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GENERALThe satisfactory performance of any modern air-

craft depends to a very great degree on the continu-ing reliability of electrical systems and subsystems.Improperly or carelessly installed wiring or improp-erly or carelessly maintained wiring can be a sourceof both immediate and potential danger. The contin-ued proper performance of electrical systems de-pends on the knowledge and techniques of the me-chanic who installs, inspects, and maintains theelectrical system wires and cables.

Procedures and practices outlined in this sectionare general recommendations and are not intended lto replace the manufacturer’: instructions and up-proved practices. ’

For the purpose of this discussion, a wire is de-scribed as a single, solid conductor, or as a strandedconductor covered with an insulating material. Fig-ure 11—1 illustrates these two definitions of a wire.

Ccmducton

ann=solid conductor

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FIGURE 11-1. Two types of aircraft wire.

The term cable, as used in aircraft electrical in-stallations, includes:

(1) Two or more separately insulated conduc-tors in the same jacket (multi-conductorcable).

CHAPTER llAIRCRAFT ELECTRICAL SYSTEMS

(2) Two or more separately insulated conduc-tors twisted together (twisted pair) .

(3) One or more insulated conductors, cov-ered with a metallic braided shield(shielded cable).

(4) A single insulated center conductor with ametallic braided outer conductor (radiofrequency cable). The concentricity of thecenter conductor and the outer conductoris carefully controlled during manufactureto ensure that they are coaxial.

‘Wire SizeWire is manufactured in sizes according to a

standard known as the AWG (American wiregage). As shown in figure 11-2, the wire diametersbecome smaller as the gage numbers become larger.The largest wire size shown in figure 11-2 is num--ber 0000, and the smallest is number 40. Largerand smaller sizes are manufactured but are not com-monly used.

A wire gage is shown in figure 11-3. This type ofgage will measure wires ‘ranging in size from num-ber zero to number 36. The wire to be measured isinserted in the smallest slot that will just accommo-date the hare wire. The gage number correspondingto that slot indicates the wire size. The slot hasparallel sides and should not be confused with thesemicircular opening at the end oi the slot. Theopening simply permits the free movement of thewire all the way through the slot.

Gage numbers are useful in comparing the diame-ter of wires, but not all types of wire or cable canbe accurately measured with a gage. Large wiresare usually stranded to increase their flexibility. Insuch cases, the total area can be determined bymultiplying the area of one strand (usually com-puted in circular mils when diameter or gage num-ber is known) by the number of strands in the wireor cable.

Factors Alfecting the Selection of Wire SizeSeveral factors must be considered in selecting

the size of wire for transmitting and distributingelectric power.

J’

4

Cage DiameterCross section

A num- (mils) Circular Squareher

000000000. 0

1

®~1O5\'J'l1h(.OO

91011121314151617I81920212223242526272829

V 3031.323334

A 353637383940

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Flculu-: 11-2. American wire gage for standard annealed afllid copper wire.

mils

2 12,000.0168,000.0133,000.0106,000.083,700.066,400.052,600.041 ,700.033,100.026,300.020,800.016,500.013, 100.010,400.08,230.06,530.05,1 80.04,1 10.03,260.02,580.02,050.01 ,620.01 ,290.01 ,020.0

810.0642.0509.0404.0320.0254.0202.0160.0127.0.101070:16625010966L525019615712590

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inches

0166.162.105.0629.0657.0521.0416.0026.0260.0206.0164.0130.0103.00s15.0064?.00s1s.0040?.0oa2s.002s6.0020a.00161.0012s.00101.000602.0006a6.000so5.0004o0.000a17.00o252.000200.0001ss.o00126.000099s.0000169.0000626.0000496.0000s94.0000s12.0000246.0000196.00001s6.000012s.000009s.000007s

4-34

Ohms per 1,000 ft.

25°C. 65°C.(=77°FJ (=149°F)

0.0500.0630.0795.100.126.159.201.253.319.403.508.641.808

1.021.281.62

. 2.042.58

» 3.254.0.95.16 ~6.518.21. I10.413.116.520.826.233.041.652.566.283.4

105.0133.0167.0 ’

' 211.0266.0335.0423.0 ’533.0673.0848.0

1,070.0

0.0577.0727.0917.116.146.184.232.292.369.465.586.739.932

1.181.481.872.362.973.754.735.967.519.48

11.915.1 .19.024.030.238.1 ‘48.060.676.496.3

121.0153.0193.0243.0307.0387.0488.0616.0776.0979.0

1,230.0

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Flcuna 11-3. A wire gage.

One factor is the allowable power loss (FR loss)in the line. This loss represents electrical energyconverted into heat. The use of large conductorswill reduce the resistance and therefore the PRloss. However, large conductors are more expensiveinitially than small ones; they are heavier and re-quire more substantial supports.

A second factor is the permissible voltage drop(IR drop) in the line. If the source maintains aconstant voltage at the input to the lines, any varia-tion in the load on the line will cause a variation inline current and a consequent variation in the IRdrop in the line. A wide variation in the IR drop inthe line causes poor voltage regulation at the load.The obvious remedy is to reduce either current orresistance. A reduction in load current lowers theamount off power being transmitted, whereas a re-duction in line resistance increases the size andweight of conductors required. A compromise isgenerally reached whereby the voltage variation atthe load is within tolerable limits and the weight ofline conductors is not excessive.

A third factor is the current-carrying ability ofthe conductor. When current is drawn through theconductor, heat is generated. The temperature ofthe wire will rise until the heat radiated, or other-wise dissipated, is equal to the heat generated bythe passage of current through the line. If the con-ductor is insulated, the heat generated in the con-ductor is not so readily removed as it would be iithe conductor were not insulated. Thus, to protectthe insulation from too much heat, the current

through the conductor must be maintained below acertain value.

When electrical conductors are installed in loca-tions where the ambient temperature is relativelyhigh, the heat generated by external sources consti-tutes an appreciable part of the total conductorheating. Allowance must be made for the influenceof external heating on the allowable conductor cur-rent, and each case has its own specific limitations.The maximum allowable operating temperature ofinsulated conductors varies with the type of conduc-tor insulation being used.

Tables are available that list the safe currentratings for various sizes and types of conductorscovered with various types of insulation. Figure11-5 shows the current-carrying capacity, in am-peres, of single copper conductors at an ambienttemperature of below 30°C. This example providesmeasurements for only a limited range of wire sizes.

Factors Aifocring Selection of Conductor MaterialAlthough silver is the best conductor, its cost

limits its use to special circuits where a substancewith high conductivity is needed.

The two most generally used conductors arecopper and aluminum. Each has characteristics thatmake its use advantageous under certain circum-stances. Also, each has certain disadvantages.

Copper has a higher conductivity; it is moreductile (can be drawn), has relatively high tensilestrength, and can be easily soldered. It is moreexpensive and heavier than aluminum.

Although aluminum has only about 60% of theconductivity of copper, it is used extensively. Itslightness makes possible long spans, and its rela-tively large diameter for a given conductivity re-duces corona (the discharge of electricity from thewire when it has a high potential) . The discharge isgreater when small diameter wire is used than whenlarge diameter wire is used. Some bus bars aremade of aluminum instead of copper where there isa greater radiating surface for the same conduct-ance. The characteristics of copper and aluminumare compared in figure 11-4. i

Frouns 11-4. -Characteristics oi copper and aluminum.Characteristic Copper Aluminum

Tensile strength (lb./£11.’) _______ _- 55,000 25,000Tensile strength for same con-

ductivity (lb.) ______________ -_ 55,000 40,000Weight for same conductivity (lb.) 100 48Cross section for same conductivity

(C. M.)_ __________________ __ 100 160Specific resistance (9/mil ft.) ____ -_ 10.6 17

435

“_____‘_ 1....-y-.._ 4_

ThermoplasticSize Rubber or asbestos, var-

Slow-burningImpregnated or weather-

thermo- cam, or asbestos asbestos Asbestos proofplastic var-cam

300 385260 330225 285195 245165 210140 180120 155105 13580 10055 70

10 40 5512 25 4014 20 30

0000 475410

510430

355 V 370305 . 325.265 .280225 240195 210170 180125 135» 100

90 100 7070 75 5550 55 4040 45 ' 30

370320275235205175150130

Frcuna ll-5. Current-carrying capacity oi wire.

Voltage Drop in Aircraft Wire and CableIt is recommended that the voltage drop in the

main power cables from the aircraft generationsource or the battery to the bus should not exceed2% of the regulated voltage when the generator iscarrying rated current or the battery is being dis-charged at a 5-min. rate. The tabulation in figure11-6 shows the recommended maximum voltagedrop in the load circuits between the bus and theutilization equipment. _

FIGURE 11-6. Recommended maximum voltage dropin load circuits.

7 hillowable voltage drop0 _ _N mill-El ~ 2 e

,y,;e,,, Continuous Iruermiztenzvoltage operation operation

14-23

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The resistance of the current return path throughthe aircraft structure is always considered negligi-ble. However, this is based on the assumption thatadequate bonding of the structure or a special elec-tric current return path has been provided and is

we-go,’ ~_-.-i , . *_'~ ,. -; —--=~rr-- .. ,~-r-r'—._¢~ Y-—-4 *

capable of carrying the required electric currentwith a negligible voltage drop. A resistance meas-urement of 0.005 ohm from the ground point of thegenerator or battery -to the ground terminal of anyelectrical device is considered satisfactory. Anothersatisfactory method of determining circuit resist-ance is to check the voltage drop across the circuit.If the voltage drop does not exceed the limit estab-lished by the aircraft or product manufacturer, theresistance value for the circuit is considered satis-factory. When using the voltage drop method ofchecking a circuit, the input voltage must be main-tained at aconstant value.

