jar 66 11

J.A.R. 66 MODULE 11a

AIRFRAME STRUCTURES GENERAL CONCEPTS

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INTRODUCTION 

An aircraft aircraftss struc structure ture is designed designed to withsta withstand nd flight flight forces forces whil whilst st remaining aerodynamically efficient. The forces are: Lift Drag Weight Thrust

An aircrafts structure is generally made of three different types of member























 Any Any loa load d or ben bendi ding ng mom moment ent acti acting ng on the the aircrafts structure could be measured in pounds, kilograms or Newton's.  The most most imp import ortant ant facto factorr is the relati relations onshi hip p between this load and the cross-sectional cross-sectional area of the member.  THIS THIS IS IS CALL CALLED ED STRE STRESS SS and and is meas measur ured ed in pounds per square inch, Newton's per square meter or another suitable unit.























 In a sim simple ple beam beam there there are two stres stresses ses trying to cause failure, stress due to t o bending and stress due to shear. In diagram (a) on the following slide, the bending moment is the greatest in the centre where the load is applied  In diag diagra ram m (b), (b), howe howeve ver, r, whi which ch use usess a cantilever structure, the bending moment is













































 In practic practicee a beam beam can can be be tape tapered red to evenly evenly distribute the stress along its length.  Shea Shearr forc forcee is gre great ates estt near near the the poin points ts of of support and least at the centre of the span in a simply supported beam.  In a cant cantile ilever ver beam beam, shear shear forc forcee does does not

























 We have have alre already ady estab establis lished hed that that a strut strut is a member in compression and a tie is a member in tension.  Struts Struts are often often hollow hollow,, wher whereas eas ties are mostly mostly solid, rods or wires.























 This This movem ovemen entt is calle called d STRA STRAIN IN..  STRA TRAIN = CH CHANG ANGE OF OF LEN LENGT GTH H ORIGINAL LENGTH A beam is a member subjected to bending, it may be























 If the beam beam is a solid solid member ember there there will will be be a plane in the middle that is neither in compression compression nor in tension.  This This is is kno known wn as as the the neut neutra rall plan planee of the the neutral axis.























 It is is a requir requireme ement nt that that all all aircr aircraft aft under under constr construct uction ion must must conform to minimum airworthiness requirements as laid down in BCAR¶s.























LOAD FACTORS  A major ajor consi consider derati ation on in aircra aircraft ft design design is the variation of loads which the











































































































































































































































































































































































































































 The The prev previo ious us slid slidee show showss a wing wing to to bod body y attachment (both old and new design).  The The orig origin inal al desi design gn rel relie iess on on the the pin pin to to accommodate accommodate the entire load. This arrangement arrangement incurs serious fatigue due to the fluctuating loads imparted on the pin.  The The orig origina inall design design must must also also posse possess ss a reinforced section of structure to accommodate accommodate the pin, therefore increasing























 The The new new design design incorp incorpora orates tes a fixe fixed d join jointt that runs the entire depth of the spar.  This This arrange arrangeme ment nt give givess us us a bette betterr load load transference and also minimises minimises fatigue at the joint.  By having having numero numerous us fas fasten teners ers (attach (attachme ment nt points). The concept of Fail-Safe is maintained.  Fatig Fatigue ue will will be be easi easily ly iden identif tifia iabl blee by the the























MULTIPLE LOAD PATH  This This metho method d incorp incorpora orates tes a num number of sm small members, members, each carrying a small percentage of the total load.  In the event event of failu failure re of of one one load load member ember,,























 This aileron attachment attachment is a typical example of a damage tolerant structure.  Thi This st structu cture al also incorporates incorporates primary and secondary structure.























 A fur furth ther er met metho hod d of achi achiev evin ing g dam damage age tolerance is by the use of crack limiting joints.  This This is is whe where re par parts ts of the the str struc uctu ture re are are designed with crack limiting joints.  These These also also ensu ensure re that that these these cracks cracks can be easily detected during normal inspection cycles.  The The foll followi owing ng slide slide illustr illustrate atess addi addition tional al

























skin























 The fail-s fail-safe afe concep conceptt is is not rest restric ricted ted to struc structur tural al components.  If we we look look at a pass passeng enger er cabin cabin window window,, you you will will note that it has to withstand the pressurisation pressurisation of the fuselage.













































ZONAL AND STATION























LOCATION NUMBERING SYSTEM (STATION NUMBERS) FUSELAGE STATIONS









































































An example of a water line



































































































































































































































































































































































FATIGUE TESTING  The fatigu fatiguee testi testing ng of of an airfr airfram amee is carr carried ied out on a structure taken off the production line. line.  It is then then subj subject ected ed to a serie seriess of tests tests desi designe gned d to simulate the airframes working life.  By this this metho method, d, an equiv equivale alent nt of many many thousa thousand ndss of actual flying hours can be reproduced on the airframe in a short period of time.























