agk 1 stress fatigue and airframe design

Stress, Fatigue and Airframe Design

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Aircraft General Knowledge

1.1. Stress, Fatigue and Airframe Stress, Fatigue and Airframe DesignDesign

2.2. HydraulicsHydraulics3.3. Flying ControlsFlying Controls4.4. Landing GearLanding Gear5.5. Pneumatic SystemsPneumatic Systems6.6. Air Conditioning and Air Conditioning and

PressurisationPressurisation7.7. Fuel SystemsFuel Systems8.8. Ice and Rain ProtectionIce and Rain Protection9.9. Basic Electric TheoryBasic Electric Theory

10.10. Direct Current ElectricityDirect Current Electricity11.11. Alternating Current ElectricityAlternating Current Electricity12.12. Internal Combustion PrinciplesInternal Combustion Principles13.13. Piston EnginesPiston Engines14.14. Jet EnginesJet Engines15.15. PropellersPropellers16.16. Integrated CircuitsIntegrated Circuits17.17. Fire and Smoke Detection and Fire and Smoke Detection and

SuppressionSuppression18.18. Oxygen and Breathing SystemsOxygen and Breathing Systems

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The forces of lift, weight, thrust and drag acting on an aircraft create stressesin the aircraft structure.Stress is formally defined as the force divided by the cross-sectional area towhich it is applied. .

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The forces of lift, weight, thrust and drag acting on an aircraft create stressesin the aircraft structure.StressStress is formally defined as the force divided by the cross-sectional area towhich it is applied. The SI unit of stress is the newton per square meter.

StrainStrain refers to a change in some spatialdimension (length, angle, or volume)compared to its original valueand stress refers to the cause of thechange (a force applied to a surface)

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Stress and System FailureStress and System Failure

Stresses can either be twisting or torsion stresses , tension, compression orshear. These stresses can act individually or together. Bending a structure,for instance, creates tension on the outside and compression on the inside.

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High compression loads can causebuckling of a structure.

Fatigue is the progressive and localisedstructural damage that occurs when amaterial is subject to cyclical loading.

When a sufficient load is applied to a metal or other structural material it willchange shape. This change in shape is called deformation.

A temporary shape change that is self-reversing after the force is removed iscalled elastic deformation. When the stress is sufficient to permanentlydeform the metal it is called plastic deformationA.

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The stress at which the structure fails is called the ultimate stress, this isthe fail point for a single application of a static load. In flight the structure isloaded and unloaded many times at levels below the ultimate stress. Inmetals, this causes cumulative damage which in turn allows the structureto fail catastrophically at a stress level well below ultimate stress.

The cumulative damage and weakening of the structure is called metalfatigue.

Composite structures also suffer from fatigue damage, but react in adifferent way.

In metal structures failures usually occur under tensile stress, for exampleon the bottom surface of a wing that is being bent upwards. The failuresusually start as cracks at the points of concentrated stress, such as rivetholes, machining marks, sharp corners and screw threads.

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Composites "soft fail" as the fibres break,and failure can usually be detected beforea catastrophic loss of strength occurs.

Combat aircraft are designed to have asafe life. Fatigue calculations are made toassess at what point the structure will fail,and the aircraft is taken out of servicebefore this point is reached.

The aircraft is then scrapped, or criticalcomponents such as wing spars arereplaced – if it is economically justified.A.

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Civil airliners are designed to be fail safe. In a fail safe design the structuralcomponents of an aeroplane are designed such that 'an evaluation of thestrength, detail, design and fabrication must show that catastrophic failure dueto fatigue, corrosion, manufacturing defects or accidental damage will beavoided throughout the operational life of the aeroplane'.

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In a fail safe design: where components are load bearing, there must be more than one;and the design must be based on the principal of 'redundancy ofcomponents’.

As with combat aircraft, fail safe components in civil aircraft have a 'safelife’, defined in either numbers of flight hours or 'cycles,. For example, a failsafe landing gear component might have a safe life defined in number oflanding gear cycles.

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Modern airliners are also designed to be damage tolerant.

A damage tolerant evaluation of a structure ensures that, should seriousfatigue, corrosion or accidental damage occur within the design servicegoal of the aeroplane, the remaining structure can withstand reasonableloads without failure or excessive structural deformation until the damageis detected.

By incorporating redundancy, crack-arrest features and parallel load pathsthe structure can be allowed to fail in some degree but continue inoperation until a periodic inspection reveals the failures and componentscan be replaced. A.

