Pull here until tight before finishing knot Cord crosses under loop Step A-Starting knot Final knot Starting knot tightened First part of final knot Step B-Intermediate half hitches tightened Step C-(Part I) Trim to 3/8" min.
Step C-(Part II) Final knot FIGURE 11-24. Single-cord lacing.
ble-cord lacing. The lacing is started at the thick end of the wire group or bundle with a bowline-ona-bight knot (A of figure 11-25). At regular intervals along the wire group or bundle, and at each point where a wire branches off, the lacing is continued using half hitches, with both cords held firmly together. The half hitches should be spaced so that the group or bundle is neat and secure. The lacing is ended with a knot consisting of a half hitch, continuing one of the cords clockwise and the other counterclockwise and then tying the cord ends with a square knot. The free ends of the lacing cord should be trimmed to approximately 3/8 in.
Lacing Branch-Offs Figure 11-26 illustrates a recommended procedure for lacing a wire group that branches off the main wire bundle. The branch-off lacing is started with a knot located on the main bundle just past the branch-off point. Continue the lacing along the branched-off wire group, using regularly spaced half hitches. If a double cord is used, both cords should be held snugly together. The half hitches should be spaced to lace the bundle neatly and securely. The lacing is ended with the regular terminal knot used in single- or double-cord lacing. The free ends of the lacing cord should be neatly trimmed.
Tying All wire groups or bundles should be tied where supports are more than 12 in. apart, Figure 11-28 illustrates a recommended procedure for tying a wire group or bundle. The tie is started by wrapping the cord around the wire group to tie a clovehitch knot. Then a square knot with an extra loop is tied, and the free ends of the cord are trimmed.
Temporary ties are sometimes used in making up and installing wire groups and bundles. Colored cord is normally used to make temporary ties, since they are removed when the installation is complete.
Whether laced or tied, bundles should be secured to prevent slipping, but not so tightly that the cord cuts into or deforms the insulation. This applies especially to coaxial cable, which has a soft dielectric insulation between the inner and outer conductor.
The part of a wire group or bundle located inside a conduit is not tied or laced, but wire groups or bundles inside enclosures, such as junction boxes, should be laced only.
CUTTING WIRE AND CABLE To make installation, maintenance, and repair easier, wire and cable runs in aircraft are broken at specified locations by junctions, such as connectors, 446
Starting knot for branch off Main bundle lacing Branch-off lacing Starting knot-Bowline-on a bight Half-hitch required at branch-off point Starting knot tightened FICURE 11-26. Lacing a branch-off.
Intermediate half hitches Stripping Wire and Cable Before wire can be assembled to connectors, terminals, splices, etc., the insulation must be stripped 2020# from connecting ends to expose the bare conductor.
Copper wire can be stripped in a number of ways depending on the size and insulation. Figure 11-27 lists some types of stripping tools recommended for various wire sizes and types of insulation.
FIGURE 11-27. Wire strippers for copper wire.
Stripper Wire Size Insulations Final knot C Hot-blade 426-44 All except asbestos Rotary, electric #26#4 All FICURE 11-25. Double-cord lacing. Bench #20-46 AII Hand pliers #26-78 All Knife #2 -#0000 AIl terminal blocks, or buses. Before assembly to these junctions, wires and cables must be cut to length. Aluminum wire must be stripped very carefully, All wires and cables should be cut to the lengths using extreme care, since individual strands will specified on drawings and wiring diagrams. The cut break very easily after being nicked.
should be made clean and square, and the wire or The following general precautions are recomcable should not be deformed. If necessary, large- mended when stripping any type of wire:
diameter wire should be re-shaped after cutting. (1) When using any type of wire stripper, Good cuts can be made only if the blades of cutting hold the wire so that it is perpendicular to tools are sharp and free from nicks. A dull blade cutting blades.
will deform and extrude wire ends. (2) Adjust automatic stripping tools care- 447
Wrap cord twice Clove hitch and Over bundle square knot FIcURE 11-28. Tying a wire group or bundle.
fully; follow the manufacturer's instructions to avoid nicking, cutting, or otherwise damaging strands. This is especially important for aluminum wires and for copper wires smaller than No. 10. Examine stripped wires for damage. Cut off and re-strip (if length is sufficient), or reject and replace any wires having more than the allowable number of nicked or broken strands listed in the manufacturer's instructions.
