of operation of an
actuator. It may, for instance, be used to slow down flap retraction or landing
gear extension.

Drawing from CAP 562

Fig 44 ONE-WAY RESTRICTOR VALVE
QUESTION: Why are flaps and landing gear likely to move too fast? (1 min)

ANSWER: The landing gear will tend to fall under its own weight and the
flaps will tend to be blown up by the airflow.

QUESTION: Would this happen to all flap systems? (1 min)

ANSWER: No. Fowler type flaps with hydraulic motor operation would not be
blown back whilst simple flaps (jack operated) would.

It is important to note that while they restrict the flow of fluid, the actual flow
rate through the valve will be related to delivery rate, delivery pressure and the
design of the valve.

Selector Valves

These may be manually or electrically operated, and can be of the four-way

type (where flap operation would require an intermediate selection), or of the
two-way type - where no intermediate positions of a service are required. The
purpose of a selector is to direct fluid to the appropriate side of the
actuator/hydraulic motor, and to provide a return path for fluid displaced from
the opposite side of that actuator/motor.

A two-way selector valve connects the pressure and return lines to alternate
sides of the actuator, without a neutral position. Selectors in open-centre
systems will trap fluid in the actuators while providing an idling circuit for the
pump. Some manually operated valves are shown in figure 45.

It is sometimes necessary to be able to hold the actuator in an intermediate

position (flaps for example). On some aircraft this is achieved by using a
selector which blocks both lines to the actuator when it is in the neutral
position, the selector being manually returned when the desired actuator
position is reached. However, as this could be distracting for the pilot at a
critical stage of flight,-afeect=o8.ck mechanism is usually used, which--------
automatically returns the selector to neutral whenever the selected position is
reached.

QUESTION: What sort of feed-back would this be? (2 mins)

ANSWER: Negative feed-back. Where the output of the system tries to cancel
the input.

-----~--
 J
R
•I
P- _R
o

P
* t
J P J'

P
(This valve has auto return)

• • For use with open centre systems

P • PRESSURE
R • RETURN
J • JACK

Drawing from CAP 562

Fig 45 MECHANICALLY OPERATED FOUR WAY SELECTOR VALVES

Electrically-Operated Selectors (Figures 46 & 47)

It is sometimes convenient to locate a selector valve at a position remote from

11" the crew compartment, and to eliminate the need for extensive mechanical
linkage the selector is normally operated electrically - or at least initiated
electrically - the actual operation is done hydraulically.

The selector shown in figure 46 is a typical electrically initiated two-way valve,

which may be used, for examp1e,--ior operation of the spoilers.

With the solenoid de-energised, the pilot valve is spring loaded against the
return seat, and fluid frofficthe system passes to both sides of the slide valve.
~·~--Since the rIght hand end of the valve is-of larger diarrieferffiari the left, the
valve moves to the left and fluid passes to the left side of the actuator. Fluid
from the opposite side of the actuator passes through the selector to the return
line.

- 57 -
With the solenoid energised, the pilot valve is held against the pressure seat
and supply pressure acts on the left-hand side of the slide valve only, the right-
hand side being open to return. The slide valve moves to the right (as shown),
and directs fluid to move the actuator in the opposite direction. The other side
of the actuator being open to return - via the valve.

TO SELECTOR SWITCH

SUPPLY
SOLENOID

RETURN SEAT

PILOT VALVE

PRESSURE SEAT

LARGE PISTON

SMAll PISTON
SLIDE VALVE

Drawing from CAP562

Fig 46 ELECTRICALLY INITIATED TWO-WAY
SELECTOR VALVE

Four-Way Selector (Figure 47)

These are used where an intermediate selection of a service is required, eg flap

circuits. They have two solenoids which control a slide in much the same way
as in the previous type.

