Aircraft Systems - Chapter 07
Aircraft Systems - Chapter 07
Aircraft Systems - Chapter 07
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 Airframe Components
Material Attachment Methods
Construction Principles
The Fuselage
The Pressure Hull
The Wings
Torsional Stresses and Flutter
The Empennage
TABLE OF CONTENTS
TABLE OF CONTENTS
TABLE OF CONTENTS
TABLE OF CONTENTS
TABLE OF CONTENTS
TABLE OF CONTENTS
Oxygen Systems
Introduction
Crew Oxygen Supply
Flight Crew Oxygen Supply
Passenger Emergency Oxygen Systems
Portable Oxygen Systems
Personal Smoke Protection
Oxygen System Safety Precautions
TABLE OF CONTENTS
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
07
The wheel and tyre assembly has to withstand the very large forces
generated on landing and support the weight of the aircraft. The
tyre absorbs some of the shock of landing and acts as the first 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 inflated to very high pressures.
If one were to fail the resulting explosion can impel pieces of the tyre at
very high speed into the aircraft structure causing significant damage,
typically to the under wing surface and the flaps.
Consequently it is vitally important that tyres are operated at the
correct pressure and the tyre assemblies are checked before every
flight 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:
JJ
JJ
JJ
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.
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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 inflation valve is incorporated into the wheel itself.
Figure 7.1
The split hub and detachable flange systems used on larger aircraft
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Tyre Construction
Aircraft tyres comprise a flexible casing which is constructed of rubber
coated rayon, cotton or nylon ply cords. These are wrapped around
beads at 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.
Sidewall
Tread
Casing Plies
Bead
Steel wire core
Figure 7.2
Tubed tyre construction
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JJ
JJ
JJ
The crown. This area holds the tread pattern and makes contact
with the surface.
The shoulder. In this area the tyre thins out from crown to
sidewall.
The sidewall. This is the weakest part of the tyre and is least able
to cope with any damage.
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
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Types of Tyre
Tyres are classified according to the way they are built and the method
used to inflate them. Some of the key defining characteristics are:
JJ
JJ
JJ
JJ
JJ
JJ
JJ
Ply rating. The ply rating give an indication of the tyres strength.
The higher the ply rating, the greater the strength of the tyre.
Tread. The tread is made from rubber and provides toughness,
durability and a good gripping surface. The tread pattern forms
flexible 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.
Tubeless. Tubeless tyres, as the same implies, have no inner tube
to 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.
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.
Radial. On a radial design the plies are laid from bead to bead,
approximately perpendicular to the centreline of the tyre.
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.
Tube Tyres. Tube tyres use an inner tube much like a bicycle tyre.
The inner tube has an inflation valve attached to it which is fed out
through a hole in the wheel. This type of tyre is usually only fitted
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 inflation valve to shear off the tube.
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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 fitted, usually during
the first five landings. A certain amount of creep is acceptable but it has
to be monitored. Excessive creep could cause damage to the inflation
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:
JJ
JJ
Figure 7.4
Creep
After each flight 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.
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Tread Patterns
Commercial transport aircraft tend to use tyres with either a ribbed
or blocked tread pattern. The tread pattern clears surface water and
provides longitudinal stability and grip.
Figure 7.5
Different types of tread
Ribbed tread 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.
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There are two other types of tyre design adapted for specific
purposes:
JJ
JJ
Figure 7.6
Maarstrand tyre
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JJ
Figure 7.7
Indications of tyre wear
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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-inflation 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
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JJ
JJ
JJ
JJ
JJ
JJ
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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
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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:
JJ
JJ
JJ
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Tubeless tyre
Plug
Fusible insert
Split hub
Inflation valve
Figure 7.10
Fusible plug
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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 efficiency.
Figure 7.11
Light aircraft disk brake
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Figure 7.12
Measuring brake wear
7.17
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Brake Piston
Brake
housing
Brake Assembly
Thrust Plate
Pressure Plate
Figure 7.13
Boeing 737 brake units
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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
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Figure 7.15
Brake wear indicator pin on a Boeing 737 showing plenty of brake pad remaining
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JJ
JJ
JJ
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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 taxi
to the stand with minimal braking will allow for a certain amount of
cooling, courtesy of the natural flow 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 flight.
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 further
applications of brake during taxiing will add more heat energy. When
the aircraft stops on stand the lack of cooling air flowing round the units
can lead to a progressive build up of heat and a brake fire.
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 fire.
To deal with the heat problem large transport aircraft are often
equipped with brake fan units to artificially 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 sufficient 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 sufficiently.
