FIGURE 11-14. Staggered splices in a wire bundle.
example, coaxial cable should never be bent to a Protection against High Temperature smaller radius than ten times the outside diameter.
Routing and Installations All wiring should be installed so that it is mechanically and electrically sound and neat in appearance. Whenever practicable, wires and bundles should be routed parallel with, or at right angles to, the stringers or ribs of the area involved. An exception to this general rule is coaxial cable, which is routed as directly as possible.
The wiring must be adequately supported throughout its length. A sufficient number of supports must be provided to prevent undue vibration of the unsupported lengths. All wires and wire groups should be routed and installed to protect them 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
(5) Damage from cargo stowage or shifting.
(6) Damage from battery acid fumes, spray, To prevent insulation deterioration, wires should be kept separate from high-temperature equipment, such as resistors, exhaust stacks, or heating ducts.
The amount of separation is normally specified by engineering drawings. Some wires must invariably be run through hot areas. These wires must be insulated with high-temperature material such as asbestos, fiber glass, or Tefion. Additional protection is also often requred in the form of conduits.
A low-temperature insulation wire should never be used to replace a high-temperature insulation wire.
Many coaxial cables have soft plastic insulation, such as polyethylene, which is especially subject to deformation and deterioration at elevated temperatures. All high-temperature areas should be avoided when installing these cables insulated with plastic or polyethylene.
Additional abrasion protection should be given to asbestos wires enclosed in conduit. Either conduit with a high-temperature rubber liner should be used, or asbestos wires can be enclosed individually in high-temperature plastic tubes before being installed in the conduit.
(7) Damage from solvents and fluids.
Protection Against Chafing Wires and wire groups should be protected against chafing or abrasion in those locations where contact with sharp surfaces or other wires would damage the insulation. Damage to the insulation can cause short circuits, malfunction, or inadvertent operation of equipment. Cable clamps should be used to support wire bundles at each hole through a bulkhead (figure 11-16). If wires come closer than 1/4 in. to the edge of the hole, a suitable grommet is used in the hole as shown in figure 11-17.
Sometimes it is necessary to cut nylon or rubber grommets to facilitate installation. In these instances, after insertion, the grommet can be secured in place with general-purpose cement. The cut should be at the top of the hole, and made at an angle of 45° to the axis of the wire bundle hole.
Protection Against Solvents and Fluids Wires should not be installed in areas where they will be subjected to damage from fluids or in the lowest 4 in. of an aircraft fuselage, except those that must terminate in that area. If there is a possibility that wire may be soaked with fluids, plastic tubing should be used to protect the wire. This tubing should extend past the exposure area in both directions and should be tied at each end. If the wire has a low point between the tubing ends, provide a ⅛-in. drain hole, as shown in figure 11-18.
This hole should be punched into the tubing after the installation is complete and the low point definitely established by using a hole punch to cut a half circle. Care should be taken not to damage any wires inside the tubing when using the punch.
Wire should never be routed below an aircraft 442
1/2 Inch Maximum with normal hand pressure FIcURE 11-15. Slack in wire bundle between supports.
Cable clamp Clearance 1/4" minimum Angle bracket with two point fastening FIcURE 11-16. Cable clamp at bulkhead hole.
battery. All wires in the vicinity of an aircraft battery should be inspected frequently and wires discolored by battery fumes should be replaced.
Protection of Wires in Wheel Well Area Wires located in wheel wells are subject to many additional hazards, such as exposure to fluids, pinching, and severe flexing in service. All wire bundles should be protected by sleeves of flexible tubing securely held at each end, and there should be no relative movement at points where flexible tubing is secured. These wires and the insulating tubing should be inspected carefully at frequent intervals, and wires or tubing should be replaced at the first sign of wear. There should be no strain on attachments when parts are fully extended, but slack should not be excessive.
