Par 11-118 Page 11-60 wires. This slack should be in addition to the drip loop and the allowance for movable equipment. Normally, wire groups or bundles should not exceed 1/2-inch deflection between support points, as shown in Figure 11-9A. This measurement may be exceeded provided there is no possibility of the wire group or bundle touching a surface that may cause abrasion. Sufficient slack should be provided at each end to:
a. Permit replacement of terminals.
b. Prevent mechanical strain on wires.
c. Permit shifting of equipment for maintenance purposes.
FIGURE 11-9A. SLACK BETWEEN SUPPORTS 11 -119. DRIP LOOP IN WIRE BUNDLE. A drip loop is an area where wire is dressed downward to a connector, terminal block, panel, or junction box. In additional to the service termination and strain relief, a trap or drip loop shall be provided in the wiring to prevent fluid or condensate from running into the above devices. (See Figure 11-9B.) Wires or groups of wires should enter a junction box or piece of equipment in an upward direction where practicable.
Where wires must be routed downwards to a junction box or unit of electric equipment, the entry should be sealed or adequate slack should be provided to form a trap or drip loop to prevent liquid from running down the wires in the box or electric unit.
Par 11-119 Page 11-61 FIGURE 11-9B. DRAINAGE HOLE IN LOW POINT OF TUBING 11 -120. POWER FEEDERS. The power feeder wires should be routed so that they can be easily inspected or replaced. They must be given special protection to prevent potential chafing against other wiring, aircraft structure, or components.
11-121. RF CABLE. All wiring needs to be protected from damage. However, coaxial and triaxial cables are particularly vulnerable to certain types of damage. Personnel should exercise care while handling or working around coaxial. Coaxial damage can occur when clamped too tightly, or when they are bent sharply (normally at or near connectors). Damage can also be incurred during unrelated maintenance actions around the coaxial cable. Coaxial can be severely damaged on the inside without any evidence of damage on the outside. Coaxial cables with solid center conductors should not be used. Stranded center coaxial cables can be used as a direct replacement for solid center coaxial.
11-122. PRECAUTIONS.
a. Never kink coaxial cable.
b. Never drop anything on coaxial cable.
c. Never step on coaxial cable.
d. Never bend coaxial cable sharply.
Par 11-122 Page 11-62
e. Never loop coaxial cable tighter than the allowable bend radius.
f. Never pull on coaxial cable except in a straight line.
g. Never use coaxial cable for a handle, lean on it, or hang things on it (or any other wire).
11-123. MOISTURE PROTECTION, WHEEL WELLS, AND LANDING GEAR AREAS.
a. Wires located on landing gear and in the wheel well area can be exposed to many hazardous conditions if not suitably protected. Where wire bundles pass flex points, there must not be any strain on attachments or excessive slack when parts are fully extended or retracted.
The wiring and protective tubing must be inspected frequently and replaced at the first sign of wear.
b. Wires should be routed so that fluids drain away from the connectors. When this is not practicable, connectors must be potted. Wiring which must be routed in wheel wells or other external areas must be given extra protection in the form of harness jacketing and connector strain relief. Conduits or flexible sleeving used to protect wiring must be equipped with drain holes to prevent entrapment of moisture.
11-124. PROTECTION AGAINST PERSONNEL AND CARGO. Wiring must be installed so the structure affords protection against its use as a handhold and damage from cargo. Where the structure does not afford adequate protection, conduit must be used, or a suitable mechanical guard must be provided.
11-125. HEAT PRECAUTIONS. Wiring must be routed away from high-temperature equipment and lines to prevent deterioration of insulation. Wires must be rated (see paragraphs 11-66 and 11-67) so that the conductor temperature remains within the wire specification maximum when the ambient temperature, and heat rise, related to current carrying capacity are taken into account. The residual heating effects caused by exposure to sunlight when aircraft are parked for extended periods should also be taken into account. Wires, such as in fire detection, fire extinguishing, fuel shutoff, and fly-by-wire flight control systems that must operate during and after a fire, must be selected from types that are qualified to provide circuit integrity after exposure to fire for a specified period. Wire insulation deteriorates rapidly when subjected to high temperatures. Do not use wire with soft polyethylene insulation in areas subject to high temperatures. Use only wires or cables with heat resistance shielding or insulation.
