Aircraft powerplant electrical systems rely on properly secured wire groups and bundles to ensure reliable operation under vibration, heat, and other operating conditions. Correct lacing and tying techniques help prevent wire movement, chafing, and insulation damage while simplifying inspection and maintenance.
The following sections describe and illustrate recommended procedures for lacing and tying wire groups and bundles using approved knots and materials.
For the purposes of this discussion, the following terms are defined:
- Tying is the securing together of a group or bundle of wires by individual pieces of cord tied around the group or bundle at regular intervals.
- 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.
The material used for lacing and tying is either cotton or nylon cord. Nylon cord is moisture- and fungus-resistant, whereas cotton cord must be waxed before use to provide these protective characteristics.
Single-Cord Lacing
Figure 1 shows the steps in lacing a wire bundle with a single cord.
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| Figure 1. Single cord lacing |
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 intervals 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 ends of the lacing cord should be trimmed to approximately 3⁄8 inch.
Double-Cord Lacing
Figure 2 illustrates the procedure for double-cord lacing. The lacing is started at the thick end of the wire group or bundle with a bowline-on-a-bight knot. [Figure 2A]
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| Figure 2. Double cord lacing |
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 inch.
Lacing Branch-Offs
Figure 3 illustrates a recommended procedure for lacing a wire group that branches off the main wire bundle.
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| Figure 3. Lacing a branch off |
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. End the lacing with the regular terminal knot used in single- or double-cord lacing, as applicable, and trim the free ends of the lacing cord neatly.
Tying
All wire groups or bundles should be tied where supports are more than 12 inches apart. Ties are made using waxed cotton cord, nylon cord, or fiberglass cord. Some manufacturers permit the use of pressure-sensitive vinyl electrical tape. When permitted, the tape should be wrapped three turns around the bundle and the ends heat-sealed to prevent unwinding of the tape.
Figure 4 illustrates a recommended procedure for tying a wire group or bundle.
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| Figure 4. Tying a wire group of bundle |
Begin the tie by wrapping the cord around the wire group and tying a clove hitch. Then, a square knot with an extra loop is tied and the free ends of the cord trimmed.
Temporary ties are sometimes used in making up and installing wire groups and bundles. Colored cord is normally used to make temporary ties, because they are removed after installation is complete.
Whether lacing or tying, bundles should be secured tightly enough to prevent slipping, but not so tightly that the cord cuts into or deforms the insulation. This precaution is especially important for coaxial cable, which has a soft dielectric insulation between the inner and outer conductor. Coaxial cables have been damaged by the use of lacing materials or by methods of lacing or tying wire bundles that cause a concentrated force on the cable insulation. Elastic lacing materials, small-diameter lacing cord, and excessive tightening deform the interconductor insulation and result in short circuits or impedance changes. Flat nylon braided waxed lacing tape should be used for lacing or tying any wire bundles containing coaxial cables.
The part of a wire group or bundle located inside a conduit is not tied or laced; however, wire groups or bundles inside enclosures, such as junction boxes, should be laced only.