021 04-00-00 Landing Gear Amendment 1

8/9/2019 021 04-00-00 Landing Gear Amendment 1

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TEXTBOOK

Landing Gear

020 00 00 00 AIRCRAFT GENERAL KNOWLEDGE

021 04 00 00 LANDING GEAR

RHLH

ELEVATOR

AILERON

TRIM

AIL

RUD

GND

12

20

32

12 10

GND

NU

ND

E

L

E

V

ROLL

FLAPS

ROLL

RUDDER

° °

SYSTEM 1 / 3

FLIGHT

CONTROL

HYDR FUEL NEXTENGINE

RNG



Landing Gear


Landing Gear Systems

The landing gear of a fixed wing aircraft supports the weight of the aircraft while it is on

the ground and is made up of the main and auxiliary landing gear. The main gear

provides the principal support. The auxiliary gear is in the form of a nose or tail wheel

installation, almost invariably the former, especially in large aircraft.

The landing gear contains shock absorbers to withstand the landing forces and the

effects of taxiing over uneven surfaces, and brakes to stop and assist in controlling the

aircraft on the ground. Most modern aircraft with tricycle (nose-wheel) layouts are

equipped with nose-wheel steering for ground manoeuvring, some very large aircraft also

have main wheel steering (known as body steering). In aircraft designed for flight at highspeeds/altitudes the landing gear is retracted into the fuselage or wings in flight, to reduce

profile drag. In low performance aircraft this is less important and the landing gear is often

fixed in the extended configuration. Most civil transport aircraft are equipped with a

double nose wheel landing gear arrangement. Main gears of smaller and medium weight

civil transport aircraft are equipped with two wheels.

The advantages of tricycle landing gear during ground Operation are:

No risk of the aircraft `nosing over' during hard braking.

Better visibility for the pilot during taxiing.

Less likelihood of ground looping during take-off or landing in cross-wind conditions.



Landing Gear


Many aircraft are equipped with a tricycle gear arrangement to achieve a stable position

on ground.

The visibility for the pilot during landing and taxiing varies with the gear location.

TAIL WHEEL

NOSE WHEEL

Apart from the advantage of increasedstability the advantages of a nosewheel tricycle landing gear

arrangement include easier loadingwith a level floor line; improved pilotvisibility making landing and taxiingeasier; aerodynamic drag on take offis reduced



Landing Gear


Landing devices may be simple or complex in construction, depending on the weight and

the category of aircraft. Any type of landing device is an airframe component. Complex

landing devices may even constitute an airframe sub-system.

Basically there are 5 main types of landing gear according to the surface from which the

aircraft is designed to operate. These types are:

- wheel-type landing gear

- float-type gear

- ski-type landing gear

- skid-type landing gear

- flotation-type gear.

Various configurations for landing gears have been adopted in the past, but many of them

where designed for special purposes.

Wheel-type Landing Gear

A wheel-type landing gear is fitted to fixed-wing and rotary-winged aircraft which are

designed to operate from hard surfaces.

Note: A ’hard surface’ may be a conventional runway or the deck of a ship.

Float-type Gear

Float-type gear may be found on fixed-wing (e.g. sea planes) or rotary-wing aircraft which

are designed to operate from the surfaces of rivers, lakes or the sea.

Ski-type Landing Gear

Ski-type landing gear is fitted to fixed-wing land planes to enable operation from surfaces

covered with ice or snow. This type of gear is normally fitted in addition to a wheel-type

gear.

Skid-type Landing Gear

Skid-type landing gear is commonly fitted to helicopters to enable operation from almost

any type of surface except water.

Flotation-type Gear

Flotation-type gear is sometimes fitted to helicopters in addition to the skids. The floats

are inflatable to enable landing on water.



Landing Gear


Landing gear Classification

Wheel-type landing gear may be classified into 2 main types:- fixed gear

- retractable gear.

Fixed Gear

The term ’fixed gear’ applies to landing gear which remains, by design, in a fixed position

to the fuselage/wing structure of an aircraft throughout the aircraft operation, whether on

ground or in flight. This type of gear is commonly fitted to light single- or twin-engined

fixed-wing aircraft designed for low airspeeds and to many types of helicopter.

Fixed landing gears are often equipped with cowlings or fairings to reduce the drag. They

may be externally braced or of the cantilever type without bracings.

The parasite drag will differ between an aircraft with a retractable landing gear and an

aircraft equipped with a fixed gear.

The advantages of this type of gear are the simplicity of design, its structural stiffness and

ease of maintenance.



Landing Gear


A fixed gear unit is made up of a main strut for taking the major loads, a fork and axle,

and a wheel. The main strut may be designed as:

- tubular spring-steel strut

- flat spring-steel strut- spring oleo strut

- oleo-pneumatic strut.

The first 3 types are fitted to light single-engined aircraft.

The oleo-pneumatic type of strut is widely used on light single- and multi-engined aircraft.

It consists of an upper strut containing hydraulic fluid and nitrogen and a lower telescopic

strut. The upper and lower struts are interconnected by a torque link. The axle is

sometimes fitted to the lowest end of the telescopic strut on a fork. Single wheels

containing brake units are normally fitted to the axles.

Both, the spring oleo strut and oleo/pneumatic strut (torque links) are used on rotary-wing

aircraft.



Landing Gear


Retractable Gear

The term ’retractable gear’ is applied to landing gear which is designed for retraction (and

stowage) in wing or fuselage compartments during flight. This type of gear is fitted to allhigh-performance fixed- and rotary-wing aircraft.

The advantage of this type of gear is the total elimination of drag caused by the gear

during climb-out and cruising phases of the flight. The climb/cruise and maximum speeds

of the aircraft will be higher for a given power/thrust rating of the engine(s).

Retractable landing gear is made up of an oleo-pneumatic shock strut, one or more

actuators and one or more axles to which one or more wheels may be mounted.

Tail-wheel/Nose-wheel Arrangements

Additional to the classification of wheel-type landing gear into ’fixed’ and ’retractable’,

such gear may be divided into ’tail-wheel’ and ’nose-wheel’ landing gear.

Tail-wheel Landing Gear

Although this type of landing gear was the conventional landing gear during the first 3

decades of aviation, it must now be regarded as obsolete for most designs.

The main wheels are located at each side of the centreline ahead of the centre of gravity.

A steerable tail wheel is located at the aft near the rudder (originally, a fixed tail skid was

used).

The advantages of the tail-wheel landing gear are as follows:

The tail wheel is:

- small,

- light in weight and

- simple in design.

- The attachment of the main-gear legs to the wings can easily be accomplished.

- A so-called 3-point landing can be carried out at high angles of attack. The

aerodynamic drag provides a retarding force, which is particularly needed when

the brakes can not be fully applied, e.g. on wet grass or on ice.