Operation of Electrically-Operated Four-Way Selector

1. With one solenoid operated (solenoidB' )the_SJJ12ply to thatsideof the

valve is shut off and the pressure that was there is allowed to return
to the reservoir. With pressure to only one side of the slide valve the
slide valve will move over to allow fluid to one side of the jack. The other
side of the jack is allowed to return via the other port.
 TERMINAL BLOCK EARTH RETURN

SOLENOID "B" SOLENOID "A"

RETURN

ANNULUS SLIDE

SLIDE VALVE PISTON

SLIDE VALVE TO/FROM JACK TO/FROM JACK

(a) Both solenoids de-energised

H~=::::;-;:~ t'=::::7ri==::::!I';=::[l- RETURN

- - - - - - ~-_._--~~--
TOJ.A.CK

(b) Solenoid "B" energised

Drawing from CAP 562

Fig 47 ELECTRICALLY INITIATED FOUR WAY
SELECTOR VALVE

- 59 -
~---------

- -- ~-------- -~---------~---~
 2. With both solenoids de-energised fluid pressure is allowed to both sides
of the slide valve. In figure 47 the effective area of the slide valve piston
on the left is greater than that of the right. This is due to the fact that
the annulus slide is away from it's body stops but resting on the slide
stops; Thus the slide valve and annulus slide move to the right until the
annulus slide rests on the body stops - the effective area then becomes
equal and the slide valve stops - in the middle.

Fluid Jettison valve

Fitted in the emergency gas system this valve permits up line fluid to be
dumped overboard on emergency down selection.

Operation. Upon selection "emergency down" gas pressure is fed into the base
of the valve to act on the piston to unseat the ball valve. UP line fluid can now
vent to atmosphere. When gas pressure is released the piston will be moved
down by it's spring and the ball valve will automatically reseat.

---~... JETTISONED FLUID

GAS
T i
PRESSURE-

Drawing from CAP-562--

Fig 48 FLUID JETTISON VALVE

----
Shuttle Valves
- ---------

These are often used in landing gear and brake systems, to enable an
emergency or alternate system to operate the same actuators as the normal
system. During normal operation, fluid flow is provided from the normal
system to the service and the emergency line / alternate supply is blocked.

--- -
- 60 -
---------

- ------~------ -_._---_._--~
 When normal system pressure is lost and the emergency system is selected (or
when the alternate system is used), the pressure moves the shuttle valve
across, allowing the emergency/alternate supply to the actuator. A typical
valve is shown in figure 49.

SERVICE

SHUTILE I
NORMAL
. . . - - - SUPPL Y

Drawing from CAP 562

Fig 49 SHUTTLE VALVE

Valve -----c=--=

Plunger

Plunger
'Down' line
Pressure
Landing Gear
RetracHon Jack
~--------
------- ---

MECHANICAL (PLUNGER IN) HYDRAULIC ('DOWN' SELECTED)

Drawing from CAP 562

Fig 50 SEQUENCE VALVES

---------~----~
------------ ---~ ~------ --- --
 Sequence Valves

Sequence valves are often fitted in landing gear circuits to ensure correct
sequencing of the landing gear and door jacks. Examples of mechanically
operated and hydraulically operated sequence valves are illustrated in
figure 50.

Sequence valves ensure that the landing gear does not extend until the doors
are open, and that the landing gear is retracted before the doors close.

Completion of the initial movement of one of the actuators results in part of the
mechanism contacting the plunger of the mechanical sequence valve, moving
the piston and allowing fluid to flow to the next actuator.

The two valves shown are operated mechanically/hydraulically. But, of course,

valves can be operated electrically using a de supply; a microswitch operated
by the moving component, and a solenoid to operate the valve.

Modulators.

A modulator is used in conjunction with the anti-skid unit in a brake system.

It allows full-flow to the brake units on initial brake application, and thereafter
a restricted flow. Figure 51 shows a modulator, the swept volume of which
would be equal to the operating volume of the brake cylinders.

ORIFICE

··{)rawiAg-from CAP 562

Fig 51 MODULATOR VALVE

During initial operation of the brakes,the-piston iSforced down the· cylinder

against spring pressure, and the brakes are applied. Subsequent fluid feed to
the brakes, necessitated by anti-skid unit operation, is through the restricting
orifice and is limited.
 This limited flow allows the anti-skid unit to completely release the brakes
when necessary, and conserves main system pressure. When the brake control
valve is released, the piston returns to its original position under the influence
of the spring and the returning fluid from the brakes.