11-126. MOVABLE CONTROLS WIRING PRECAUTIONS. Clamping of wires routed near movable flight controls must be attached with steel hardware and must be spaced so that failure of a single attachment point cannot result in interference with controls. The minimum separation between wiring and movable controls must be at least 1/2 inch when the bundle is displaced by light hand pressure in the direction of the controls.
11-127. FLAMMABLE FLUIDS AND GASES. An arcing fault between an electrical wire and a metallic flammable fluid line may puncture the line and result in a fire. Every effort must be made to avoid this hazard by physical separation of the wire from lines and equipment containing oxygen, oil, fuel, hydraulic fluid, or alcohol. Wiring must be routed above these lines and equipment with a minimum separation of 6 inches or more whenever possible. When such an
Par 11-127 Page 11-63 arrangement is not practicable, wiring must be routed so that it does not run parallel to the fluid lines. A minimum of 2 inches must be maintained between wiring and such lines and equipment, except when the wiring is positively clamped to maintain at least 1/2-inch separation, or when it must be connected directly to the fluid-carrying equipment. Install clamps as shown in Figure 11-10. These clamps should not be used as a means of supporting the wire bundle.
Additional clamps should be installed to support the wire bundle and the clamps fastened to the same structure used to support the fluid line(s) to prevent relative motion.
FIGURE 11-10. SEPARATION OF WIRES FROM PLUMBING LINES 11 -128.–11-134. [RESERVED.]
Par 11-135 Page 11-64 SECTION 10. SERVICE LOOP HARNESSES (PLASTIC TIE STRIPS)
11-135. GENERAL. The primary function of a service loop harness is to provide ease of maintenance. The components, mounted in the instrument panel and on the lower console and other equipment that must be moved to access electrical connectors, are connected to aircraft wiring through service loops. Chafing in service loop harnesses is controlled using the following techniques.
11-136. SUPPORT. Only string ties or plastic cable straps in accordance with paragraph 11-158 should be used on service loop harnesses. A 90°or “Y” type spot tie should be installed at the harness breakout point on the harness bundle. Ties should be installed on service loop harnesses at 4- to 6-inch intervals.
11-137. ANTI-CHAFING MATERIAL. When service loops are likely to be in contact with each other, expandable sleeving or equivalent chafe protection jacket material must be installed over service loop harnesses to prevent harness-to-harness chafing. The sleeve should be held in place with string ties at 6- to 8-inch intervals. Harness identification labels should be installed, with string tie, within 3 inches of the service loop harness installation.
11-138. STRAIN RELIEF. The strain relief components may be installed to control routing where close clearance exists between termination and other components or bulkheads. Strain relief components provide support of the service loop harness at the termination point. Connector strain relief adapters, heat-shrinkable boot, or a length of heat shrinkable tubing should be installed. The heat-shrinkable boots will provide preselected angles of wire harness termination when heat is applied. Heat-shrinkable tubing should be held at the desired angle until cool.
11-139. “SERVICE LOOP.” Primary support for service loop harness(es) should be a cushion clamp and a connector at the harness termination. Service loop harnesses should be inspected for the following:
a. Adequate Length. Components should extend out from their mounting position a distance that permits rotating and unlocking (or locking) the electrical connector. Usually a distance of 3 to 6 inches, with all other components installed, should be sufficient.
b. Bundle Breakout Point.
(1) Bundle breakout point should be adequately supported with string tie.
(2) Service loop must maintain a minimum bend radius of 3 times the harness diameter.
(3) The breakout point should be located directly behind, beside, below, or above the component so that the service loop harness does not bind other components.
(4) Plastic ties should not be used between the service loop breakout and the electrical connector when they are likely to chafe against adjacent wire.
Par 11-139 Page 11-65
c. Service Loop Routing. The service loop harness should be routed directly from the breakout point to the component. The harness should not contact moving mechanical components or linkage, and should not be wrapped or tangled with other service loop harnesses.
d. Service Loop Harness Termination. Strain relief should be provided at the service loop harness termination, and is normally provided by the connector manufacturer’s back shell, heat-shrinkable boot, or tubing.