(3) Make sure insulation is clean-cut with no frayed or ragged edges. Trim if necessary.
(4) Make sure all insulation is removed from stripped area. Some types of wires are supplied with a transparent layer of insulation between the conductor and the primary insulation. If this is present, remove it.
(5) When using hand-plier strippers to remove lengths of insulation longer than 3/4 in., it is easier to accomplish in two or more operations.
(6) Re-twist copper strands by hand or with pliers, if necessary, to restore natural lay and tightness of strands.
A pair of hand wire strippers is shown in figure
11-29. This tool is commonly used to strip most types of wire.
The following general procedures describe the steps for stripping wire with a hand stripper.
(Refer to figure 11-30.)
(1) Insert wire into exact center of correct cutting slot for wire size to be stripped.
Each slot is marked with wire size.
(2) Close handles together as far as they will go.
(3) Release handles, allowing wire holder to return to the "open" position.
(4) Remove stripped wire.
FIcURE 11-29. Light-duty hand wire strippers.
Solderless Terminals and Splices Splicing of electrical cable should be kept to a minimum and avoided entirely in locations subject to extreme vibrations. Individual wires in a group or bundle can usually be spliced, provided the completed splice is located so that it can be inspected periodically. Splices should be staggered so that the bundle does not become excessively enlarged. Many types of aircraft splice connectors are available for splicing individual wires. Self-insulated splice connectors are usually preferred; however, a noninsulated splice connector can be used if the splice is covered with plastic sleeving secured at both ends.
Solder splices may be used, but they are particularly brittle and not recommended.
Electric wires are terminated with solderless terminal lugs to permit easy and efficient connection to and disconnection from terminal blocks, bus bars, or other electrical equipment. Solderless splices join electric wires to form permanent continuous runs.
Solderless terminal lugs and splices are made of copper or aluminum and are preinsulated or uninsulated, depending on the desired application.
Terminal lugs are generally available in three types for use in different space conditions. These are the flag, straight, and right-angle lugs. Terminal lugs are "crimped" (sometimes called "staked" or "swaged") to the wires by means of hand or power crimping tools.
The following discussion describes recommended 448
Insulation grip Barrel Wire insulation Color-coded Tongue insulation Stripped wire Select correct hole to match FIcURE 11-31. Preinsulated terminal lug.
wire gauge methods for terminating copper and aluminum wires using solderless terminal lugs. It also describes the method for splicing copper wires using solderless splices.
Copper Wire Terminals Copper wires are terminated with solderless, preinsulated straight copper terminal lugs. The insulation is part of the terminal lug and extends beyond its barrel so that it will cover a portion of the wire insulation, making the use of an insulation sleeve unnecessary (figure 11-31).
In addition, preinsulated terminal lugs contain an insulation grip (a metal reinforcing sleeve) beneath Blades remain the insulation for extra gripping strength on the open until wire wire insulation. Preinsulated terminals accommois removed date more than one size of wire; the insulation is usually color-coded to identify the wire sizes that can be terminated with each of the terminal lug sizes.
Crimping Tools Hand, portable power, and stationary power tools are available for crimping terminal lugs. These tools crimp the barrel of the terminal lug to the conductor and simultaneously crimp the insulation grip to the wire insulation.
Hand crimping tools all have a self-locking ratchet that prevents opening the tool until the crimp is complete. Some hand crimping tools are equipped with a nest of various size inserts to fit different size terminal lugs. Others are used on one terminal lug size only. All types of hand crimping tools are checked by gages for proper adjustment of crimping jaws.
Figure 11-32 shows a terminal lug inserted into a hand tool. The following general guidelines out- FIcURE 11-30. Stripping wire with hand stripper. line the crimping procedure.
Locator Tongue horizontal Barrel butts flush against tool stop
- Stop FICCRE 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 the terminal lug barrel butts flush against the tool stop.
(3) Insert the stripped wire into the terminal lug barrel until the wire insulation butts flush against the end of the barrel.
(4) Squeeze the tool handles until the ratchet releases.