Flow Control Valve

A flow control valve may be fitted in a hydraulic system to maintain a constant

flow of fluid to a particular component - similar to a throttling valve. It is
frequently found upstream of a hydraulic motor which is required to operate at
a constant speed. A typical flow control valve is shown in figure 52 and
consists of a body and a floating valve.

VALVE SEAT

~Em'5:m=ms.mm:sta I ~~~~
• IN

DAMPER UNIT FLOATING VAL.VE VALVE HEAD

Drawing from CAP 562

Fig 52 FLOW CONTROL VALVE

Flow through the valve head is restricted by an orifice, which creates a

pressure drop across the valve head. At normal supply pressure and constant
demand, the pressure drop is balanced by the spring and the valve is held in
an intermediate position - the tapered land on the valve partially restricting
flow through the valve seat, and maintaining a constant flow through the
outlet.

If inlet pressure rises, or demand increases, the pressure differential across the
_ yal~e head also iI!~!ea.~esjandmovesthe valve to the left to reduce the sizeof
the aperture and maintain a constant flow.- --------

The spring loading is increased by the valve movement, and again balances the
pressure drop. Similarly, if inlet pressure drops or demand decreases, the
valve takes up a new position, to the right, so as to maintain a constant flow.

--_._-----~
Hydraulic Jacks or Actuators

The purpose of a hydraulic jack or actuator is to convert hydraulic pressure

and flow into linear motion.

t t
T t t
~ I ~
,
jU~~1 11
l. ~

U H
~

It
J.

SINGLE ACTING DOUBLE ACTING EQUAL AREA DOUBLE ACTING UNEQUAL AREA

Drawing from CAP 562

Fig 53 TYPES OF JACK

combined elastomerl slipPer

SCRAPER

Fig 54 JACKS - EXAMPLE OF SEAL LOCATIONS

Jacks are the main component in a hydraulic system for the conversion of
hydraulic power to mechanical power. In one form or another they are found in
landing gear circuits, flap circuits, spoiler circuits, powered flying control
units, nose wheel steering circuits etc. There are two basic types in use
depending upon the requirements of the system they actuate.
 The most common is the unequal area type which is adequate for most
requirements. However, due to the ram, piston area on one side is less than
on the other and thus extension and retraction forces (and speeds) will differ.

Where it is necessary to generate the same force or same speed in both

directions an equal area jack is used. In this case the ram on both sides of the
piston reduces both areas by the same amount.

The type of jack will also dictate the speed of operation. Given the same fluid
supply in each case an unequal area jack will move in faster than it will move
out. With an equal area jack it's speed of operation is the same in both
directions.

QUESTION: Which one of the jacks shown in the previous drawing (figure 53)
would be fitted in a power steering circuit? (5 mins)

ANSWER: One double acting equal area jack - similar to the A320 or two
double acting unequal area jacks - similar to the B747.

QUESTION: Which way should the unequal area jack move to retract the
landing gear? (3 mins)

ANSWER: It should move out or to the right in the previous drawing. This
will mean that it uses its largest area, and we know that:
FORCE = PRESSURE X AREA - so the more area the more force.

QUESTION: Which way are the jacks fitted on the landing gear of your aircraft?
"... Are they different and if so do you know why?

.---'"
----_ .. _----- _..
_._~

Drawing from CAP 562

Fig 55 THERMAL RELIEF VALVE

... - 65 -
1tE! . . ~ . _-----
.....•.:::..•.-' .:-=

-It-·--~
 Thermal Relief Valve

To relieve slow pressure rises due to thermal expansion of the fluid a thermal
relief valve is fitted between the pressure line it is to protect and a return line.
Similar in principle to a pressure relief valve but it incorporates a restrictor
which ensures that when it opens it will relieve only some of the pressure.
Works on pressure only.

