Cable shielding is a key component in protecting critical and essential air-plane systems from the damaging effects of lightning strikes, high-intensityradiated fields, and other potentially harmful environmental hazards. The advent of fly-by-wire airplanes such as the 777 has further increasedthe importance of this protection. Boeing has developed a portable loopresistance tester for airplane maintenance personnel to use when testingcable shields and shield connections in airplane wire bundles. The loopresistance tester has proved to be a significant improvement over previoustest methods, offering operators substantial time and labor savings togetherwith a decreased likelihood of error. The tester is portable, operable by oneperson, usable on a fueled airplane, and allows totally non-intrusive testingfor both overall loop and joint resistance measurements.

LOOP RESISTANCE TESTER

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PRODUCTS & SERVICES

ERIK GODOPRINCIPAL ENGINEER

COMMERCIAL AVIONICSSYSTEMSBOEING COMMERCIALAIRPLANES GROUP

MATT TAORMINA

PRINCIPAL ENGINEER

ELECTRICAL SYSTEMS

BOEING COMMERCIALAIRPLANES GROUP

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irplanes in flight are susceptible to various environmentalhazards including lightning and high-intensity radiated

fields (HIRF). Both these conditions can impose sudden, serious damage to critical and essential airplane systems such aselectronic engine controls, high lift devices, and primary flightcontrols, and can affect safety of flight. Protection from theseconditions is built into Boeing airplanes through shieldedenclosures and shielded wiring, which are grounded to airplanestructure. Airplanes also operate under the constant extremes ofpressure and temperature while exposed to moisture, shock,and vibration. These degrade the integrity of shielding systems,requiring operators to periodically test the shields and theirconnections. Understanding the benefits of using the Boeingloop resistance tester (LRT) includes knowledge of the following:

1. Causes and results of environmental damage to airplane systems.

2. Mitigation of damage to airplane systems.

3. Shielding system concepts and testing.

4. Loop resistance tester description and operation.

a

CAUSES AND RESULTS OFENVIRONMENTAL DAMAGE TO AIRPLANE SYSTEMS

Critical and essential airplane systemsare vulnerable to damage from envi-ronmental factors. These include light-ning strikes, HIRF, moisture, extreme

change in pressure, extreme range oftemperature, vibration, and shock.

A lightning strike can cause directphysical damage to an airplane and,through circulating current coupling,can indirectly affect the function ofcritical and essential systems. It occurs

only about once every 3,000 hr (aboutonce a year) on a commercial airplane.This rate is frequent enough for alightning strike to be considered almostinevitable. Lightning produces a currentin the airplane skin, generating voltagesacross joints in the skin and structure.These currents couple, or connect, tointernal airplane wiring by way of theelectrical and magnetic fields that aregenerated by current flow (fig. 1).These electromagnetic fields are createdat the airplane surface, inducing volt-ages inside the airplane that cancause damage to electrical equipmentor cause it to malfunction indirectly.The resulting effects, known as light-ning indirect effects, range fromtripped circuit breakers to computermalfunction to physical damage ofinput or output circuits in electronicequipment. HIRF is generated by variousradio frequency (RF) emissions such ashigh-power radio and television signalsand radar. It is similar to the electro-magnetic fields induced by lightningand can also affect the proper func-tioning of critical and essential systems.Low-intensity RF can originate frompersonal electronic devices (PED) suchas laptop computers and cell phonesused by passengers in flight (see“Electromagnetic Interference FromPassenger-Carried Portable ElectronicDevices” on p. 12 in this issue).

1

LIGHTNING STRIKE EFFECTS

FIGURE

1

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These low-intensity devices can alsoaffect critical and essential systems.Electromagnetic interference from PEDsis suspected as the cause of manyunexplained flight control upsets.