(5) Remove the completed assembly and examine it for proper crimp.
Some types of uninsulated terminal lugs are insulated after assembly to a wire by means of pieces of transparent flexible tubing called "sleeves." The sleeve provides electrical and mechanical protection at the connection. When the size of the sleeving used is such that it will fit tightly over the terminal lug, the sleeving need not be tied; otherwise, it
• 1" approx.
Tight or shrunk sleeve Lacing cord Loose sleeve FIGURE 11-33. Insulating sleeve.
should be tied with lacing cord as illustrated in figure 11-33.
Aluminum Wire Terminals The use of aluminum wire in aircraft systems is increasing because of its weight advantage over copper. However, bending aluminum will cause "work hardening" of the metal, making it brittle This results in failure or breakage of strands much sooner than in a similar case with copper wire.
Aluminum also forms a high-resistant oxide film immediately upon exposure to air. To compensate for these disadvantages, it is important to use the most reliable installation procedures.
Only aluminum terminal lugs are used to terminate aluminum wires. They are generally available in three types: (1) Straight, (2) right-angle, and
(3) flag. All aluminum terminals incorporate an inspection hole (figure 11-34) which permits checking the depth of wire insertion. The barrel of aluminum terminal lugs is filled with a petrolatumzinc dust compound. This compound removes the oxide film from the aluminum by a grinding process during the crimping operation. The compound will also minimize later oxidation of the completed connection by excluding moisture and air. The compound is retained inside the terminal lug barrel by a plastic or foil seal at the end of the barrel.
Splicing Copper Wires Using Preinsulated Splices Preinsulated permanent copper splices join small wires of sizes 22 through 10. Each splice size can be used for more than one wire size. Splices are usually color-coded in the same manner as preinsulated small copper terminal lugs. Some splices are insulated with white plastic. Splices are also used to reduce wire sizes (figure 11-35).
Crimping tools are used to accomplish this type of splice. The crimping procedures are the same as those used for terminal lugs, except that the crimping operation must be done twice, once for each end of the splice.
EMERGENCY SPLICING REPAIRS Broken wires can be repaired by means of crimped splices, by using terminal lugs from which the tongue has been cut off, or by soldering together and potting broken strands. These repairs are applicable to copper wire. Damaged aluminum wire must not be temporarily spliced. These repairs are for temporary emergency use only and should be replaced as soon as possible with permanent repairs. Since some manufacturers probibit splicing, 450
Protectiv rer Cover hole to prevent forcing petroleum Petrolatum compound out compound Removing cover Half full of compound Check to see if wire Is properly inserted Stripped wire FIcURE 11-34, Inserting aluminum wire into aluminum terminal lugs.
the applicable manufacturer's instructions should always be consulted.
Splicing with Solder and Potting Compound When neither a permanent splice nor a terminal lug is available, a broken wire can be repaired as follows (figure 11-36) :
(1) Install a piece of plastic sleeving about 3 in. long, and of the proper diameter to fit loosely over the insulation, on one piece of the broken wire, Thinner wire Heavy wire doubled over Cover with viny tube tied at both ends FICURE 11-35. Reducing wire size with a permanent splice.
(2) Strip approximately 1-1/2 in. from each broken end of the wire.
(3) Lay the stripped ends side by side and twist one wire around the other with approximately four turns.
(4) Twist the free end of the second wire around the first wire with approximately four turns, Solder wire turns together, using 60/40 tin-lead resin-core solder.
(5) When solder is cool, draw the sleeve over the soldered wires and tie at one end. If potting compound is available, fill the sleeve with potting material and tie securely.
(6) Allow the potting compound to set without touching for 4 hrs. Full cure and electrical characteristics are achieved in 24 hrs.
CONNECTING TERMINAL LUGS TO TERMINAL BLOCKS Terminal lugs should be installed on terminal blocks so that they are locked against movement in the direction of loosening (figure 11-37).
1 1/2" 1 1/2" Approx. Approx.
Step 1 Step 2 Step Step УОДА Step 5 Potting dispenser FIGURE 11-36. Repairing broken wire by soldering and potting.
