Aircraft Systems - Chapter 07

8/10/2019 Aircraft Systems - Chapter 07

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TABLE OF CONTENTS

Aircraft Systems

CHAPTER 01 AIRFRAME DESIGN AND MATERIALS

Introduction

Certification Standards

Design Concept

Loads and Stresses

Ultimate Load and Limit Load

Fatigue

Material Properties

Composites

Summary of Material Properties

Corrosion

Maintenance Methods

CHAPTER 02 MAJOR AIRFRAME COMPONENTS

The Major Air frame Components

Material Attachment Methods

Construction Principles

The Fuselage

The Pressure Hull The Wings

Torsional Stresses and Flutter

The Empennage


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CHAPTER 03 SUBSIDIARY AIRFRAME STRUCTURES

Introduction

The Floor

Doors and Hatches

Flight Deck Windows

Passenger Cabin Windows

CHAPTER 04 HYDRAULIC PRINCIPLES

Introduction

Hydraulic Principles Hydraulic Power

Hydraulic Fluid Requirements

Advantages and Disadvantages of Hydraulic Systems

A Elementary Hydraulic System

CHAPTER 05 HYDRAULIC SYSTEMS

Overview

Actuators

Basic System Components

Hydraulic Circuits

Light Aircraft Systems

Large Aircraft System Components

Large Aircraft Systems Emergency and Auxiliary Power Sources


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TABLE OF CONTENTS

CHAPTER 06 THE LANDING GEAR

Types of Landing Gear

Retractable vs Fixed landing Gear

Light Aircraft Fixed Gear Systems

Landing Gear Configurations

Shock Absorption

Main Gear Components

Nose Gear Assemby

Retraction and Extension

Landing Gear Locks Control and Operation

Speed Limits

Emergency Extension

Nose Wheel Steering

Nose Wheel Shimmy

CHAPTER 07 WHEELS, TYRES AND BRAKES

Wheel Construction

Tyre Construction

Tyre Inspection

Aquaplaning

Tyre Overheat Protection

Wheel Brakes Light Aircraft Wheel Brakes

Large Aircraft Brake Systems

Heat Dissipation

Anti-skid Systems

Emergency Brakes

Parking Brake


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TABLE OF CONTENTS

CHAPTER 08 PRIMARY FLIGHT CONTROLS

Overview

Manual Control System

Power Operated Controls

Partially Powered Controls

Fully Powered Controls

Trimming Systems

Rudder Limiter

Blow-back System

Fly-by-wire Control System Control Locks

CHAPTER 09 SECONDARY FLIGHT CONTROLS

The Principal Secondary Controls

Activation Methods - Light Aircraft

Activation Methods - Large Aircraft

Flap Load Relief

Flap Asymmetry Protection

Auto-slats

Flaps and Slats Alternate Operation

Speed Brakes and Spoilers

Mechanical Locking and Blow-back


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TABLE OF CONTENTS

CHAPTER 10 PNEUMATICS AND AIR CONDITIONING

Introduction

Light Aircraft Pneumatic Sources

Gas Turbine Air Supplies

The Source of Bleed Air

The Bleed Tapping

Pneumatic System Architecture

Overheat Detection and Warning

Bleed Air Controls and Indictions

Duct Construction

CHAPTER 11 CABIN CONDITIONING AND PRESSURISATION

Introduction

Air Conditioning

Air Distribution

Cabin Pressurisation

Pressurisation Control

Pressurisation System Components

Pressurisation System Controls and Indicators

The Pressurisation Schedule

Cabin Decompression


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TABLE OF CONTENTS

CHAPTER 12 ANTIICING, DEICING AND RAIN PROTECTION

Introduction

Countering the Effects of Icing

Aircraft Systems Requiring Ice Protection

Ice Detection Systems

Fluid Based Ice Protection System

Thermal Ice Protection Systems

Turbo-prop Ice Protection Systems

Windscreen Ice Protection

Rain Repellent and Rain Removal Systems

CHAPTER 13 FUEL SYSTEMS

Introduction

Fuel Tanks

Piston Engine Fuel

Light Aircraft Fuel Systems

Turbine Engine Fuel

Turbine Engine Fuel Systems

Fuel Heating

Fuel Contents Gauging

Manual Fuel Contents Checking

Fuel Flow Indicators

Fuel Jettison Refuelling


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TABLE OF CONTENTS

CHAPTER 14 FIRE AND SMOKE PROTECTION AND DETECTION