Another environmental condition thatdamages airplane systems is moisturefrom rain and salt air, which continuallybathes external wiring and connectors.Wide pressure swings from altitudechanges, in conjunction with extremesof temperature, literally force moistureinto connectors and junctions. Smallair cavities in connector backshells aresusceptible to internal condensationeven if completely sealed to outsideair. Climbout from airports where tem-peratures reach above 100°F (38°C) toflight at 40,000 ft, where temperaturescan fall below –67°F (–55°C), causesthese air pockets to shrink and drawmoist outside air inside.Descent from low-pres-sure 40,000-ft altitudesthrough clouds andrain forces moist airinto connector cav-ities when outsidepressure builds atlower altitudes.This effect can occureven in pressurizedareas of the airplane.Airframe vibrations andshocks from landings andturbulent flight can loosen fasteners and connectors, creatingadditional paths for moisture to enter.

This inevitable infusion of moisturecauses corrosion. Corrosion degradeselectrical ground paths through achemical interaction between metal andanother element, usually air (oxygen),water, salt, or chemicals such as Skydrol.The shield grounding techniques usedon Boeing airplanes involve metal-to-metal contact at junctions. The presenceof oxygen or water causes an oxide toform between the contacting surfaces.The oxide is an insulator, which limitsthe flow of electrical current. Graduallythe resistance across the junctionincreases and, over time, the electricaljunction can be completely broken.Degradation of this type results in ahigher resistance path to ground, whichallows greater coupling of lightning or

RF currents to internal wiring. Thisdegradation is not evident to mainte-nance personnel, and extreme instancesof corrosion or loosening of connectorscan completely unground the shield.

MITIGATION OF DAMAGE TOAIRPLANE SYSTEMS

If electronic equipment needs to beoperated in a region subject to chang-ing electromagnetic fields, and if thecurrents generated by these fields areconsidered harmful, the recommendedapproach to mitigating the harmfuleffects is to shield and ground theelectronic equipment and the intercon-necting wiring. As a result, electricalcurrents generated by lightning or HIRFthen circulate through the equipmentenclosure to ground without affectinginternal circuitry. This enclosure practice

extends to interconnectingwiring through the use

of cable shielding;that is, the shield isthe enclosure thatis grounded. Otherdamage mitigationconsiderationsinclude the loca-tion of the equip-

ment and wiring,use of effective

wiring, use of good

transient voltages below those levels.

Lightning strikes vary in intensity andduration. When designing airplanes,Boeing assumes that all lightningstrikes are of high intensity and dura-tion and, therefore, a certain amountof current must be shunted to groundto minimize the amount that is indi-rectly coupled to internal wires. If theresistance of the shield circuit rises,less current is shunted to ground andthe likelihood of damage to internalequipment increases. Measuring shieldpath resistance directly indicates whatvoltage level will be reached andallows testing limits to be establishedfor determining when corrective actionmust be taken.

A new shielded cable properlyinstalled will exhibit a certain amountof resistance in the shield circuit. Bymonitoring this resistance, mainte-nance personnel can determine theability of the shield to protect internalwiring. Any increase in resistanceindicates that a problem is occurringin the circuit, such as corrosion at ajunction or loose hardware. When theresistance reaches a certain level,maintenance personnel must take cor-rective action, usually by cleaning theaffected junctions, securing looseconnections, or replacing the cable.

The U.S. Federal Aviation Administra-tion (FAA) has mandated maintenanceactions for certain critical systems toensure that the shielding used onthem provides continued protectionover the life of the airplane. Theseactions include operator-scheduledmaintenance and Boeing validations ofthis maintenance. Operator maintenanceincludes visual inspections for corrosionand loose hardware, and electricaltesting of the ground path for minimumresistance. Boeing validation involvessample testing of in-service airplanesto verify the effectiveness of scheduledmaintenance and to investigate addi-tional system integrity.

2

When designing airplanes, Boeing assumes that all lightning

strikes are of high intensity and duration.

23

grounding practices, and buildingequipment to withstand transients. Allthese tactics are incorporated into thedesign of Boeing airplanes and theinstalled equipment.

