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of 18Dickinson, 9th Joint FAA/DoD/NASA Aging Aircraft Conference

DEVELOPMENTS IN EWIS AND LASER WIRE MARKING ANDSTRIPPING TECHNOLOGY IN RELATION TO THE AEROSPACE

REGULATORY AND STANDARDS FRAMEWORK

Peter H. Dickinson B.Sc, Ph.D, - CEOJonathan Golding B.Sc - International Sales & Marketing Manager

Spectrum Technologies PLCWestern Avenue

Bridgend CF31 3RTWales

United Kingdom

Abstract:

The development and growth in size and complexity of aerospace Electrical WiringInterconnection Systems (EWIS) is traced over the last 50 years. The technologies used formarking and stripping of the wire insulation are reviewed with reference to the issues arising fromthe continuous process of wire development and design, and the reduction in wire dimensions andchanges in constructions. The latest developments in wire constructions are reviewed along withthe state-of-the-art in laser wire marking and stripping technology. These developments areconsidered in light of recent recommendations from electrical standards groups and from the USAging Transportation Systems Rulemaking Advisory Committee (ATSRAC). The impact of therecently published Federal Aviation Authority (FAA) Notice of Proposed Rule making (NPRM)on EWIS in relation to wire processing methods for manufacture and maintenance is alsoreviewed.

Introduction:

Just over one hundred years ago Orville Wright lifted into the air for the first sustained mannedpowered aeroplane flight at Kitty Hawk, North Carolina, and launched a revolution intransportation. While there was some 700 ft of wiring used on the Wright Flyer almost all of thiswas used for mechanical bracing of the structure and the control systems; electrical wiring as weknow it was virtually absent. In 2005 the Airbus A380 took to the skies - amount of electricalwiring: over 500 miles (800 km) including some 190 miles of flight test wiring – in all almost22,000 times the length of Orville’s first flight.

Over the last five decades, in particular, the size of aircraft electrical wiring interconnect systems(EWIS) has rocketed. At the same time, the need to minimise weight on aircraft has driven thedevelopment of thin wall fluoropolymer insulated wires and the introduction of lighter, higherperformance electrical interconnect systems. However, the use of these modern materials andconstructions has created significant problems with regard to the traditional mechanical methodsused to process these wires during the course of manufacture and maintenance of the interconnectsystems. Mechanical processes of marking and stripping wire can lead to serious damage andconsequences, and there has been a major push to develop new technology to replace long-established hot stamp marking in particular.


Laser technology has been applied to these problems and UV laser marking is now the acceptedstandard for aerospace wire marking [1]. Recent developments have resulted in the introductionof new lower cost technology and products enabling its uptake across the broadest range of users,particularly for those involved in lower volume production and aircraft maintenance and overhaulactivities. The introduction of the latest generation of wires employing aluminium alloyconductors, as used on A380, poses increased challenges in the area of stripping; again lasertechnology offers a solution.

Discussion:

Since the introduction of the first commercial jetliner, over 50 years ago, aircraft electricalsystems have become evermore complex, larger and heavier. The growth in the size ofinterconnect systems has been driven by a number of factors including:

• The increasing number of avionics systems• The development of fly-by-wire control technology• The introduction of in flight entertainment (IFE) systems• The dramatic increase in the size of passenger aircraft

In addition aircraft lifetimes are becoming evermore extended; with current life extensionprogrammes the active lifetime of some airframes, such as the B52, are now being projected at,remarkably, close on a century. All of these factors have combined to require lighter, higherperformance wiring and interconnect systems. This in turn places increased demands on theprocess technology used to manufacture and maintain the interconnect systems to ensure that theyfunction safely and correctly over the lifetime of the aircraft.

Most people, even within the aerospace industry, are unaware of the sheer size and complexity ofthe electrical wiring interconnect system on a modern commercial jetliner, as it is hidden awaywithin the structure. As an example Figure 1 illustrates the EWIS for the Boeing 777, graphicallydemonstrating just how significant it is: the full outline of the aircraft is clearly visible althoughmade up only of the wiring system.