Throttling Valve

A specialised restrictor valve which automatically regulates the flow-rate in

inverse proportion to the pressure of the supply to maintain a constant speed
of operation. These valves may be one-way or two-way (as shown).

Figure 56 shows the valve in operation. When too much fluid tries to flow
through the valve the piston is pushed so that the metering needle enters the
outlet orifice, thus restricting the fluid.

The valve is so designed that the outlet fluid flow rates are within the normal
operating parameters. Similar to a flow control valve.

NORMAL FLUID FLOW

f

t
HIGltrLUID PRESSURE
Drawing from CAP 562
Fig 56· THROTTLING VALVE

~--------

- 66--
 Pressure Relay Valve

A pressure relay is a component which transmits fluid pressure to a direct

reading pressure gauge (Bourden tube type) or to a pressure transmitter which
electrically indicates pressure on an instrument in the flight deck. In some
cases both types of indication are provided, the direct reading gauge
being fitted in the hydraulic equipment bay, adjacent to the relay. Transits
fluid pressure but not fluid flow.

During normal operation the piston acts as a separator, transmitting fluid

pressure to the gauge side. If a leak develops on the gauge side, the piston
moves to the gauge end of the cylinder, and the valve seats in the cylinder
head, thus preventing leakage from the system.

The valve also permits bleeding when a new gauge, or gauge line, is fitted.

~==~flrOfJ
~ _ --- TO
GAUGE

PISTON VALVE

Fig 57 PRESSURE RELAY VALVE

Quick Disconnect Couplings

In positions where it is necessary to frequently disconnect a coupling for

servicing purposes, a self-sealing, quick disconnect coupling is fitted. The
coupling enables the line to be disconnected without loss of fluid, and without
the need for subsequent bleeding.

Pressure Release Valves

Pressure.release'valves are.fitted to enable p~~~l.l.r~ to be released from the

system for servicing purposes. The valves are manually operated, an-d consist--
of a valve body with an inlet and outlet port, the passage between the two
being blocked by a spring-loaded valve. Operation of an external lever opens
the valve against spring pressure, and allows fluid to flow from the
accumulator or pressure line back to the reservoir.

- 67 - . ~-- __ ~ _

- ~---,-_._._-- - - ------------------~--"-'
~------~
Drain Cocks/Sampling valves

Drain cocks are generally simple manually operated spherical valves, and are
located in the hydraulics bay at the lowest point in the system. They are
marked to indicate direction of flow, and are used to drain the system, when it
is necessary to do so, in order to replace the fluid, or in some systems to
change certain components. A sampling valve allows fluid to be taken for
analysis.

Fire Shut-Off Valve

Electrically operated fire shut-off valves are mounted between each reservoir
and each of the engine-driven pumps (within a fire zone). These make it
possible to cut-off the supply of fluid quickly (in less than two seconds) in the
event of a fire in the engine.

SAMPLING

::::::::;::;;~;:::;:;;;::::::;:::::" -.; , ".-.- . .

;:;:,.:::.;;:::: ••.• ::~.:.:-.- ..:.>:.'.--.'.". ,"

\
\.,
FLUID SAMPLING

Fig 58FLllID_ SAMPLIllCLVALVE_OF THE A300

The shut-off valves are normally controlled from the hydraulic panel by the
PUMPS ON-DUMP-SHUT VALVES switches. The shut-off valves close when the
associated switches are placed in the SHUT VALVES position. When the
switches are at either of the other two positions, the valves are open.

__- -6.8 -
In the event of an engine fire, the fire control handle is pulled and the fire shut-
off valve will close irrespective of the PUMPS control selector position.

Fig 59 FIRE SHUT-OFF VALVES - POSITION

Hydraulic Motors

The purpose of a hydraulic motor is to convert hydraulic power into rotary

motion, usually by the swashplate principle.

These motors which are reversible, may be ope-rated continuously,

intermittently, or stalled without damage, when driven at rated pressure. They
are used in systems that require powerful and smooth operation, eg spoilers,
flaps etc.