Boeing has studied data from a largenumber of lightning strikes to charac-terize the voltage and current generated.These data were used to determinethe level of transient (short-duration)voltages that might affect the airplaneand the installed electronic equipment.If electronic equipment is designedand certified to withstand certainlevels of transient voltages, the equip-ment enclosure and cable shieldingmust perform at a level to keep the

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Airplane designers must basetheir techniques for shieldgrounding on the assumptionthat moisture is inevitable andthat the use of corrosion-resistant materials and sealantsis mandatory. Using corrosion-resistant materials involvesseveral compromises, however.An example is corrosion-resistantstainless steel (CRES), but it isheavy and does not conductelectric current easily. Also,CRES is only corrosion resistantand not corrosion proof.Consequently, connectors aremade of lighter materials suchas aluminum, which is a goodconductor of electricity. Becausealuminum corrodes quickly insalt air, it is usually platedwith nickel and cadmium foradditional protection. However,time and exposure eventuallywill cause all materials to cor-rode. This is the main reasonthe FAA has mandated specificmaintenance actions on certainairplane models for monitoringexposed shield paths to keepthem functioning over the lifeof the airplane.

SHIELDING SYSTEM CONCEPTS AND TESTING

Shields perform other functionsbeyond providing lightning and HIRFprotection. A well-known example islow-frequency hum on airplane audiocircuits usually traceable to the 400-Hzpower system. The traditional solutionis to install a shield and ground it atone end, which has proved to be veryeffective against this type of low-frequency interference. Grounding theshield at both ends is typical for light-ning protection, but this tactic wasfound to be ineffective against thelow-frequency hum. One of the effectsof a lightning strike is the generationof changing electromagnetic fieldsinside the airplane hull. These fieldsare at much higher frequencies thanthe 400-Hz power interference. Undersuch conditions, shields grounded atonly one end might not be effective.In some cases this can also result in

the shield acting as an antenna, thusmaking the surge voltages even larger than they would be if the conductorswere unshielded.

No single point on an airplane can beconsidered “ground,” so the entire air-plane structure is typically used as aground. If a shield is grounded at bothends of a cable, circulating currents gointo the structure and return throughthe shield ground path at the otherend, creating a loop. The circulatingloop current cancels the magnetic fieldthat produces common mode voltages.This concept is directly counter to thereason for establishing a single-pointground and calls for shields to begrounded at both ends. However,installing dual-shielded cable with theinner shield grounded at one end andthe outer shield grounded at both endseliminates hum while maintaininglightning protection.

If circulating currents mitigate the

effects of electromagneticfields, keeping the shieldingloop resistance low will maxi-mize the protection. Testing forthis low resistance is a directmeasure of the protection.Initial methods developed tomeasure resistance in shieldinginvolved disconnecting the cableconnector and measuring theresistance with an ohmmeter.Because ohmmeter testinginvolves injecting a fixed current into the path and meas-uring the amount of voltagethat results, the path must beisolated from any other path tomeasure a particular resistance.If the path is not broken, theohmmeter current can flow inmultiple directions, making itimpossible to accuratelymeasure the resistance. Thisintrusive approach disturbsconnectors, is time consuming,and does not ensure a goodreconnection. When testing isdisturbed in this manner, asystem functional test mustfollow to verify that the systemstill performs as intended.

Initial attempts to testshielding circuits nonintrusively includ-ed using a laboratory network analyzerwith wraparound probes to couple avoltage to the shield and sense theresulting flow of current. Althoughaccurate, this configuration was difficultto set up, required long probe leads,was not suitable for field use because itwas not portable or waterproof, andrequired 115 V ac power.

LOOP RESISTANCE TESTERDESCRIPTION AND OPERATION

To avoid encountering these circum-stances during resistance testing,Boeing developed the LRT to provide anonintrusive testing technique thatwill not disturb connectors, is faster,and will ensure the soundness of theoverall circuit.

To properly use the LRT and achievethe best results possible, maintenancepersonnel should understand the fol-lowing about the device: LRT elements,

4

3

LRT in 777

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LRT technique, loop troubleshooting,joint mode, sensing of coupler closureand joint probe connection, mainte-nance requirements, safety features,and calibration and certification.