In Figure 2 we can see how the size of interconnect systems has grown over the last 50 years aswe trace the development of civil aircraft from the Douglas DC8, which first flew in 1958, to theAirbus A380, which had its maiden flight in 2005 and will enter service in 2007. It is instructiveto see how wire usage has increased over that period. The DC8, the world’s largest passengerplane until the introduction of the Boeing 747 in the early ‘70s, had some 40 miles of wiring onboard and carried 130 plus passengers; i.e. about 0.31 miles of wiring per passenger. Theproduction version of the A380 will have about 320 miles of wiring and carries 555 passengers;i.e. about 0.58 miles per passenger. The change is a net increase in wire usage per passenger ofsome 87%. It is assumed that a significant portion of this increase can be related to theintroduction of fly-by-wire technology and IFE systems.

During this same period we can see how the industry has responded to the need to minimise thegrowth in weight of aircraft EWIS by switching to the use of smaller gauge wires, as exemplifiedin Figure 3, which shows the changes in interconnect wiring employed on fighter aircraft.Fighters in particular strive for maximum performance and at the same time are densely packedwith avionics, driving the need for high performance light weight, interconnect systems to thelimit. This general trend is equally mirrored in commercial aircraft.


At the same time as the industry has moved to the use of smaller gauge wires, the wireconstructions themselves have developed through the use of higher performance materials toenable smaller diameter conductors and thinner wall insulations for given gauges. Figure 4 showshow this trend has developed over nearly 50 years.

To facilitate this progression, a range of new insulation materials and constructions has evolved.From the 1950s to the late 1970s extruded wire insulations were almost universally in use, withmaterials ranging from glass braid and rubber through silicon rubber and PVC to cross-linkedethylene-tetrafluoro-ethylene (ETFE); the latter still being in wide use. However from the mid tolate 1970s onwards tape wrap wire constructions came into widespread use. In particularpolyimide insulations were developed such as MIL81381 and Boeing’s BMS13-51, while inEurope there was a slight divergence with the development of polyimide insulated wires withfluoropolymer topcoats, such as Airbus’ CF low temperature wire, or with PTFE outer tapes suchas Airbus’ higher temperature DK wire; the latter also being referred to generically as a TK wire,for Teflon-Kapton®.

Due to the well documented problems encountered with polyimide wires, various research studieswere initiated, notably the study funded through the US Air Force Materials Directorate, WrightLaboratory [2], undertaken by the McDonnell Douglas Corporation in the early 1990s. As a resulta new generation of tape wrapped wires has been developed, the so-called composite or hybridwires, which lead to the creation of the MIL 22759/80 through 92 family of wires. These use asandwich tape of PTFE/Polyimide/PTFE, sometimes referred to as TKT for Teflon-Kapton-Teflon.

Boeing introduced commercial variants of TKT with their BMS 13-60 wire, while Airbusdeveloped an improved TK version of their original DK wire, with the introduction of the newerDM and now DR wires. In all of these wires the outer surface is a PTFE tape, which is onlycapable of being ink marked with any success by means of hot stamp. However, wall thicknesshas also continued to shrink, down to as little as 150 m (6 mil), making hot stampingincreasingly untenable with these new wires.

The A380 project has placed yet more demands on the components and systems used, with theneed to drive weight down even further. This has resulted in the introduction of the new AirbusAD wire with the use for the first time of an aluminium alloy conductor for the general airframeinterconnect wiring. The progression of wire insulation types and constructions since the 1950s isillustrated in Table 1.


Table 1. Evolution of wire types: 1950s to date

DATE WIRE TYPE CONSTRUCTION NOTES1950s PREN Glass braid & rubber

Tersil Silicon rubber & polyethylene terephalateglass braid

Fepsil Silicon rubber & glass braid + FEP1960s Efglas Glass braid & PTFE

Nyvin PVC glass & nylonPoly-X Alkene amide polymer

1970s Stilan Polyether ketoneMinyvin PVC glass nylonTefzel XL ethylene tetrafluoroethylene

Extruded insulationtechnology

1980s MIL 81381 Polyimide tape Introduction of tapewrap insulations

BMS 13- 51 Polyimide tape (B)

DK PTFE & polyimide tape (A)