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The S/N CurveThe S/N Curve

The relationship between repeatedapplications of stress and the gradualdegradation of the safe stress level is givenby the S/N Curve.

This is a plot of the number of times anddegrees of stress applied, and shows whatpercentage of the original ultimate stresswill cause catastrophic failure for a givenfatigue history.

The designers now assess the stress and the S/N forecast for the aircraft -taken from the forecast flight profiles, weights and loading - and design astructure that should be safe for the life of the aircraft. If the aircraft isoperated in an entirely different manner from the original design, for instanceshort haul instead of long haul, then this will affect the life of the aircraft andthe planned servicing and spares holdings.

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Reducing FatigueReducing Fatigue

It should be clear from all this that an aircraft's fatigue life can be greatlyextended if stress levels are kept low. This means smooth flying, avoidinghigh g, avoiding turbulence and going easy on the power. Weight is acritical factor. Increasing aircraft all-up weight by 1% can increase fatiguelife consumption by 5%.

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Certification RequirementsCertification Requirements

The design requirements for aircraft are set out in:

CS25 For aircraft with a maximum take off mass 5700kg or more

CS23 For light aircraft

Very similar, if not identical, documents exist in other states. In the US,for instance, FAR25 mirrors CS25 and FAR23 matches CS23

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Level of CertificationLevel of Certification

The table opposite shows the certification safety objectives associated withfailure conditions.

There is a direct link between the probability of a failure, and the severityof the effects. The more severe the effect of a failure, then the probabilityof that failure happening must be more remote.

To keep within these limits, systems are often duplicated, or triplicated onaircraft.

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Classificationof Failure

Conditions

No safetyeffect

Minor Major Hazardous Catastrophic

Probability perFlight Hour

No probabilityrequirement

<10-3 <10-5 <10-7 <10-9

Effect onAeroplane

No effect onoperationalcapabilities orsafety

Slightreduction infunctionalcapabilities orsafety margins

Significant reduction in Functional capabilities or safety margins

Large reduction inFunctional capabilities or safety margins

Normally withhull loss

Effect onOccupantsexcluding

Flight Crew

InconveniencePhysicaldiscomfort

Physical distress,Possibly includinginjuries

Serious or fatal injury to a small number of passengers or cabin crew

Multiplefatalities

Effect on FlightCrew

No effect onflight crew

Slight increasein workload

Physical discomfort or a significant increase in workload

Physical distress orExcessive workloadimpairs ability to perform tasks

Fatalities orincapacitation

QualitativeProbability

No probabilityrequirement

Probable RemoteExtremelyremote

Extremelyimprobable

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IntroductionIntroduction

The ideal material for aircraft structures would have the following properties:

Low density High strength High stiffness Good corrosion resistance Good fatigue performance High operating temperature Ease of fabrication Low cost

None material has all these properties.A. T

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In the construction of an aircraft a variety of materials may be used to meetthe requirements of a particular structure.

Early aircraft used wood with fabric covering for the main structure withmetal fittings at critical points.

In the 1920s and "30s steel and aluminium replaced the wooden frame butthe fabric covering remained.

Advances in metallurgy eventually led to aluminium alloys that were lightyet had similar properties to steel. Engine design advanced in parallel and,as more thrust became available, all-metal aircraft were eventually built.A.

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AluminiumAluminium and and AluminiumAluminium AlloysAlloys

Aluminium is less dense than steel, has good corrosion resistance but isrelatively weak. Aluminium alloys are stronger but have worse corrosionresistance.

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Duralumin is a common aluminium alloy in aircraft structures which wasinvented in Germany in the 1930's. It may contain about 3% or 4% copper,½ to 1% manganese, ½ % to 1 ½ % magnesium, and, in some formulations,a little silicon.

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Duralumin alloys are relatively soft and workable in the normal state. Theymay be rolled, forged, extruded, or drawn into a variety of shapes andproducts. After heat treatment and ageing, these alloys are comparable tosoft steel in strength and hardness. Once manufactured, Duralumin shouldnot be heated above 120°C. This makes Duralumin unsuitable for weldingand restricts its use for aircraft operating above the speed of sound wheresurface temperatures can rise above this level. Although restricted byoperating temperatures Duralumin has good thermal conductivity, andbeing metal also conducts electricity.