Introduction

Smoke Detection

Fire Detection and Protection Systems

Fire Detection

Fire Warning Indicators

Fire Suppression

Engine Fire Extinguishers

Cargo Compartment Extinguishers

CHAPTER 15 OXYGEN SYSTEMS

Introduction

Crew Oxygen Supply

Flight Crew Oxygen Supply

Passenger Emergency Oxygen Systems

Portable Oxygen Systems

Personal Smoke Protection

Oxygen System Safety Precautions


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TABLE OF CONTENTS


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CHAPTER 7: WHEELS, TYRES AND BRAKES

Wheel Construction

Tyre Construction

Tyre Inspection

Aquaplaning


Wheel Brakes

Light Aircraft Wheel Brakes


Heat Dissipation

Anti-skid Systems

Emergency Brakes

Parking Brake


10/40Issue 1 7.

Wheels, Tyres and Brakes

Introduction

The wheel and tyre assembly has to withstand the very large forcesgenerated on landing and support the weight of the aircraft. The

tyre absorbs some of the shock of landing and acts as the rst shock

absorbing system for all loads felt through the landing gear assembly

when manoeuvring on the ground. The integral braking system also has

to withstand the very high temperatures generated when bringing a

heavy aircraft to a halt on landing.

The tyres of heavy transport aircraft are inated to very high pressuresIf one were to fail the resulting explosion can impel pieces of the tyre a

very high speed into the aircraft structure causing signicant damage,

typically to the under wing surface and the aps.

Consequently it is vitally important that tyres are operated at the

correct pressure and the tyre assemblies are checked before every

ight for general wear and obvious signs of damage.

Well start this chapter by looking at the wheel assembly before going

on to look at tyres and braking systems.

Wheel Construction

Wheels are usually cast or forged from aluminium alloy or magnesium

alloy. There are 3 main types:

J Well based: usually tted to light aircraft with tubed tyres

J Detachable ange: to allow for easier tyre replacement.

J Split hub or divided. Allows the tyre to be mounted onto one half

of the wheel with the other half bolted to it, forming the complete

wheel assembly. This type is used on large aircraft.

07


11/40Issue 1 7.

AGK - Systems Wheels, Tyres and Brake

Modern large aircraft tyre assemblies run on tubeless tyres which allow

for a lighter wheel assembly and lower heat generation.

Tubeless tyres require sealing rings to ensure a gas tight seal and

prevent loss of nitrogen from the assembly.

The ination valve is incorporated into the wheel itself.

Figure 7.1

The split hub and detachable flange systems used on larger aircraft


12/40Issue 1 7.


Tyre Construction

Aircraft tyres comprise a exible casingwhich is constructed of rubber

coated rayon, cotton or nylon ply cords. These arewrapped around

beadsat each edge of the tyre.

The core of the bead is a series of steel wires which reinforce the tyre

and hold its circular shape.

Tread

Sidewall

Casing Plies

Bead

Steel wire core

Figure 7.2

Tubed tyre construction

The casing plies make up the strength of the tyre and comprise the tyre

carcass. The rubber tread is then moulded to the carcass to form the

complete tyre.Tyres are normally inated with nitrogen which helps to absorb

shocks, support the weight of the aircraft, maintain the tyre shape and

determine the size of the surface contact area.


13/40Issue 1 7.


The tyre is divided into the following four zones:

J The crown.This area holds the tread pattern and makes contact

with the surface.

J The shoulder.In this area the tyre thins out from crown to

sidewall.

J The sidewall. This is the weakest part of the tyre and is least able

to cope with any damage.

J The bead.This is the strong rim of the tyre which engages with the

rims on the wheel to form an airtight seal.

Figure 7.3

Regions of a tyre


14/40Issue 1 7.