A= AirbusB = BoeingTeflon/Kapton “TK”wire

CF Polyimide tape + PTFE/FEP topcoat (A)1990s DM PTFE & polyimide tape (A)

BMS 13- 60 PTFE/Polyimide/PTFE sandwich tape (B)MIL22759/80-92

PTFE/Polyimide/PTFE sandwich tape

DMS 2426 PTFE/Polyimide/PTFE sandwich tape (B)

Teflon/Kapton/Teflon“TKT”, hybrid orcomposite wires

2000s DR Enhanced PTFE & polyimide tape (A)AD PTFE & polyimide tape + Al alloy

conductor (A)

We can see from this table the continuous progression in wire materials and designs that is stillongoing. Yet, despite these continuing advances in wire technology, for much of its existence theaerospace industry has relied solely on the same mechanical technologies for the key processesemployed in the manufacture of electrical wiring systems. Mechanical methods were and still arethe main means of stripping the insulation from wire and cable, while from its introduction in1941, hot stamp wire marking has, until fairly recently, been the mainstay of the industry for wirecoding.

It is often said by (electrical) engineers that when designing a new aircraft, the last thing that getsthought of is the EWIS. If this is true, then improving the manufacturing processes to match thechanges and developments in wiring systems often receives even less consideration. Nevertheless,the introduction of fluoropolymer thin wall insulations has created significant challenges forcoding wires, while the new aluminium conductors are exacerbating the issues surroundingmechanical stripping processes.

The first indications that mechanical processes might need reviewing arose as long ago as theearly 1960s. Researchers at Lockheed in California began investigating problems with electricalbreakdown and fires in wiring systems and came across the phenomenon of arc tracking [3].


Throughout the 1960s and 1970s evidence came to light of the damage that could be done towiring by hot stamp marking, amongst other things, which could create sites for later breakdownof the wiring in service.

In the 1980s a number of serious incidents occurred that raised the profile of hot stamping as anissue and set aerospace companies looking for alternative technologies. These included severaloccasions where hot stamping had breached the insulation and the wiring was later subjected tofluid contamination. This included an incident in 1986 when a Monarch Airlines Boeing 757 wasforced to divert during a flight from the Canary Islands to the UK after arc tracking occurredaround a hot stamp site on wiring located under a leaking toilet compartment. The UnitedKingdom Civil Aviation Authority (CAA) subsequently put out a notice regarding this event,advising of the need to take greater care when hot stamping [4]. Cases of wiring damaged by hotstamp have been uncovered in a number of industry investigations; Figure 5 shows someexamples. It is clear from such investigations that the maximum recommended limit for the depthof penetration of hot stamp marks of 10% of the insulation thickness has often been exceeded.Further as the remaining users of hot stamp tend to be the smaller maintenance, repair andoverhaul (MRO) and subcontract shops, who have less facilities and resources to check on thequality of their hot stamping, it would be reasonable to assume that the problems could well begetting worse rather than better.

The aerospace industry began looking for new solutions to replace hot stamp from the early 1980sand various new ink based technologies were developed, including ink jet wire marking and “softkiss” ink marking. A key problem, however, has been the move at about the same time to the useof PTFE (Teflon) and similar materials for the wire insulation. Ink based systems are doomed tofailure in terms of meeting the industry’s requirements for mark permanence when dealing withTeflon insulated wires due to their ultimate non-stick characteristics.

While laser technology was also investigated in the early 1980s, the initial developments usedinfrared lasers, as the only industrial technology then available. However, infrared lasers areessentially sources of directed thermal radiation and create marks via thermal mechanisms.Neodymium infrared solid state lasers operating at their fundamental wavelength of 1.06 m, thenas now, are widely used for marking in general, but were, unsurprisingly, found to causeunacceptable damage to aerospace wires [5].

In 1987 an Airbus working group was looking for new wire marking technology. At this timeBritish Aerospace’s (now BAE Systems) corporate research centre was studying the industrialapplications of a new class of industrial lasers operating at short wavelengths in the ultraviolet(UV). As an Airbus partner BAE initiated a study into laser marking of wires and discovered thathigh peak power pulsed UV lasers created a novel, permanent and benign mark on fluoropolymerwiring insulations [6]. As a non-contact marking method UV laser is able to mark both singlecore wires and multi-core cables; see Figures 6 and 7.