LRT elements. The LRT comprises four elements: adrive current coupler, a sense currentcoupler, joint probes, and an instrumentassembly containing the LRT batteryand processor. The battery allows theLRT to be used continuously for eighthr without recharging.

LRT technique.The LRT technique is patterned afterthe laboratory network analyzerapproach that induces a voltage onthe shield through a wraparound probeand measures the current flow throughanother similar probe. The shieldshown in figure 2 acts as a conductiveloop, going through the structure,connector, cable shield, connector, andback to structure. When a coupler isconnected to this loop, the couplerforms the output winding and magneticcore of a transformer, and the shieldloop forms the input winding of thetransformer. When a voltage is forcedon the coupler winding, a voltage is

then forced around the shield loop.The impedance of the loop can thenbe found by measuring the currentaround the loop. To keep the coupleroperating as closely as possible toideal conditions, the LRT uses twocouplers. A drive coupler drives thevoltage around the loop, and a sensecoupler senses the current flowing inthe loop. The maintenance personplaces both couplers on the shieldedbundle and presses the start switch tostart a loop measurement (fig. 3).

The LRT approach is to numerically“demodulate” the voltage and currentwaveforms to create a phasor repre-sentation of the loop voltage and loopcurrent, then take the real part oftheir complex ratio. To create thephasor representation, the voltage andcurrent waveforms are multiplied bysine and cosine waveforms andsummed. The ratio of the voltage andcurrent phasors produces the complexloop impedance, the real part of whichis the loop resistance (fig. 4).

VOLTAGE SHIELD CONDUCTIVE LOOP

FIGURE

2

COUPLERS ATTACHED TO MEASURE CURRENT

FIGURE

3

Take average, get phasor

Take real part -> resistance

0.05

0.150.05 + j0.15

0.071 + j0.118 71 milliohms

Divide voltage by currentGet impedance

=0.05 + j0.15

0.071 + j0.1181.12 + j0.25

Multiply voltage and currentwaveforms by sine and cosine

X =

=X

PHASOR REPRESENTATION EQUATION

FIGURE

4

Applied voltage induces current

Shielded cable

Backshell to plug

Receptacle to secondary structure Bracket to primary structure

AC current applied

AC current measured

Coupler 1 Coupler 2

LRT

Receptacle to equipment

Equipmentto structure

Plug to receptacle

Loop troubleshooting. Once a shield is found to have aresistance value that is larger thanallowable, the problem must be isolatedand fixed. Maintenance personnel cando this by disconnecting the cable inquestion and testing it with a bondingmeter. The cable connector must berotated to remove it, however, and thecorrosion may be cut through in thisprocess. If this happens the cablemay appear to be in good condition,making it impossible to find the problemarea. The cable may be returned toservice and will continue to corrode,potentially jeopardizing the systemwhile the airplane is in operation.The LRT was developed to allow main-tenance personnel to test for a poorconnection while the cable is stillinstalled. The LRT is also helpful whena problem can be fixed without removingthe cable because many cables are dif-ficult and costly to remove and replace.

Joint mode. To troubleshoot a bad loop,the LRT is used in jointmode. In this mode,instead of measuring theloop voltage and the loopcurrent, the LRT measuresthe voltage across a joint(joint voltage) and loop

current. Given the jointvoltage and loop current,

the joint impedance can bemeasured in the same way that

the loop impedance was measured.The joint voltage is measured by aseparate pair of joint probes that aredirectly connected across the joint ofinterest (fig. 5). After placing bothcouplers on a bundle and performing aloop measurement, the maintenanceperson switches the LRT to joint mode,and places the joint probes across thejoint of interest. Once the joint probesmake a connection, a joint measure-ment is started. By measuring the jointsin the loop, the maintenance personcan find the bad joint connectionwithout having to remove the cable.