Instructions For Use of Electric Wire ChartThe charts in figures 11-7 and 11-8 apply to

copper conductors carrying direct current. Curves1, 2, and 3 are plotted to show the maximum am-pere rating for the specified conductor under thespecified conditions shown. To select the correctsize of conductor, two major requirements must bemet. First, the size must be suflicient to prevent anexcessive voltage drop while carrying the requiredcurrent over the required distance. Secondly, thesize must be suificient to prevent overheating of thecable while carrying the required current. Thecharts in figures 11—7 and 11-8 can simplify thesedeterminations. To use these charts to select theproper size of conductor, the following must beknown:

436

WIRELENGTHINFEET

12so149o¥as!7o‘so

1 1201 210 \ so“\ 1‘ 100‘ 115 1 25 1 IJVIIII 7 1 - I 1__. ;——_L J1 ;__'1 1

120‘ 9 18 II'l11,r.ni/Izwinb ,1 11 1

CONTINUOUS ‘cmcurr AVOLTAGE

‘115 200 14 2a‘13001: 100 200

ELECTRIC WIRE cmm" - V ( '/n4n_7'

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20 1s 16 14 12 10 s 6 4 2 I 1/0 2/03/u 4/0WIRE SIZE

FIGURE 11-7. Conductor chart, continuous flow. (Applicable to copper conductors.)

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WIRE SIZE

F1611!!!-1 ll-8. Collduclor chart, intermittent flow.

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(1) The conductor length in feet.

(2) The number of amperes of current to becarried.

(3) The amount of voltage drop permitted.

(4-) Whether the current to be carried will beintermittent or continuous, and if continu-ous, whether it is a single conductor infree air, in a conduit, or in a bundle.

Assume that it is desired to install a 50-ft. con-ductor from the aircraft bus to the equipment in a28-volt system. For this length, a 1-volt drop ispermissible for continuous operation. By referringto the chart in figure 11-7, the maximum number offeet a conductor may be run carrying a specifiedcurrent with a 1-—volt drop can be determined. Inthis example the number 50 is selected.

Assuming the current required by the equipmentis 20 amperes, the line indicating the value of 20amperes should be selected from the diagonal lines.Follow this diagonal line downward until it inter-sects the horizontal line number 50. From tl'1ispoint, drop straight downward to the bottom of thechart to find that a conductor between size No. 8and No. 10 is required to prevent a greater dropthan 1 volt. Since the indicated value is betweentwo numbers, the larger size, No. 8, should be se-lected. This is the smallest size conductor whichshould be used to avoid an excessive voltage drop.

To determine. that the -conductor size is suilicientto preclude overheating, disregard both the num-bers along the left side of the chart and the hori-zontal lines. Assume that the conductor is to be asingle wire in free air carrying continuous current.Place a pointer at the top of the chart on thediagonal line numbered 20 amperes. Follow this lineuntil the pointer intersects the diagonal line marked“curve 2.” Drop the pointer straight downward tothe bottom of the chart. This point is between num-bers 16 and 18. The larger size, No. 16, should beselected. This is the smallest size conductor accepta-ble for carrying 20-ampere current in a single wirein free air without overheating.

If the installation is for equipment having onlyan intermittent (max. 2 min.) requirement forpower, the chart in figure ll-8 is used in the samemanner.

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Conductor InsulationTwo fundamental properties of insulation materi-

als (for example, rubber, glass, asbestos, or plastic)are insulation resistance and (dielectric strength.These are entirely difierent and distinct properties.

Insulation resistance is the resistance to currentleakage through and over the surface of insulationmaterials. Insulation resistance can be measuredwith a megger without damaging the insulation, anddata so obtained serves as a useful guide in deter-mining the general condition of the insulation.However, the data obtained in this manner may notgive a true picture of the condition of the insula-tion. Clean, dry insulation having cracks or otherfaults might show a high value of insulation resist-ance but would not be suitable for use.

Dielectric strength is the ability of the insulatorto withstand potential difference and is usually ex-pressed in terms of the voltage at which the insula-tion fails because of the electrostatic stress. Maxi-mum dielectric strength values can be measured byraising the voltage of a test sample until the insula-tion breaks down.

Because of the expense of -insulation and its stifi-ening effect, together with the great variety of phys-ical and electrical conditions under which the con-ductors are operated, only the necessary minimuminsulation is applied for any particular type oicable designed to do a specific Ijob.

The type of conductor insulation material varieswith the type of installation. Such types of insula-tion as rubber, silk, and paper are no longer usedextensively in aircraft systems. More common todayare such materials as vinyl, cotton, nylon, Teflon,and Rockbestos.

Identifying Wire and CableAircraft electrical system wiring and cable may

be marked with a combination of letters and num-hers to identify the wire, the circuit it belongs to,the gage number, and other information necessaryto relate the wire or cable to a wiring diagram.Such markings are called the identification code.

There is no standard procedure for marking and-identifying wiring; each manufacturer normally de-velops his own identification code. One identifica-tion system (figure 11—~9) shows the usual spacing

t

1

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439 ,

in marking a wire. The number 22 in the coderefers to the system in which the wire is installed,e.g., the VHF system. The next set of numbers,.013, is the wire number, and the 18 indicates thewire size.

pr.

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Ftcunn 11-9. Wire identification code.

Some system components, especially plugs andjacks, are identified by a letter or group of lettersand numbers added to the basic identification num-ber. These letters and numbers may indicate thelocation of the component in the system. Intercon-nected cables are also marked in some systems toindicate location, proper termination, and use.

In any system, the marking should be legible, andthe stamping color should contrast with the color ofthe wire insulation. For example, black stampingshould be used with light-colored backgrounds, orwhite stamping on dark-colored backgrounds.

Wires are usually marked at intervals of not morethan 15 in. lengthwise and within 3 in. of eachjunction or terminating point. Figure 11-10 showswire identification at a terminal block.

Coaxial cable and wires at terminal blocks andjunction boxes are often identified by marking orstamping a wiring sleeve rather than the wire itself.For general purpose wiring, a flexible vinyl sleev-ing, either clear or white opaque, is commonly used.For high-temperature applications, silicone rubberor silicone fiber glass sleeving is recommended.Where resistance to synthetic hydraulic fluids or

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Fiona: 11-10. Wire identification at a terminal block.

other solvents is necessary, either clear or whiteopaque nylon sleeving can be used.

While the preferred method is to stamp thehidetbtification marking directly on the wire or on thesleeving, other methods are often employed. Figure11-11 shows two alternate methods: one method uses

Pressure-sensitive tape

@._IllIllSleeve markertied in place \

xvz/5‘ A-‘E \ \\,

F10-out ll-II. Alternate methods of identifyingwire bundles.

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a marked sleeve tied in place; the other uses apressure-sensitive tape.

Electrical Wiring InstallationThe following recommended procedures for in-

stalling aircraft electrical wiring are typical of thoseused on most aircraft. For purposes oi this discus-sion, the following definitions are applicable:

(1) Open wiring-—-any wire, wire group, orwire bundle not enclosed in conduit.

(2) Wire group—-two or more wires going tothe same location tied together to retainidentity of the group..

(3) Wire bundle-—two or more wire groupstied together because they are going inthe same direction at the point where thetie is located.

(4) Electrically protected wiring--wires whichinclude (in the circuit) protection againstoverloading, such as fuses,~circuit break-ers, or other limiting devices.

(5) Electrically unprotected wiring-—wires(generally from generators to main busdistribution points) which do not haveprotection, such as fuses, circuit breakers,or other current-limiting devices. ,

Wire’ Groups and BundlesGrouping or bundling certain wires,‘such as elec-

trically unprotected power wiring and wiring goingto duplicate vital equipment, should be avoided.

Wire lnmdles should generally be less than 75wires, or 1-1/2 to 2 in. in diameter where practica-ble. When several wires are grouped at junctionboxes, terminal blocks, panels, etc., identity of thegroup within a bundle (figure 11-12) can be re-tained.

Bundle tie 61'0"? lie Bundle tie

FIGURE 11-12. Group and bundle ties.

Twisting WiresWhen specified on the engineering drawing, or

when accomplished as a local practice, parallelwires must sometimes be twisted. The following arethe most common examples:

(1) Wiring in the vicinity of magnetic com-pass or flux valve.

(2) Three-phase distribution wiring.(3) Certain other wires (usually radio wiring)

as specified on engineering drawings.Twist the wires so that they will lie snugly

against each other, making approximately the num-ber of twists per foot as shown in figure 11-13. Al-ways check wire insulation for damage after twist-ing. If the insulation is torn or frayed, replace theWIIB.

FIGURE ll-13. Recommended number of twists per foot.

Wire Size

#22 #20 #18 1516 #51 #12 {poi its is in2 Wires 10 10 9 e 7 7 6%3 Wires 10 10 3% 7 6% 6 5%

Spliced Connections in Wire Bundlesspliced connections in wire groups or bundles

should be located so that they can be easily in-spected. Splices should also be staggered (figure11-14) so that the bundle does not become exces-sively enlarged. All noninsulated splices should becovered with plastic, securely tied at both ends.