- When brakes are applied, the vertical load on the main gear increases.

- This reduces the risk of skidding.



Landing Gear


The following disadvantages are the reason why the tail-wheel landing gear has almost

completely been replaced by the nose-wheel gear:

- Violent braking tends to tip the airplane onto its nose.

- The braking force acts ahead of the centre of gravity. It thus has a destabilizingeffect when the airplane is moving at a certain angle of yaw relative to its track.

This may cause a ground loop.

- During a so-called 2-point landing a ’tail down’ moment is created by the impact

force on the main landing gear. This results in an increase of lift which will cause

the airplane to bounce.

- On the ground, the attitude of the wings makes taxiing difficult under strong

winds.

- The steep ground angle inclines the cabin floor. In case of airliners this is

uncomfortable for passengers. Additionally, loading/unloading operations are

inconvenient.

- In the ’tail down’ attitude the inclination of the fuselage will limit the pilot’s view

over the nose of the airplane,

- During the initial take-off run the drag is high until the tail can be raised.

Nose-wheel Landing Gear

The main wheels of a nose-wheel (or: tricycle) landing gear are located at each side of

the centreline behind the centre of gravity. A free-swivel or steer-able nose wheel is

mounted on the centreline in the forward section of the fuselage (in many smaller

airplanes just behind the propeller).

The advantages and disadvantages of the nose-wheel gear are roughly the opposite of

those for the tail-wheel type. The principal advantages are as follows:

- The breaking forces act behind the centre of gravity.

- They have a stabilizing effect, thus enabling the pilot to make full use of the

brakes.

- On the ground, the fuselage (and consequently the cabin floor) of the aircraft are

(almost) levelled.

- The pilot’s view is good.

- The nose wheel is a safeguard against turning over (protection of the

propeller(s)).

- During the initial part of the take-off the drag is low.

- During a 2-point landing the main gear creates a positive pitching moment.

Because of the steady increase in landing speeds of modern airplanes these advantages

have compensated the following disadvantages by far:

The nose-wheel unit must take 20 % to 30 % of the airplane’s weight in a ’steady braking’

condition. Therefore, it must be comparatively heavy.



Landing Gear


The landing gear must be fitted at a position where special structural provisions have

been made.

In the case of retractable nose-wheel gears on light airplanes, it may be difficult to find

any stowage space at all in the compartment.

Retractable Wheel-type Landing Gear / Operation

Main Landing Gears

Retractable landing gears were developed to eliminate as much as possible the drag

caused by the exposure of the landing gear to airflow during flight. Usually, the landing

gear is completely retractable. There are airplanes in which a portion of the wheels is still

exposed to the airstream after the gear is retracted (semi-retractable landing gear).

There are almost as many different gear retraction systems as there are different airplane

designs. The direction of retraction may be fore or aft, sideways or a combination of

either. The main wheels are retracted inboard whenever possible, because outboard

retraction places the weight of the gear further out in the wings. This, in turn, will increase

the wing lateral mass moment (known as the ’lateral inertia’). The increase of the lateral

inertia must be compensated by larger and more powerful ailerons to maintain a specified

rate of roll or acceptance of a reduced rolling rate for a given wing configuration.

Manual retraction used on smaller airplanes is the simplest type of retraction system.

Manual retraction can be accomplished by direct hand lever action or by use of a

hydraulic hand pump. No alternate (emergency) retraction method is required with

manual systems.

The landing gears of large airplanes are power-operated. This is normally accomplished

with hydraulic power, sometimes with electric or pneumatic power.

In addition to the normal operating system, it is necessary to provide emergency systems

to ensure that the landing gear can be lowered if the primary system fails. Emergency

systems consist of stored air or gas which can be directed into actuating cylinders or

mechanical systems that can be operated manually.

The operation of the landing gear doors is made by linkages with the gear struts or

braces so that the doors follow the gear extension and retraction. Mechanical locks

engage the landing gear automatically in the ’down’ and ’up’ positions to ensure that the

gear will hold its position.

Landing gear systems are normally provided with safety switches or locks which make it

impossible to retract the gear when the airplane is on the ground.

General, onmodern jettransportaircraft, thelanding gearcan beextendedmechanically incase of acompletehydraulicsystem failure.



Landing Gear


Most retraction mechanisms are derived from the 4-bar linkage. A suitable pivot point is

required for the leg. At the same time, the linkage provides the required wheel position

and allows adequate length of the leg. The mechanical parts of the retraction system

must be designed in such a way that minimum space is required when the landing gear isretracted.

Note: Due to the fact that the majority of modern aircraft have a retractable landing gear

with nose wheel, only this type will be discussed more detailed in the this Lesson.

Retractable Main Landing Gear wi th Single Wheel

The main landing gear (MLG) of the ’direct’ type includes an oleo-pneumatic shock

absorber for each leg. It supports an aluminium alloy wheel (hub) with a tubed tyre and a

hydraulic disk-brake.

The inner cylinder of the shock absorber is prevented from rotating inside the outer

cylinder by torque links.



Landing Gear


As for the auxiliary leg, each main leg is actuated and locked by a slanted-strut hydraulic

jack.

The tricycle landing gear improves the performance envelope of the aircraft because of

the following reasons:- The auxiliary leg is mounted laterally within the front box structure of the

fuselage. It is retracted rearwards.

- Both main legs are mounted on the fuselage. They are retracted rearwards into

appropriate fairings.

Retractable Main Landing Gear with Double Wheels

The main landing gear with double wheels has 2

twin wheels with tubeless tyres and multiple disk

brakes. Such gears are usually of the ’direct’

type as well. This means, they have a built-in

oleo-pneumatic shock absorber. The bracing is

provided by 2 struts.

Operation of the gear, i.e. extending and

retracting, is by means of a double-acting

hydraulic actuating cylinder. The ’landing gear

up’ and ’landing gear down’ locking is

mechanical by means of spring boxes.

Unlocking of the gear from the ’up’ and ’down’

positions is done by means of single-acting

hydraulic actuating cylinders.

Usually, the 2 main landing gears of a civil

airliner are mounted laterally at the wings.

Many main landing gears retract towards the

nose into the engine nacelle under the

combined action of struts and a hydraulically

controlled actuating cylinder.

A mainlanding gearis said to belocked downwhen thestrut islocked by anover-centremechanism



Landing Gear


Retractable Main Landing Gear with Trailing Arm

The main landing gear shown here is of ’trailing arm’ type. It has an independent oleo-

pneumatic shock absorber. It is equipped with twin wheels with tubeless tyres andmultiple disk brakes. The bracing is provided by 2 struts.

Side loads imposed on an undercarriage unit on loading are absorbed by the side load

strut or link.

The gear is operated by means of a double-acting hydraulic cylinder.

Locking the gears is done mechanically:

- locking in the ’up’ position by means of an up-lock box,

- locking in the ’down’ position by 2 spring boxes.