Types of Tyre

Tyres are classied according to the way they are built and the method

used to inate them. Some of the key dening characteristics are:

J Ply rating. The ply rating give an indication of the tyres strength.

The higher the ply rating, the greater the strength of the tyre.

J Tread.The tread is made from rubber and provides toughness,

durability and a good gripping surface. The tread pattern forms

exible channels which expel any water between the tyre and

the ground surface. The most common tread pattern for modern

transport aircraft is the ribbed tyre.

J Tubeless.Tubeless tyres, as the same implies, have no inner tubeto contain the gas. Instead an airtight lining is vulcanised to the

underside of the beads. This forms a gas tight seal against the

wheel rim. Having no inner tube reduces weight and allows the tyre

to run cooler.

J Bias (or cross-ply). On a cross-ply design the plies are laid in

pairs and set so that the adjacent cords of adjacent plies are at 90

to one another.

J Radial. On a radial design the plies are laid from bead to bead,

approximately perpendicular to the centreline of the tyre.

J Retread. Aircraft tyres can be remoulded several times with a new

crown when the tread pattern is worn to limits. This is done by heat

bonding new rubber to the carcass.

J Tube Tyres. Tube tyres use an inner tube much like a bicycle tyre.The inner tube has an ination valve attached to it which is fed out

through a hole in the wheel. This type of tyre is usually only tted

to older aircraft types and light aircraft. A major disadvantage of the

tubed tyre is that any movement of the tyre around the wheel (tyre

creep) can cause the ination valve to shear off the tube.


15/40Issue 1 7.


Tyre Creep

The sudden acceleration of the tyre on landing can cause it to slip

around the wheel. This phenomenon is known as tyre creep.

Creep is greatest just after a new tyre has been tted, usually during

the rst ve landings. A certain amount of creep is acceptable but it has

to be monitored. Excessive creep could cause damage to the ination

valve.

The usual method is to paint a red bar across a portion of the tyre

sidewall and the wheel. This is known as a creep mark. Creep marks

should be:

J 1in wide for tyres of 24in or less in diameter

J 1.5in wide for tyres over 24in in diameter

Figure 7.4Creep

After each ight the creep mark is checked for movement. Provided

there is some overlap between the mark on the tyre and the mark on

the wheel the amount of creep is within limits. If the creep marks no

longer align the tyre must be re-set on the wheel.


16/40Issue 1 7.


Tread Patterns

Commercial transport aircraft tend to use tyres with either a ribbed

or blockedtread pattern. The tread pattern clears surface water and

provides longitudinal stability and grip.

Figure 7.5

Different types of tread

Ribbedtread patterns are most common for commercial aircraft using

concrete or tarmac runways. Blocked treads are used for all-weather

tyres on rough runways or unmade runway strips.


17/40Issue 1 7.


There are two other types of tyre design adapted for specic

purposes:

J Maarstrand tyre. The Maarstrand tyre is a single tyre used on

castoring nose wheels. It has two ridges to provide two areas of

contact with the ground. This reduces the risk of shimmying. The

tyre wear is within limits provided that there is no evidence of

surface contact in the centre section.

J Chined tyre. The chined tyre has a strip of rubber moulded onto

one or both sides of the tyre shoulder. The chine deects water

to the side of the tyre diverting it away from the centreline and

preventing excess water from being ingested by the engines.

Figure 7.6

Maarstrand tyre


18/40Issue 1 7.1


Tyre Speed Rating

Tyres are speed ratedto give a tyre limiting speed. The speed rating

must match or exceed the maximum anticipated ground rolling speed o

the aircraft. The correct rating must be used to guarantee that the tyre

can withstand the forces generated by takeoff and landing. Exceedingthe tyre limiting speed is likely to result in tyre failure.

Tyre Wear

Tyres must be inspected for signs of wear. The following limitations

must be strictly observed.

J Ribbed treads. The minimum tread depth is 2mm, measured from

bottom of the groove. The tyre may have a tyre wear marker bar.This is moulded into tyre between the treads and indicates 2mm

depth. The tyre is on its limit when the marker bar is at the same

height as the rib.

J Blocked treads. This type of tyre is within limits provided that the

block pattern is still visible.