Terminal blocks are normally supplied with studs secured in place by a plain washer, an external tooth lockwasher, and a nut. In connecting terminals, a recommended practice is to place copper terminal lugs directly on top of the nut, followed with a plain washer and elastic stop nut, or with a plain washer, split steel lockwasher, and plain nut.
Aluminum terminal lugs should be placed over a plated brass plain washer, followed with another plated brass plain washer, split steel lockwasher, and plain nut or elastic stop nut. The plated brass washer should have a diameter equal to the tongue width of the aluminum terminal lug. Consult the manufacturer's instructions for recommended di mensions of these plated brass washers, Do not place any washer in the current path between two aluminum terminal lugs or between two copper terminal lugs. Also, do not place a lockwasher directly against the tongue or pad of the aluminum terminal.
To join a copper terminal lug to an aluminum terminal lug, place a plated brass plain washer over the nut which holds the stud in place; follow with the aluminum terminal lug, a plated brass plain washer, the copper terminal lug, plain washer, split steel lockwasher, and plain nut or self-locking, allmetal nut. As a general rule use a torque wrench to tighten nuts to ensure sufficient contact pressure.
Manufacturer's instructions provide installation torques for all types of terminals.
BONDING AND GROUNDING Bonding is the electrical connecting of two or more conducting objects not otherwise adequately connected. Grounding is the electrical connecting of a conducting object to the primary structure for a return path for current. Primary structure is the main frame, fuselage, or wing structure of the aircraft, commonly referred to as ground, Bonding and grounding connections are made in aircraft electrical systems to:
(1) Protect aircraft and personnel against hazards from lightning discharge.
(2) Provide current return paths.
(3) Prevent development of radio-frequency 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 Procedures The following general procedures and precau tions are recommended when making bonding or grounding connections:
(1) Bond or gronud parts to the primary aircraft structure where practicable.
(2) Make bonding or grounding connections so that no part of the aircraft structure is weakened.
(3) Bond parts individually if possible.
(4) Install bonding or grounding connections against smooth, clean surfaces.
(5) Install bonding or grounding connections so that vibration, expansion or contraction, or relative movement in normal serv.
ice will not break or loosen the connec tion.
FIGURE 11-37. Connecting terminals to terminal block.
Cadmium plated steel screw Clamp Copper terminal Aluminum Cylindrical surface washers Cadmium plated Cadmium plated steel lockwasher steel locknut A. Copper jumper connection to tubular structure.
Cadmium plated steel screw Clamp washer Aluminum
- Structure Aluminum washer Aluminum alloy or corrosion-resisting Cadmium plated steel conduit steel lockwasher Cadmium plated locknut B. Bonding conduit to structure.
Cadmium plated steel screw Clamp Aluminum washer Aluminum terminal Cylindrical surface Aluminum washer Cadmium plated steel: locknut steel lockwasher Cadmium plated C, Aluminum jumper connection to tubular structure, FicURE 11-38. Hardware combinations used in making bonding connections.
(6) Install bonding and grounding connec- (2) Nut plates are used where access to the tions in protected areas whenever possible. nut for repairs is difficult. Nut plates are Bonding jumpers should be kept as short as prac- riveted or welded to a clean area of the ticable. The jumper should not interfere with the structure (figure 11-40).
operation of movable aircraft elements, such as sur- Lockwasher face controls; normal movement of these elements Screw or bolt - Limited to 4 should not result in damage to the bonding jumper. Washer Washer Electrolytic action can rapidly corrode a bonding.
connection if suitable precautions are not observed.
Aluminum alloy jumpers are recommended for most cases; however, copper jumpers can be used to bond together parts made of stainless steel, cadi- Rivet or weld. • Nut-plate mum-plated steel, copper, brass, or bronze. Where FIcURE 11-40. Nut plate bonding or grounding to a contact between dissimilar metals cannot be flat surface.
avoided, the choice of jumper and hardware should be such that corrosion is minimized, and the part most likely to corrode will be the jumper or associated hardware. Figure 11-38 illustrates some proper hardware combinations for making bonding connections. At locations where finishes are removed, a protective finish should be applied to the completed connection to prevent corrosion.