The type of brakeunit found on mosttransportaeroplanes is amultiple disk brake

To prevent thelanding gear fromcollapsing whenthe aircraft isparked on theground, lockingpins wi th flags areused



Landing Gear


The gear is unlocked from the ’up’ and ’down’ positions by means of single-acting

hydraulic cylinders.

During emergency operation, an assisting actuator allows extension of the gear down to

the ’locked’ position.



Landing Gear


Nose Landing Gears

Here you can see an example of a retractable nose landing gear (NLG).

The operation of these gears is similar to that of main gears. The gears move in a straight

line in fore and aft direction.



Landing Gear


Important for the operation of the nose gear are the torque links and the shimmy damper.

The torque links hold the nose wheel aligned.

Torque links come under most stress when tight turns are made during taxiing

The shimmy damper is a hydraulic snubbing unit which reduces the tendency of the

wheel to oscillate to both sides.

Shimmy dampers are usually designed as piston or vane types. Both may be modified to

provide power steering as well as shimmy damper action.

NLG Centring Device

Nose gear shock struts are provided with an upper locating cam attached to the upper

cylinder and a mating lower locating cam attached to the lower cylinder. These cams line

up the wheel-and-axle assembly in the ’straight ahead’ position when the shock strut is

fully extended. This prevents the nose wheel from being cocked to one side when the

nose gear is retracted. Possible structural damage to the aircraft is prevented.

The mating cams also keep the nose

wheel in the ’straight ahead’ position

prior to landing when the strut is fully

extended.

Nose wheel shimmy may be described as a possible damagingvibration of the nose wheel when moving on the ground

Landing gear torque links are used to prevent rotation of thelanding gear piston in the oleo strut



Landing Gear


Generally, nose gear struts are equipped with a locking (or disconnect) pin to enable full

turning of the aircraft when it is on the ground or in the hangar. Disengagement of this pin

will allow the wheel fork spindle to rotate 360°, thus enabling the aircraft to be turned in a

very small area.Nose (as well as main gear) shock struts are usually provided with jacking points and

towing lugs. The jacks should always be placed under the correct points. When towing

lugs are provided, the towing bar should be attached only to these lugs.

Energy Absorption

Various methods have been used to absorb the kinetic energy of the airplane when

touching down (i.e. to absorb the so-called ’landing shocks’).

Shock Cord

A shock cord is a simple and cheap way of controlling wheel deflection to absorb vertical

velocity. The energy is absorbed by the friction developed between elastic components.

Axle displacement permitted by the shock cord plus tyre deflection reduces vertical

impact velocity. This means that the shock cord itself does not absorb any impact energy.



Landing Gear


Rubber Disc

The principle of rubber discs is similar to that of shock cords. They absorb some energy

by developing internal friction (by compressing the rubber block inside). Because of the

contact between the rubber block and the wall of the container, surface friction may bedeveloped as well.



Landing Gear


Oleo-Pneumatic Shock Struts

The landing gear shock struts are designed to absorb the shock loads of landing and

taxiing over uneven ground, preventing them from being transmitted to the airframe. The

upper (outer) cylinder of the strut is attached to the airframe and contains a lower (inner)

cylinder which is free to slide up or down (and rotate in the case of the nose strut) within

the outer cylinder. The cylinders are partially filled with oil and pressurised with

compressed air or nitrogen. This compressed gas absorbs the shocks of normal taxiing

and balances the weight of the aircraft when it is stationary on the ground, so that the

inner cylinder takes up an approximate mid-stroke position.

The purpose of the oil and the nitrogen in an oleo-pneumatic strut are:

- the oil supplies the damping function and

- the nitrogen supplies the spring function

Under the increased shock of landing the inner cylinder moves up, shortening the strut

length. To prevent excessive upward movement, transference of oil from lower to upper

cylinder is progressively restricted by either a metering pin or a snubber valve. As the

volume of the gas space in the upper cylinder decreases with the upward movement, thegas pressure increases to Balance the upward force.

Nitrogen and

a viscousliquid are thedampingelement(s) ina landinggear shockabsorberused onlarger aircraft.



Landing Gear


To prevent aircraft bounce on landing, the shock absorber damping on rebound is greater

than the damping on compression.

It is important that the struts are inflated to the correct gas pressure. If the pressure is too

high the shock absorption is reduced, if the pressure is too low the strut extension will be

inadequate, leading to `bottoming' and complete loss of shock absorption under shock

loading. Checking the strut gas pressures is a job for an engineer, however a check of the

amount of the inner cylinder which is visible (the amount of extension) is a good indication

to the pilot on a walk around inspection that the gas pressure is approximately right.

Tables or graphs may be available which enable the pilot to determine the appropriate

extension for a given weight and loading configuration.

Landing Gear BracingDuring take-off, landing and taxiing, the landing gear is subject to considerable side-for/aft

loads. To prevent damage, breaking or possible collapsing of the leg, additional support is

provided by fitting side Ioad struts/stays and drag struts to the gear assembly. Torsion

links maintain the wheel alignment with the longitudinal axis of the aircraft. They join the

inner and outer cylinders but allow the shock strut to compress for damping.

Main Landing Gear Wheel Configuration

A scissor is a component found on landing gears. Its function is toprevent any rotation of the oleo strut in the undercarriage shock

absorber.

During a walk round pre-flight inspection, a main oleo leg is observed tobe lower than the other leg with no sign of leakage and the aircraft is

parked on a plan level you shou ld check the load extension requirementand have the leg charged with air.

Heavy aircraft need to spread the

weight of the aircraft over a wide

area to achieve an acceptable

pavement loading. The four basic

configurations of landing gear

wheel arrangement are single,

double, tandem and bogie.



Landing Gear


Lowering the gear using the free fall system will result in the main landing gear doors

remaining open.

Mechanically Driven Doors

An overheat / fire warning may be provided in the Wheel / Undercarriage bay.



Landing Gear


Gear Retraction

When the gear retracts, the movement of the NLG leg is transmitted to the rear door by a

push-pull rod (rear-door rod) and to the split-lever assembly by a second push-pull rod

(front-door rod).The rear-door rod acts directly on the rear door which closes as soon as the NLG leg is

retracted. The front-door rod is the input rod to the split-lever assembly. It acts indirectly

on the front door through the assembly.

The split lever provides a link between the NLG leg and the front door. Actually, it is a

bellcrank whose input and output cranks can be manually separated.

During initial movement, the NLG leg drives the split lever input rod forward. At the same

time, it drives the output crank of the bellcrank down. The front-door rod is pushed down

and the door is opened.

When the NLG leg has moved through approx. 45° the dead point of the split lever input

rod’s travel is reached and the bellcrank reverses its direction of movement. The output

crank now starts to pull on the door. When the gear is fully retracted the door is fully

closed. In this situation, it is pressed firmly against a fuselage stop.