Following on from this discovery the first prototype UV laser wire marker was developed in early1988, using a xenon chloride UV gas laser, see Figure 8, and based upon this a production systemwas then developed; this system was subsequently the first system used anywhere in a productioncapacity when put into use manufacturing wing harnesses for the A320. These initial systems andsubsequent gas laser based systems were designed for high speed marking to support high volumeproduction requirements and resultantly were relatively large and expensive compared to theneeds of smaller wire processors.


To meet the requirement for smaller lower cost systems suited to the lower volume productionneeds of aircraft maintenance organisations, solid state laser technology was introduced in 1996.However, even these first solid state laser systems were relatively expensive compared to hotstamp, thereby limiting their uptake to larger aircraft maintenance, repair and overhaul (MRO)organisations and those organisations obligated to implement UV laser technology.

During the last several years the industry has moved forward in developing controls andrecommendations on hot stamp wire marking as follows:

• In June 2002 the SAE published AIR 5575 “Hot Stamp Wire Marking Concerns forAerospace Vehicle Applications” [7], noting: “ … the time has come for military services andcivil regulatory agencies to accelerate the phase-out of hot stamping in aircraft wiringapplications”.

• In October 2002 ATSRAC’s Working Group 6 report was published [8], counselling:“….airframer markings: Hot stamp printing is not recommended. Alternative identificationmethods to mark directly on the wire are: Laser Printing preferably, Dot Matrix or Ink JetPrinting”.

• In August 2003 Revision B of SAE AS50881 “Wiring Aerospace Vehicle” [9] was releasedwith the unambiguous directive: “Hot stamp marking of wire and electrical/optical cable shallnot be used.”

• In September 2004 the latest version of the US (now tri-service) military aircraft wire harnessinstallation and repair maintenance manual [10] was released, including the following notice:“WARNING: Hot stamp marking directly on the wire or cable is not authorized for anyapplication”.

Most recently, on 6 October 2005, the Federal Aviation Administration published its long awaitedEnhanced Airworthiness Program for Airplane Systems/Fuel Tank Safety (EAPAS/FTS) with itsNotice of Proposed Rule Making (NPRM) on EWIS [11]. The recommendations are extensive butinclude specific comments on the use of hot stamp as follows: “Section 25.1711 ComponentIdentification: EWIS. … Proposed § 25.1711 ….. Paragraph (d) would require that the meansused to identify an EWIS component does not have an adverse effect on the component’sperformance throughout its design life. Certain wire marking methods have the potential todamage the wire’s insulation. Hot-stamp marking is one such method. According to SAE (Societyof Automotive Engineers) aerospace information report AIR5575 “ … the hot-stamp markingmethod is not well suited for today’s generation of aircraft wiring. … hot stamp wire markingprovides many opportunities for error. The controls, methods, and guidance necessary to achievesatisfactory performance with hot stamp marking are often not made available to operators insmaller wire shops.’’ The FAA concurs with this Assessment”. In summary the NPRMdiscourages the use of hot-stamp marking and advocates following the recommendations of theSAE and ATSRAC on the use of modern non-aggressive marking technology. It placesrequirements on any user to undertake stringent testing in accordance with SAE recommendedpractice ARP5369, ‘‘Guidelines for Wire Identification Marking Using the Hot Stamp Process’’or equivalent.

ARP5369 notes that it is “not intended to encourage the use of hot stamp marking or to endorsethe related process”, and “methods which do not deform the wire or cable insulation such as UVlaser are encouraged”. In practice, the recommendations on testing given in ARP5369, which theFAA’s NPRM imply would be mandatory, will add significantly to the procedures that smallershops must follow when employing hot stamp wire marking. In particular shops would have to:


1. Ensure that they control material adequately with regard to foil selection; ensurecompatibility with the marking equipment and wire insulation; control storage and shelflife and consult with the manufacturer as necessary

2. Ensure proper control of the process with regard to variables including: typewheels,marking temperature, pressure and dwell time

3. Consult with the equipment manufacturer for specific adjustments for each type of wireinsulation to be marked

ARP5369 sets out the initial requirements for hot stamp Process Qualification: “… tests designedfor qualifying the marking characteristics should be undertaken whenever new or replacementequipment or marking foils are introduced and the first time that marking a new wire or cabletype is initiated.” The tests shown in Table 2 below must be carried out whenever new markingmaterials or wire types are introduced to the marker and additionally at regular intervalsthroughout the continued use of the equipment at intervals concomitant with the usage level.