11-140.–11-145. [RESERVED.]
Par 11-146 Page 11-66 SECTION 11. CLAMPING
11-146. GENERAL. Wires and wire bundles must be supported by using clamps meeting AS21919, or plastic cable straps in accessible areas if correctly applied within the restrictions of paragraph 11-158. Clamps and other primary support devices must be constructed of materials that are compatible with their installation and environment, in terms of temperature, fluid resistance, exposure to ultraviolet (UV) light, and wire bundle mechanical loads. They should be spaced at intervals not exceeding 24 inches. Clamps on wire bundles should be selected so that they have a snug fit without pinching wires, as shown in Figures 11-11 through 11-13.
CAUTION: The use of metal clamps on coaxial RF cables may cause problems if clamp fit is such that RF cable’s original cross-section is distorted.
a. Clamps on wire bundles should not allow the bundle to move through the clamp when a slight axial pull is applied. Clamps on RF cables must fit without crushing and must be snug enough to prevent the cable from moving freely through the clamp, but may allow the cable to slide through the clamp when a light axial pull is applied. The cable or wire bundle may be wrapped with one or more turns of electrical tape when required to achieve this fit. Plastic clamps or cable ties must not be used where their failure could result in interference with movable controls, wire bundle contact with movable equipment, or chafing damage to essential or unprotected wiring. They must not be used on vertical runs where inadvertent slack migration could result in chafing or other damage. Clamps must be installed with their attachment hardware positioned above them, wherever practicable, so that they are unlikely to rotate as the result of wire bundle weight or wire bundle chafing. (See Figure 11-11.)
b. C lamps lined with nonmetallic material should be used to support the wire bundle along the run. Tying may be used between clamps, but should not be considered as a substitute for adequate clamping. Adhesive tapes are subject to age deterioration and, therefore, are not acceptable as a clamping means.
c. The back of the clamp, whenever practical, should be rested against a structural member. Stand-offs should be used to maintain clearance between the wires and the structure.
Clamps must be installed in such a manner that the electrical wires do not come in contact with other parts of the aircraft when subjected to vibration. Sufficient slack should be left between the last clamp and the electrical equipment to prevent strain at the terminal and to minimize adverse effects on shock mounted equipment. Where wires or wire bundles pass through bulkheads or other structural members, a grommet or suitable clamp should be provided to prevent abrasion.
d. When wire bundle is clamped into position, if there is less than 3/8-inch clearance between the bulkhead cutout and the wire bundle, a suitable grommet should be installed as indicated in Figure 11-14. The grommet may be cut at a 45-degree angle to facilitate installation, provided it is cemented in place and the slot is located at the top of the cutout.
11-147. WIRE AND CABLE CLAMPS INSPECTION. Inspect wire and cable clamps for proper tightness. Where cables pass through structure or bulkheads, inspect for proper clamping and grommets. Inspect for sufficient slack between the last clamp and the electronic equipment
to prevent strain at the cable terminals and to minimize adverse effects on shock-mounted equipment.
FIGURE 11-11. SAFE ANGLE FOR CABLE CLAMPS
DANGEROUS ANGLES
SAFE ANGLES FIGURE 11-12. TYPICAL MOUNTING HARDWARE FOR AS21919 CABLE CLAMPS A5-21911
CABLE CLAMI OR
EQUIVALENT MS-27009/NAS 603 SCREW AN 960 AN 960 WASHER,
WASHER, PLAIN PLAIN AN 9368
LOCKWASHER, EXTERNAL TEETH MS 21044 NUT, MS 35338
SELF-LOCKING LOCKWASHER, AN 340 NUT SPLIT Par 11-147 Page 11-67
Par 11-147 Page 11-68 FIGURE 11-13. INSTALLING CABLE CLAMP TO STRUCTURE
FIGURE 11-14. CLAMPING AT A BULKHEAD HOLE A. CUSHION CLAMP AT MS21919 BULKHEAD HOLE.
CABLE CLAMP
CLEARANCE 3/8" MINIMUM WIRES LESS THAN 3/8 INCH
FROM HOLE EDGE
GROMMET
ANGLE BRACKET WITH
TWO POINT FASTENING MS21919
CABLE CLAMP B. CUSHION CLAMP AT
BULKHEAD HOLE WITH MS35489 GROMMET.
