Automated eddy current Inspection of aircraft wheels Talk originally given at Italian NDT conference, Ferrera, 1993 Joseph M. Buckley formerly at Hocking NDT Ltd. Abstract The Hazards resulting from failure of an aircraft wheel during landing, or explosion in flight, are self-evident. After a number of accidents regular Non-Destructive Inspection of wheels has been mandatory for many years. This is most often done using the eddy current technique and traditionally has been performed manually using pencil or special purpose shaped probes. While effective in the hands of a skilled operator it is always difficult to confirm that such an inspection has been performed adequately, adequately, and operator fatigue and l oss of concentration may become a problem if it is desired to inspect more than the most critical part of the wheel. A number of attempts have been made to automate the process, but most have been bulky or expensive or both. A machine machine is now now available available which can be be installe installed d easily, easily, requ require ires s onl only y A.C A.C.. pow power er and and occ occup upie ies s les less s tha than n one one cubi cubic c meter of space, Inno Innova vati tive ve "Aut "Autot otra rak" k" soft softwa ware re foll follow ows s the the whee wheell prof profil ile e with withou outt com compl plex ex prog progra ramm mmin ing g and and allo allows ws oper operat ator ors s wit with h minim inima al ND NDT ex experien ience to ca carry ou out ef effective ive an and rep repeatab atable le insp inspec ecttions ions of a wid wide va varie riety of wheels eels,, ch checkin cking g the full surface of the wheel in a few minutes.
Figure 1: 1: Aircraft wheel outboard half showing major crack
Wheel Wheel failures failures while while the aircraft aircraft is on the ground ground are expe expens nsiv ive, e, inco inconv nven enie ient nt and and emb embar arra rass ssin ing. g. The The res resul ults ts of wheel failure in flight can be tragic. One such case occurred in the the late late 1970 1970's 's when when the the whee wheell of of an L101 L1011 1 airc aircra raft ft expl explod oded ed at high high-a -alt ltit itud ude. e. Part Parts s of of the the whee wheell wen wentt thr throu ough gh the pa passenger ca cabin, ca causin sing imm immediat iate de decompression ion an and loss loss of life life.. The aim of wheel inspection is to prevent all such incidents from happening.
INTRODUCTION Wheel overview In this paper I will describe the problems that have been found with aircraft wheels, some of the inspection techniques which have been used in the past, and how we at Hocking attempted attempted to address address these problems problems in creating a machine that was simple and cost -effective while meeting the highest performance specifications. Modern Aircraft wheels have a very exacting task to perform. A few wheels wheels must transm transmit it the landing landing and braking braking forces forces of an aircraft weighing typically 200 tons at 200 kph. Much design work has been expended to create wheel assemblies that will carry out this function thousands of times without failure. Normally this is achieved. However, as with any part suffering repetitive stress, small imperfections or damage may grow into cracks. Usually these will cause an air leak and thus become apparent due to wheel deflation. In more serious cases a wheel may break on landing. Figure 1. shows a wheel that disintegrated disintegrated during taxiing.
Figure 2: 2: Exploded view of typical aircraft wheel
A typical typical aircraft aircraft wheel wheel assembly assembly consists consists of the the following following main parts: ! An 'outbo 'outboard' ard' wheel wheel half containin containing g the the inflatio inflation n valve. valve. ! An 'inboard 'inboard'' wheel wheel half half containin containing g the brake assembly. assembly. ! A tubeless tubeless tyre, tyre, inflated inflated to a high pressure pressure ! Bearing assemblies, one in each half. ! High-tensile fasteners holding the two wheel halves together, with a rubber seal. In typical airline use a tyre may last up to 300 landings. When the tyre is replaced an opportunity exists to inspect the wheel. It is vital that any inspection technique used can locate small defects that may, during those 300 landings, grow to a potentially dangerous size. As tyre tyre techno technology logy has improved improved,, the the interva intervall between between tyre replacement has increased, meaning that smaller defects in the wheel must be reliably located.
O:\after_s\foils\wheetalk HOCKING NDT Ltd. 21st July 1993 1993
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Summary of Inspection requirements
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Reliability is very operator dependant.