 After After each each perio period d of of test testing ing is comple complete. te. The structure is scrutinised for signs of failure or material degradation.  The The resu results lts of the tests, tests, togeth together er with with the the data available for the structural materials, enable an accurate prediction to be made on how long, in terms of aircraft life and total fatigue, the aircraft can safely remain in service.  The The airc aircra raft ft , or or par parts ts of of it, it, is is the then n























 Some Some aircra aircraft, ft, in parti particul cular ar milita military ry aircr aircraft aft,, use a fatigue indexing system.  The The saf safee life life or or fati fatigue gue life life of a partic particula ular r aircraft is given in terms of a fatigue index.  A hig highe herr ffat atig igue ue ind index ex may may be be giv given en to to aircraft if the airframe has been subjected to further tests or if major structural parts have been replaced.























 The The fati fatigue gue meter eter will will meas measure ure the the varying varying loads imposed imposed on the airframe by the action of manoeuvres, landings or atmospheric conditions and converts these into ³G´ loads, which are recorded by the fatigue meter.  On land landin ing, g, the the ³G´ ³G´ load load figu figure ress are are recorded and a calculation of the amount of























STRUCTURAL SURVEYS  Struct Structura urall surve surveys ys and NDT NDT prog program ramm mes are carried out to ensure the continuing integrity of an airframe.  As the aircra aircraft ft is main maintain tained ed to an approv approved ed maintenance schedule, these surveys provide a continuous assessment assessment of the

























 With With lon long g life life airc aircra raft ft,, a mor moree in-de in-dept pth h inspection may be required by the manufacturers. This This may may take take the the f

f a stru structu ctu l























STRUCTURAL SURVEYS  Inform Informatio ation n for for struct structura urall surv surveys eys can be found in CAA Airworthiness notice 89.























STRUCTURAL REASSESSMENT  This in involves:The identif identificati icati

of structural structural parts parts which which























± The establi establishm shment ent of a procedure procedure for developing developing programmes that provide a high probability of detecting fatigue damage before























 A pro progr gram amm me such such as thi thiss is norm normal ally ly directed at older types of aircraft, as these are normally the aircraft with the highest number of flight cycles and are therefore most most likely to experience initial init ial fatigue

























 The The areas areas of the struct structure ure that that the the insp inspect ection ion

























NDT INSPECTION

























 The The use use of NDT NDT equi equipm pment ent and and methods ethods are



































































































































































































































































































































































 Bondi Bonding ng strips strips and static static discha discharge rge wicks wicks should be examined for burning and disintegration.  All contro controll surf surface acess includ including ing flaps, flaps, spoil spoilers ers and tabs should be inspected for damage at their hinge bearings.  Unsat Unsatisf isfact actory ory bondin bonding g may have have allo allowed wed static discharge and tracking across the bearings, causing burning, break-up or























 A check check for for roughn roughnes esss and and cont control rol surfa surface ce resistance to movement movement at each bearing will usually indicate damage at such points.  In add addit itio ion, n, the the foll follow owin ing g inspe inspect ctio ions ns should be carried out:-























 Exam Examinatio ination n of of engin enginee cowli cowlings ngs for signs signs of burning or pitting.  Exam Examin inat atio ion n of fus fusel elag agee skin skin and and rivet rivetss generally for signs of burning and pitting.  If the landin landing g gear gear was extend extended ed at the the tim time of strike, examine the lower parts for static























 The The insp inspect ection ionss outl outline ined d shou should ld be be follo followed wed by functional checks of the radio and radar equipment, instruments, compasses, electrical circuit and flying controls, in accordance with the relevant chapters of the approved maintenance manual.  On som some aircra aircraft ft a bondi bonding ng resist resistanc ancee check check on radomes may also be specified.























FLIGHT THROUGH SEVERE TURBULANCE  If an aircra aircraft ft has flown flown throug through h condit conditio ions ns of severe turbulence, the the severity may be difficult to establish.  An indica indicatio tion n may may be found found in the the acce acceler lerom omete eter r or fatigue meter fitted to some aircraft.  Howeve However, r, these these inst instrum rument entss are are desig designed ned to recor record d steady loads and force peaks and recorded data

























 Gene Genera rall lly y if a rea readi ding ng tha thatt exce exceed edss ±0.5 ±0.5g g and +2.5g is recorded on a transport t ransport aircraft, then some damage may be found. With With othe otherr typ

of air

ft (e.

Aeroba Aerobatic, tic,























 Seve Severe re tur turbu bule lenc ncee may may cause cause exce excess ssiv ivee vertical or lateral forces on the aircraft structure and the effects may be increased by the inertia of heavy components such as engines, fuel tanks, water tanks and cargo.























AIRCRAFT GROUNDING  The The most most commo common n type of ground ground used used at all principal airports is the grid system, in























WARNING!!  Do not use head-s head-sets ets or handle handle electr electrica icall connections to the aircraft during severe























 Static Static ground grounding ing of aircra aircraft ft is achiev achieved ed by attaching approved grounding cables to specified locations on the landing gear assemblies.  The The exac exactt locat location ionss will will be speci specifie fied d in the

jar 66 11

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