Landing Gear


Hydraulically Operated Doors

The inner main landing gear door forms a separate sub-system of the landing gear

extension and retraction system together with the door actuator and the door up-lock. Thedoor is controlled from the gear control unit according to the position of the landing gear.

The door is closed when the gear is down and locked. The door can be opened

mechanically for ground maintenance.

Extension and Retraction

Raising and lowering the retractable landing gear of an aircraft is, almost invariably,

achieved by hydraulic systems. Since the landing gear is essential to safe landing of the

aircraft, it is of vital importance that there should be an alternate means of extending the

gear in the event of hydraulic system failure. In many aircraft an emergency pneumatic

system can be selected to actuate the landing gear. An alternative method of lowering the

landing gear in an emergency is a gravity extension or free-fall system. In this type of

system, hydraulic pressure to the retraction system is shut off, the raise and lower lines

are open to return and the gear and door up-locks are mechanically released. The gear

extends under the influence of gravity and the down locks are engaged by spring action.

A selector valve directs hydraulic system pressure to the appropriate side of the operating jacks (up or down) - at the same time connecting the other side of the jacks to the

reservoir return line.

A back-up tothe normallanding gearextensionsystem maybe a highpressure airfrom airstoragebottles

If there is afluid leakage

from a jackan oil sealhas failed



Landing Gear


Up selection

The selector valve spool moves to the right, directing hydraulic pressure to the up lines,

releasing the main landing gear (MLG) and nose landing gear (NLG) down locks.

Pressure is also applied to the MLG and NLG operating jacks to retract the gears. As theNLG reaches the fully retracted Position the spring-loaded NLG up lock engages with a

spigot on the torsion links. As the MLG reaches the fully-retracted position the spring-

loaded MLG up lock engages the detent on the MLG shock strut and a pinsle on the main

gear opens the sequence valve (SV1). This directs up line pressure to the inner door jack,

which operates to close the inner door.

Down selection

The selector valve spool moves to the left, directing hydraulic pressure to the down lines,

releasing the NLG up lock and pressurising the NLG operating jack to extend the nose

gear. Before the main gear is extended the inner doors must be opened by the door jack.

When the door reaches the full open position a pintle contacts sequence valve 2 (SV2),

opening it and allowing hydraulic pressure to release the MLG up lock and extend the

main gear. It will be noted that a one way restrictor valve is fitted in the up line between

the selector valve and the MLG jack. This restricts the flow of fluid returning from the jack

to the reservoir during MLG extension, thus limiting the rate of travel of the heavy main

gear which, with gravity added to hydraulic force, would otherwise be excessive. During

retraction the one-way restrictor permits full fluid flow.

Accidental retraction of the landing gear, due to inadvertent UP selection with the aircraft

on the ground, is prevented by weight-on safety switches, commonly called squat

switches, (which isolate the selector switch when aircraft weight is on the wheels) or by

mechanical locking devices inserted by ground maintenance staff.

In some aircraft, there is a protection device to avoid the landinggear being inadvertently retracted on the ground. It consists of a

latch located in the landing gear lever



Landing Gear


Landing Gear Control and Indication

The landing gear control panel contains a selector handle with a knob in the shape of a

wheel for UP or DOWN and indications of landing gear position - down and locked,

travelling (unlocked) and up and locked. The gear operating handle may be locked whenthe gear is down and the aircraft weight is on the wheels. It will only be unlocked when

the aircraft is airborne and its weight is off the wheels. In order to compare Operator

demands wich aircraft conditions (airborne or on the ground) a logic circuit is provided.

Three-position selector handles (UP/OFF/DOWN) are often used for additional safety.

The OFF position permits depressurisation of the landing gear retraction system in flight.

Landing gear position is displayed by means of one indicator for each gear (nose, left

main, right main) and often takes the form of three lights which illuminate green when the

gears are down, and locked in the down position. Whilst the gears are travelling from UP

to DOWN, or vice versa, a red light

illuminates to indicate `gear unlocked'.

This sometimes takes the form of a flashing red light in the gear operating handle. When

the gear is locked up, all lights are extinguished. The three gear indicators show:

up and locked

unlocked (travelling)

down and locked

The unlocked condition is accompanied by a red warning light.

The landing gear lever has threepositions. When the landing gearlever is placed in the OFF position thelanding gear hydraulic system isdepressurised

The illumination of the green lampindicator corresponding to a landinggear means that the landing gear islocked down



Landing Gear


Landing Gear Warning System

The warning system provides visual and aural warning that an unsafe landing condition

exists. The red landing gear warning lights come on and a warning horn sounds

whenever the gear is not down and locked and the aircraft is not safe to land. Undercertain conditions the warning horn can be cancelled. The landing gear red lights

illuminate when the throttles are retarded. The warning horn sounds with a combination of

flap extension and throttles retarded.

Nose wheel Steering

Practically all tricycle aircraft incorporate a steering mechanism for the nose wheel.

Larger aircraft invariably do, and some very large ones have also main (body) gear

steering. In most aircraft, exceptions exist specially for light aircraft, the nose-wheel

steering commands are transmitted hydraulically to a yoke, or steering arm, attached to

the nose-wheel shock strut. Light aircraft are steered by push pull rods connected to the

rudder pedals.

Rotation of the pilot's steering wheel is transmitted by cables to a steering drum which,

through a system of pulleys and linkages, moves the hydraulic control valve. This directs

hydraulic pressure to the steering jacks which rotate the yoke attached to the lower

cylinder of the NLG shock strut. As the yoke rotates, its motion is transmitted through the

pulley system to return the control valve to the neutral position when the desired degree

of turn has been reached by the nose wheel. In many transport aircraft the steering wheel

or tiller is only used for large steering inputs, small inputs being transmitted through the

rudder pedals. For aircraft ground towing, when the nosewheel must be able to caster

freely through wide angles, the connection between upper and lower shock strut

cylinders, provided by the torsion links, is broken by the removal of a quick-release pin. It

is an essential pre-taxiing check to ensure that this pin is engaged. Nose-wheels,

especially single wheels, have a natural tendency to ‘caster’, that is a natural self-centring

stability. Camping is required however to prevent oscillation about the centre line, a

condition known as 'shimmy'.

In the event of an approach to land being made with the thrott le leversretarded towards idle and the flaps down and the gear up, the warning

given to the pilot wil l be a horn

An essentialrequirement ina Nose WheelSteeringSystem is afollow-upmechanism



Landing Gear


Nose-wheel shimmy would exert considerable force on the steering mechanism and

make steering the aircraft difficult. The castering tendency is snubbed by means of a

shimmy damper. This is usually in the form of a hydraulic cylinder containing a piston and

filled with hydraulic fluid. The piston rod is connected to a fixed part of the airframe andthe cylinder is attached to the nose-wheel leg. A small orifice in the piston allows

restricted flow of fluid from one side to the other, dampening piston movement. More

complex dampers are sometimes used in large transport aircraft, consisting of a system

of rotary vanes and known as a vane-type damper.