Manual Eddy current inspection using a pencil probe. A standard standard high-freque high-frequency ncy eddy eddy current current crack detector, such as the HOCKING Locator UH, UH, is used with a pencil probe which is scanned around the surface of the wheel.
Figure 3: 3: Areas requiring eddy current inspection
The The mo most high highly ly stre stress sse ed are area a of of th the wh wheel is the the rim rim and tyre tyre seat seat (a) (a) and and it is here here that that most most prob proble lems ms have have occu occurre rred. d. Most Most whee wheell man manuf ufac actu ture rers rs req requi uire re that that this this be be insp inspec ecte ted d at at ever every y tyr tyre e cha chang nge. e. When When eddy eddy curr curren entt is is use used d a typi typica call reference defect size is 0.75mm deep and 1.50 mm long.
Vari Variou ous s aid aids s ma may be be use used d in in ach achie ievi ving ng Figure 4 a reg regul ular ar scan scan patt patter ern. n. A tur turnt ntab able le eith either er powe powere red d or or man manua uall hel helps ps regu regula larr rot rotat atio ion n and and allo allows ws the the oper operat ator or to conc concen entr trat ate e on scan scanni ning ng and and obse observ rvin ing g the the instrument response. A suitable coating (such as penetrant developer) may be applied to the wheel before scanning, then then the probe probe will will leave leave a 'tra 'trail' il' allowi allowing ng the opera operator tor to see wher where e he has has scan scanne ned. d. Alter Alterna nati tive vely ly a suit suitab able le guid guide e bloc block k with with seve severa rall pro probe be hole holes s may may be used used to ensu ensure re regu regula lar r scan scanni ning ng of the the bea bead d sea seatt are area. a.
The barrel barrel (b) is also also prone prone to som some e probl problems ems,, parti particula cularly rly on the the inbo inboar ard d half half wher where e the the brak brakes es are are atta attach ched ed.. While While not not ofte often n man manda dato tory ry,, man many y ope opera rato tors rs,, par parti ticu cula larl rly y tho those se with with auto automa mate ted d mac machi hine nes, s, are are car carry ryin ing g out out insp inspec ecti tion ons s her here e also also and finding cracks.
Advantages
Other parts of the wheel, such as boltholes(c), the counterbore around boltholes(d), and the ventilation holes(e) also require inspection. This is normally carried out when the wheel is is st stripped do down an and fu fully ov overhauled. Ai Airline pr practice and wheel manufacturer's recommendations vary, but this is typically carried out every three to five tyre changes.
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In addition, supplementary inspections such as conductivity or hardness testing are normally carried out when there is any indication that the wheel has overheated.
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A phase plane eddy current instrument such as the HOCKING Phasec 1.1 is used with a specially designed probe to scan the bead seat area of the wheel
Dye Penetrant Inspection
: Advantages ! !
Conceptually simple method- limited training required Low capital investment - cost savings for small volume operators.
Advantages !
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Very time consuming Inspection is adversely affected by dirt, oil or other contamination Won't always find tight cracks- Dye may not penetrate cracks closed by residual stress
Inspection of the entire bead seat re region in in a single ro rotation. Can distinguish between crack Figure 5 and corrosion on the impedance plane display. Consistent scan can be easily achieved. Unaffected by dirt, paint, oil, etc.
Disadvantages !
Disadvantages
Time consuming, it is usually impractical to inspect more than the bead seat area. On ameter type instrument it can be difficult to dist isting inguish ish sma small cra cracks cks fro from sur surface rou roughness or corrosion. No guarantee that all areas have been inspected Inspection qu quality is is de dependant en entirely on on op operator sk skill and and inte integr grit ity. y. Oper Operat ator or fati fatigu gue e is a majo majorr conc concer ern. n. No record of results.
Manual Eddy current inspection using a contoured 'bead seat' probe.
MANUAL INSPECTION METHODS
The wheel is stripped of paint, cleaned and coated with penetrant. After an interval the penetrant is removed and the wheel co coated wi with a developer. Sm Small traces of of dy dye re retained by cracks in the wheel are visible against the contrasting background of the developer.