Figure 7.7

Indications of tyre wear


19/40Issue 1 7.1


Tyre Inspection

It is extremely important that tyres are carefully inspected for damage

and that the tyre is operated at the correct pressure. Damage or

under-ination can lead to tyre failure or tread separation. Worse still

a tyre burst can result in serious damage to the aircrafts structure. In

extreme cases, as in the Concorde accident in Paris, it can result in loss

of the aircraft.

Figure 7.8

Tyre Inspection


20/40Issue 1 7.1


When inspecting tyres for damage look out for:

J Cuts.Any cuts to the cords or sidewall require the tyre to be

replaced.

J Bulges.Unusual bulges especially around sidewall areas are a sign

of impending tyre failure.

J Foreign object damage. Ensure no stones, fasteners, glass or

metal are embedded in the tyre.

J Contamination.Ensure no hydraulic oil or engine oil has been

spilled onto the tyre.

J Creep. Check that the creep marks are overlapping.

J Wear. If the tyre is worn beyond the acceptable limit it must be

replaced.

J Pressure. Over ination may lead to blow out and excess wear.

Under ination leads to tyre creep, excess wear and possible tyre

failure. Commercial aircraft tyres are normally inated with nitrogen.


21/40Issue 1 7.1


Aquaplaning

Aquaplaning is a phenomenon in which a wedge of water builds up

at the front of the tyre and, as speed increases, starts to lift the tyre

off the surface. A fully aquaplaning tyre will have no contact with the

surface and may even stop rotating altogether.

Aquaplaning may result in reduced or no braking ability, loss of

directional control and damage to the tyre from superheated steam

generated by the friction forces between water, tyre and surface.

Figure 7.9

Aquaplaning


22/40Issue 1 7.1


Aquaplaning only occurs above a certain speed, dictated by tyre

pressure. The formulae for calculating the minimumaquaplaning speed

are:

9P

where P is the tyre pressure in psi

and:

34P

where P is the tyre pressure in kg/cm2

The risk of aquaplaning can be minimised by:

J Ensuring tyre pressures and tread wear are correct.

J Using an anti-skid system.

J Avoiding ooded runways or large patches of standing water on a

runway.


If a wheel or tyre heats up excessively the heat is transferred to

the internal gas which then expands causing a dramatic rise in tyre

pressure. Eventually the pressure may rise to the point where the tyre

fails.

The primary cause of tyre overheating is braking. The very large

amounts of energy absorbed by the brake system on landing arereleased as heat. While some of this radiates away to atmosphere, a lot

of heat conducts through the brake assembly into the wheel and tyre.

A tyre burst from overheating can be very dangerous and possibly fatal

to any ground servicing staff who happen to be nearby. Consequently

each aircraft wheelis equipped with a fusible plugwhich is designed to

burst before the tyre fails.


23/40Issue 1 7.1


When the wheel assembly reaches a pre-determined temperature, the

fusible material in the plug melts allowing the pressurised air to escape

to atmosphere. Plugs come in various temperature settings and are

colour coded for identication.

Tubeless tyre

Plug

Split hub

Inflation valve

Fusible insert

Figure 7.10

Fusible plug


24/40Issue 1 7.1


Wheel Brakes

Wheel brakes produce friction at the wheel assembly to slow or stop the

rotation of the wheel. Light aircraft use a simple single disc type brake

but large transport aircraft require multiple discs to deal with the forces

generated. Most most modern transport aircraft use hydraulic power

to operate the brakes. However, the Boeing 787 uses an electrically

actuated system to reduce weight and increase braking efciency.

Light Aircraft Wheel Brakes

Simple braking systems comprise a steel disk xed to the wheel. A

brake unit or calliperis equipped with friction pads operated by an

hydraulic piston. When the brake pedals are pressed, hydraulic pressure

from foot pedal pressure transmits hydraulic pressure to the calliper

piston which squeezes the friction pads onto the disc.

Figure 7.11

Light aircraft disk brake


25/40Issue 1 7.1


Brake Wear Indication

The friction pads themselves are made from an ablatable material

which erodes with use. Eventually they wear down to the point where

they must be replaced. To check the amount of wear, some light aircraft

systems are equipped with brake wear indicators.