UIQ PU! NIP

Slack in Wiring BundlesSingle wires or wire bundles should not be in-

stalled with excessive slack. Slack between supportsshould normally not exceed a maximum of 1/2 in.deflection with normal hand force (figure 11-15).However, this may be exceeded if the wire bundle isthin and the clamps are, far apart. Slack shouldnever be so great that the wire bundle could abradeagainst any surface. A suflicient amount of slackshould be allowed near each end of a bundle to:

(1) Permit easy maintenance.(2) Allow replacement of terminals.(3) Prevent mechanical strain on the wires,

wire junctions, and supports.(4) Permit free movement of shock and vibra-

tion-mounted equipment.(5) Permit shifting of equipment for purposes

of maintenance.

Bend RudiiBends in wire groups or bundles should be not

less than 10 times the outside diameter of the wiregroup or bundle. However, at terminal strips, wherewire is suitably supported at each end of the bend,a minimum radius of three times the outside diame-ter of the wire, or wire bundle, is normally accepta-ble. There are, of course, exceptions to these guide-lines in "the case of certain types of cable; for

I

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FIGURE 11-14. Staggered splices in a wire bundle.

example, coaxial cable should never be bent to asmaller radius than ten times the outside diameter.

Routing and InstallationsAll wiring should be installed so that it is me-

chanically and electrically sound and neat in ap-pearance. Whenever practicable, wires and bundlesshould be routed parallel with, or at right angles to,the stringers or ribs of the area involved. An excep-tion to this general rule is coaxial cable, which isrouted as directly as possible.

The wiring must be adequately supportedthroughout its length. A suflicient number of sup-ports must be provided to prevent undue vibrationof the unsupported lengths. All wires and wiregroups should be routed and installed to protectthem from:

(1) Chafing or abrasion.(2) High temperature.(3) Being used as handholds, or as support

for personal belongings and equipment.(4) Damage by personnel moving within the

aircrait.(5) Damage from cargo stowage or shifting.(6) Damage from battery acid fumes, spray,

or spillage.(7) Damage from solvents and fluids.

Protection Against ChafingWires and wire groups should be protected

against chafing or abrasion in those locations wherecontact with sharp surfaces or other wires woulddamage the insulation.lDamage to the insulationcan cause short circuits, malfunction, or inadvertentoperation of equipment. Cable clamps should beused to support wire bundles at each hole through abulkhead (figure ll-16). If wires come closer than1/4 in. to the edge of the hole, a suitable grommetis used in the hole as shown in figure ll-17.

Sometimes it is necessary to cut nylon or rubbergrommets to facilitate installation. In these in-stances, after insertion, the grommet ‘can be securedin place with general-purpose cement. The cutshould be at the top of the hole, and made at anangle of 45° to the axis of the wire bundle hole.

Protection against High Temperature

To prevent insulation deterioration, wires shouldbe kept separate from high-temperature equipment,such as resistors, exhaust stacks, or heating ducts.The amount of separation is normally specified byengineering drawings. Some wires must invariablybe run through hot areas. These wires must beinsulated with high-temperature material such asasbestos, fiber glass, or Teflon. Additional protec-tion is also often requred in the form of conduits.A low-temperature insulation wire should never beused to replace a high-temperature insulation wire.

Many coaxial cables have soft plastic insulation,such as polyethylene, which is especially subject todeformation and deterioration at elevated tempera-tures. All high-temperature areas should be avoidedwhen installing these cables insulated with plasticor polyethylene.

Additional abrasion protection should be given toasbestos wires enclosed in conduit. Either conduitwith a high-temperature rubber liner should beused, or asbestos wires can be enclosed individuallyin high-temperature plastic tubes before being in-stalled in the conduit.

Protection Against Solvents and Fluids

Wires should not be installed in areas where theywill be subjected to damage from fluids or in thelowest 4-in. of an aircraft fuselage, except thosethat must terminate in that area. Ifthere is a possi-bility that wire may be soaked with fluids, plastictubing should be used to protect the wire. Thistubing should extend past the exposure area in bothdirections and should be tied at each end. If thewire has a low point between the tubing ends, pro-vide a %~in. drain hole, as shown in figure 11-18.This hole should be punched into the tubing afterthe installation is complete and the low point defi-nitely established by using a hole punch to cut ahalf circle. Care should be taken not to damage anywires inside the tubing when using the punch.

Wire should never be routed below an aircraft

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FIGURE 11-15. Slack in wire bundle between supports.

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FIGURE 11-I6. Cable clamp at bulkhead hole.

battery. All wires in the vicinity of an aircraftbattery should be inspected frequently and wiresdiscolored by battery fumes should be replaced.

Protection of Wires in Wheel Well AreaWires located in wheel wells are subject to many

additional hazards, such as exposure to fluids,pinching, and severe floating in service. All wirebundles should be protected by sleeves of flexibletubing securely held at each end, and there shouldbe no relative movement at points where flexibletubing is secured. These wires and the insulatingtubing should be inspected carefully at frequentintervals, and wires or tubing should be replaced atthe first sign of wear. There should be no strain onattachments when parts are fully extended, butslack should not be excessive. "

Routing PrecautionsWhen wiring must be routed parallel to combusti-

ble fluid or oxygen lines for short distances, asmuch fixed separation “as possible should be main-tained. The wires should be on a level with, orabove, the plumbing lines. Clamps should be spacedso that if a wire is broken at a clamp it will notcontact the line. lmlere a 6-in. separation is notpossible, both the wire bundle and the plumbingline can be clamped to the same structure to preventany relative motion. If the separation is less than 2in. but more than 1/2 in., a polyethylene sleeve maybe used over the wire bundle to give further protec-tion. Also two cable clamps back-to-back, as shownin figure 11-19, can be used to maintain a rigidseparation only, and not for support of the bundle.No wire should be routed so that it is locatednearer than 1/2 in. to a plumbing line. Neithershould a wire or wire bundle be supported from a

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FIGURE 11-18. Drain hole in low point of tubing.

plumbing line that carries flammable fluids or oxy-gen.

Wiring should be routed to maintain a minimumclearance of at least 3 in. from control cables. Ifthis cannot be accomplished, mechanical guardsshould be installed to prevent contact between wir-ing and control cables.

Installation of Cable ClampsCable clamps should be installed with regard to

the proper angle, as shown in figure 11-20. Themounting screw should be above the wire bundle. Itis also desirable that the back of the cable clamprest against a structural member where practicable.

Figure 11-21 shows some typical mounting hard-ware used in installing cable clamps.

Care should be taken that wires are not pinchedin cable clamps. Where possible, mount the cabludirectly to structural members, as shown in figure11-22.

Clamps can be used with rubber cushions tosecure wire bundles to tubular structures as shownin figure 11-23. Such clamps must fit tightly, butshould not be deformed when locked in piece.

IACING AND TYING WIRE BUNDLESWire groups and bundles are laced or tied with

cord to provide ease of installation, maintenance,and inspection. This section describes and illus-trates recommended procedures for lacing and tyingwires with knots which will hold tightly under allconditions. For the purposes of this discussion, thefollowing terms are defined:

(1) Tying is the securing together of a group

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FIGURE 11-20. Proper mounting angles for cable clamps.

Sale Angles

or bundle of wires by individual pieces ofcord tied around the group or bundle atregular intervals.

(2) Lacing is the securing together of a groupor bundle of wires by a continuous pieceof cord forming loops at regular intervalsaround the group or bundle.

(3) A wire group is two or more wires tied orlaced together to give identity to an indi-vidual system.

(4) A wire bundle is -two or more wires orgroups tied or laced together to facilitatemaintenance.

The material used for lacing and tying is eithercotton or nylon cord. Nylon cord is moisture- andfungus-resistant, but cotton cord must be waxedbefore using to give it these necessary protectivecharacteristics.

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Single-Cord lacingFigure 11-24 shows the step in lacing a wire

bundle with a single cord. The lacing procedure isstarted at the thick end of the wire group or bundlewith a lcnot consisting of a clove hitch with an extraloop. The lacing is then continued at regular inter-

vals with hall hitches along the wire group or bun-dle and at each point where a wire or wire groupbranches oh’. The half hitches should be spaced sothat the bundle is neat and secure. The lacing isended by tying a knot consisting of a clove hitchwith an extra loop. After the knot is tied, the free

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FIGURE ll-22. Mounting cable clamp to structure.

ends of the lacing cord should be trimmed to ap-proximately 3/8 in.Double-Cord lacing

Figure 11-25 illustrates the procedure for dou-

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Flaunt: 11-24. Single-cord lacing.

ble-cord lacing. The lacing is started at the thickend of the wire group or bundle with a b0wline-on-a~bight knot (A of figure 11-25). At regular inter-vals along the wire group or bundle, and at eachpoint where a wire branches ofi, the lacing is con-tinued using half hitches, with both cords heldfirmly together. The half hitches should ‘be spacedso that the group or bundle is neat and secure. Thelacing is ended with a knot consisting of a halfhitch, continuing one of the cords clockwise and theother counterclockwise and then tying the cord endswith a square knot. The free ends of the lacing cordshould be trimmed to approximately 3/8 in. '

lacing Branch-Offs 'Figure 11-26 illustrates a recommended proce-

dure ior lacing a wire group that branches off themain wire bundle. The branch-oil lacing is startedwith a knot located on the main bundle just past thebranch-ofi point. Continue the lacing along thebranched-oil wire group, using regularly spacedhalf hitches. If a double cord is used, both cordsshould be held snugly together. The half hitchesshould be spaced to lace the bundle neatly andsecurely. The lacing is ended with the regular termi-nal knot used in single- or double-cord lacing. Thefree ends of the lacing cord should be neatlytrimmed.