Table 2. ARP5369 Hot stamp process qualification requirements

Specimens:• 15 specimen lengths of 3 ft minimum.• Divided into 3 sample groups 5 specimens each after abrasion resistance test.Test Name Test Method Estimated time and costAbrasionResistance

All 15 specimens tested.Precision test equipment required.

Time required approx 4 minutes perspecimen, totalling approx 60 mins.Cost* $650

ThermalAging

One sample group 5 specimenstested.Thermal oven chamber required.

166 hrs in ovenThen mandrel testedThen wet dielectric test required with 5%sodium chloride solution.Cost* $535

FluidImmersion

One sample group 5 specimenstested.Test fluids required and suitablehandling apparatus.

24 hr test @ 70°C.One specimen from the group immersed ineach of the 5 fluids.Followed by legibility test.Cost* $370

WetDielectrictest.

One sample group 5 specimenstested.Immersion equipment required.

30 mins to perform test.

Cost* $190*Test costs quoted by qualified international test laboratory already set up with relevant testequipment

ARP5369 also sets out Quality Assurance provisions that should be followed during routine useof hot stamp marking equipment, as shown in Table 3 below. NB: critically, regarding frequencyof inspection, ARP5369 notes that the examinations included within Table 3 should be performedevery time a reel of wire is changed over and set up on the machine. Table 3 also shows theestimated time and cost for undertaking these inspections and where relevant a comparison withthe UV laser wire marking method.


Table 3. ARP5369 Hot stamp quality assurance provisions

Frequency of Inspection: “… as a minimum, these examinations should be performed whenever adifferent reel of wire or cable is put on the machine.”

Test Name Test Method Estimated time and cost UV Lasercomparison.

Legibility Reading of marked characterswithout additionalmagnification.

Minimal. Routine practice formost electrical wireterminations / processes

Similar timeand cost

Mandrel WrapExamination

5 cycles of wrapping themarked wire around acorresponding size ofmandrel.Visual examination of wireinsulation requiringmagnification between 6 and10 times.

12 inch of wire required.9 precision mandrels requiredat $450 (est)• 1 minute to perform test.• 1 minute to inspect and

record result.Magnification equipment -$300 (est)

Not required

Dielectric Test 100% of all marked wires andcables.In line spark tester or a wetdielectric test.

If available on existingsystem, no additionalprocessing time.If not fitted with in linedielectric tester, approx.$2800 to purchase (ifavailable for model ofequipment).

Not required

Such levels of testing and inspection both for initial and follow on qualification and routinequality assurance are likely to be well beyond those currently practised by almost all present usersof hot stamp equipment. By contrast no such testing is required of non-contact UV laser markingmethods, offering significant time and cost savings compared to hot stamp marking.

UV laser wire marking is clearly now the industry standard with airframers and with most largesuppliers and MRO operations, as well as with large parts of the military maintenance base. It canbe seen from the above discussion on standards and industry regulations and recommendationsthat there is also a clear desire to phase out hot stamp as quickly as possible throughout theremainder of the industry, but in the post 9/11 world the aerospace sector has faced severefinancial constraints and the needs of the lower tiers of industry for new wire marking systemshave been reduced to two primary requirements:

• UV laser marking technology in line with accepted standards• Lowest possible cost equipment

This requirement for small, low cost UV laser wire markers is most urgent to replace hot stampmarkers for MRO and low volume wire harness production operations, in particular.

A study of the requirements of the commercial and military MRO sector has resulted in thedevelopment of a systems requirement that has been brought to fruition in a compact bench topUV laser wire marking system. This employs an advanced, high efficiency, solid state laser,providing all the essential characteristics of UV laser marking associated with larger systems, butpackaged in a unit that meets the above need for low cost of acquisition while offering acceptableperformance for maintenance needs as a direct replacement to MRO hot stamp markers; seeFigure 9.