The brakes should be checked for wear after every ight, with the

brakes applied. A rule or special gauge may be needed measure the

gap between the disc and brake housing.

Figure 7.12

Measuring brake wear

On conventional steel disc brakes, excessive heat diminishes brakingefciency resulting in a condition known as brake fade. Brake fade

usually starts to occur at the end of a landing run. Despite standing

on the pedals the aircraft becomes very difcult to stop. Overheated

brakes may judder (chatter) when applied or make high pitched

squealing sounds. They may also cause the pads to stick to the discs

(draggingor binding) further increasing brake temperature.


26/40Issue 1 7.1



Large transport aircraft use multi disc and multi piston brakes to cope

with the much greater amounts of energy that must be absorbed.

On the multi disc system a series of pads are arranged concentricallyaround both sides of a stator assembly.

In between each pair of stators is a rotor segment. The rotor segment

is physically engaged with the wheel assembly via the torque tubeand

so rotates with the wheel.

A torque platecarries the operating pistons. When hydraulic pressure

is applied to the pistons they squeeze the entire assembly of rotors and

stators together between thepressure plateand the thrust plate.

Brake Assembly

Brake Pis

Brak

hous

Rotors and Stators

Pressure Plate

Thrust Plate

Figure 7.13

Boeing 737 brake units


27/40Issue 1 7.1


The discs are segmented and held together relatively loosely to allow

for thermal expansion. The rotors are constructed from steel alloy. The

brake pads are usually made from ceramic material.

Automatic Brake Adjuster

Automatic brake adjusters ensure correct clearance between the

rotating assemblies when the brakes are in the off position.

Figure 7.14

Automatic brake adjuster


28/40Issue 1 7.2


Brake Wear Indication

A wear indicator pin passes through brake housing. As the brake pads

wear down the pin starts to retract into the housing. A return spring

returns the indicator pin to the original position when the brakes are

selected off.

Brake wear is measured by the position of the pin. Usually the pads

must be replaced when end of the pin is ush with the adjuster

housing.

Figure 7.15

Brake wear indicator pin on a Boeing 737 showing plenty of brake pad remaining


29/40Issue 1 7.2


Carbon Braking Systems

The multi disc system is a very heavy bit of kit when built with

traditional materials so the latest generation of transport aircraft

tends to be equipped with carbon brakes. These have a number of

advantages:

J Weight. Carbon brakes are lighter, resulting in weight savings of

about 50% over steel brakes. On the Boeing 737 NG for example

the carbon brake system is 300kg lighter than the steel design.

J Performance. The carbon composite material retains its efciency

at all temperatures and at high or low speeds. It can absorb twice

as much heat as steel and so is much less prone to brake fade.

J Durability. Carbon brakes last longer; anything up to 4100

landings per overhaul. This is the number of landings each brake

can handle before the heat sinkstack of carbon disks has to be

refurbished. This represents a 30% to 175% improvement over the

service life of steel brakes.

J Economy. Although more expensive to t, carbon brakes are less

expensive than steel brakes over their service lifetime.


30/40Issue 1 7.2


Heat Dissipation

One of the biggest problems for any multi-disk system is getting rid of

the enormous amount of heat energy generated on landing.

A landing at normal speeds and weights, followed by a gentle taxito the stand with minimal braking will allow for a certain amount of

cooling, courtesy of the natural ow of air around the units and the

time available for cooling to take place. The brakes should then cool

further whilst the aircraft is on stand being prepared for ight.

The biggest problem however, comes from a high speed landing

at heavy weight followed by a short, brisk taxi to the stand. The

brakes will already be extremely hot after the landing roll and furtherapplications of brake during taxiing will add more heat energy. When

the aircraft stops on stand the lack of cooling air owing round the units

can lead to a progressive build up of heat and a brake re.

Furthermore, if the aircraft then departs the stand after a short

turnover it may begin its next take-off with the brakes already very hot

If the crew subsequently needs to reject the take-off the brakes may

become seriously overheated and fade or catch re.To deal with the heat problem large transport aircraft are often

equipped with brake fan units to articially ventilate the brake pack.