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Tying

All Wire groups or bundles should be tied wheresupports are more than 12 in. apart. Figure 11-28illustrates a recommended procedure for tying awire group or bundle. The tie is started by wrap-ping the cord around the wire group, to tie a clove-hitch knot. Then a square knot with an extra loopis tied, and the free ends of the cord are trimmed.

Temporary ties are sometimes used in making upand installing wire groups and bundles. Coloredcord is normally used to make temporary ties, sincethey are removed when the installation is complete.

Whether laced or tied, bundles should be securedto prevent slipping, but not s0 tightly that the cordcuts into or deforms the insulation. This appliesespecially to coaxial cable, which has a soft dielec-tric insulation between the inner and outer conduc-to_r.

Thepart of a wire group or bundle located insidea conduit is not tied or laced, but wire groups orbundles inside enclosures, such as junction boxes,should be laced only. p

CUTTING WIRE AND CABLE 'To make installation, maintenance, and repair

easier, wire and cable runs in aircraft are broken atspecified locations by junctions, such as connectors,

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terminal blocks, or buses. Before assembly to thesejunctions, wires and cables must be cut to length.

All wires and cables should be cut to the lengthsspecified on drawings and wiring diagrams. The cutshould be made clean and square, and the wire orcable should not be deformed. If necessary, large-diameter wire should be re-shaped after cutting.Good cuts can be made only if the blades of cuttingtools are sharp and free from nicks. A dull bladewill deform and extrude wire ends.

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, FIGURE 11-2V6. Lacing a branch-off.

Stripping Wire and CableBefore wire can be assembled to connectors, ter-

minals, splices, etc., the insulation must be strippedfrom connecting ends to expose the bare conductor.

Copper wire can be stripped in a number of waysdepending on the size and insulation. Figure 11-27lists some types of stripping tools recommended forvarious wire sizes and types of insulation.

FIGURE ll-27. Wire strippers for copper wire.

l U lStripper T l Z Z lvlrfszlf l l lm}1ZuZ»§ lHot-blade §26—-114 All except asbestosRotary, electric §26—#4 AllBench #20-—§6 AllHand pliers #26-—f8 AllKnife #2 -+#0000 All

Aluminum wire must be stripped very carefully,using extreme care, since individual strands willbreak very easily after being nicked.

The following general precautions are recom-mended when stripping any type of wire:

(1) When using any type of wire stripper,hold the wire so that it is perpendicular tocutting blades.

(2) Adjust automatic stripping tools care-

44-7 .

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FIGURE 1]~2B. Tying

fully; follow the manufacturer’s instruc-tions to avoid nicking, cutting, or other-wise damaging strands. This is especiallyimportant for aluminum wires and forcopper wires smaller than No. 10. Exam-ine stripped wires for damage. Cut ofl andre-strip (if length is sufficient), or rejectand replace any wires having more thanthe allowable number of nicked or brokenstrands listed in the manufacturer’s in-slructions.

(3) Make sure insulation is clean-cut with nofrayed or ragged edges. Trim if necessary.

(4) Make sure all insulation is removed fromstripped area. Some types of wires aresupplied with a transparent layer of insu-lation between the conductor and the pri-mary insulation. lf this is present, removeit.

(5) When using hand-plier strippers to removelengths of insulation longer than 3/4 in.,it is easier to accomplish in two or moreoperations.

(6) Re-twist copper strands by hand or withpliers, if necessary, to restore natural layand tightness of strands.

A pair of hand wire strippers is shown in figure11-29. This tool is commonly used to strip mosttypes of wire.

The following general procedures describe thesteps for stripping wire with a hand stripper.(Refer to figure 11-30.)

(1) Insert wire into exact center of correctcutting slot for wire size to be striPPed.Each slot is marked with wire size.

(2) Close handles together aspiar as they willgo.

(3) Release handles, allowing wire holder toreturn to the “open” position.

(4) Remove stripped wire.

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Solderless Terminals and SplicesSplicing of electrical cable should be kept to a

minimum and avoided entirely in locations subjectto extreme vibrations. Individual wires in a groupor bundle can usually be spliced, provided the com-pleted splice is located so that it can be inspectedperiodically. Splices should be staggered so that thebundle does not become excessively enlarged. Manytypes of aircraft splice connectors are available forsplicing individual wires. Self-insulated spliceconnectors are usually preferred; however, a nonin-sulated splice connector can be used if the splice iscovered with plastic sleeving secured at both ends.Solder splices may be used, but they are particu-larly brittle and not recommended.

Electric wires are terminated with solderless ter-minal lugs to permit easy and elficient connection toand disconnection from terminal blocks, bus bars,or other electrical equipment. Solderless splices joinelectric wires to form permanent continuous runs.Solderless terminal lugs and splices are made oicopper or aluminum and are preinsulated or uninsu-lated, depending on the desired application.

Terminal lugs are generally available in threetypes for use in different space conditions. Theseare the flag, straight, and right-angle lugs. Terminallugs are “crirnped” (sometimes called “staked” or“swaged”) to the wires by means of hand or powercrimping tools.

The following discussion describes recommended

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wire

Frown: 11-31. Preinsulated terminal lug.

methods for terminating copper and aluminumwires using solderless terminal lugs. It also de-scribes the method for splicing copper wires usingS0l(l6l'l6SS splices.

Copper Wire TerminalsCopper wires are terminated with solderless,

preinsulated straight copper terminal lugs. The in-sulation is part of the terminal lug and extendsbeyond its barrel so that it will cover a portion ofthe wire insulation, making the use of an insulationsleeve unnecessary (figure 11-31).

in addition, preinsulated terminal lugs contain aninsulation grip (a metal reinforcing sleeve) beneaththe insulation for extra gripping strength on thewire insulation. Preinsulated terminals accommo-date more than one size of wire; the insulation isusually color-coded to identify the wire sizes thatcan be terminated with each of the terminal lugsizes.

Crimping ToolsHand, portable power, and stationary power tools

are available for crimping terminal lugs. These toolscrimp the barrel of the terminal lug to the conduc-tor and simultaneously crimp the insulation grip tothe wire insulation.

Hand crimping tools all have a self-lockingratchet that prevents opening the tool until thecrimp is complete. Some hand crimping tools areequipped with a nest of various size inserts to fitdiflerent size terminal lugs. Others are used on oneterminal lug size only. All types of hand crimpingtools are checked by gages for proper adjustment ofcrimping jaws.

Figure 11-32 shows a terminal lug inserted intoa hand tool. The following general guidelines out-line the crimping procedure.

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FIGURE 11-32. Inserting terminal lug into hand tool.

(1) Strip the wire insulation to proper length.(2) Insert the terminal lug, tongue first, into

hand tool barrel crimping jaws until theterminal lug barrel butts flush against thetool stop.

(3) Insert the stripped wire into the terminallug barrel until the wire insulation buttsflush against the end of the barrel.

(4) Squeeze the tool handles until the ratchetreleases.

(5) Removethe completed assembly and exam-ine it for proper crimp. '

Some types of uninsulated terminal lugs are insu-lated after assembly to a wire by means of pieces oftransparent flexible tubing called “sleeves.” Thesleeve provides electrical and mechanical protectionat the connection. When the size of the sleevingused is such that it will fit tightly over the terminallug, the sleeving need not be tied; otherwise, it

mil 1!! approx‘

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Loose sleeve 4

Frcuna ll-33. Insulating sleeve.

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should be tied with lacing cord as illustrated infigure 11-33. '

Aluminum Wire TerminalsThe use of aluminum wire in aircraft systems is

increasing because of its weight advantage overcopper. However, bending aluminum will cause“work hardening” of the metal, making it brittle.This results in failure or breakage of strands muchsooner than in a similar case with copper wire.Aluminum also forms a high-resistant oxide filmimmediately upon exposure to air. To compensatefor these disadvantages, it is important to use themost reliable installation procedures.

Only aluminum terminal lugs are used to termi-nate aluminum wires. They are generally availablein three types: (1) Straight, (2) right-angle, and(3) flag. All aluminum terminals incorporate aninspection hole (figure 11-34) which permitschecking the depth of wire insertion. The barrel ofaluminum terminal lugs is filled with a petrolatum-zinc dust compound. This compound removes theoxide film from the aluminum by a grinding processduring the crimpingoperation. The compound willalso minimize later oxidation of the completedconnection by excluding -moisture and -air. The com-pound is retained inside the terminal lug barrel bya plastic or foil seal at the end of the barrel.

Splicing Copper ‘Wires Using Prainsululed Splice:Preinsulated permanent copper splices join small

wires of sizes 22 through 10. Each splice size canbe used for more than one wire size. Splices areusually color-coded in the same manner as preinsu-lated small copper terminal lugs. Some splices areinsulated with white plastic. Splices are also used’ toreduce wire sizes (figure 11-35). -

Crimping tools are used to accomplish this typeof splice. The crimping procedures are the same asthose used for terminal lugs, except that the crimp-ing operation must be done twice, once for each endof the splice.

EMERGENCY SPI-ICING REPAIRSBroken wires can be repaired by means of

crimpecl splices, by using terminal lugs from whichthe tongue has been cut ofi, or by soldering to-gether and potting broken strands. These repairsare applicable tocopper wire. Damaged aluminumwire must not be temporarily spliced. These repairsare for temporary emergency use only and shouldbe replaced as soon as possible with permanentrepairs. Since some manuiacturers prohibit splicing,

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the applicable manufacturer's instructions should al-ways be consulted. _ _

Splicing with Solder and Potting CompoundWh neither a permanent splice nor s terminal

lug is available, a broken wire can be repaired asfollows (figure 11-36):

(1) Install a piece of plastic sleeving about 3in. long, and of the proper diameter to fitloosely over the insulation, on one pieceof the broken wire.