Duralumin has poor resistance to corrosion. To overcome this, a thin layerof pure aluminium or a corrosion-resistant aluminium alloy is used to coverthe Duralumin core. These special laminated sheets are called Alclad and itis in this form that Duralumin is most used for aircraft construction.

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In summary, the advantages of Duralumin are:

Low density High strength High stiffness Fatigue tolerant Ease of fabrication Good thermal conductivity Low cost The disadvantages are: Poor corrosion resistance Low operating temperatureA.

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Magnesium AlloysMagnesium Alloys

Magnesium alloys are less dense than aluminium but have very lowoperating temperatures and a high susceptibility to corrosion. Magnesiumalloys should only be used where they can be easily inspected.

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Titanium AlloysTitanium Alloys

Titanium alloys are expensive and difficult to work but are extremely strongand will sustain operating temperatures up to 400°C. Titanium is used forengine fire-walls and other critical components such as helicopter rotorheads. Titanium can be welded with electron beams.

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MonelMonel

Monel is an alloy of copper and nickel with small amounts of iron andmanganese. Monel alloy‘ s high resistance to corrosion, its low coefficientof expansion and its high strength make it useful for certain applicationslike the exhaust system for aircraft engines.

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Honeycomb MaterialsHoneycomb Materials

The conventional method of fixing metal structures is riveting or bolting. Inthe 1940's epoxy adhesive metal-to-metal bonding came into use, and isstill widely used in aircraft construction.

A spin-off from this was the development of "honeycomb" materials, wherea cellular fill is bonded between two sheets of metal to give a light but stiffstructure.

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Honeycomb MaterialsHoneycomb Materials

The core itself is weak but stabilises the outer skins and produces a light yetstrong torsional structure. The main function of the core material is tostabilise the covering sheets. A honeycomb or sandwich structure isunsuitable for absorbing concentrated loads, and extra protection isrequired if the structure is to be subjected to such loads.

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CompositesComposites

One of the latest materials is a composite of fibres reinforced with apoymer matrix (also known as resin or filler). The fibres can be glass orkevlar, for example, but the most used are carbon fibres, which have thebest tensile strength to weight ratio.

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CompositesCompositesThe fibres can be laid in a random pattern, which gives a material with thesame bending strength in any direction, or in one particular direction togive great bending strength along the fibre run but a much lower strengthacross the run. In this way the bending response of the material can betailored exactly to the designers‘ needs , a fact which becomes importantwhen we deal with the wing bending response of swept wing aircraft.

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CorrosionCorrosion

Corrosion results from the fact that most metals will try to revert totheir natural and more stable state.

Although there are a number of reactions that can take place betweenmetals and their environment they may broadly be divided into twocategories, oxidationoxidation and electrolyticalelectrolytical.

Oxidation, or dry corrosion, is the reaction between a metal and itsenvironment without the intervention of an electrolyte.

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CorrosionCorrosion

Electrolytical, or wet corrosion, requires an electrolyte which conductselectricity, such as impure water. A potential difference exists betweendissimilar metals of two surfaces, or two areas of the same surface, theelectrolyte provides the current path. One of the areas becomes anodic(+) and the other area becomes cathodic (-). The anodic area usuallycorrodes while the cathodic are has material added to it.If a structure is subjected to corrosion and fatigue then stress corrosion,

or stress corrosion cracking can occur.

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CorrosionCorrosion

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The structure of the conventional aeroplane can be broken down intothree major subsections. The fuselage carries the crew, cargo andpassengers, the wings provide lift and the empennage (the fin and thetailplane) provide stabilisation.

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As aircraft design has advanced, these distinctions have become blurred inthe search for efficiency. Most passenger aircraft rely on the fuselageshape to provide some of the lift, stabilising surfaces have been combinedinto V tails, and extreme designs such as the American B2 combine all thefeatures into a flying wing.

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Where the functions of control surfaces are combined so are the names.Elevons are combination ailerons and elevators fitted to the outer wing,tailerons are the same things but fitted to the tail. Flaperons are acombination flap and aileron used mostly on light aircraft and fighters.

Fuel is usually carried in the wing but is often carried in fuselage tanks,particularly on large aircraft and, occasionally, even in the fin.

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Early aircraft were built of a fabric covered wooden frame. Three or four longwooden struts called longerons ran the length of the fuselage.

The longerons were held apart bycompression struts or bracing strutsforming individual "bays‘ which were inturn cross braced with tie-bars or wire.This is also known as trussconstruction. The frame of a structurelike this carries the entire structuralload, the fabric skin is purely foraerodynamic efficiency.A.