The use of solder to attach bonding jumpers should be avoided. Tubular members should be bonded by means of clamps to which the jumper is attached. The proper choice of clamp material minimizes the probability of corrosion. When bonding jumpers carry a substantial amount of ground return current, the current rating of the jumper should be adequate, and it should be determined that a negligible voltage drop is produced.
Bonding and grounding connections are normally made to flat surfaces by means of through-bolts or screws where there is easy access for installation.
Other general types of bolted connections are as follows:
(1) In making a stud connection (figure 11-39), a bolt or screw is locked securely to the structure, thus becoming a stud.
Grounding or bonding jumpers can be removed or added to the shank of the stud without removing the stud from the struc- Bonding and grounding connections are also made to a tab riveted to a structure. In such cases it is important to clean the bonding or grounding surface and make the connection as through the connection were being made to the structure. If it is necessary to remove the tab for any reason, the rivets should be replaced with rivets one size larger, and the mating surfaces of the structure and the tab should be clean and free of anodic film.
Bonding or grounding connections can be made to aluminum alloy, magnesium, or corrosion-resistant steel tubular structure as shown in figure 11-41, which shows the arrangement of hardware for bonding with an aluminum jumper. Because of the ease with which aluminum is deformed, it is necessary to distribute the screw and nut pressure by means of plain washers.
Hardware used to make bonding or grounding connections should be selected on the basis of mechanical strength, current to be carried, and ease of installation. If connection is made by aluminum or copper jumpers to the structure of a dissimilar material, a washer of suitable material should be installed between the dissimilar metals go that any corrosion will occur on the washer, which is expendable.
ture.
Plate alund Screw is bolt Lockwasher Washer Washer Plain nex Cylindrical surface Cadium-plated Wanker steel Jockmot Cadimum-plated Tenkaal steel lockwasher (timit to 4) netnut FIcURE 11-37. Bonding or grounding connections to a FIcURE 11-39. Stud bonding or grounding to a flat surface. cylindrical surface.
Hardware material and finish should be selected aircraft. Each class of connector has slightly differon the basis of the material of the structure to ent construction characteristics. Classes A, B, C, which attachment is made and on the material of and D are made of aluminum, and class K is made the jumper and terminal specified for the bonding of steel.
or grounding connection. Either a screw or bolt of (I) Class nthe proper size for the specified jumper terminal should be used. When repairing or replacing exist- (2) Class es ing bonding or grounding connections, the same sed type of hardware used in the original connection nt should always be used. be Testing Grounds and Bonds The resistance of all bond and ground connections should be tested after connections are made before re-finishing. The resistance of each connec- - Class tion should normally not exceed 0.003 ohm. Resistance measurements need to be of limited nature only for verification of the existence of a bond, but should not be considered as the sole proof of satisfactory bonding. The length of jumpers, methods, and materials used, and the possibility of loosening the connections in service should also be consid ered. (4) Class st-
CONNECTORS Connectors (plugs and receptacles) facilitate maintenance when frequent disconnection is required. Since the cable is soldered to the connector inserts, the joints should be individually installed and the cable bundle firmly supported to avoid damage by vibration. Connectors have been particularly vulnerable to corrosion in the past, due to condensation within the shell. Special connectors with waterproof features have been developed which may replace non-waterproof plugs in areas where mositure causes a problem. A connector of the same basic type and design should be used when replacing a connector. Connectors susceptible to corrosion difficulties may be treated with a chemically inert waterproof jelly. When replacing connector assemblies, the socket-type insert should be used on the half which is "live" or "hot" after the connector is disconnected, to prevent unintentional grounding.
Types of Connectors Connectors are identified by AN numbers and are divided into classes with the manufacturer's variations in each class. The manufacturer's variations are differences in appearance and in the method of meeting a specification. Some commonly used connectors are shown in figure 11-42. There are five basic classes of AN connectors used in most misture.
(5) Class K-A fireproof connector used in areas where it is vital that the electric current is not interrupted, even though the connector may be exposed to continuous open flame. Wires are crimped to the pin or socket contacts and the shells are made of steel. This class of connector is normally longer than other classes of connectors.
Connector Identification Code letters and numbers are marked on the coupling ring or shell to identify a connector. This code (figure 11-43) provides all the information necessary to obtain the correct replacement for a defective or damaged part.