5:-_;::1_iZ'_II3TI'liIlIICI' WIIB wire

doubled over

Strip approximately 1-I/2 in. from eachbroken end of the wire.

(3) Lay the stripped ends side by side andtwist one wire around the other with ap-proximately four turns.

(4) Twist the free end of the second wirearound the first wire with approximatelyfour turns. Solder wire turns together,using 60/40 tin-lead resin-core solder. lWhen solder is cool, draw the sleeve over

the soldered wires and tie at one end. Iipotting compound is available, fill thesleeve with potting, material and tie se-curely.Allow the potting compound to set withouttouching for 4 hrs. Full cure and electricalcharacteristics are achieved in 24 hrs.

couuecrmoi TERMINAL was to remnant“ * '* ‘ BLOCKSOwe” with vinyl tube Terminal lugs should be installed on terminal

“ed at both ems I blocks so that they are locked against movement inFIGURE 11-35. Reducing wire size with a permanent splice. the direction of loosening (figure 11-37).

1 1/2" l I/2”Approx. Approx.

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Step 2 i i

Step 3

w I" Step 4 .

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Potting dispenser

FICURE 11-36. Repairing ‘broken wire _soldering and potting. '_ ,

Terminal blocks arenormally supplied with studssecured in place by a plain washer, an extemaltooth lockwasher, and a nut. In connecting termi-nals, a recommended practice is to place copperterminal lugs directly on top of the nut, followedwith a plain washer and elastic stop nut, or with aplain washer, split steel lockwasher, and plain nut.

Aluminum terminal lugs should be placed over aplated brass plain washer, followed with anotherplated brass plain washer, split steel lockwasher,and plain nut or elastic stop nut. The plated brasswasher should have a diameter equal to the tongueWidth of the aluminum terminal lug. Consult themanufacturer's instructions for recommended di-mensions of these plated brass washers. Do notplace any washer in the current path between twoalmninum terminal lugs or between two copper ter-minal lugs. Also, do not place a lockwasher directlyagainst the tongue or pad of the aluminum terminal.

To join a copper terminal lug to an aluminumterminal lug, place a plated brass plain washer overthe nut which holds the stud in place; follow withthe aluminum terminal lug, a plated brass plainwasher, the copper tenninal lug, plain washer, splitsteel lockwasher, and plain nut or self-locking, all-metal nut. As a general rule use a torque wrench totighten nuts to ensure suflicient contact pressure.

452

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Manufacturer’s instructions provide installationtorques for all types of terminals.BONDING AND. GROUNDINQ

Bonding is the electrical connecting of two or__more conducting -objects not otherwise adequatelyconnected. Grounding is the electrical connecting ofa conducting object to the primary structure for areturn path for current. Primary structure is themain frame, fuselage, or wing structure of the air--craft, commonly referred to as ground. Bondingand grounding connections are made in aircraftelectrical systems to:

(1) Protect aircraft and personnel against haz-V ards from lightning discharge.

(2) Provide current return paths.(3) Prevent development of radio-frequcy

potentials.(4) Protect personnel from shock hazards.(5) Provide stability of radio transmission

and reception.(6) Prevent accumulation of static charge.

General Bonding and Grounding ProceduresThe following general procedures and precau-

tions are recommended when making bonding orgrounding connections:

(1) Bond or gronud parts to the primary air-craft structure where practicable.

(2) Make bonding or grounding connectionsso that no part of the aircraft structure isweakened.

(3) Bond parts individually if possible.(4) Install bonding or grounding connections

against smooth, clean surfaces.V (5) Install bonding or grounding connections

so that vibration, expansion or contrac-tion, or relative movement in normal serv-ice will uot break or loosen the connec-tion. i

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_F10uan 11-37. Connecting terminals to terminal block.

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(6) Install bonding and grounding connec-tions in protected areas whenever possible.

Bonding jumpers should be kept as short as prac-ticable. The jumper should not interfere with theoperation of movable aircraft elements, such as sur-face controls; normal movement of these elementsshould not result in damage to the bonding jumper.

Electrolytic action can rapidly corrode a bondingconnection if suitable precautions are not observed.Aluminum alloy jumpers are recommended for mostcases; however, copper jumpers can be used tobond together parts made of stainless steel, cadi-mum~plated steel, copper, brass, or bronze. Wherecontact between dissimilar metals cannot beavoided, the choice of jumper and hardware shouldbe such that corrosion is minimized, and the partmost likely to corrode will be the jumper or asso-ciated hardware. Figure 11-38 illustrates someproper hardware combinations for making bondingconnections. At locations where finishes are re-moved, a protective finish should be applied to thecompleted connection to prevent corrosion.

The use of solder to attach bonding jumpersshould be avoided. Tubular members should beBonded by means of clamps to which the jumper isattached. The proper choice of clamp material mini-mizes the probability of corrosion. When bondingjumpers carry a substantial amount of ground re-turn current, the current rating of the jumpershould be adequate, and it should be determinedthat s negligible voltage drop is produced.

Bonding and grounding connections are normallymade to flat surfaces by means of through-bolts orscrews where there is easy access for installation.Other general types of bolted connections are asfollows:

(1) In making a 'stud connection (figure11-39), a bolt or screw i locked securelyto the structure, thus becoming a stud.Grounding or bonding jumpers can be re-moved or added to the shank of the studwithout removing the stud from the struc-ture.

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(2) Nut plates are used where access to thenut for repairs is dillicult. Nut plates areriveted or welded to a clean area of thestructure (figure 11-40).

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7Bonding and grounding connections are also

made to a tab riveted to a structure. In such cases itis important to clean the bonding or groundingsurface and make the connection as through theconnection were being made to the structure. If it isnecessary to remove the tab for any reason, therivets should be replaced with rivets one size larger,and the mating surfaces of the structure and the tabshould be clean and free of anodic film.

Bonding or groimding connections can be madeto aluminum alloy, magnesium, or corrosion-resis~tant steel tubular structure as shown in figure11-4-1, which shows the arrangement of hardwarefor bonding with an aluminum jumper. Because ofthe ease witlrwhich aluminum is deformed, it isnecessary to distribute the screw and nut pressureby means of plain washers. A

Hardware used to make bonding or groundingconnections should be selected on the basis of me-chanical strength, current to be carried, and ease ofinstallation. If connection is made by aluminum orcopper jumpers to the structure of a dissimilar ml-terial, a washer of suitable material should be in-stalled betweenthe dissimilar metals so that anycorrosion will occur on the washer, which is eu-pendablc. ’

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cylindrical surface.

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Hardware material and finish should be selectedon the basis of the material of the structure towhich attachment is made and on the material ofthe jumper and terminal specified for the bondingor grounding connection. Either a screw or bolt ofthe proper size for the specified jumper terminalshould he used. When repairing or replacing exist-ing bonding or grounding connections, the sometype of hardware used in the original connectionshould always be used.

Testing Grounds and Bonds .The resistance of all bond and ground connec-

tions should be testedafter connections are madebefore re-finishing. The resistance of each connec-tion should normally not exceed 0.003 ohm. Resist-ance measurements need to be of limited natureonly for verification of the existence of a bond, butshould not beconsidered as the sole proof of satis-factory bonding. The length of jumpers, methods,and materials used, and the possibility of looseningthe connections in service should also be consid-ered.

CONNECTORS 'Connectors (plugs and receptacles) facilitate

maintenance when frequent "disconnection is re-quired. Since the cable is soldered to the connectorinserts, the joints should be individually installedand the cable bundle firmly supported to avoiddamage by vibration. Connectors have been particu-larly vulnerable to corrosion in the past, due tocondensation within the shell. Special connectorswith waterproof features have been developedwhich may replace non-waterproof plugs in areaswhere mositure causes a problem. Aiconnector ofthe same basic type and design should be usedwhen replacing a connector. Connectors susceptibleto corrosion difiiculties may be treated with a chem-ically inert waterproof jelly. When replacingconnector assemblies, the sockebtype insert shouldbe used on the half which is “live” or “hot” afterthe connector is disconnected, to prevent uninten-tional grounding.

Typos of Connectors _Connectors are identified by AN numbers and

are divided into classes with the rnanufacturer’s var-iations in each class. The manufacturefs variationsare difierences in appearance and in the method ofmeeting a specification. Some commonly usedconnectors are shown in figure 11-4-2. There arefive basic classes of AN connectors used in most

aircraft. Each class of connector has slightly difier-ent construction characteristics. Classes A, B, C,and D are made of aluminum, and class K is madeof steel.

(I) Class A—--Solid, one-piece back shell, gen-_ eral-purpose connector.

' (2) Class B--Connector back shell separatesinto two parts lengthwise. Usedprimarily where it is importantthat the soldered connectors be

readily accessible. The back shell- is held together by a threaded

ring or by screws.(3) Class C-—A_ pressurized connector with

I inserts that are not removable.C Similar to a class A connector

in appearance, but the insidesealing arrangement is some-times difierent. It is used onwalls of bulkheads of pressur-ized equipment.

(4) Class D~Moisture- and vibration-resistant connector which has a seal-ing grommet in the back shell.Wires are threaded throughtight-fitting holes in the grom-met, thus sealing against mois-

. ture.(5) Class K——A fireproof connector used in

areas where it is vital that theelectric current is not inter-rupted, even though the con-

~ nector may be exposed to con-tinuous open flame. Wires arecrimped to the pin or socketcontacts and the shells are madeoi steel. This class of con-nector is.normaIly longer than

. other classes oi connectors.