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IntroductionIntroductionLater versions of the braced fuselage used steel rather than wood. Thestructure was bolted together in the early years rather than welded aswelding reduced the strength of the joint. As lighter and more easily weldedsteel became available in the late 1930s welded structures became thenorm.

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The The MonocoqueMonocoque FuselageFuselage

A monocoque structure relies on the shape of the outer skin of the aircraft forstrength. The ideal cross section is circular and the shape is maintained bycircular formers. The stressed skin structure can only be built from light,strong and easily worked materials. Early aircraft used plywood, later aircrafteither use alloys of aluminium, magnesium and titanium or compositematerials.

The disadvantage of puremonocoque structures is that theydepend too heavily on their shapefor strength. Any damage ordeformation weakens the structureand can eventually lead to failureA.

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The SemiThe Semi--MonocoqueMonocoque FuselageFuselage

Including some of the features of theframe structure with some of thefeatures of a pure monocoque structuregives a design where the skin only takespart of the load and allows fuselages tobe other than circular in cross section.

The semi-monocoque structure usesbulkheads and formers to support theload bearing skin and the longitudinalstringers are more robust to take some ofthe tensile and compression load. Framestructures like the cabins of light aircraftcan be included in the design

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The SemiThe Semi--MonocoqueMonocoque FuselageFuselage

As the skin carries more of the load in monocoque and semi-monocoquestructures it becomes necessary to move away from a simple uniformsheet skin. Early semi-monocoque aircraft like the Ju52 used corrugatedskins for extra strength. In more modern aircraft large areas of skin aremachined on the underside in complex patterns to carry the varying loads.For some very convoluted patterns chemical etching has replaced millingto form the internal skin profile.

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The Reinforced Shell FuselageThe Reinforced Shell Fuselage

The final development of the semi-monocoque structures is called thereinforcedreinforced shellshell. The basic structure is stressed skin with skin shapebeing defined by frames, bulkheads and stringers but now reinforcedwith longerons. The purpose of the stringers, in fuselage construction, isto assist the skin to absorb longitudinal compressive loads. Whereopenings are made for windows and doors a frame is fitted around thehole which is bolted, riveted or welded to the frame.

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The Reinforced Shell FuselageThe Reinforced Shell Fuselage

Where weak points such as windowopenings occur the structure aroundthem is strengthened with additionalstraps called doublers. Doublers can beeither riveted or bonded to the mainstructure.

The floor panels are normallysuspended on cross beams.A.

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PressurisationPressurisation LoadsLoads

Modern transport aircraft havepressurised cabins to allow passengersand crew to breathe normally at highaltitudes without having to wearbreathing apparatus. The pressure insidethe cabin is maintained at a higherpressure than outside. Doors andwindows are designed to open inwards, sowhen the aircraft is pressurised they areforced into their apertures forminggastight seals.

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The loads caused by pressurisation can be split into axial or longitudinalloads and hoop or radial stresses. The pressurisation loads are appliedonce each flight and it makes little difference whether the aircraft staysat height for one hour or ten. Because of this, the number ofpressurisations, called the pressurisation cycles, are recorded along withthe flight time.

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If an aircraft suffers a tailstrike on take-off or landing apart from theobvious possibility of fuselage damage, the aft pressure bulkhead mayalso be damaged.Pressurisation loads are one reason why it is not always straightforwardto convert a long haul aircraft to short haul operations - the aircraft mayend up with a significantly shorter life.

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The wings of a modern aircraft are of a cantilever design, that is to say theyare self-supporting and do not require external bracing or wires. Thestructure is stressed skin with a rigid beam called a spar running the fulllength of the wing, many aircraft have two or three spars in the wing. Thespar may be designed to run the full length of the wing, full span, or may beassembled in half span sections and bolted together. Wing spars can begirders or box section.

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The aerodynamic shape of the wing ismaintained by ribs in the classic aerofoilshape which run fore and aft and stringerswhich run along the wing. Ribs correspondto the frames used to define the shape ofa stressed skin fuselage. Cavities in thewing are designed to carry fuel, either inflexible tanks or sealed into the structureitself.

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The forces on the skin are transmittedby the ribs and stringers to the sparswhich ultimately take the load. Figureshows a wing with threespars, ribs and stringers. The skinbetween the spars has been corrugatedto increase its strength and would haveanother layer of thin metal over the top.The spars and the stressed skin nowform a box section called a torsion boxthat is very rigid and resistant totwisting.