Disadvantages !
One point which should be borne in mind is that modern aircraft wheels are expensive items and are designed to withstand thousands of landing cycles. They will be serviced many times during this long and arduous life, and will ine inevita itably su sustain mu much min mino or su surface damage su such as as scra scratc tche hes, s, mino minorr dent dents, s, and corr corros osio ion n due due to the the cond condit itio ions ns they operate in. They will inevitably gather residues of oil, dirt and rubber. This is perfectly normal and any inspection technique must take it into account, In the competitive world of air travel operators cannot afford to scrap wheels that are perfectly serviceable.
Low capital investment - operators will probably have a suitable eddy current instrument already. Unaffected by moderate oil or dirt, removal of firm paint is un unnecessary Good results if used carefully An excellen excellentt backup backup metho method d for investigat investigating ing indications and checking other parts of the wheel.
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Only inspects bead seat Region Sensitivity limited (but adequate for some current standards) as a large area of metal is inspected at once. Can be difficult to identify exact position of defect. Specific probe required for each wheel type.
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General problems of manual inspection ! ! !
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Effective inspections are very time consuming. A high level of of operator operator skill is required. required. Operator fa fatigue ca can se seriously af affect th the qu quality of of inspection It is di difficult to to distinguish be between sm small cr crack indications and general surface noise and corrosion
Financial ially the la labour de dependance of of ma manual in inspection meth method ods s mea means ns that that the the cos costt for for larg large e vol volum ume e ope opera rato tors rs is a sign signif ific ican antt fac facto torr in in wis wishi hing ng to auto automa mate te the the pro proce cess ss.. Fro From ma safe safety ty poin pointt of of vie view, w, whil while e man manua uall insp inspec ecti tion on has has fou found nd many many larg large e de defect fects, s, sma small fla flaws are are like likely ly to be be ove overl rloo ooke ked d. Eve Even n aft after manual nual insp inspec ecti tion on wheel heel fail failur ure es ha have stil stilll occ occu urre rred.
The Hocking Approach - WheelScan 700 In 1989 Hocking NDT began began devel developm opment ent of a new wh wheel in inspection system, primarily for the RAF. (B (British Royal Ai Air Force) The intention was to address these disadvantages, an and to to crea create te a mac machi hine ne whic which h woul would d be be the the natu natura rall choi choice ce of all all med mediu ium m and and high igh vol volum ume e air aircr craf aftt mainte intena nan nce opera erators tors..
AUTOMATED EDDY CURRENT INSPECTION Construction
To addres ress these hese prob roblem lems many appro proaches ches have ave been een tried. So Some ha have be been ve very co complex, us using mu multichannel probe arrays or specially programmed industrial robots. Perhaps one of the more effective was that developed by Britis British h Airw Airway ays s in the the 1970 1970's 's.. Varia Variati tion ons s on this this des desig ign n have have been been mark market eted ed by a numb number er of of comp compan anie ies. s. Thi This s empl employ oyed ed a pneu pneuma mati tic c ram ram to lift lift the the whe wheel el whic which h was was then then rota rotate ted d at at consta constant nt speed. speed. A prob probe e was was then then appl applied ied by a grav gravity ity/sp /sprin ring g load loaded ed arm arm and and scan scanne ned d acro across ss the the whe wheel el on a lead leadsc scre rew. w. Results were recorded on a chart recorder. General advant advant ages !
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Cons Consis iste tent nt scan scan patt patter ern n ove overr ent entir ire e fla flang nge, e, bead beadse seat at and tubewell area Considerable improvement in effective sensitivity. Detection sensitivivity is the eqaul of pencil probe insp inspec ecti tion on,, but but nois noise e can can be con consid sider erab ably ly impr improv oved ed due due to even rotation. Mini Minima mall ope opera rato torr ski skill ll requ requir ired ed once once equi equipm pmen entt set set up. up. Probes ar are Universal - work wi with any wh wheel Print inted (C (Chart) rt) Re Record fro from m ea each in inspecti ction. Minim inima al ope opera rato torr tim time e - extre xtrem mely ely cos costt eff effe ectiv ctive e for for large volume use
General disadvantages ! !