C. CUSHION CLAMP AT
BULKHEAD HOLE WITH MS21266 GROMMET.
ANGLE BRACKET WITH
TWO POINT FASTENING
11-148.-11-154. [RESERVED.] Par 11-147 Page 11-69
Par 11-155 Page 11-70 SECTION 12. WIRE INSULATION AND LACING STRING TIE
11-155. GENERAL. Insulation of wires should be appropriately chosen in accordance with the environmental characteristics of wire routing areas. Routing of wires with dissimilar insulation, within the same bundle, is not recommended, particularly when relative motion and abrasion between wires having dissimilar insulation can occur. Soft insulating tubing (spaghetti) cannot be considered as mechanical protection against external abrasion of wire, since at best, it provides only a delaying action. Conduit or ducting should be used when mechanical protection is needed.
11-156. INSULATION MATERIALS. Insulating materials should be selected for the best combination of characteristics in the following categories:
a. Abrasion resistance.
b. Arc resistance (noncarbon tracking).
c. Corrosion resistance.
d. Cut-through strength.
e. Dielectric strength.
f. Flame resistance.
g. Heat distortion temperature.
h. Impact strength.
i. Mechanical strength.
j. Resistance to fluids.
k. Resistance to notch propagation.
l. Smoke emission.
m. Special properties unique to the aircraft.
n. For a more complete selection of insulated wires refer to SAE AS4372, Performance Requirements for Wire, Electric, Insulated Copper or Copper Alloy, and SAE AS4373, Test Methods for Insulated Electric Wire.
11-157. STRIPPING INSULATION. Attachment of wire, to connectors or terminals, requires the removal of insulation to expose the conductors. This practice is commonly known as stripping. Stripping may be accomplished in many ways; however, the following basic principles should be practiced.
a. Make sure all cutting tools used for stripping are sharp.
Par 11-157 Page 11-71
b. When using special wire stripping tools, adjust the tool to avoid nicking, cutting, or otherwise damaging the strands.
c. Damage to wires should not exceed the limits specified in Table 11-15.
d. When performing the stripping operation, remove no more insulation than is necessary.
TABLE 11-15. ALLOWABLE NICKED OR BROKEN STRANDS Maximum Allowable Nicked and Broken Strands Wire Size Conductor Material Number of Strands Per Conductor Total Allowable Nicked and Broken Strands 24-14 Copper 19 2 nicked, none broken 12-10 or 37 4 nicked, none broken 8-4 Copper Alloy 133 6 nicked, 6 broken 2-1 665-817 6 nicked, 6 broken 0-00 1,045-1,330 6 nicked, 6 broken 000 1,665- 6 nicked, 6 broken 0000 2,109- 6 nicked, 6 broken 8-000 Aluminum All numbers of strands None, None
11-158. LACING AND TIES. Ties, lacing, and straps are used to secure wire groups or bundles to provide ease of maintenance, inspection, and installation. Braided lacing tape per A-A-52080 is suitable for lacing and tying wires. In lieu of applying ties, straps meeting MS17821-4-9 or MS17822-1-9B may be used in areas where the temperature does not exceed 120 °C. Straps may not be used in areas of SWAMP such as wheel wells, near wing flaps, or wing folds. They may not be used in high-vibration areas, where failure of the strap would permit wiring to move against parts which could damage the insulation and foul mechanical linkages or other moving mechanical parts. They also may not be used where they could be exposed to UV light, unless the straps are resistant to such exposure.
a. Lacing. Lace wire groups or bundles inside junction boxes or other enclosures. Single cord-lacing method, shown in Figure 11-15, and tying tape, meeting specification A-A-52080, may be used for wire groups of bundles 1 inch in diameter or less. The recommended knot for starting the single cord-lacing method is a clove hitch secured by a double-looped overhand knot as shown in Figure 11-15, step a. Use the double cord-lacing method on wire bundles 1-inch in diameter or larger as shown in Figure 11-16. When using the double cord-lacing method, employ a bowline on a bight as the starting knot.