Note that when the control valve is in the neutral (mid) position, the steering jacks are

connected to the return line and therefore the nosewheel is free to caster. This is

necessary to permit directional control by rudder at high speeds whilst on the ground.

Turning Radius

An important feature of nose-wheel steering is its effect upon aircraft turning radius, and

particularly wing tip clearance. When the aircraft is turned by means of the nose-wheel,

the path followed by the outer wing tip will pass well ahead of the Crack of the nose-

wheel, and therefore of the pilot, whose seat position is also forward of the nose-wheel.

Consequently the pitot must allow sufficient clearance from ground obstructions to

accommodate this. On the Embraer 170 for example, both outer wing and tailplane tip

radius are greater than nose radius.

Factors which will affect the turning radius of an aircraft are taxi speed, gross weight,

centre of gravity position, nosewheel steering angle, distance between nose and main

wheel centres and track width between main wheels.

R. 16.49

R. 16.17

R. 5.80

R. 5.28

R. 10.97

17.05 m

R. 11.25

R. 15.02

76

The cause ofrather v iolentvibration of the

nose wheel,described asshimmy, isoften tyreimbalance andlooseness ofthe nose wheelsupportmechanism



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Wheel Braking Systems

Almost without exception, aircraft wheel brakes are of the disc type, only a few light

aircraft remain fitted with drum brakes. Larger aircraft use multiple disc brakes, whereaslight general aviation aeroplanes often require only single disc brakes. To avoid disc

distortion, very large aircraft often use a variation of the multiple disc system known as a

segmented rotor brake.

On light aircraft an independent or unboosted brake system may be used. A brake pedal

attached to each rudder pedal permits the application of differential braking. The applied

brake pressure is proportional to pedal pressure. The brakes may be locked on by

applying a parking brake.

The single disc brake comprises a polished steel disc which is keyed to the landing gear

wheel. When the brake is applied, hydraulic pressure on a piston forces high friction pads

to clamp on either side of the disc. The greater the force applied, the greater the braking

friction.

Multiple disc brakes use a number of discs mounted parallel to each other with more

hydraulically operated pads, to increase the braking friction. This is necessary when

stopping a large, heavy aircraft. The brake plate is splined to the aircraft wheel and

If the gaspressure in abrakeaccumulator istoo low theenergy forbraking is low



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rotates with it. The torque plate carrying the brake operating pistons is attached to the

axle and is stationary.

The majority of large aircraft have main landing gears that retract inwards towards the

aircraft centre-line. To reduce the stress on the wheel spinning before gear retractiontakes place, the main wheels are braked. This braking function is achieved by an auto

retract brake system. Nose gears that retract fore/aft usually have de-spin friction pads

contacted by the tyres when the gear reaches the up Position.

Operation

We can divide between following braking systems:

- Directly Controlled Brake System, and

- Powered Brake System.

Directly Controlled Brake System

A typical hydraulic brake system comprises 2 separate brake circuits, each affecting an

individual wheel. The pressure duct is routed from the main brake cylinder via a shuttle

valve and a parking brake valve to the disc brake assembly.

When the pressure within the brake housing is increased, the brake pistons will be

pressed against the inner fixed disc. When applying brake force during rotation of the

wheel, friction is generated - this in turn causes the braking effect.

A shuttle valve is built into each hydraulic brake circuit. These valves prevent hydraulic

pressure from entering the other duct, when the pilot’s or co-pilot’s brake pedal is

actuated. 2 spring-loaded return pins compensate for the wear of brake discs and serve

for measuring the rate of wear. They also ensure free motion of the discs.

If the brakingsystem shuttlevalve gets jammed in thenormalposition mainbraking wouldnot beavailable.



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Powered Brake System

As these systems vary between aircraft type we will take as example the powered brake

system of the DH8.The brake system is controlled by the pilot's and co-pilot's brake pedals and is powered

by the No. 1 hydraulic system.

The system consists of left and right hydraulic brake control valves, a dual skid control

valve for each pair of main wheels, a quantity limiting valve and shuttle valve for each

main wheel, and four disc brake units, one at each main wheel.

The brake control valves are mounted on the wing front spar. Each valve is controlled by

a lever and cable system from the interconnected pilot's and co-pilot's left and right brake

pedals. The linkage attached to the base of the brake pedals is spring-loaded to return

the pedals to the off position when released. Two spring rods are designed to relieve

braking loads.The hydraulic lines are routed from each brake control valve into the appropriate nacelle

to the dual skid control valve. In addition, two lines directly from the No. 1 main hydraulic

system are routed to each dual skid control valve. From the dual skid control valve a

separate line for each wheel is routed through a quantity limiting valve and shuttle valve

to the wheel brake unit.

On large

aeroplanesequippedwith powerbrakes, themain sourceof power isderived fromtheaeroplane'shydraulicsystem



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When a brake pedal is pressed, the associated tie rod operates the brake control lever,

either by pushrod directly from the pilot's brake pedal, or through the interconnect cable

from the co-pilot's brake pedal. Operation of the pilot's brake pedals does not operate the

co-pilot's brake pedals, but operation of the co-pilot's brake pedals will drive the pilot's

brake pedals. Movement of the brake control lever extends the lever spring and pulls the

control cable to actuate the lever of the related brake control valve. The brake control

valve acts as a variable pressure reducing valve to vary the pressure according to the

amount of brake pedal movement, thereby providing progressive braking.

Operation of the valve control lever allows pressurized fluid from the No. 1 hydraulic

system to be directed through the dual skid control valve to an independent outlet for

each brake. From the dual skid control valve, pressure is delivered independently through

a quantity limiting valve and shuttle valve to the associated brake unit.

Each quantity limiting valve is a hydraulic fuse which closes to shut off the line to the

brake unit if the fluid passing through the valve exceeds a predetermined quantity. Thus,

fluid loss is reduced in the event that excessive leakage occurs downstream of the valve.

When the brake pedal is released, the lever spring returns the pedal to the off position. A

spring in the brake control valve returns the valve to the off position, brake pressure is

released to return and the valve inlet port closes.



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Brake Energy Capacity

Stopping a high speed aircraft either after landing or on a rejected take-off involves the

conversion of considerable kinetic energy into heat at the brake units and main wheels.

This energy may be expressed in foot-pounds or joules. An aircraft may have a brakelimitation chart to provide flight crew and maintenance personnel a means of determining

how to deal with hot brakes safely and effectively.