SHAFT BEARINGS

,...... ..
DISPLACEMENT
I.NGLE
0<..

VA.LVE PLATE INLET SLOT

Fig 60 BENT AXIS PISTON TYPE HYDRAULIC MOTOR

- 69 -

--~~~---
Although the basic principle of operation is common in all types of hydraulic
motor, each aircraft system will employ its own specific design of motor.

Pressure fluid enters the motor through a valve block (figure 60). It is directed
to the individual cylinder bores through a valve plate. The non-rotating valve
plate provides the timing for the motor.

Pressure builds up in the cylinder bores until the resulting force on the drive
shaft overcomes the resisting torque. Motor speed is determined by the supply
fluid flow-rate, the load it can take is governed by the supply pressure.

The pressure of the fluid is transmitted to the pistons, and from the piston
rods to the drive shaft, overcoming the torque resistance of the load connected
to the driveshaft.

Fluid under pressure enters via the inlet slot to those pistons on their outward
movement. The pressure exerted on the piston will push it out - but it can
only go out if it moves up (in the drawing). This it does, while at the same time
moving round. This causes the whole cylinder block to rotate in the direction
shown in figure 60.

The universal link keeps the drive shaft and cylinder block in alignment since
the piston rods have ball joints on both ends. The drive between the cylinder
block and the drive shaft is maintained by the universal link. The link
however, does not transmit load torque.

The bent-axis motor obtains its high torque efficiency by minimising frictional
losses of the moving parts.

Each piston rod has pressure-lubricated ball joints at both the piston and
drive shaft ends.

"""",,,,",,""

~~---------._-
 EMERGENCY/STANDBY SYSTEMS

One or more of the following systems/components may be fitted to provide

standby/ emergency operation:

1. Accumulators.
2. Electrically driven pumps - ac and de.
3. Duplicate/triplicate systems.
4. Duplicate/triplicate components (PFCUs, yaw dampers, A/P servos etc).
5. Power transfer units from one system to another.
6. Manual reversion (PFCUs).
7. Gravity freefall (U/C down).
8. Gas operation.
9. Ram air turbines (RAT).
10. Air driven pumps.
11. Separate system with a de driven pump (u/c down on some aircraft).

YEllOW
KYOUUL le PUNP

UP LOCl, _ _-"i,.T'R>t..\
UNIT

....,.~
HSENIU

Fig 61 HYDRAULIC RAM AIR TURBINE (HYRAT)

A RAT System
-------~-- -----

This variable pitch ram air-driven propeller is housed inside the wing root and
the housing is closed by two mechanically operated doors.
 EXTENTION lEG

RAM AIR TURBINE

PUMP

.... FAN

Fig 62 RAT LOCATION

. Two control handles at the first and second officers positions permit extension
of the ram air turbine.

The RAT leg is secured in the retracted position by an uplock which is

mechanically operated by a control cable. This runs from the flightdeck to the
inboard wing using tumbuckles, fairleads, cable quadrants etc.

When the RAT is released the ejection jack extends under it's spring pressure
to thrust the RAT into the airstream. The latter stage of the jack extension is
retarded by the hydraulic damping devices to avoid high impact at full
extension of the unit. A down-locking pin secures the RAT in the extended
position.

The doors are mechanically opened and locked by the RAT movement.

RAT Warming
- ~ - - ' - - - ~ ' - ~ - ~ ~ --- -------- .. _-- -----------
With the RAT in the stowed position the test selector is'set to provide a
permanent supply of warming fluid.

This maintains the power pump and the gerotor pump (an Airbus term) at the
correct operational temperature.
 The flow is controlled within the control module and by the flow-sensitive valve
in the leg.

The return line is provided with a filter monitored by a clogging indicator.

Operation. As soon as the RAT is down-locked the turbine is simultaneously

unlocked by withdrawal of the locking rod and immediately commences to
rotate.

After the momentary off-loading effect provided by the flow sensitive valve in
the airborne test selector, the power pump operates to supply full pressure to
the yellow system.

