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.
b. Tying. Use wire group or bundle ties where the supports for the wire are more than 12 inches apart. A tie consists of a clove hitch, around the wire group or bundle, secured by a square knot as shown in Figure 11-17.
c. Plastic Ties. See paragraph 11-220 and Table 11-22.
FIGURE 11-15. SINGLE CORD LACING
PULL HERE UNTIL TIGHT
BEFORE FINISHING KNOT
CORD CROSSES
UNDER LOOP STEP a - STARTING KNOT
FINAL KNO
STARTING KNOT—
TIGHTENED
STEP & - INTERMEDIATE HALF HITCHES FIRST PART OF FINAL STEP c - (PART I) KNOT TIGHTENED TRIM TO 3/8" MIN STEP c - (PART II) FINAL KNOT Par 11-158 Page 11-72
Par 11-158 Page 11-73 FIGURE 11-16. DOUBLE CORD LACING
Par 11-158 Page 11-74 FIGURE 11-17. MAKING TIES
11-159. INSULATION TAPE. Insulation tape should be of a type suitable for the application, or as specified for that particular use. Tape should be used primarily as a filler under clamps and as secondary support. Non-adhesive tape may be used to wrap around wiring for additional protection, such as in wheel wells. All tape should have the ends tied or otherwise suitably secured to prevent unwinding. Tape used for protection should be applied so that overlapping layers shed liquids. Drainage holes should be provided at all trap points and at each low point between clamps. Plastic tapes, that absorb moisture or have volatile plasticizers that produce chemical reactions with other wiring, should not be used. (Refer to AS50881.)
11-160.–11-166. [RESERVED.]