Such systems are eminently suitable for marking and processing up to 4000 ft of wire per 8 hourshift if required, certainly more than enough for typical low volume users. Recent industry casestudies have also shown that such systems are demonstrably more cost effective than hot stampsystems, even before the addition of the extra test and quality assurance provision requirements ofARP5369 for hot stamping noted above.

An analysis of the production of a typical job for a 40 wire harness consisting of 3 different wiregauges showed that, taking machine set up as well as wire processing time into account, plusquality assurance testing required for hot stamp, on a like for like basis for an average 13 ft (4m)wire segment, it can take about four times longer to produce a wire on a basic hot stamp markercompared to a UV laser wire marker. In fact the study showed that it took an average of 180seconds to prepare each 13 ft (4m) wire on a KIP-20 hot stamp compared to 45 seconds on aCAPRIS 50-100 [12]. Using the fully burdened hourly cost for a mechanic of $35.74 used in theFAA’s regulatory analyses contained within the EAPAS document, the actual cost of producingthe harness in this example would be:

• Hot stamp: $107.22• UV laser: $ 17.87

This should remove any doubt about the cost effectiveness of UV laser wire markers, should itexist, for most low volume users. Even users processing as little as a few hundred feet of wire perday should find UV laser technology gives an acceptable pay back given the added requirementsof the NPRM. For those situations where users are processing less than this amount then there isalways the fallback of heat shrink sleeve ID methods.

While laser technology has become the mainstay of wire coding for aircraft production,mechanical technology has continued as the main method of wire stripping. Yet here as well,mechanical technology has its limitations. In particular, there are strict rules regarding the extentto which conductor damage can be tolerated. This is to ensure that connections do not degradeand become potential failure points during service due to increased resistance and overheating.The difficulty with mechanical stripping is that it can lead to just this problem.

When using mechanical strippers, the use of worn blades or the wrong tool can damage theconductor, as a result of nicks, scrapes or the complete loss of conductor strands. To avoid thisand ensure optimum performance, it is essential to maintain calibration and ensure the correct setup on a wire stripper. With worn or incorrectly set blades it becomes difficult to strip the wire,particularly the inner layers of modern tape wrap wires. Furthermore operators can and do makemistakes in using the wrong tool for the job: using a stripper designed for a smaller gauge wirecan result in conductor damage. In any of these cases, if the operator remains unaware of theproblem it is likely that the damage caused will go unnoticed to the point where the damagedconductor is crimped into the connector pin, where the damage may then be exacerbated by theenvironment, vibration and general wear and tear, leading eventually to failure.

Such situations are of particular concern in MRO operations where operating conditions can beless than ideal. Operators are often working in confined spaces, resultantly they tend to use alarge number of individual hand tools, which increases the potential for use of the incorrect tool.In addition there is usually very limited access to the wire ends and there is often time pressure tocomplete the job. All of these can lead to errors and the problems noted above.

Laser technology has been successfully applied to wire stripping in the form of long wavelengthinfrared (IR) carbon dioxide (CO2) lasers operating at 10.6 µm. The laser beam is absorbedstrongly by the insulation and vaporises it, but does not damage the conductor because it reflects


the laser beam like a mirror; see Figure 10. Advantageously the laser beam can be scanned invarious ways to create transverse and longitudinal slits, which facilitate the removal of insulationfrom larger gauge wires in particular; see Figure 11. Because laser stripping is a non-contactprocess and results are practically independent of the wire type and material, it is well suited todealing with even the new AD aluminium wires being introduced by Airbus; see Figure 12. Thenon-contact characteristics of laser stripping also means that it can be applied to stripping jacketson irregular multi-core cables as well as coax cables.

A further advantage of laser wire stripping is that as a non-contact method it is insensitive to thevariations in hardness of insulation materials and makes easy work of slicing through theheterogeneous layers of modern composite wires that combine relatively soft PTFE layers with arelatively tougher polyimide layer. Mechanical systems on the other hand are best at dealing witha homogeneous material.