Sensing of coupler closure and jointprobe connection.The LRT can sense if a coupler is notclosed adequately to complete a read-ing. This could occur if (1) somethingis stuck between the jaws of the coupler and the coupler doesn’t closecompletely, or (2) the maintenanceperson put the coupler in a locationthat is hard to see and the coupler isclosed on a bolt or some other piece

of hardware. The LRT can detect theseoccurrences and notify the maintenanceperson before a bad reading is given.The notification is in the form of anerror message on the LRT display. TheLRT also detects if the joint probes areconnected to the joint before a readingis taken in joint mode.

Maintenance requirements. The LRT was developed originally foruse on the 777, the first Boeing com-mercial fly-by-wire airplane (see “777Fly-By-Wire Maintenance” on p. 28).Experience gained with the LRT identified many inherent anomalies inshielding systems. These anomaliesexist undetected in other airplanemodels. Consequently, in 1997 the FAAreleased Flight Standards InformationBulletin for Airworthiness #97-16A onthe subject of lightning/HIRF protectionmaintenance. The bulletin containedguidelines for FAA inspectors to usefor ensuring that in-service airplanesmaintain continued airworthinessagainst lightning and HIRF hazards.Each operator was requested to pro-vide a lightning/HIRF protectionmaintenance program to the appropriateFAA district office for review andapproval. This activity was required forall models including earlier airplaneswith mainly analog electrical/electroniccontrols and displays although not allaspects of the bulletin would apply tothe earlier airplanes. The bulletinresulted in additional airline mainte-nance requirements, increasing thevalue of the LRT as a tool to help

JOINT VOLTAGE MEASUREMENT

FIGURE

5

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Joint probes

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operators reduce the time required toaccomplish the maintenance.

As of early 2000, the LRT currently wasnot mandated for use in the lightning/HIRF maintenance program for the737-600/-700/-800/-900 models. Alightning/HIRF maintenance programis being developed for the 757-300,and one will also be developed for the767-400 extended-range airplane laterin 2000. It has not been determinedwhether these programs and others willrequire use of the LRT. Similar programsfor the 717 and MD-11 airplanes call forvisual inspection of shielding systems.In all cases, the LRT provides thefastest, most accurate verification thata particular shield circuit is fully oper-ational and will provide maximumprotection against lightning and HIRFincidents.

Safety features. Boeing designed the LRT foruse in certain hazardouslocations, such as on afueled airplane, as well asnonhazardous locations.Safety features includeadded series resistance tothe couplers, added seriesresistance to the battery to

limit fault currents, and minimizedcapacitance in higher voltage circuits.The LRT is classified by theUnderwriters Laboratories Inc. asintrinsically safe to the propane classof fuels (class 1, division 1, group Dfuels), which includes jet fuel. Becausethe LRT can be used in depressions inthe airplane where fuel vapors maycollect, it carries the most stringentsafety classification. This means theLRT can also be used inside fuel tanksto check bonding.

Calibration and certification. As a piece of test equip-ment, the LRT must be calibrated and certifiedfor accuracy at regularintervals. Because the LRTis nonintrusive, a new setof loop resistance standards forcalibration and certification neededto be developed for its use (fig. 6).Large loop resistances (>1 ohm) areeasy to calibrate because standardresistors can be connected in a loop.As the loop resistance gets smaller,

the connection between the resistorsbecomes a large portion of the loopresistance. The lowest value LRT loopstandard is only about 5 milliohms, sothe joints made in connecting the looptogether (typically about 1 milliohm)become a large portion of the loop.In addition, the loop standards for theLRT needed to be measurable by cali-bration equipment so the loop standardsthemselves could be certified andtraceable to standards set by the U.S.National Institute for Standards andTechnology. For joint standards, the LRTuses standard current shunts (fig. 7).The required calibration/certificationinterval for the LRT is one year.

Calibration procedures areprovided in the LRT

ground equipmenttechnical manual.

In the past, the commercial aviation industry has not identified cableshielding systems as candidates for regular testing throughout the life ofan airplane. However, as fly-by-wire systems continue to be more widelyused, it is highly likely that operators will be required to test the relatedshielding systems in airplanes. Historical obstacles to testing these systemshave included the cost and difficulty of testing, the lack of testing opportu-nities during airplane production, and the ability to conduct only visualtesting in the field. The LRT developed by Boeing offers operators a solutionto these obstacles. It provides the ability to test cable shielding for allcritical and essential systems, including fly-by-wire systems, and as a resultcan help operators increase safety of flight.