The specific purpose of the chart is to avoid in-flight fires and to ensure adequate brake

capacity at all times for a rejected take-off. The chart determines the amount of energy to

be absorbed by the brakes by considering the aircraft gross weight, indicated airspeed

and density altitude at the time the brakes are applied. A condition zone for a braking

event may be classified as normal, caution or danger.

Anti -Skid Systems

The function of the wheel brakes is to convert the kinetic energy of the aircraft into heat

energy, through the friction in the brakes. If the wheel stops rotating (locks) and the tyre

skids on the runway, the brakes have ceased to function and the energy transfer is now

only between tyre and runway. Furthermore, on a wet runway directional control of the

aircraft may be lost. To prevent the wheels locking during braking, transport aircraft

braking systems include skid control, or anti-skid systems. The principle of Operation of

such a system requires a device to measure wheel rotational speed (the skid-control

generator) and to apply the brakes in proportion. As rotational speed diminishes, braking

force is reduced sufficiently to just prevent wheel-locking.

The operating princ iple of an anti sk id system is as follows:The brake pressure will be decreased on the slower turning wheels

A wheel brakeanti-skid unit issensitive toangulardeceleration



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The skid-control generator consists of a small DC or AC generator mounted in the wheel

axle. The voltage output of the generator (and frequency in the case of AC) will be directly

proportional to wheel rotary speed. This is fed as a signal to the skid control unit which

compares it with the pilot's braking demands. If there is no wheel skid developing, thebraking action is proportional to the pilot's pressure on the brake pedals. If a skid is

developing, the skid control unit activates valves to release some of the brake actuating

pressure to prevent the skid developing further.

Clearly, the anti-skid sensing system can only function if the wheels are rotating in the

first place. If, for example, the aircraft were to touch down with wheel brakes applied, the

wheels would skid and the anti-skid system would have no way of sensing this, so the

wheels would remain locked. A protection circuit in the control unit prevents the brakes

from being applied during the landing approach. This circuit is called touch down

protection or touch down control.

Should any wheel lock fully when the aircraft is rolling, as can happen on a patch of ice,

the anti-skid system will release the brakes fully on that wheel until it spins back up. This

is known as locked-wheel skid control and is only functional at aircraft speeds above 15

to 20 mph. In the case of failure of the anti-skid system a warning light or caption is

activated on the flight deck and the brake system becomes fully manual.

The output of the wheel speed sensor is fed to a deceleration rate controller, which

compares actual deceleration rate of wheel rotation with a preset reference value. If the

wheel deceleration rate (spin-down) is within limits there is no output from the rate

controller and a permanent magnet holds a flapper valve in its mid-position. This allows

hydraulic pressure from the spool valve to escape equally from the jets on either side of

the flapper and maintains equal pressure at either end of the spool valve, centralising it.

In this position, hydraulic pressure from the pilot's brake pedals is transmitted directly to

the wheel brake cylinders. If the deceleration rate exceeds the preset value the rate

controller produces an output signal which biases the permanent magnet field and causes

the flapper valve to tilt. This results in an increased pressure on the left side of the spool

valve and a decrease on the right, which causes the spool to move over, relieving the

brake cylinder pressure and releasing the brakes, thus preventing the wheels

decelerating to a locked condition. Once the deceleration rate is back within limits the

controller will restore normal brake operation.

The modern anti-skid processes are based on the use of acomputer whose input data is:

- idle wheel speed (measured);- braked wheel speed (measured);- desired idle wheel t rain slipping rate

With the anti-

skid systemunserviceable,brakeapplicationshould bemade with anON and OFFbraking action



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Parking Brake

The purpose of the parking brake is to hold the aircraft stationary while the pilot is not

operating the brake pedals. Applying the parking brake routes brake hydraulic pressure to

the wheel brakes to hold them firmly ON so long as brake hydraulic pressure is available.Operation varies with aircraft types, but in general the procedure is to depress the two

brake pedals fully and apply the parking brake, which holds the brake control valves in

the fully ON position. It should be noted that, in many aircraft, application of the parking

brake (even partial application) cuts out the anti-skid system by closing the return line

from the anti-skid valves.

With parking brake applied a warning indication is illuminated on the flight deck, together

with anti-skid failure warnings. Parking brakes usually only operate main wheel brakes,

and in aircraft with multiple main wheels often only some of the main wheel brakes are

operated by the parking brake.

Normal, Alternate and Reserve Systems

The power for operating the wheel braking system is provided by the aircraft hydraulic

system. Normal braking hydraulic pressure is supplied by one of the main hydraulic

systems (for example system A) and alternate braking by another (for example system

B).

The brake hydraulic system(s) always include an accumulator and a non-return valve. In

the event of loss of hydraulic supply the non-return valve prevents pressure loss from the

brake system to main system and the accumulator holds sufficient reserve pressure for a

number of brake applications. In many modern transport aircraft reserve braking is also

available by connecting an electric hydraulic pump to a reserve supply of fluid in the event

of loss of main hydraulic systems. In some cases emergency brake operation employs

pneumatic pressure.

In a hydraulic braking system, the accumulator i s anaccumulator designed to restore brake energy in the event of a

hydraulic failure



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Indications

Flight deck brake system indications are usually of brake system (accumulator) pressure,

and brake temperature. Warning indications of failure of normal supply and the anti-skid

system are always provided and may include aural as well as visual warnings. Aircraftwith electronic instrument systems may have tyre pressures displayed. The system has a

pressure transducer in each wheel which sends a signal corresponding to tyre pressure

to a computer for display on a page showing landing gear information.

Brake wear is indicated at the brakes. A protruding wear indicator pin shows brake life

remaining, the pin retracts as wear progresses. Where no wear indicator is provided,

wear of the brake pads can be determined by measuring the distance between brake

piston and disc with the brakes applied. Brake temperature indications are usually

numerical, increasing with increased temperature. Above a certain value a warning

indication is activated, since brake efficiency decreases with increasing temperature.

On some aircraft with 'conventional' displays a brake temperature monitoring system is

fitted which includes temperature sensors at each wheel, which feed to a central monitor

and warning unit on the flight deck. The monitor has a single temperature gauge and an

illuminating selector button for each wheel. The monitor is calibrated to a predetermined

temperature level and the gauge normally displays the highest of the brake temperatures.

If any of the individual wheel brake temperatures exceed the predetermined temperature

the selector button associated with that wheel will illuminate. Pressing any of the selector

buttons will cause the gauge to indicate the temperature of the brake unit on that

particular wheel.

Auto Brakes

Modern large transport aircraft incorporate an auto-braking facility which enables the pilot

to pre-select various deceleration rates. The maximum auto-brake deceleration rate is

less than that available from manual braking. Anti-skid protection is maintained during

auto-brake operation. The system is armed by selecting a deceleration rate, but it will only

apply the brakes when the engine thrust levers are at IDLE. The auto-brake system then

maintains the selected deceleration rate in conjunction with the aerodynamic speed

brakes and thrust reversers. It will continue to provide braking to a complete stop or until

disarmed.