Routing Precautions When wiring must be routed parallel to combustible fluid or oxygen lines for short distances, as much fixed separation as possible should be maintained. The wires should be on a level with, or above, the plumbing lines. Clamps should be spaced so that if a wire is broken at a clamp it will not contact the line. Where a 6-in. separation is not possible, both the wire bundle and the plumbing line can be clamped to the same structure to prevent any relative motion. If the separation is less than 2 in. but more than 1/2 in., a polyethylene sleeve may be used over the wire bundle to give further protection. Also two cable clamps back-to-back, as shown in figure 11-19, can be used to maintain a rigid separation only, and not for support of the bundle.
No wire should be routed so that it is located nearer than 1/2 in. to a plumbing line. Neither should a wire or wire bundle be supported from a Wires less than 1/4-inch from hole edge Approved grommet 27220 FIGURE 11-17. Cable clamp and grommet at bulkhead hole.
Drainage hole 1/8-inch diameter at lowest point in tubing. Make the hole after installation is complete and lowest point is firmly established FIcURE 11-18. Drain hole in low point of tubing. FicURE 11-19. Separation of wires from plumbing lines.
plumbing line that carries flammable fluids or oxy.
gen.
Wiring should be routed to maintain a minimum clearance of at least 3 in. from control cables. If this cannot be accomplished, mechanical guards should be installed to prevent contact between wiring and control cables.
Installation of Cable Clamps Cable clamps should be installed with regard to the proper angle, as shown in figure 11-20. The mounting screw should be above the wire bundle. It is also desirable that the back of the cable clamp rest against a structural member where practicable.
Figure 11-21 shows some typical mounting hardware used in installing cable clamps.
Care should be taken that wires are not pinched in cable clamps. Where possible, mount the cables directly to structural members, as shown in figure
11-22.
Clamps can be used with rubber cushions to secure wire bundles to tubular structures as shown in figure 11-23. Such clamps must fit tightly, but should not be deformed when locked in place.
LACING AND TYING WIRE BUNDLES Wire groups and bundles are laced or tied with cord to provide ease of installation, maintenance, and inspection. This section describes and illustrates recommended procedures for lacing and tying wires with knots which will hold tightly under all conditions. For the purposes of this discussion, the following terms are defined:
(1) Tying is the securing together of a group Safe angles FIGURE 11-20. Proper mounting angles for cable clamps.
or bundle of wires by individual pieces of cord tied around the group or bundle at regular intervals.
(2) Lacing is the securing together of a group or bundle of wires by a continuous piece of cord forming loops at regular intervals around the group or bundle.
(3) A wire group is two or more wires tied or laced together to give identity to an individual system.
(4) A wire bundle is two or more wires or groups tied or laced together to facilitate maintenance.
The material used for lacing and tying is either cotton or nylon cord. Nylon cord is moisture- and fungus-resistant, but cotton cord must be waxed before using to give it these necessary protective characteristics.
Single-Cord Lacing Figure 11-24 shows the step in lacing a wire bundle with a single cord. The lacing procedure is started at the thick end of the wire group or bundle with a knot consisting of a clove hitch with an extra loop. The lacing is then continued at regular inter- Screw vals with half hitches along the wire group or bundle and at each point where a wire or wire group branches off. The half hitches should be spaced so that the bundle is neat and secure. The lacing is ended by tying a knot consisting of a clove hitch with an extra loop. After the knot is tied, the free Cable clamp
- Plain
• Plain washer washer Lockwasher Nut Lockwasher split Nut extemal teeth self-locking FIGURE 11-21. Typical mounting hardware for cable clamps.
MS 21919 Cable clamps ends of the lacing cord should be trimmed to approximately 3/8 in.
Double-Cord Lacing Figure 11-25 illustrates the procedure for dou- Tubular structure "Angle" member "Z" member Correct Angle bracket
• Clamp Cable clamp Wire is pinched in clamp Incorrect FICURE 11-23. Installing cable clamp to FIGURE 11-22. Mounting cable clamp to structure. tubular structure.
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.