Clearly, it is very important that you monitor brake temperatures

carefully. Additionally manufacturers may specify a minimum brake

hold-over time to allow sufcient cooling between landing and the next

take-off.

ATC must be informed in the event of brake overheat. Consult the ops

manual before moving the aircraft. Avoid approaching hot brake units.

If you have to, make sure you approach either from the front or rear.

Do not allow refuelling until the brakes have cooled sufciently.


31/40Issue 1 7.2


Brake Temperature Indication

Brake temperature will be displayed either on the ECAM or EICAS

screens or on a dedicated display unit as shown below. The unit

defaults to display the hottest brake temperature. Pressing the caption

for each individual unit displays its temperature. A OVHT warning willshow if brake temperature is above the acceptable limit.

INNER

OVHT

INNER

OVHT

OUTER

OVHT

OUTER

OVHT

MAX

L MAX R

TEST

O

F

F

Figure 7.16

The brake temperature gauge by default displays the temperature of the hottest brake

Some indication systems use numbers to represent levels oftemperature. Higher numbers indicate higher temperature levels.

2 21 3

1 11 2

BRAKE TEMP

Figure 7.17

Brake temperature gauges


32/40Issue 1 7.2


Anti-Skid Systems

Maximum retardation from wheel braking is achieved when the

maximum braking force is applied to a rotating wheel without stopping

it. If the wheel locks the tyre will skid over the surface of the runway.

Skidding produces signicantly lessretardation than a properly braked

wheel.

The problem for the pilot is knowing how much pressure to apply to the

brakes. Too little pressure and he may not slow the aircraft adequately.

Too much pressure and he may lock the wheel. A locked wheel not only

produces less friction but the skid itself very quickly wears away the

tyre crown. At best this results in ruined tread requiring a new tyre.

But it might also cause the tyre to be weakened to the point where it

bursts.

The solution to this dilemma is the anti-skid system. An anti-skid

system works by monitoring wheel rotation. If a spinning wheel starts

to slow down quickly the system interprets this as an impending

skid. It then intervenes to release brake pressure and then quickly

reapply it. The process happens very quickly, several times a second,

but ultimately it ensures that, no matter how much brake force is

demanded by the pilot, the wheels never lock.

Anti-skid systems provide skid protection when braking on normal, dry

runways and on wet runways. They will also provide skid protection on

runways contaminated with snow and ice.

However, its important to understand the crucial difference between

skid protection and retardation. An anti-skid system prevents the tyrefrom skidding but it cant compensate for a general lack of friction

on the runway surface. In very slippery conditions the anti-skid will

operate continuously because every time brake pressure is reapplied

the wheel quickly slows down. The effect is very little braking action

and can end in an embarrassing encounter with the runway overrun

area.


33/40Issue 1 7.2


Mechanical Anti-Skid System

Mechanical anti-skid systems are nowadays only seen on older aircraft.

The most common is the DunlopMaxaretunit. The system comprises

an hydraulic valve regulated by a spring loaded, clutched ywheel. The

ywheel is mounted inside a drum which is tightly connected to theinside of the wheel hub. In normal operation the rotation of the wheel

causes the drum and ywheel to rotate at the same speed.

When the wheel starts to slow down, the natural inertia of the ywheel

causes it to overrun. When the position of the internal ywheel exceeds

60relative to the drum, the unit presses forward onto the hydraulic

valve which then opens to dump hydraulic pressure to the brake unit.

With brake pressure released the wheel (and the drum with it) then

speed up again. When drum and ywheel speeds once again match,

the hydraulic valve is released allowing brake pressure to be restored.

The unit becomes sensitive enough to be active above about 20kt. Once

active it is capable of operating at up to 10 times per second.

From brake control valve

Maxaret unit

To reservoir

Figure 7.18

Maxaret System


34/40Issue 1 7.2


Electronic Anti-Skid System

Electronic anti-skid control uses a small generator driven by the wheel

assembly. The electrical signal it produces varies with wheel speed. This

signal is sent to the anti-skid controller which cross-references it with

the signals being received from the other wheel units.