If an anti-skid or auto-brake system fault develops, the auto-brakes disarm automatically

and a warning light is displayed on the instrument panel. Brake application by the pilot will

also disarm the auto brake system.

The Auto Brake System (ABS) is being disconnected afterlanding by pilot action

The auto brakesystemmodulateshydraulicpressure to the

brakes to obtaina constantdecelerationduring thelanding roll

Light aircraftbrake pad wearis measured byindividual padthickness



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The auto-brake system includes a rejected take-off (RTO) selection, which can only be

armed with the aircraft on the ground. With RTO selected, the auto-brake system applies

maximum brake if the engine thrust levers are retarded to idle above a certain aircraft

speed (typically 80 to 90 knots).With RTO (rejected take-off) selected and armed, the brakes will in addition be

automatically applied if reverse thrust is selected at any time.

Fading / Loss o f braking action

This occurs in drum type brakes when they are hot and is due to expansion of the drum

away from the brake shoes. Disc brakes are designed to resist brake fade; however this

may occur with a fully worn brake during a severe rejected take-off.

Dragging / Failure of the brakes to release completely

This is caused by a variety of factors including weak or broken return springs, distorted

discs and air in the brake hydraulic system.

Chattering or Squealing

Instead of maintaining an even friction the brake friction varies during one revolution of

the wheel. This causes a 'charter' sound to come from the segmented brake discs. If the

frequency of this chattering is high enough the brakes emit a squealing noise. The causes

are warped or glazed discs or deposits of brake lining material on the discs, leading to

uneven friction.

Overheating

The function of the brakes is to convert kinetic energy into heat energy. The greater the

kinetic energy to be converted, the greater the heat generated in the brakes. The major

single cause of brake overheating is high taxiing speeds. Overheating brakes may cause

the disc to warp or cause the friction material to break up and adhere to the disc. Aircraft

with very hot brakes should not be parked with the brakes applied to prevent fusing of the

heat pack.

Brake "fade"is ineffectivebraking dueto heating



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Aquaplaning

Aquaplaning is caused by a layer of water beneath the tyre, which can build up into a

wedge and lift the tyre away from contact with the runway, thereby negating the effects of

braking. It can occur in water depths as little as 0.1 of an inch and is dependent uponaircraft speed and tyre pressure.

A simple formula has been derived which states that the minimum speed for initiation of

aquaplaning is approximately:

9 x √ (tyre pressure in lb/in²)

Thus, for a tyre pressure of 200 psi the aquaplaning speed is (9 x √ 200 which is 127

knots)

The formula which gives the minimum speed (Vp) at which aquaplaning may occur is:

Vp = 9 x sqrt(P) where P is kg/cm3 and Vp is in knots

Now try an example:

An aircraft has a tyre pressure of 225 psi, its minimum aquaplaning speed will be ...

The correct answer will be 135 kts.

There are three distinct types of aquaplaning:

Dynamic

This is due to standing water where the tyre is lifted off of the runway and completely

supported by the water.

Viscous

This occurs when the runway is damp and provides a very thin film of water which cannot

be penetrated by the tyre. Viscous aquaplaning can occur at, or persist down to, much

lower speeds than simple dynamic aquaplaning. Viscous aquaplaning is particularly

associated with smooth surfaces such as the touch-down zone of the runway which is

smoothed by rubber deposits.

When a landinggear wheel ishydroplaning,

it's frictionfactor is equalto 0.1



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Reverted rubber

When reverted rubber aquaplaning occurs the affected tyre(s) become tacky and takes

on the appearance of uncured rubber. It is normally the consequence of a long skid

occurring on a wet runway, during which the friction between the tyre and the wet surfaceboils the water and reverts the rubber. As a consequence a seal is formed which delays

water dispersal. The resulting steam then prevents the tyre from making contact with the

runway surface.



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Wheels and Tyres

Wheels

Aircraft wheels are usually constructed from forgings or castings made from aluminium or

magnesium alloy to minimise aircraft weight. The most critical part of a wheel is the bead

seat area which is rolled, pre-stressing its area, thus increasing its strength to protect

from tensile loads applied by the tyre.

Most large aircraft use tubeless tyres mounted on split-hub wheels.

The two halves of the wheel are separated for installation and removal of the tyre.

Obviously when assembled they make an airtight seal to contain the tyre pressure. The

surface condition of the wheel flange is also a vital factor in preventing air escaping from

the tyre. The high pressure used to inflate large aircraft tyres makes the structural

integrity of the wheels extremely important. Corrosion and cracking are conditions to be

guarded against. Aircraft wheel hubs are made from aluminium alloy, but in some cases

magnesium alloy, a material prone to rapid corrosion. Aircraft tyres must withstand

aircraft loads of many tons at speeds up to 250 mph. Consequently their construction is

designed to withstand these loads whilst the tyre is constantly flexing during wheel

rotation.

Ai rcraft wheelsare usuallymade ofmagnesium oraluminium



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Bias-ply Aircraft Tyre Construct ion

The tread is made of rubber which is compounded for toughness and durability. The tread

pattern is designed according to the aircraft’s operational requirements. Nowadays, the

circumferential ribbed tread is widely used to provide good traction under varying runwayconditions.

The sidewall consists of a protective layer of flexible, weather-resistant rubber covering

the outer carcass ply. It extends from the tread edge to the bead area.

Tread reinforcement is obtained by one or more layers of nylon fabric which strengthen

and stabilize the tread area for high-speed operation. The reinforcement also serves as a

reference for the buffing process in re-treadable tyres.



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Reinforcing plies of nylon or aramid fabric, called ’breakers’, are placed under the tread

rubber to protect the carcass plies and to strengthen and stabilize the tread area. They

are an integral part of the carcass construction.

Alternate layers of rubber-coated nylon fabric, so-called ’plies’, provide the strength of atyre. They completely encompass the tyre body. The carcass plies are wrapped around

the wire beads and back against the tyre sidewalls (ply turn-ups).

The beads are high tensile strength steel wires embedded in rubber. They anchor the

carcass plies and provide firm mounting surfaces on the rim.

A wedge of rubber called ’apex strip’ is fixed to the top of the bead bundle. It serves as a

filler.

The flippers are layers of rubberized fabric. They fix the bead wires to the carcass and

improve the durability of the tyre.

The chafers (protective layers of rubber and/or fabric) are located between the carcass

plies and the rim to prevent chafing.

The inner bead edge closest to the tyre centre line is called ’bead toe’. The bead heel is

the outer bead edge which touches the wheel flange. In tubeless tyres, the inner layer of

low permeability rubber (’inner liner’) acts as a built-in tube and prevents air from seeping

through casing plies. For tube-type tyres a thinner rubber liner is used to prevent tube

chafing against the inside ply.