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Rela Relati tive vely ly high high init initia iall cos costt disc discou oura rage ges s sma small ll oper operat ator ors s Permanently installed - difficult to move for field use. (Nor (Norma mall lly y ins insta tall lled ed in Whee Wheell sho shop p pro proce cess ss)) eve even n ifif itit could be moved, it requires an air supply. Gene Genera rall lly y sin singl gle e pur purpo pose se;; the the Eddy Eddy curr curren entt mac machi hine ne cannot be used separately. Can Can only only be be used used wit with h the the whee wheell flan flange ge dow down n - impo impose ses s constraints on other processes or requires manhandling. Because it still uses an amplitude type crack detector it can be difficult to distinguish between cracks and minor surface denting or corrosion, particularly as: The spring loaded arm tends to skip off the surface if it is rough or corroded, Although this can be controlled by increasing the pressure this causes increased probe wear. It can be quite difficult to find the crack after inspection.
A majo ajor req require uirem ment of the the Figure 6 : 6 : Hocking Wheelscan 700 military wa was transportability. It was vital that the machine could be easily deployed to meet rapidly changing needs. To meet meet thi this s requ requir irem emen entt seve severa rall thin things gs wer were e cruc crucia ial: l: wei weigh ght, t, bulk, bulk, stre streng ngth th,, lack lack of of spec specia iall supp supply ly req requi uire reme ment nts s and and eas ease e of setu setup. p. To achi achiev eve e the the nece necess ssar ary y str stren engt gth h wit witho hout ut excess excessive ive weigh weightt we we used used a rug rugged ged alumin aluminium ium alloy alloy frame frame.. By makin making g ful fulll use use of mode modern rn elec electr tron onic ic and and mec mecha hani nica call technology we were able to reduce the volume to less than one cubic metre. By the same methods we ensured that the only supply required is from a standard power socket 110 or 220 220 V 50 50 or or 60 60 Hz, Hz, ava avail ilab able le in any any wor works ksho hop p or or off offic ice e in in the the world. Instrumentation
To imp impro rove ve the the per perfo form rman ance ce and and con conve veni nien ence ce of of the the eddy eddy current instrumentation several significant features have been been inco incorp rpor orat ated ed.. While While an impe impeda danc nce e pla plane ne disp displa lay y giv gives es good se segregation of of di different ty types of of in indication itit re requires considerable ex experience to to int interpret. We We wi wished to ma make th the inst instru rum ment ent as as sim simp ple as possi ossib ble. le. We We so solved lved this this by providing a second 'liftoff' trace on the chart recorder, allowing results to be easily understood: if it shows up mostly on the main trace it's a crack, if it shows up more on the lifto liftoff ff trac trace e it' it's s pro proba bably bly corro corrosio sion n or or pro probe be lift-o lift-off ff.. To ensu ensure re a cor corre rect ct test test the the ins instr trum umen entt aut autom omat atic ical ally ly sets sets itself to the correct phase angle at the beginning of each test and and per perfo form rms s a cali calibr brat atio ion n sca scan n ove overr a refe refere renc nce e def defec ectt (0.5mm slot) The calibration scan is repeated at the end of the the test test.. prov provid idin ing g a per perma mane nent nt rec recor ord d of the the tes testt sens sensiti itivit vity y
Figure 7: 7: Typical traces from Hocking Wheelscan
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The The cha chart rt reco record rds s sho show w ano anoth ther er majo majorr adv advan anta tage ge of the the auto automa mate ted d appr approa oach ch:: if the the hel helix ix is is set set to be smal smalle lerr than than the the prob probe e fie field ld then then a def defec ectt will will be seen seen on seve severa rall succ succes essiv sive e scan scans, s, this this give gives s a very very high high degr degree ee of conf confid iden ence ce that that a defect will not be missed.