Connector IdentificationCode letters and numbers are marked on the cou-

pling ring or shell to identify a connector. Thiscode (figure 11-43) provides all the informationnecessary to obtain the correct replacement for adefective or damaged part.

Many special-purpose connectors have been de-signed for use in aircraft applications. These in-clude subminiature and rectangular shell connec-tors, and connectors with short body shells or split-shell construction.

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9 9 gonna amusement numbe, (1) Locate the proper position of the plug in.“/N relation to the receptable by aligning the

\ l Contact style (socket) key of one part with the groove or keyway\_ Insert rotation of the other part.

(2) Start the plug into the receptacle with aFIGURE 1143- AN °°'"1e°*°1' marking light forward pressure and engage the

' ’ ~\ _ ** TYP° (5""*i3"" Plug) The following procedures outline one recom-9 ' C1855 mended method of assembling connectors to recept-

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threads oi the coupling ring and recepta-cle.

(3) Alternately push in the plug and tightenthe coupling ring until the plug is com-pletely seated.

(4) Use connector pliers to tighten couplingrings one sixteenth to one eighth turn be-yond fingertight if space around theconnector is too small to obtain a goodfinger grip.

(5) Never use force to mate connectors to re-ceptacles. Do not hammer a plug into itsreceptacle; and never use a torque wrenchor pliers to lock coupling rings.

A connector is generally disassembled from a re-ceptacle in the following manner:

(1) Use connector pliers to loosen couplingrings which are too tight to be loosenedby hand.

(2) Alternately pull on the plug body and un-screw the coupling ring until the connec-tor is separated.

(3) Protect disconnected plugs and receptacleswith caps or plastic bags to keep debrisfrom entering and causing faults.

(4) Do not use excessive force, and do notpull on attached wires.

CONDUIT

Conduit is used in aircraft installations for themechanical protection of wires and cables. It isavailable in metallic and nonmetallic materials inboth rigid and flexible form.

lmien selecting conduit size for a specific cablebundle application, it is common practice to allowfor ease in maintenance and possible future circuitexpansion by specifying the conduit inner diameterabout 25% larger than the maximum diameter oithe conductor bundle. The nominal diameter of arigid metallic conduit is the outside diameter.Therefore, to obtain the inside diameter, subtracttwice the tube wall thiclmcss.

From the abrasion standpoint, the conductor isvulnerable at the conduit ends. Suitable fittings areafiixed to the conduit ends in such a manner that asmooth surface comes in contact with the conductorwithin the conduit. When fittings are not used, theconduit end should be flared to Prevent wire insula-tion damage. The conduit is supported by clampsalong the conduit run.

Many of the common conduit installation prob-lems can be avoided by proper attention to the

following details:(1) Do not locate conduit where it can be

used as a handheld or footstep.(2) Provide drain holes at the lowest point in

a conduit run. Drilling burrs should becarefully removed from the drain holes.

(3) Support the conduit to prevent chafingagainst the structure and to avoid stress-ing its end fittings.

Damaged conduit sections should be repaired toprevent damage to the wires or wire bundle. Theminimum acceptable tube bend radii for rigid con-duit as prescribed by the manufacturer’s instruc-tions should be followed carefully. Kinlced or wrin-lcled bends in a rigid conduit are normally notacceptable.

Flexible aluminum conduit is widely available intwo types: (1) Bare flexible and (2) rubber-cowered conduit. Flexible brass conduit is normallyused instead of flexible aluminum conduit, wherenecessary to minimize radio interference. Flexibleconduit may be used where it is impractical to userigid conduit, such as areas that have motion be-tween conduit ends or where complex bends arenecessary. Transparent adhesive tape is recom-mended when cutting flexible tubing with a hack-saw to minimize fraying of the braid.

ELECTRICAL EQUIPMENT lNS'I'Al.l.ATIONThis section provides general procedures and

safety precautions for installation of commonly usedaircraft electrical equipment and components. Elec-trical load limits, acceptable means of controlling ormonitoring electrical loads, and circuit protectiondevices are subjects with which mechanics must befamiliar to properly install and maintain aircraftelectrical systems.

Electrical loud limitsWhen installing additional electrical equipment

that consumes electrical power in an aircraft, thetotal electrical load must be safely controlled ormanaged within the rated limits of the affected com-ponents of the aircraft’s power-supply system.

Before any aircraft electrical load is increased,the associated wires, cables, and circuit protectiondevices (fuses or circuit breakers) should bechecked to determine that the new electrical load(previous maximum load plus added load) does notexceed the rated limits of the existing wires, cables,or promotion devices.

The generator or alternator output ratings pre-scribed by the manufacturer should be compared

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with the electrical loads which can be imposed onthe affected generator or alternator by installedequipment. When the comparison shows that theprobable total connected electrical load can exceedthe output load limits of the generator(s) or alter-nator(s), the load should be reduced so that anoverload cannot occur. When a storage battery ispart of the electrical power system, ensure that thebattery is continuously charged in flight, exceptwhen short, intermittent loads are connected such asa radio transmitter, a landing-gear motor, or othersimilar devices which may place short-time demandloads on the battery.

Controlling or Monitoring thoilectricul loudPlacards are recommended to inform crewmem-

bers of an aircraft about the combination of electri-cal loads that can safely be connected to the powersource. . » »

In installations where the ammeter is in the bat-tery lead, and the regulator system limits the maxi-mum current that the generator or alternator candeliver, a voltmeter can be installed on the systembus. As long as the ammeter does not read “dis-charge” (except for short, intermittent loads suchas operating the gear and flaps) and the voltmeterremains at “system voltage," the generator or alter-nator will not be overloaded.

In installations where the -ammeter is in the gen-erator or alternator lead, and the regulator systemdoes not limit the maximum current that the genera-tor or alternator can deliver, the ammeter can beredlined at 100% of the generator or alternatorrating. Ii the ammeter reading is never allowed toexceed the red line, except for short, inermittentloads, the generator or alternator will not be over-loaded.» A

Where the use of placards or monitoring devicesis not practicable or desired, and where assuranceis needed that the battery in a typical small aircraftgenerator/battery power source will be charged inflight, the total continuous connected electrical loadmay be held‘ to approximately 80% of the totalrated generator output capacity. (When more thanone generator is used in parallel, the total ratedoutput is the combined output of the installed gen-erators.)

When two or more generators are operated inparallel and the total connected system load canexceed the rated output of one generator, meansmust be provided for quickly coping with the sud-den overloads which can be caused by generator or

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engine failure. A quick load reduction system, or aspecified procedure whereby the total load can bereduced to a quantity which is within the ratedcapacity of the remaining operable generator(s),can be employed.

Electrical loads should be connected to inverters,alternators, or similar aircraft electrical powersources in such a manner that the rated limits of thepower source are not exceeded, unless some type ofefiective monitoring means is provided to keep theload within prescribed limits.

Circuit Protection DevicesConductors should be protected with circuit

breakers or fuses located as close as possible to theelectrical power source bus. Normally, the manufac-turer of the electrical equipment specifies the fuseor circuit breaker to be used when installing equip-ment. '

The circuit breaker or fuse should open thecircuit before the conductor emits smoke. To accom-plish this, the time current characteristic oi theprotection device must fall below_that of the asso-ciated conductor. Circuit protector characteristicsshould be matched to obtain the maximum utiliza-tion of the connected equipment.

Figure 11-44 shows an example of the chart usedin selecting the circuit breaker and fuse protectionfor copper conductors. This limited chart is applica-ble to a specific set of ambient temperatures andwire bundle sizes, and is presented as a typicalexample only. It is important to consult such guidesbefore selecting a conductor for a specific purpose.For example, a wire run individually in the openair may be protected by the circuit breaker of thenext higher rating to that shown on the chart.

Wire AN Circuit ‘ Fuse amp.gage copper breaker amperage

22 5 520 7.5 518 10 1016 15 1014 20 1512 30 2010 40 308 50 506 80 70

100 70125 100

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Ficum-: ll-44. Wire and circuit protector chart.

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All re-settable circuit breakers should open thecircuit in which they are installed regardless of theposition of the operating control when an overloador circuit fault exists. Such circuit breakers arereferrozl to as “trip-free.” Automatic re-set circuitbreakers automatically re-set themselves. Theyshould not be used as circuit protection devices inaircraft. -

SwitchesA specifically designed switch should be used in

all circuits where a switch malfunction would behazardous. Such switches are .of rugged construc-tion and have sufficient contact capacity to break,make, and carry continuously the connected loadcurrent. Snap-action design is generally preferred toobtain rapid opening and closing of contacts re-gardless of the speed of the operating toggle orplunger, thereby minimizing contact arcing.

V The nominal current rating of the conventionalaircraft switch is usually stamped on the switchhousing. This rating represents the continuous cur-rent rating with the contacts closed. Switchesshould be derated from their nominal current ratingfor the following types of circuits: '

( 1) High rush-in circuits-—Circuits containingincandescent lamps can draw an initialcurrent which is 15 times greater than thecontinuous “current. Contact burning orwelding‘ may occur when the switch isclosed.

' (2) Inductive circuits-—l\/lagnetic energystored in solenoid coils or relays is re-leased and appears as an arc when thecontrol switch is opened.

(3) Motors-—Direct-current motors will drawseveral times their rated current duringstarting, and magnetic energy stored intheir armature and field coils is releasedwhen the control switch is opened.

The chart in figure 11-45 is typical of thoseavailable for selecting the proper nominal switchrating when the continuous load current is known.This selection is essentially a dcrating to obtainreasonable switch efiiciency and service life.