Speed Governing. Hydraulic pressure supplied by the gerotor pump for

operation of the speed governing mechanism passes via the turbine shaft
sleeve assembly into the piston valve. As the turbine rotates, centrifugal force
causes the governor weights to pivot outwards and move the piston valve
against it's spring pressure. This allows high pressure fluid to pass through
ports in the sleeve and shaft and flow to the annular chamber.

Here the pressure acts on the cylinder which is moved forward against it's
spring load, turning the blades toward the fine pitch position.

Low pressure fluid, displaced from the space forward of the cylinder, passes via
the cylinder guide and central ducts of the piston valve and drive shaft to
return to the gerotor pump. A consequent increase of turbine speed occurs
until the pre-determined governing speed is reached. At this stage, the
governor weight force equals the opposing piston valve spring force.

Any tendency of the turbine to change speed is corrected by variation of the

position of the governor weights and piston valve. Which thus maintains a
balance of pressures to keep the speed constant.

In the event of an excessive speed occurring, due to malfunctioning of the

mechanism, the spring loaded dump valves will open under centrifugal force
and high pressure fluid will be bled from the cylinder chamber into the low
pressure cavitytobalance the pressure. The cylinderwitt thusmove to
coarsen the blade pitch and reduce the speed.

Figure 63 shows the RAT system schematic.There is nO heed to_fyffiember the

details, but you should be able to follow the general principles.

RAT Test

It is possible to test the RAT pump performance on the ground. A special test
selector allows the pump to be used as a motor to drive the pro-peller.

---------- - 73 -

---------~
- -- -- -- ------ --- ------- -- -- - - - - - - - - - - - '
 PRESSURE
Iiii!liililiilll----~_I _
TANK_ .. -

PISTON VALVE
GOVERNOR WEIGHT

--
RELIEF
VALVE

----------
HIGH PRESSURE _

LOW PRESSURE (::::)::::\d

HIGH PRESSURE ~
(GEROTOR PUMP)

Fig 63 RAT SYSTEM SCHEMATIC

-~------
 TEST

®
ON nPUMP
N
OHM t~SPIN
10I1II I ~~A-:'
- I 1111.", OllPRESS
R.P.M.

Fig 64 RAT TEST

The following equipment is necessary for the test:

Yellow hydraulic ground supply (output flow 60 l/min at rated pressure).

- Test rig connected to the circuit socket.
Aircraft electrical power on.

The RAT leg must be extended and positively downlocked - the area must be
cleared for high-speed propeller rotation - position warnings and rope-off area.

1. Start the test rig and select switch to ON. Check that the red light
illuminates. . ... - - -

2. Set the test switch to "SPIN". This energises the test selector solenoid.
The pressureandr.etumJines are crossed-Q.yer to .sup£ly the E~!llP wi~ _
fluid pressure and make it work as a motor.

3. Check the speed of the propeller on the rpm indicator.

4. When normal speed is reached (5300rpm) release the test switch. The
test selector solenoid is de-energised and the motor is returned to pump
mode with the pressure / return lines configured for pump operation.
 5. Power is now supplied to the pump by the propeller inertia, and pressure
output is monitored by a pressure switch which activates the green test
rig light if the delivered pressure is correct. This lasts for a period of
about 2 secs until blade inertia is absorbed.

6. Reset the RAT.

7. Reconfigure the aircraft.

8. Record all work done and sign paperwork.

Reset Procedure

1. Reset the control handle.

2. Check that the filter clogging indicator is "IN".
3. Set the blades perpendicular to allow the blade locking pin to engage the
spline.
4. Pull back and lock the door control pins.
5. Release the downlocking pin and push RAT leg upward (two operators are
required), uplock mechanically by the UPLOCK UNIT.
6. Release the door control pins.
7. Release the door lock.
8. Close the doors -ensure they fit flush.
9. Record all work done and sign the appropriate documents.