Many special-purpose connectors have been designed for use in aircraft applications. These include subminiature and rectangular shell connectors, and connectors with short body shells or splitshell construction.
AN3100 AN3102 wall receptacle AN3101 box receptacle cable receptacle AN3106 AN3107 straight plug AN3106 ‚straight plug MCK disconnect plug AN3108 AN3106 angle plug angle plug FIcURE 11-42. AN connectors.
Standard Installation of Connectors Type (straight plug) The following procedures outline one recom- Class mended method of assembling connectors to recept- Size (in 1/16-inch) acles.
Contact arrangement number (1) Locate the proper position of the plug in relation to the receptable by aligning the 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 a FICURE 11-43. AN connector marking. light forward pressure and engage the 456
threads of the coupling ring and recepta-
(3) Alternately push in the plug and tighten the coupling ring until the plug is completely seated.
(4) Use connector pliers to tighten coupling rings one sixteenth to one eighth turn beyond fingertight if space around the connector is too small to obtain a good finger grip.
(5) Never use force to mate connectors to receptacles. Do not hammer a plug into ite receptacle; and never use a torque wrench or pliers to lock coupling rings.
A connector is generally disassembled from a receptacle in the following manner:
by hand.
(2) Alternately pull on the plug body and unscrew the coupling ring until the connector is separated.
(3) Protect disconnected plugs and receptacles with caps or plastic bags to keep debris from entering and causing faults.
(4) Do not use excessive force, and do not pull on attached wires.
CONDUIT Conduit is used in aircraft installations for the mechanical protection of wires and cables. It is available in metallic and nonmetallic materials in both rigid and flexible form.
When selecting conduit size for a specific cable bundle application, it is common practice to allow for ease in maintenance and possible future circuit expansion by specifying the conduit inner diameter about 25% larger than the maximum diameter of the conductor bundle. The nominal diameter of a rigid metallic conduit is the outside diameter.
Therefore, to obtain the inside diameter, subtract twice the tube wall thickness.
From the abrasion standpoint, the conductor is vulnerable at the conduit ends. Suitable fittings are affixed to the conduit ends in such a manner that a smooth surface comes in contact with the conductor within the conduit. When fittings are not used, the conduit end should be flared to prevent wire insulation damage. The conduit is supported by clamps along the conduit run.
Many of the common conduit installation problems can be avoided by proper attention to the following details:
(1) Do not locate conduit where it can be used as a handhold or footstep.
(2) Provide drain holes at the lowest point in a conduit run. Drilling burrs should be carefully removed from the drain holes.
(3) Support the conduit to prevent chafing against the structure and to avoid stressing its end fittings.
Damaged conduit sections should be repaired to prevent damage to the wires or wire bundle. The minimum acceptable tube bend radii for rigid conduit as prescribed by the manufacturer's instructions should be followed carefully. Kinked or wrinkled bends in a rigid conduit are normally not Flexible aluminum conduit is widely available in two types: (1) Bare flexible and (2) rubber-cov.
ered conduit. Flexible brass conduit is normally used instead of flexible aluminum conduit, where necessary to minimize radio interference. Flexible conduit may be used where it is impractical to use rigid conduit, such as areas that have motion between conduit ends or where complex bends are necessary. Transparent adhesive tape is recommended when cutting flexible tubing with a hacksaw to minimize fraying of the braid.
ELECTRICAL EQUIPMENT INSTALLATION This section provides general procedures and safety precautions for installation of commonly used aircraft electrical equipment and components. Electrical load limits, acceptable means of controlling or monitoring electrical loads, and circuit protection devices are subjects with which mechanics must be familiar to properly install and maintain aircraft electrical systems.
Electrical Load Limits When installing additional electrical equipment that consumes electrical power in an aircraft, the total electrical load must be safely controlled or managed within the rated limits of the affected components of the aircraft's power-supply system.
Before any aircraft electrical load is increased, the associated wires, cables, and circuit protection devices (fuses or circuit breakers) should be checked to determine that the new electrical load (previous maximum load plus added load) does not exceed the rated limits of the existing wires, cables, or protection devices.
The generator or alternator output ratings prescribed by the manufacturer should be compared 457