One type of spar is called an ‘I’beam spar'. This has two horizontal members,called "girders', joined by a vertical component, called 'the web, together, theyform an I shape in cross section.

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Wing LoadsWing Loads

The main loads on a wing are bendingbendingloadsloads, incorporating both tensiontension andcompressioncompression, both in flight and on theground.In flight the weight of the aircraft issupported by the lift of the wings, onthe ground by the landing gear.Although the normal loads tend to bendthe wing upwards, on landing and inturbulence the wing flexes downwardsso the spar has to take bending loads inboth directions. The load is at amaximum at the wing root.A.

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The primary bending and shear loads arecontrolled by observing ‘g, and loadinglimits, in particular thethe MaximumMaximum ZeroZeroFuelFuel MassMass (MZFM)(MZFM)..

This manufacturer's limit ignores the effect of fuel load in the wings. It ensuresthat the value of maximum bending of the wing at the wing root is notexceeded at the designed maximum load factor ('g,) of the aircraft. Although itis mainly concerned with the weight of the fuselage the MZFM is defined forconvenience as the maximum permissible aircraft weight disregarding fuel. Itshould never be exceeded in flight or on the ground.

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Wing loading may also be reduced by up rigging the ailerons, by causing themto produce less lift in their 'at rest, position.

TwistingTwisting or torsionaltorsional loads caused by shifts in the centre of gravity and centreof pressure and by control surface deflection are also present as are shearshear loadson the centre section of the main spar, particularly if half span spars are boltedto a centre box section.

The forces on the spar are affected by theamount of fuel in the wing. Some aircraft havespecial fuel management procedures whichkeep fuel in the outboard tanks to balance thelift and reduce the fatigue loads on the wingroot.

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AerodynamicAerodynamic FlutterFlutterAll three loads can be extremely high if the wing or control surfaces sufferfrom aerodynamic flutter, an undamped oscillation caused byaerodynamic imbalance.

The mass of the wing affects the likelihood of flutter, a lighter wing beingmore susceptible to high frequency flutter. Fuel may therefore be retainedin the outboard tanks not only to reduce the stress on the spar but also toreduce the onset of flutter.

The position of the engines can also reduce flutter by providing abalancing mass forward of the main wing, this tends to move the flexuralaxis closer to the centre of pressure and reduce the imbalance.A.

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The empennage serves two functions. It provides directional stability andcontrol in yaw and longitudinal stability and control in pitch.Traditionally the empennage is split into a vertical surface, the fin, with therudder attached and a horizontal stabilizer or tailplane with the elevatorsattached.

Directional stability is provided by the fin.Tailplane position is affected by the stallcharacteristics of the aircraft and thedesire to keep the tailplane away from thewing vortices at low speeds.

High tail planes keep out of the vortex butincrease the possibility of a deep stall.

V tails combine the functions of bothsurfaces.

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Fuselage mounted

Cruciform

T-tail

Flying tailplane

Tailplane mounted

Twin tailboom

Wing mountedA. T

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Two different methods of aircraftmaintenance may be employed:

“hard time”, where a component is replacedafter a set amount of hours, cycles oroperations; or

“on condition” when a component is onlyreplaced when it is deemed to beunserviceable or out of limits.

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Maintenance of aircraft is a comprehensive, ongoing process. The entireaircraft needs to be examined, maintained, and have the necessary partsreplaced to uphold the safety standards.

Aircraft are required to be maintained after a certain period of calendar timeor flight hours or flight cycles.

Also, some aircraft articles havea specific life (flight cycle) limit,and need to be replacedimmediately upon reaching themaximum use requirements.Besides the aircraft articles thatare due for replacement, allother parts need to be checkedfor faults or faulty performance.

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Here are just some of the routine maintenancetasks performed by an AMT:

cleaning aircraft and components application of corrosion prevention compound lubricating parts draining and trouble shooting fuel systems checking and servicing hydraulics andpneumatic systems replacing components inspecting for general wear and tear

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A newer field of aircraft maintenance is working in avionics, which dealswith electronic systems. These parts are vital for navigation andcommunications, and include radar, instruments, computer systems,radio communications, and global positions systems (GPS).

A strong knowledge of wiring and technical skills is required for workingin avionics maintenance

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agk 1 stress fatigue and airframe design

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