QUESTION: The next section deals with seal shapes. Before reading on, can
you remember what seal material goes with what fluid? (5 mins)

ANSWER: Check your answers with page 7. Remember the CAA ask this sort
of question in the multi-choice paper. -'
HYDRAULIC SEALS

To prevent leakage of fluid past pistons, piston rods, etc, special seals are
fitted. The material used in the manufacture of seals depends on the type of
-------------fluid'tlSed in-the system, whilethe.shape.of.a seaLisgoverned by the pres~ure_~ _
of the fluid and the-purpose of the seal, Hydraulic seals are made ofnatural
rubber, synthetic rubber, or other synthetic material. Modern seals are made
to meet the fluid specification (MIL SPEC or local or national) to cope with the
temperature range, operating conditions and fluid types.

76 -
 When replacing a seal it must be replaced by a seal of identical shape, material
and part number. This may be indicated on the seal and/ or on the package.
Check the AMM, components manual and/ or the IPC. Some seals may have
coloured dots or strips painted on them for identification purposes.

Care must be taken when replacing seals as they can easily be damaged.
Special tools are provided for there removal/fitment.

Square Section Seal. This provides sealing in both directions and can be used
as a piston ring seal. It is either located in a square groove in the piston head
or supported by tufnol rings on either side.

Fig 65 SQUARE SECTION SEAL

"0" Section Seal. This seal is used to provide a fluid tight joint for contact
surfaces where no movement takes place (ie between the mating faces of two
halves of a component where a fluid tight joint is required), especially where
the shape of the joint is irregular. If used on moving surfaces with pressures
above 1500psi they should be backed with backing rings to prevent distortion.

Fig 66 ROUND OR "0" SECTION SEAL

~ --~._-----
-

Bonded Seal. Consists of a metal washer with a rubber seal bonded to its inner
surface, the rubber being slightly thicker than the metal. It is usually fitted
with a hollow bolt that is torque loaded and is used for sealing end caps, banjo
unions, etc where no movement takes place.

~;;~- ---
~----- ~~--_.
~._~-­
~."------

--~
~-~--
 RUBBER . l -......-..r
METAL
Fig 67 BONDED SEAL

Wiper Ring/ Scrapper Ring. This is not a seal as such, but fitted to prevent
dirt/ debris from entering the seal assembly on a piston ram, undercarriage
oleo leg etc.

-:
WIPER EDGE

Fig 68 WIPER RING

Duplex Seal. Consists of a hard rubber square section ring with a soft rubber
square section ring bonded to its inner face. Resists both high and low
pressures and is suitable for fluid/ gas components.

HARD RUB8ER
serF-T- RU-BBCR--- --- -.-----~­

Fig 69 DUPLEX SEAL

Chevron Seal. If used singly will withstand pressure in one direction only. If
used backed as a pair will withstand pressures in both directions.

__-~7B---- ~ __

.... _-----_.. _ - _ . _ - - - - - -
The drawing shows a single chevron seal designed to withstand pressure in the
direction shown. It is normally supported by back-up spacers and spreader
packing pieces between it and the component.

/ SPACERS OR PACKING .

CHEVRON SEAL,...

Fig 70 CHEVRON SEAL

"V" Ring/"U" Ring Seals. Similar to the chevron seal, though the "U" ring seal
has a shape similar to a "U" rather than a "V". Normally used without backing
pieces and are fitted singly.

_ _ AIR CHARGING VALVE

MOTOR SWITCH SWITCH OPERATING COLLAR

SELECTOR LEVER
I
FILLER

FILTER ~~.,~

FLUID LEVEL
ACTUATOR

Drawing from CAP 562

Fig 71 POWER PACK

. - 79 .;
 POWER PACK

Light aircraft may be fitted with a power-pack, an example of which is shown

in figure 71.

When the selector lever is operated the pressure from the accumulator starts to
move the actuator. As the accumulator ram moves up, the switch operating
collar moves away from the motor switch and the pump commences it's
pumping cycle. The actuator will extend until full travel is reached. Further
pump action will recharge the accumulator until the fully charged position is
reached and the switch-operating collar will operate the motor switch and stop
the pump.

"""""""""""

....~ - - ~ ~ -