A tubeless tyre is a tyre:

- which requires solid or branched wheels;

- which does not burst in the event of a tire puncture;

- which eliminates internal friction between the tube and the tire

Compared to a tube tyre a tubeless tyre offers following advantage:

Tubeless tyres are cooler in operation when subjected to high speeds and high loads.

Radial Aircraft Tyre Construct ion

The following components are of the same materials and provide the same functions as

those of the bias-ply tyre:

- sidewall

- breakers

- plies

- chafer

- bead heel

- inner liner.

Compared to a

tyre fitted with aninner tube, atubeless tyrepresents thefollowingcharacteristics:- lower risk ofbursting;- betteradjustment towheels



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Radial tyres feature a rigid belt and a flexible carcass to provide an increasing number of

landings and a reduction of the rolling resistance. The efficient use of high strength

materials results in lighter-weight tyres with improved performance.

The tread is made of rubber, compounded for toughness and durability. It is similar inform and function to the bias-ply tyre tread.

The carcass plies are layers of rubber-coated fabric which run radially from bead to bead.

The carcass plies provide the tyre strength in the sidewall area. The belt plies form a

composite structure which stiffens the tread area for increased landings. The belt plies

provide the tyre strength in the tread area.

The beads are hoops of high tensile strength steel wire which anchor the carcass plies

and provide a firm mounting surface on the wheel.

The overlay is a layer of reinforcing fabric placed on top of the belts to aid in high speed

operation.

The chippers are layers of rubber coated fabric applied at a diagonal angle which improve

the durability of the tyre in the bead area.



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Tyre Markings

All commercial aircraft tyres are clearly marked with the following information:

- manufacturer

- size- load rating

- speed rating

- skid depth

- manufacturer’s part number

- serial number

- Manufacturer’s plant identification plus TSO (’Technical Standard Order’) marking

and AEA code (which defines new tyre casing and tread construction). In

addition, tyres are marked with the ply rating and other markings as required by

airframe manufacturers or other organizations.

Military tyres carry markings required by the appropriate military specification.

Re-treaded tyres are marked in the shoulder with the following information:

- size

- ply rating

- speed category

- retread plant and/or country of re-treading

- retread level (’R-level’)

- date of re-treading

- retread AEA code (if appropriate).



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Creep

During braking and when the wheels spin up on touch-down the tyres may ‘creep' around

the wheel, which leads to wear of the tyre head and can damage the inner tube in tubed

tyres (fitted to some light aircraft). White creep indication marks are painted on tubed andtubeless tyres at the wheel rim (flange). These marks are one inch in width for tyres of up

to 24 inches outside diameter and one and a half inches in width for tyres over 24 inches

outside diameter. The tendency of the tyre to creep is greater when the tyre is newly

fitted, and/or when the tyre pressure is too low.

Temperature

Build up of high wheel temperature during prolonged braking could result in overheating

of the tyre head, and an increase in tyre inflation pressure which could result in explosive

fracture of the wheel. Fusible plugs are fitted in high performance aircraft wheels. These

plugs melt at a predetermined temperature and release tyre pressure.

A green fusible plug is designed to deflate the tyre if a temperature of 177°C is reached.

Wear

If a tyre is over-inflated it suffers excess wear on the crown. If the tyre is under-inflated itwears on its shoulders. Locked wheels and the spin up on touch-down cause scuffing of

the tread.

It is recommended that tyres be removed when wear has reached the limits defined

below:

Patterned tread tyres may be used until the tread is worn to the depth of the pattern.

Ribbed tyres with marker tie bars may be used until worn to the top of the tie bars.

Ribbed tyres without marker tie bars may be worn to within 2mm of the bottom of the

wear indicator groove.

The function o fa fusible plugis to protectthe tyre againstexplosion dueto excessivetemperature

Tyre creep may be described as the circumferential movement of thetyre in relation to the wheel flange.

It is most li kely occurs when the tyre is newly fitted.

It can be measured by painting marks on the tyre and wheel rim.

On a modern aeroplane, to avoid the risk of tyre burst fromoverheating, due for example to prolonged braking during anaborted take-off, there is a hol low bolt screwed into the wheel

which melts at a given temperature (thermal fuse) and deflates thetyre.

On an aircraftlanding gear,

an underinflated tyrewill wear at theshoulders

A ribbed tyre isworn to itslimits when it isworn to 2 mmfrom the bottomof the wearindicatorgrooves

A red band

painted on thewall of a tyre isa balance mark



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Twin contact tyres may be used until the centre of the crown shows sign of contacting the

ground.

Cuts

Foreign objects on the runway/taxiway can cut into the tyre tread.

Contamination

Leakage of oil (especially some hydraulic oils) and solvents onto tyres will destroy the

rubber casing.

Tyre Checking Procedures - Tyre Inflation

Tyre inflation pressure is given in the aircraft operating manual. This is always the

inflation pressure with the wheel not supporting the aircraft weight. When tyre pressure is

adjusted with aircraft weight on wheels an allowance of 4% should be added to the rated

inflation pressure. A tolerance of 5% to 10% above this loaded inflation pressure is

generally specified and tyre pressures up to this maximum are permitted. Tyres should be

inflated with nitrogen for safety. If tyre pressures increase as a result of heating, due to

prolonged taxiing or heavy braking, the excess pressure should not be released as this

could result in under-inflation at normal temperatures.

When checking the pressure of cold tyres which are at ambient temperature, any tyre

which is more than 10% below loaded inflation pressure should be rejected, together with

the companion tyre on the same axle. Any tyre which is between 5% and 10% below

loaded inflation pressure should be re-inflated to the correct pressure and checked at the

next daily check; if the pressure is again more than 5% low the tyre should be rejected.

When it is necessary to check the pressure of tyres that are still hot following a landing

the pressure of each tyre should be checked and noted and compared with the pressures

of the other tyres on the same undercarriage leg. Any tyre with a pressure 10% or more

below the maximum recorded on the same leg should be re-inflated to that maximum

pressure, but should be rejected if a similar loss is apparent at the next check. A typical

tyre pressure for a commercial aircraft is in the range 150-250 PSI. Aircraft with electronic

instrument systems (glass cockpit) may include a tyre pressure indication system.

A highpressure tyrewouldtypically beinflated tobetween 120and 250 psi

Tyre wear can be reduced while taxiing by taxiing at 50 knots or less

The ratedpressure ofan aircrafttyre is theunloadedpressurewhen the tyre

is cold

Wheninflating atyre fitted toan aircraft,the pressureindicated onthe gaugeshould read4% above the

ratedinflationpressure.

021 04-00-00 Landing Gear Amendment 1

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Transcript of 021 04-00-00 Landing Gear Amendment 1