The controller estimates the speed of the aircraft based on the

measured wheel speed and then predicts the slip ratio based on the

measured wheel speed and estimated aircraft speed. The controller

signals the pressure control valve to momentarily release pressure to

the brake when the difference between the predicted slip ratio and the

desired slip ratio reaches a pre-determined value.

PARK BRAKE

PARKING BRK OFF

PULL & TURN

Anti

skid

control

Anti

skid

control

Anti

skid

control

Anti

skid

controlTacho Tacho Tacho Tacho

Brake

unit

Brake

unit

Brake

unit

Brake

unit

Skidcontrolvalve

Skidcontrolvalve

Skidcontrolvalve

Skidcontrolvalve

Brakecontrolvalve

Brakecontrolvalve

Inertial reference

Left

outer

wheel

Left

inner

wheel

Right

inner

wheel

Right

outer

wheel

Independent anti skid control

for all wheels

Parking

brake

valve

Anti skid

returnBrake

hydraulic

supply

Captains pedals First officers pedals

Left Right Left Right

Parking Brake:

Applies pressure to all brakes

Closes parking brake valve

Figure 7.19

Electronic anti-skid protection


35/40Issue 1 7.2


Pressure is then reapplied at a lower pressure depending on the time

interval of the skid. Brake pressure then rises steadily in search of the

maximum braking force before the next skid begins to develop.

Changeover circuits couple the valves of all brake units so that the loss

of a speed signal from one unit doesnt affect system operation.

Touchdown and Bounce Protection

Modern anti-skid systems usually incorporate touchdown and bounce

protectionfeatures.

This system prevents brake pressure being applied at the wheel before

touchdown even if the pilot inadvertently applies pressure to the brake

pedals.

If the aircraft bounces after initial touchdown the system releases any

brake pressure, allowing the wheel to spin up again before the second

touchdown.

Automatic Braking

Sophisticated anti-skid systems have automatic braking or auto-brake.

This brings the aircraft to a halt after landing, or after a rejected take-

off, with no intervention from the pilot.

A two-position three-way solenoid valve is energised after the wheels

spin-up. The valve feeds hydraulic pressure to the adaptive pressure

control valves which adjusts the amount of pressure fed to the brake

units.

The amount of pressure fed by the control units is determined by a

setting on the ight deck. The settings are either numbered or labelled.

For example 1 or MIN sets the lowest level of retardation; 3 or

MAX guarantees to have the passengers pinned against their lap

straps!

The system is automatically overridden if the pilot manually applies the

brakes or sets the power levers to TOGA (take-off or go-around).


36/40Issue 1 7.2


The system requires a number of prerequisites before it can be armed

(meaning that the system is ready to operate):

J Auto-brake must be selected ON and a deceleration level selected.

J The anti-skid system must be on and serviceable.

J The power levers must be set below a certain value.

J The hydraulic system must be functional.

J The brake pedals must not be depressed.

1

2

3

MAXOFF

RTO

AUTO BRAKE

ANTI-SKID

ANTI SKID

INOP

ANTI BRAKE

DISARM

Figure 7.20

Boeing 737 Auto Brake

The system operates when the aircraft is on the ground and the wheels

have spun-up.


37/40Issue 1 7.2


Anti-Skid Inoperative

Anti-skid braking systems greatly enhance overall braking performance

so much so that if the system is lost the required landing distance can

double, particularly on contaminated runways.

An amber warning caption illuminates if a fault is detected in the

anti-skid system. Faulty anti-skid carries a number of operational

considerations including the type and length of runway that you are

permitted to land on.