Hazardous errors in switch operation can beavoided by logical and consistent installation. Two-position “on-off” switches should be mounted sothat the “on” position is reached by an upward orforward movement of the toggle. When the switchcontrols movable aircraft elements, such as landinggear or flaps, the toggle should move in the same

Nominal system Type of load Deratiugvoltage factor

24 v. d.c. Lamp 824 v. d.c. Inductive

(Relay-Solenoid)24- v. d.c. Resistive (Heater)24- v. d.c. Motor12 v. d.c. Lamp *12 v. d.c. Inductive

(Relay-Solenoid) 212 v. d.c. Resistive (Heater) 112 v.‘ d.c. "Motor 2

FIGURE ll-45. Switch derating factors.

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direction as the desired motion. Inadvertent opera-tion of a switch can be prevented by mounting asuitable guard over the switch.

RelaysRelays are used as switching devices where a

weight reduction can be achieved or electrical con-trols can be simplified. A relay is an electricallyoperated switch and is therefore subject to dropoutunder low system voltage conditions. The foregoingdiscussion of switch ratings is generally applicableto relay contact ratings. l S

AIRCRAFT LIGHTING SYSTEMS‘Aircraft lighting systems provide illumination for

both‘ exterior and interior use. Lights on the exte-rior provide illumination for such operations aslanding at night, inspection of icing conditions, andsafety from midair collision. Interior lighting pro-vides illumination for instruments, cockpits, cabins,and other sections occupied by crewmembers andpassengers. Certain special ights, such as indicatorand warning lights, indicate the operational statusof equipment.

Exterior lightsPosition, anti-collision, landing, and taxi lights

are common examples of aircraft exterior lights.Some lights, such as position lights and anti-colli-sion lights, are required for night operations. Othertypes of exterior lights, such as wing inspectionlights, are of great benefit for specialized flyingoperations.

Position LightsAircraft operating at night must be equipped

with position lights that meet the minimum require-ments specified by the Federal Aviation Regula-tions. A set of position lights consist of one red, one

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green, and one white light. Position lights are some-times referred to as “navigation” lights. On manyaircraft each light unit contains a single lampmounted on the surface of the aircraft (A of figure11-46). Other types of position light units containtwo lamps (B of figure 11-46), and are oftenstreamlined into the surface of the aircraft struc-ture '

The green light unit is always mounted at theextreme tip of the right wing The red unit ismounted in a similar position on the left wing Thewhite unit is usually located on the vertical stabi-lizer in a position where it is clearly visible througha wide angle from the rear of the aircraft

The wingtip lamps and the tail lamps are con-trolled by a double-pole, single-throw switch in thepiIot’s compartment. On “dim”, the switch connectsa resistor in series with the lamps Since the resistordecreases current flow, the light intensity is re-duced. On “bright”, the resistor is shorted out ofthe circuit, and the lamps glow at full brilliance

On some types of installations a switch in thepilot’s compartment provides for steady or flashingoperation of the position lights. For flashing opera-tion, a flasher mechanism is usually installed in theposition light circuit. It consists essentially of amotor-driven camshaft on which two cams aremounted and a switching mechanism made up oftwo breaker arms and two contact screws. Onebreaker arm supplies d.c. current. to the wingtiplight circuit through one contact screw, and theother breaker arm supplies the tail light circuitthrough theother contact screw. When the motorrotates, it turns the camshaft through a set of reduc-tion gears and causes the cams to operate the

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' FIGURE 11-48. Single-circuit position light circuitry without flasher.

breaker which opens and closes the wing and taillight circuits alternately. Figure 11-47 is a simpli-fied schematic diagram of a navigation light circuitwhich illustrates one type of position light circuitry.

The schematic diagram of another type of posi-tion iight circuitry is shown in figure 11-4-8. Con-trol of the position lights by a single on-ofi toggleswitch provides only a steady illumination. There isno flasher and no dimming rheostat.

There are, of ‘course, manyivariations in the posi-tion light circuits used on diflerent aircraft. Allcircuits are protected by fuses or circuit breakers,and many circuits include flashing and dimmingequipment. Still others are wired to energize a spe-cial warning light dimming relay, which causes allthe cockpit warning lights to dim perceptihly whenthe position lights are illuminated.

Small aircraft are usually equipped with a simpli-fied control switch and circuitry. In some cases, onecontrol knob or switch is used to turn on severalsets of lights; for example, one type utilizes a con-trol knob, the first movement of which turns on theposition lights and the instrument panel lights. Fur-ther rotation of the control knob increases the in-tensity of only the panel lights. A flasher unit isseldom included in the position light circuitry ofvery light aircraft, but is used in small twin-engineaircraft.Anti-collision lights

An anti-collision light system may consist of oneor more lights. They are rotating beam lights whichare usually installed on top of the fuselage or tail in

such a location that the light will not ailect thevision of the crewmember or detract from the con-spicuousness of the position lights. In some casesone of the lights is mounted on the underside of thefuselage.

The simplest means of installing an anti-collisionlightis to secure it to a reinforced fuselage skinpanel, -as shown in figure 11-49.

Anti-collision light

Fuselage skin

Existing stringer

Existing stringer

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Light mountingfairing

Mounting"318

Vertical stabilizer

FIGURE 11-50. Typical anti—col1ision light installationin a vertical stabilizer.

An anti-collision light is often installed on top ofthe vertical stabilizer if the cross section of thestabilizer is large enough to accommodate the in-stallation, and if aircraft flutter and vibration char-acteristics are not adversely affected. Such installa-tions should be located near a spar, and formersshould be added as required to stiflen the structurenear the light. Figure 11-50 shows a typical anti-collision light installation in a vertical stabilizer.

An anti-collision light unit usually consists of oneor two rotating lights operated by an electric motor.The light may be fixed, but ‘mounted under rotatingmirrors inside a protruding red glass housing. Themirrors rotate in an arc, and the resulting flash rateis between 4-0 and 100 cycles per minute. (Seefigured 11-51.) The anti-collision light is a safetylight to warn other aircraft, especially in congestedareas.

landing lights 'Landing lights are installed in aircraft to illumi-

nate runways during night landings. These lightsare very powerful and are directed by a parabolicreflector at an angle providing a maximum range ofillumination. Landing lights are usually locatedmidway in the leading edge of each wing or stream-

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Anti-collision lights Oscillating mirrors‘ 0 4

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4-Vertical stabilizer Q ° Q 0 Q Q“

FIGURE 11-51. Anti-collision light.

lined into the aircraft surface. Each light may becontrolled by a relay, or it may be connected di-rectly into the electric circuit.

Since icing of the lamp lenses reduces the illumi-nation quslity of a lamp, some installations useretractable landing lamps (figure 11-52) . When thelamps are not in use, a motor retracts them intoreceptacles in the wing where the lenses are notexposed to the weather.

As shown in figure 11-53, one type of retractablelanding light motor has a split-field winding. Twoof the ‘field winding terminals connect to the twoouter terminals of the motor control switch throughthe points of contacts C and D, while the centerterminal connects to one of two motor brushes. Thebrushes connect the motor and magnetic brake sole-

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1i from against the motor shaft, allowing the motor to

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noid into the electric circuit. The points of contactC are held open by the geared quadrant of thelanding lamp mechanism. The points ofcontact Dare held closed by the tension of the spring to theright of the contacts. This is a typical arrangementof a landing lamp circuit when the landing lamp isretracted and the control switch is in the “off”position. No “current flows in the circuit, and nei-ther the motor nor the lamp can he energized.

When the control‘ switch is placed in the upper,or “extend,” position (figure 11-53), current fromthe battery flows through the closed contacts of theswitch, the closed contacts of contact D, the centerterminal of the field winding, and the motor itself.Current through the motor circuit energizes thebrake solenoid, which withdraws the brake shoe

turn and lower the lamp mechanism. After -the lampmechanism moves about 10°, contact A touches andrides along the copper bar B. In the meantime,relay F‘ is energized, and its contacts close. Thispermits current to flow throughlthe copperlbar B,contact A, and the lamp. When the lamp mechanism

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is completely lowered, the projection at the top ofthe gear quadrant pushes the D contacts apart,opens the circuit to the motor, and causes the de-en-ergized brake solenoid to release the brake. Thebrake is pushed against the motor shaft by thespring, stopping the motor and completing the low-ering operation. .

To retract the landing lamp, the control switch isplaced in the “retract” position (figure IIWS3). Themotor and brake circuits are completed through thepoints of contact C, since these contacts are closedwhen the gear quadrant is lowered. This actioncompletes the circuit, the brake releases, the motorturns (this time in the opposite direction) and thelanding light mechanism is retracted. Since switch-ing to “retract” breaks the circuit to relay F, therelay contacts open, disconnecting the copper barand causing the landing lamp to go out. When themechanism is completely retracted, contact points Copen, and the circuit to the motor is again broken,the brake applied, and the motor stopped.

‘Retractable landing lights that can be extended toany position of theirextension are employed onsome aircraft. Landing lights used on high-speedaircraft are usually equipped with an airspeed pres-sure switch which prevents extension of landinglights at excessive &l1'SP8B(l5.' Such switches alsocause retraction of landing lights if the aircraftexceeds a predetermined speed. , L

Many large aircraft are equipped with four land~ing lights, two of which are fixed and two retracta-ble. Fixed lights are usually located in either thewing root areas or just outboard of the fuselage inthe leading edge of each wing. The two retractablelights are usually located in the lower outboardsurface of each wing, and are normally controlledby separate switches. On some aircraft, the fixed

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FIGURE 11-54. Fixed landing light and taxi light.