SUMMARY

STANDARD CURRENT SHUNTS

FIGURE

7

LOOP CERTIFICATIONSTANDARDS

FIGURE

6

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The flight control systemfor the 777 airplane is dif-

ferent from those on otherBoeing airplane designs. Rather

than have the airplane rely on cablesto move the ailerons, elevator, and rudder, Boeingdesigned the 777 with fly-by-wire technology. As a

result, the 777 uses wires to carry electrical signalsfrom the pilot control wheel, column, and pedals toa primary flight computer (PFC). The PFC combinesthese pilot inputs with inertial data and air datafrom the air data inertial reference system to pro-duce flight control surface commands. The PFC thensends the commands, also in the form of electrical

777FLY-BY-WIREMAINTENANCE

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signals carried by wires, to the actuator control elec-tronics, which in turn control the hydraulic actuatorsthat move the control surfaces. Though the 777 doesnot have direct cable connections from the pilotcontrollers to the hydraulic actuators for most sur-faces, for redundancy it has a cable control pathfrom the wheel to one pair of flight spoilers and aredundant cable control input to the stabilizer.

The 777 fly-by-wire flight control system providesall functions necessary for manual control of theairplane in the pitch, roll, and yaw axes. The PFCcontrol laws provide basic maneuver control, stabilityaugmentation, and envelope protection functions.The use of a full authority fly-by-wire systemrequires special care from a design and maintenanceperspective, but allows a greater range of enhancedcontrol functions. One such function is the maneuvercommand pitch control law that optimizes handlingqualities; suppresses any transients, or short-durationvoltages (flight path upsets), caused by configurationchanges or turbulence; and reduces weight by pro-viding stability augmentation that allows the use ofa smaller horizontal tail. The envelope protectionfunctions enhance safety in all axes by helping thepilot avoid normal operational envelope exceedances.

Electrical signals are susceptible to voltage transientscaused by lightning and high-intensity radiated fields(HIRF). The airplane critical flight control system, aswell as all lightning/HIRF critical and essential sys-tems, must be protected from these voltages for thelife of the airplane. Boeing provides the initial protec-tion in the airplane structure; shielding all cabling isadditional protection. Operators are responsible formaintaining the protection by adhering to groundingpractices for all components and inspecting theintegrity of the shielding and shielding connections.Boeing develops scheduled maintenance requirements

for continuous airworthiness using Air TransportAssociation maintenance steering group (MSG)revision 3 processes (MSG-3).

Establishing the requirements begins with extensivemeetings of a working group that includes the origi-nal equipment manufacturer, operators, potentialoperators, the Joint Aviation Authorities, and theU.S. Federal Aviation Administration (FAA). TheMSG outlines the initial minimum maintenance andinspection requirements for development of anapproved continuous airworthiness maintenanceprogram for the airframe, engines, systems, andcomponents. Operators use these requirements asthe basis for developing their own continuous air-worthiness maintenance programs. The resultingmaintenance tasks that operators must complete arethen published in a maintenance review board reportthat Boeing submits to the FAA. Following approval,Boeing includes the tasks in the maintenance planningdata document that is distributed to operators.

In addition to the scheduled maintenance that oper-ators must accomplish on the 777, the FAA requestedthat Boeing develop and implement a lightning/HIRFprotection assurance plan to help operators monitor theshield protection system over the life of the airplane.This plan tests certain critical and essential cables onsix different in-service airplanes every four years todetect failed connectors, failed grounds, or otherinstallation problems not found by operator-scheduledmaintenance activity. The results of these periodicBoeing “validation” tests are compared to conditionsthat existed at the time of airplane delivery to deter-mine if any degradation is occurring that might indi-cate an impending failure. In addition to flight controlsystems, the tests are developed to check enginecontrol circuits, high lift systems, and some ARINC 629circuits that are internal to the pressure hull.