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OFF
TEST
L
OVHT
OVHT
OUTER
INNER
MAX
MAX
R
OVHT
OVHT
INNER
OUTER
Figure 7.16
The brake temperature gauge
by defaultTemperature
displays the temperatureGauge
of the hottest brake
Brake
Some indication
systems
use
numbers
toorrepresent
levels of
Displays the
temperature
of the
hottest brake,
any other
brake
as
selected
by
pushing
the
respective
OVHT
button.
temperature. Higher numbers indicate higher temperature levels.
BRAKE TEMP
Figure 7.17
Brake temperature gauges
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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 significantly less retardation 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 tyre
from 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.
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To reservoir
Maxaret unit
Figure 7.18
Maxaret System
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PARKING BRK
Captains pedals
Left
OFF
Right
PARK BRAKE
Parking Brake:
Applies pressure to all brakes
Closes parking brake valve
Anti skid
return
Brake
control
valve
Anti
Left
skid
outer
control wheel
Skid
control
valve
Brake
unit
Brake
unit
Tacho
Tacho
Left
Right
Brake
hydraulic
supply
Parking
brake
valve
Skid
control
valve
Brake
control
valve
Anti
Left
skid
inner
wheel control
Anti
Right
skid
inner
control wheel
Skid
control
valve
Skid
control
valve
Brake
unit
Brake
unit
Tacho
Tacho
Anti
Right
skid
outer
wheel control
Inertial reference
Figure 7.19
Electronic anti-skid protection
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JJ
JJ
JJ
JJ
AUTO BRAKE
ANTI BRAKE
DISARM
2
1
OFF
3
MAX
RTO
ANTI-SKID
ANTI SKID
INOP
Figure 7.20
Boeing 737 Auto Brake
The system operates when the aircraft is on the ground and the wheels
have spun-up.
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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.
YAW DAMPER
REVERSER
UNLOCKED
START
VALVE OPEN
LOW OIL
PRESSURE
REVERSER
UNLOCKED
A/T LIM
Brake
Pressure
Gauge
START
VALVE OPEN
OIL FILTER
BYPASS
LOW OIL
PRESSURE
NOSE
GEAR
OIL FILTER
BYPASS
15
UP
C
RL
TAT
FLAPS
40
ENG
OIL
PRESS
MAN SET
NOSE
GEAR
10
25
30
LEFT
GEAR
RIGHT
GEAR
LEFT
GEAR
RIGHT
GEAR
SPEED
BRAKE
PSI
N1
100
% RPM
50
0
12
10
12
10
LE FLAPS
TRANSIT
LE FLAPS
EXT
OIL
TEMP
LANDING GEAR
2
4
100
50
100
EGT
200
100
ANTI-SKID
200
ANTI SKID
INOP
% FULL
ON
VIB
3
N2
4
1
2
PSI X 1000
4
0
OFF
OIL QTY
% RPM
3
HYD BRAKE
PRESS
UP
4
1
OFF
Gear Selector
DN
AUTO BRAKE
A
HYD
PRESS
ANTI BRAKE
DISARM
PSI
x 1000
FF/FU
10
PULL
TO
SET
N1
x 1000
12
10
2
8
2 1 0
12
2 1 0
2
8
PULL
TO
SET
N1
QTY
% FULL
RF 88%
1
OFF
RTO
3
MAX
LANDING GEAR
LIMIT (IAS)
OPERATING
EXTEND 270K-82M
RETRACT 235K
EXTEND 320K-82M
15-195K
25-190K
30-185K
40-158K
PUSH
LB
FUEL USED
RESET
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EDP
eng 1
PTU
pump
PTU
motor
Green
Reservoir
EDP
eng 1
Elec
pump
Elec
pump
Yellow
Reservoir
EDP
eng 2
PTU
motor
PTU
PTU
pump
Elec
pump
Hand
pump
EDP
eng 1
RAT
PTU
Cargo
doors
Brakes
P
P
RAT = Ram Air Turbine
EDP= Engine Driven Pump
PTU = Power Transfer Unit
= Non return valve
P
Landing
gear
= Priority valve
= Control valve
Figure 7.22
Duplicated hydraulic supplies to the braking system
7.30
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D
EE
SP
E
AK
BR
DOWN
FLAP
ARMED
FLAP
UP
0
1
FLIGHT
DETENT
STAB
TRIM
APL
NOSE
DOWN
UP
TAKE-OFF
STAB
TRIM
TAKE-OFF
APL
NOSE
DOWN
10
5
10
10
15
15
15
APL
NOSE UP
APL
NOSE
UP
25
30
HORN
CUTOUT
PARKING
BRAKE
40
FLAP
DOWN
PULL
STAB TRIM
MAIN
ELEC
NORMAL
AUTO
PILOT
CUT
OUT
Parking Brake
Lever
Parking Brake
Warning Light
Landing Gear
Warning Horn
Cutout Switch
Figure 7.23
Location of parking brake lever
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