Yet despite these advantages, while laser stripping has been in use for well over ten yearsthroughout the electronics industry it has not been widely taken up within the aerospace industry.This is largely because the equipment design and cost has not been optimised to meet aerospaceindustry needs. The smallest bench mounted laser wire strippers previously available have beenrelatively large and expensive, and notably have been configured for batch processing for theelectronics industry where multiple wires are ganged in parallel for stripping.

To meet the needs of the aerospace sector new, improved laser technology and products are nowin development. In these a high speed system applies the beam individually to single wires. Thisenables a single piece of equipment to be configured to strip all wire sizes from 26 to 12 AWG.Importantly, there should be no need to vary the set up on the system. Therefore any wire can beoffered up to a single wire stripping port and all that is required is to set the strip length. The lasersystem needs no equivalent to the calibration of mechanical systems. Instead of the blades of themechanical system that have to be replaced or sharpened at regular intervals the laser stripperuses a low maintenance industrial CO2 laser, which is sealed off for up to 10,000 hours. Thisapproach offers high quality laser stripping while removing all the issues associated withmechanical stripping and advantageously reducing equipment maintenance and calibration costs.

At present new bench top systems are in development, suited to both manufacturing andmaintenance needs in the workshop environment. Future developments offer the potential for yetfurther miniaturisation for MRO applications, including portable systems for use at the formboardand on board aircraft.


Conclusion:

The performance and integrity of aircraft EWIS is now recognised to be of paramount importancealongside other aircraft systems. The performance of interconnect systems depends not only onthe materials and wire constructions used, but also on the use of appropriate process technology,correctly applied. Over the last several decades wire constructions have advanced with therequirements for lighter, higher performance interconnect systems. Tape wrap wires have beenintroduced along with the use of fluoropolymer materials that have generated particular problemsfor ink markers. Traditional mechanical wire marking and stripping technologies suffer particularproblems that can result in damage to the wiring, which in turn can lead to serious failures of theelectrical wiring interconnect system.

Hot stamp has been replaced by UV laser as the main means of wire marking with airframemanufacturers and major suppliers and with many of the larger maintenance organisations.Industry recommendations and controls are continuing to be tightened to phase out hot stampcompletely. Many airframe manufacturers and end users have already banned the use of hotstamp marking. The FAA’s EAPAS and NPRM proposals are likely to require residual hot stampusers to undertake ever more time consuming and stringent process analyses and qualityassurance tests to ensure compliance. The cost of non-aggressive UV laser technology haspreviously limited its uptake particularly in the maintenance, repair and overhaul (MRO) market.However, in the last few years the cost of UV laser technology has halved and new compact, lowcost laser wire markers have recently been introduced to the market and offer cost effectivesolutions for most low volume users, down to those processing a few hundred feet of wire andcable per day.

While mechanical stripping remains the main means of removing wire insulation, it too suffersproblems with potential damage to the wire conductor, which may also lead to interconnectsystem failures. This issue is widely recognised within the MRO community in particular.

Laser wire stripping has been developed as an alternative non-contact method of wire strippingthat completely avoids all the issues with mechanical methods and can strip the latest thin wallinsulations effectively and safely, even from aluminium conductors.

Laser based wire processing products offer the generic advantages of laser processing, namely:enhanced process quality and lack of damage, combined with simplified universal set upprocedures, thus enabling a wide range of wire gauges and types to be processed on a singlesystem, with much reduced maintenance and calibration requirements. All of this eases theproduction task for operators, removing them as a source of error. The bottom line is that, evenfor smaller users, laser technology provides improved process times and reduced costs, giving anexcellent payback that makes it cost effective compared to older mechanical technologies whileoffering improved quality and importantly enhanced safety for the future.