ensu ensure res s tha thatt the the whee wheell is is cor corre rect ctly ly cent centre red, d, pres presse ses s sta start rt and and come comes s back back 2 minu minute tes s late laterr when when the the tes testt is comp comple lete te.. Few Few ope opera rato tors rs req requi uire re such such a spe speed ed,, but but 20 halfhalf-wh whee eels ls per per hour hour are are eas easil ily y ach achie ieva vabl ble e for for the the bas basic ic insp inspec ecti tion on.. CONCLUSION
To allow the auxiliary inspections to be carried out easily up to two two add addit itio iona nall pro probe bes s can can be perm perman anen entl tly y con conne nect cted ed,, for for exam exampl ple e a bolt boltho hole le prob probe e and and a gen gener eral al purp purpos ose e penc pencil il prob probe. e. By push pushin ing g a butt button on the the ope opera rato torr sel selec ects ts the the cor corre rect ct prob probe e and and a set set of of prep prepro rogr gram amme med d para parame mete ters rs,, allo allowi wing ng an an imme immedi diat ate e tes testt with withou outt spe spend ndin ing g tim time e set setti ting ng up the the pro probe be.. When When the the aux auxili iliar ary y test test is is comp comple lete ted d the the butt button on is is pres presse sed d to reve revert rt to auto automa mati tic c ope opera rati tion on.. Thu Thus s the the whee wheels lsca can n is the the onl only y eddy current machine required in the wheel shop. Additiona Additionall features features include include the the ability ability to to operat operate e at frequencies fr from 10 100 kH kHz to to 3 MHz, an and th the 'F 'FlawStop' mode of operation where the scan stops automatically when a fla flaw w is is ind indic icat ated ed allo allowi wing ng the the ope opera rato torr to to inv inves esti tiga gate te it. it. Probe Guidance
Perhaps the most innovative feature of the wheelscan is the 'Autotrak' probe control system. We wished to allow the probe to move freely over variations in the wheel, while minimising any tendency to 'bounce' over rough areas. We solved this by making the probe mechanism pressure sensitive. At the beginning of the scan the probe moves towards the wheel centreline until it encounters resistance. It then moves up (or down) the wheel wheel at a constant selectable helix. Because the probe moves only as the driving motors move any fast surface variations will be damped out, while giving a minimum static pressure and thus reducing probe wear.
Figure 8: 8: Scan paths on Hocking
Wheelscan 700
As the probe probe moves moves under under proces processor sor control control the orient orientation ation is unimportant, the system can test as well flange up or flange down. The probe will automatically follow the wheel shape, without any additional programming. Operation
Many otherwise excellent pieces of equipment are let down by a poor 'user interface'. We have tried to ensure that the task of the operator operator is understanding the application. The equipment should do its job unobtrusively. All paramet parameters ers relatin relating g to the the eddy eddy current current inspection inspection are set set on the 'wheelscan E' eddy current instrument. Normally this is just frequency, gain and alarm level. It unlikely that these will be changed from one wheel to another. On the wheelscan base unit the operator can set wheel lift height (as convenient to the wheel geometry), inspection speed and helix. For a given wheel the operator need set only the start (flange end) and finish heights as measured from the roller table surface,
As I said said ear earlie lierr we we set set out out to to add addre ress ss the the tech techni nica call and and prac practi tica call pro probl blem ems s of of the the exis existi ting ng tech techno nolo logy gy.. We We fee feell tha thatt we have have now now ans answe were red d all all of the the tec techn hnic ical al obje object ctio ions ns for for airc aircra raft ft ope opera rato tors rs con conte temp mpla lati ting ng aut autom omat ated ed whe wheel el ins inspe pect ctio ion. n. Cost Cost savi saving ngs, s, reli reliab abili ility ty and and saf safet ety y imp impro rove veme ment nts s mak make e auto automa mate ted d insp inspec ecti tion on the the onl only y sens sensib ible le pol policy icy for for larg larger er comp compan anie ies. s. At Hocking Hocking we were were very proud proud of our our new new machin machine e when when we we launched launched it, it, but as with any produc product, t, it's it's the reaction reaction of the customers th that co counts. Thre Three e yea years rs late laterr a cust custom omer er list list cove coveri ring ng the the map map from from Seattle to Sydney suggests that they share our opinion!