HYD BRAKE

PRESS

3

2

1

0

4

PSI X 1000

SPEEDBRAKE

12

10

8

6

4

2

012

10

8

6

4

2

0

12

10

8

6

4

2

012

10

8

6

4

2

0

MAN SET

FF/FUx 1000

% RPM

N2

EGTC

% RPMN1

PULLTO

SETN1

PULLTO

SETN1

PPH

PUSHLB FUELUSED

RESET

100

500

100

500

100200

0

100200

0

02

34

5

1 02

34

5

1

43

21

0

43

21

0

TAT C

ENGOIL

PRESS

OILTEMP

OIL QTY

VIB

HYDPRESS

QTY

PSI

C

% FULL

PSIx 1000

% FULL

RF 88%

A B

1

2 3

10

15

25

3040

UP

FLAPS

RL

YAW DAMPER

1

2

3

MAXOFF

RTO

AUTO BRAKE

ON

OFF

ANTI-SKID

ANTI SKIDINOP

ANTI BRAKE

DISARM

1 2

1 2

LANDING GEAR

LIMIT (IAS)

FLAPS LIMIT (IAS)

OPERATING

EXTEND 270K-82MRETRACT 235KEXTEND 320K-82M

1-230K2-230K5-225K10-210K

15-195K25-190K30-185K40-158K

230K ALT FLAP EXT

L

A

N

D

I

NG

G

E

A

R

UP

OFF

DN

LE FLAPS

TRANSIT

LE FLAPS

EXT

REVERSER

UNLOCKED

REVERSER

UNLOCKEDA/T LIM

STARTVALVE OPEN

LOW OIL

PRESSURE

OIL FILTER

BYPASS

STARTVALVE OPEN

LOW OIL

PRESSURE

OIL FILTER

BYPASS NOSE

GEAR

NOSE

GEAR

LEFT

GEAR

RIGHT

GEAR

LEFT

GEAR

RIGHT

GEAR

Autobrake and Anti-sk

Controls

Gear and Flap

Limits Placard

Gear Selector

Brake

Pressure

Gauge

Figure 7.21

Location of gear and brake controls on a Boeing 737


38/40Issue 1 7.3


Auto-Retract or Flight Brake

Some aircraft, are equipped with an auto-braking system for the main

wheels. On retraction the wheels are automatically braked to a halt

before they enter the wheel wells.

Emergency Braking Systems

Commercial transport aircraft will always be equipped with an alternate

hydraulic source for braking. The example system shown here provides

separate hydraulic supplies to the brakes from the Yellow and Green

systems.

EDPeng 1

EDPeng 1

Elecpump

Elecpump

Handpump

EDPeng 1 RAT

Elecpump

EDPeng 2

PTUpump

PTUmotor

PTUmotor

PTUpump

Brakes

Landinggear

Cargodoors

High lift devices

Flying controls

P

P

P

P

BlueReservoir

GreenReservoir

YellowReservoir

PTU PTU

RAT

EDP

PTU

= Ram Air Turbine

= Engine Driven Pump

= Power Transfer Unit

= Non return valve

= Priority valve

= Control valve

Figure 7.22

Duplicated hydraulic supplies to the braking system

As a last resort a brake accumulator provides up to six applications of

brake should all hydraulic pressure be lost.


39/40Issue 1 7.3


The Parking Brake

The parking brake allows brake pressure to the wheel brakes to be

applied and held applied.

The parking brake is usually set using a lever. On some systems theparking brake lever must be applied simultaneously with the toe brakes

to engage the system.

When the parking brake is set to on it overrides all other braking

systems including anti-skid and touch-down protection. For this reason

you must neverset the parking brake on in ight.

Furthermore, the parking brake should not be set on if your brakes are

excessively hot (more than 500C on the Boeing 737). Applying parkingbrake in these circumstances increase the risk of the rotors welding to

the stators.


40/40


0

5

10

15

APL

NOSEUP

STAB

TRIMAPL

NOSE

DOWN

T

AKE-OFF

0

5

10

15APL

NOSE

UP

STAB

TRIM

APL

NOSE

DOWN

TAKE-OF

F

STAB TRIM

NORMALMAIN

ELEC

AUTO

PILOT

CUT

OUT

PARKING

BRAKE

PULL

FLAP

SPEEDBRAKE

FLIGHT

DETENT

UP

ARMED

DOWN

FLAP

UP

0

1

2

5

15

10

25

30

40

HORN

CUTOUT

FLAP

DOWN

Parking Brake

Lever

Parking Brake

Warning LightLanding Gear

Warning Horn

Cutout Switch

Figure 7.23

Location of parking brake lever

Aircraft Systems - Chapter 07

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Transcript of Aircraft Systems - Chapter 07