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landing light is mounted in an area with a taxilight, as shown in figure 11-54.

Taxi lightsTaxi lights are designed to provide illumination

on the ground while taxiing or towing the aircraftto or from a runway, taxi strip, or in the hangararea.

Taxi lights are not designed to provide the de-gree of illumination necessary for landing lights;150- to 250-watt taxi lights are typical on manymedium and heavy aircraft.

On aircraft with tricycle landing gear, either sin-gle or dual taxi lights are often mounted on thenon-steerable part of the nose landing gear. As il-lustrated in figure 11-55, they are positioned at anoblique angle to the center line of the aircraft toprovide illumination directly in front of the aircraftand also some illumination to the right and left ofthe aircraft’s path. On some aircraft the dual taxilights are supplemented by wing-tip clearance lightscontrolled by the same circuitry.

Taxi lights are also mounted in the recessed areasof thewing leading edge, often in the same areawith a fixed landing light.

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Many small aircraft are not equipped with anytype of taxi light, but rely on the intermittent use ofa landing light to illuminate taxiing operations. Stillother aircraft utilize a dimming resistor in the land-ing light circuit to provide reduced illumination fortaxing. A typical circuit for dual taxi lights isshown in figure 11-56.

Some large aircraft are equipped with alternatetaxi lights located on the lower surface of the air-craft, aft of the nose radome. These lights, operatedby a separate switch from the main taxi lights,illuminate the area immediately in front of andhelow the aircraft nose.

Wing Inspection LightsSome aircraft are equipped with wing inspection

lights to illuminate the leading edge of the wings topermit observation of icing and general condition ofthese areas in flight. On some aircraft, the winginspection light system (also called wing ice lights)consists of a 100-watt light mounted flush on theoutboard side of each nacelle forward of the wing.These lights permit visual detection of ice formationon wing leading edges while flying at night. Theyare‘ also often used as floodlights during groundservicing. They are usually controlled through arelay by an “on-off” toggle switch in the cockpit.

Some wing inspection light systems may includeor be supplemented by additional lights, sometimescalled nacelle lights, that illuminate adjacent areassuch as cowl flaps or the landing gear. These arenormally the same type of lights and can be con-trolled by the same circuits.

MAINTENANCE AND INSPECTION OF LIGHTINGSYSTEMS

Inspection of an aircraft’s lighting systems nor-mally includes checking the condition and securityof all visible wiring, connections, terminals, fuses,and switches. A continuity light or meter can beused in making these checks, since the cause ofmany troubles can often be located by systemati~cally testing each circuit for continuity.

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1 A" B. c. V D.FIGURE 11~5?. Continuity testing with a continuity tester.

All light covers and reflectors should be keptclean and polished, Cloudy reflectors are oftencaused by an air leak around the lens.

The condition of the sealing compound aroundposition light frames should be inspected regularly.Leaks or cracks should be filled with an approvedsealing compound.

Care should be exercised in installing a new bulbin a light assembly, since many bulbs fit into asocket in only one position and excessive force cancause an incomplete or open circuit in the socketarea.

Circuit testing, commonly known as troubleshoot-ing is a means of systematically locating faults inan electrical system. These faults are usually ofthree kinds:

(1) Open circuits in which leads or wires arebroken.

(2) Shorted circuits in which grounded leadscause current to be returned by shortcutsto the source of power.

(3) Low power in circuits causing lights toburn dimly and relays to chatter. Electri-cal troubles may develop in the unit or inthe wiring. If troubles such as these arecarefully analyzed and systematic stepsare taken to locate them, much time andenergy not only can be saved, but damageto expensive testing equipent often canbe avoided. (For a more extensive treat-ment oi circuit testing than the summaryprovided here, refer to Chapter 8, Air-

frame and Powerplant Mechanics GeneralHandbook, AC 65~9A.)

The equipment generally used in testing lightingcircuits in an aircraft consists of a voltmeter, testlight, continuity meter, and ohmrneter.

Although any standard d.c. voltmeter with flexi-ble leads and test prods is satisfactory for testingcircuits, portable voltmeters especially designed fortest purposes are usually used.

The test lamp consists of a low wattage aircraftlight. Two leads are used with this light.

Continuity testers vary somewhat. One type eon-sists of a small lamp connected in series with twosmall batteries (flashlight batteries are very suita-ble) and two leads. (See A of figure 11-57.) An-other type oi continuity tester contains two batter-ies connected in series with a d.c. voltmeter and twotest leads. A completed circuit will be registered bythe voltmeter.

Whenever generator or battery voltage is availa-ble, the voltmeter and the test light can be used incircuit testing, since these sources of power willactivate the test light and the voltmeter.

If no electrical power is available (the circuit isdead), then the continuity tester is used. The self-contained batteries of the continuity tester forcecurrent through the circuit, causing the continuitymeter to indicate when the circuit being tested iscompleted. When using the continuity meter, thecircuit being tested should always be isolated fromall other circuits by removing the fuse, by openingthe switch, or by disconnecting the wires.

Figure 11-57 illustrates techniques which may be

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used in checking circuits. The continuity tester con-tains a light to serve as an indicator. When the testleads are touched together, a complete circuit isestablished and the indicator light illuminates.“Then the leads are brought into contact with aresistor or other circuit element, as shown in B offigure 11-57, and the light does not illuminate, thenthe circuit being tested is open.

For the open test to be conclusive, be sure theresistance of the imit tested is low enough to permitthe lamp to light. In a test in which the resistanoeistoo high, usually more than 10 ohms, connect avoltmeter in the circuit in place of the lamp. If thevoltmeter pointer {ails to deflect, an open circuit isindicated.

The test for shorts (C oi figure 11-57) shows thecontinuity tester connected across the terminals of aswitch in the “open” position. If the tester lamplights, there is a short circuit in the switch.

To determine whether a length, of wire isgrounded at some point between’, its terminals,connect the wire at each end and hook one test clipto the wire at one end and ground the other testclip (D of figure ll-57) . If the wire is grounded,the lamp will light. To locate the ground, checkback at intervals toward the other end. The lightingof the lamp will indicate the section of the wire thatis grounded.

The ohmmeter, although primarily designed tomeasure resistance, is useful for checking continu-ity. With an ohmmeter, the resistance of a lightingcircuit can be determined directly by scale. Since anopen circuit has infinite resistance, a zero readingon the ohmmeter indicates circuit continuity.

As illustrated in figure 11—58, the ohmmeter usesa battery as the source oi voltage. There are fixedresistors, which are of such value that when the testprods are shorted together, themeter will read fullscale. The variable resistor, in parallel with themeter, and the fixed resistors compensate forchanges in voltage of the battery. The variable re-sistor provides for zero adjustment on the metercontrol panel.

On the meter there may be several scales, whichare made possible by various values of resistanceand battery voltage. The desired scale is selected bya selector switch on the face of the ohmmeter. Eachscale reads low resistances at the upper end. Greaterresistance in a circuit is indicated by less deflectionof the indicator on the scale. .

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When using an ohmmeter to check continuity,connect the leads across the circuit. A zero ohmreading indicates circuit continuity. For checkingresistance, a scale should he chosen which will con-tain the resistance oi the element being measured.In general, a scale should be selected on which thereading will fall in the upper half of the scale.Short the leads together and set the meter to readzero ohm by the zero adjustment. If a change inscales is madeianytime, remember to re-adjust themeter to zero ohm.

When making circuit tests with the ohmmeter,never attempt to check continuity or measure theresistance in a circuit while it is connected to asource of voltage. Disconnect one end of an elementwhen checking resistance, so that the ohrnmeter willnot read the resistance of parallel paths.

The following summary of continuity testing oflighting circuits is recommended, using either anohmmeter or any other type of continuity tester.

T (1) Check the fuse or circuit breaker. Be sureit is the correct one for the circuit beingtested.

(2) Check the electrical unit (light).(3) If fuse or circuit breaker and light are in

good condition, check at the most accessi-ble point for an open or short in thecircuit.

(4) Never guess. Always locate the trouble inthe positive lead of a circuit, the operating

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FIGURE 11-59. Colitinuity testiluz with a voltmeter.

unit, or the negative lead before removing‘any equipment or wires.

A voltmeter with long flexible leads provides ssatisfactory, though different, method of checkingthe continuity of lighting system wiring in an air-craft. The voltage to be checked by the voltmeter isfurnished by the storage battery in the aircraft.

The following procedure indicates the steps forcontinuity checking by a voltmeter in a circuitwhich consists of a 24-volt battery, a fuse, a switch,and a landing lamp.

(1) Draw a simple wiring diagram of the cir-cuitry to bentested, as shown in figure1l—59.

(2) Check the fuse by touching the positivevoltmeter lead to the load end of the fuseand the negative lead to ground. If thefuse is good, there will be an indicationon the voltmeter. If it is burned out, itmust be replaced. If it bums out again,

the circuit is grounded. Check for theground at the lamp by removing theconnector and replacing the fuse; if itburns out, the short is in the line. How-ever, if the fuse does not burn out thistime, the short is within the lamp.If the fuse tests good, the circuit has anopen. Then with the negative clip of thevoltmeter connected to ground, move thepositive clip from point to point along thecircuit, following the diagram as a guide.Test each unit and length of wire. Thefirst zero reading on the voltmeter indi-cates that there is an open circuit betweenthe last point at which the voltage wasnormal and the point of the first zeroreading. In the illustration of figure1]-59. open circuits are caused by anopen fuse, an open lamp filament, and anopen lamp-to-ground connection.