Figure 1 Boeing 777 electrical wiring interconnect system

Figure 2 Development of aircraft electrical interconnect systems

0

100

200

300

400

500

600

1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

B747-100 A300

A320

A340

A380

B777

B747-400

B727 B737

B767

MD80

B757 MD11

DC8

Tota

l len

gth

of E

WIS

inte

rcon

nect

wiri

ng- k

m


Figure 3 Changing wire gauge usage – fighter aircraft

1960 - F4 1970 - F15 1980 - F16 1990 - F18

Figure 4 Reduction in like for like wire size over last 50 years

Dimensions shown in mm (drawing not to scale)

Conductor 0.78 0.78 0.61 0.48diameter

Wall

Late 1950s 1960s 1970s 1980s-1990s

Wires available since 1990 have an effective cross-sectional area of 17% compared to theirequivalent in the late 1950s. This is due to improvements in insulating materials and refiningprocesses for producing lower resistance copper alloy. Their mass is approximately 30% ofearlier wires. NB For comparison with the wall thickness of current wire insulations note that thediameter of a human hair is ~ 0.05 mm.

0

5

10

15

20

25

30

35

20 22 20 22 24+ 20 22 24+ 20 22 24+

Wir

e le

ngth

- km

0.56 0.25 0.20 0.15


Figure 5 Example wires exhibiting hot stamp damage

a) Example of hot stamped wire - NTSB report

Hot Stamp Mark Areas

0.2 mm

0.07mm

b) Failed hot stamp marked wiring from in service aircraft - US Air Force


Figure 6 UV laser marked multi-core cable

Figure 7 UV laser marked wire - x -section

Note: laser generates mark by photochemically induced colour change - typical markdepth = 10 -20 m (< 1 mil)

Laser mark


Figure 8 World’s first prototype UV (excimer) laser wire marker - 1988(Courtesy BAE Systems)

Figure 9 Compact 4th generation low cost bench top UV laser wire marker


Figure 10 Laser stripped – single conductor wire O.D. 12.7mm, 0.5 inches,Teflon over Kapton

Figure 11 Laser stripping allows cross cuts and longitudinal cuts

Figure 12 Laser stripped Airbus AD aluminium conductor wire – no damage


References

1. SAE ARP5468A “Ultraviolet (UV) Lasers for Aerospace Wire Marking”; 2006;SAE AS5469 “Wire and Cable Marking Process, UV Laser” - in preparation

2. New Insulation Constructions for Aerospace Wiring Applications, Vols 1 & 2,Materials Directorate, Wright Laboratory, USAF, WL-TR-91-4066; Jun 1991

3. David Elliott, Lockheed, Wright Canyon; Nov 1962.4. UK Civil Aviation Authority Note 11-22 Appendix 24-35. Rex Beach, “Evaluation of circuit identification Marking on Aircraft Wires”,

5th Aerospace Electrical Interconnect System Conference; Oct 1989; Orlando, FL6. S. W. Williams, P.C. Morgan. “Excimer Laser Printing of Aircraft cables”; ICALEO:

International Congress on the Application of Lasers and Electro Optics; 30 Oct – 4Nov, 1988; Santa Clara, CA.

7. SAE AIR 5575 “Hot Stamp Wire Marking Concerns For Aerospace VehicleApplications”, June 2002.

8. ATSRAC Working Group 6 Final Report; Oct 2002.9. SAE AS50881 Revision B, “Wiring Aerospace Vehicle”; Aug 200310. NAVAIR 01–1A–505–1 / TO 1–1A–14 / TM 1–1500–323–24–1 Wire, Cable, And

Harness Marking Installation And Repair Practices Aircraft Electrical And ElectronicWiring; 1 Sept 2004

11. US Dept of Transportation, Federal Aviation Administration, 14 CFR Parts 1, 25, 91etc, Enhanced Airworthiness Program for Airplane Systems/Fuel tank Safety(EAPAS/FTS); Proposed Advisory Circulars; Proposed rules & Notices; 6 Oct 2005

12. Detailed cost analyses available c/o the authors

SAE: Society of Automotive Engineers SAE International, 400 Commonwealth Drive,Warrendale, Pennsylvania 15096, USA.

Acknowledgements

We would like to thank Boeing Commercial Airplane for use of the B777 EWIS drawing.In addition we would also like to thank Jean-Luc Ballenghien of Airbus, Tom Harris andDarrel Santala of Boeing, John Tulloch and Glen Jackson of BAE Systems, SteveAllwood of QinetiQ, and Bob Dennish of Omega Enterprises/Tyco-Raychem forproviding background information for this paper.

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developments in ewis and laser wire marking and … · was used for mechanical bracing of the...

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