3.3 Harness Fabrication, Routing, and Lacing Standards

3.3 Harness Fabrication, Routing, and Lacing Standards

Core Aviation Standard: In aviation, an electrical wiring harness is an integral airframe structure. FAA Advisory Circular AC 43.13-1B, Chapter 11, Sections 8 through 12, gives acceptable practices for routing, separation from fluid lines, clamping intervals, and bend radii to prevent chafing, fire propagation, and structural fatigue throughout the life of the aircraft.

Aircraft move, flex, vibrate, and experience intense thermal expansion in flight. Wiring harnesses that are improperly routed or clamped will quickly contact moving flight controls, rub against sharp sheet-metal bulkheads, or accumulate dripping hydraulic fluids. Following strict fabrication and installation standards prevents catastrophic in-flight electrical failures.

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Harness Routing Rules and Separation from Fluid Plumbing

Wiring harness routing must anticipate potential mechanical failures elsewhere in the airframe. The most critical routing rule in aviation electrical engineering concerns proximity to fluid-carrying plumbing.

The Gravitational Separation Rule

Electrical wire bundles must always be routed ABOVE fluid lines carrying flammable liquids (aviation gasoline, Jet-A, hydraulic fluid, alcohol de-ice fluid), oxygen lines, and lavatory waste lines.

ROUTING SEPARATION GEOMETRY (AC 43.13-1B):

       +-------------------------------------+
       |   ELECTRICAL WIRING HARNESS BUNDLE  |  <--- (ROUTED ON TOP)
       +-------------------------------------+
                          |
                          |  6" Preferred Clearance
                          |  (2" minimum; 1/2" only if positively clamped)
                          v
       =======================================
          COMBUSTIBLE FLUID / HYDRAULIC LINE    <--- (ROUTED ON BOTTOM)
       =======================================
          (Leaking fluid drips downwards away from wiring)

• Physical Rationale: Gravity causes leaking liquids to drip downwards. If electrical wiring is routed beneath fluid lines, dripping fuel or hydraulic fluid pools directly on the wire bundle. If wire insulation is compromised or an arc occurs, the pooled flammable fluid ignites immediately, feeding an intense, uncontained in-flight fire.
• Separation Distances (paragraph 11-126):
  • Preferred: Route wiring above these lines with a minimum separation of 6 inches or more whenever possible.
  • When 6 inches is not practicable: Route the wiring so it does not run parallel to the fluid lines, and keep a minimum of 2 inches between wiring and the lines or equipment.
  • Closer than 2 inches: Allowed only when the wiring is positively clamped to keep at least 1/2 inch of separation, or when it must connect directly to the fluid-carrying equipment.
• No Support from Fluid Lines: The separation clamps shown in AC 43.13-1B Figure 11-10 should not be used to support the wire bundle. Install additional clamps to support the bundle, fastened to the same structure that supports the fluid line to prevent relative motion.

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Minimum Bend Radius Standards

Bending a wire bundle too sharply causes severe physical damage: outer conductor strands stretch and undergo tensile thinning, inner strands compress and buckle, and insulation cold-flows over time, leading to internal dielectric breakdown. In coaxial and digital databus cables, sharp bends crush the dielectric core, altering characteristic impedance and causing RF signal reflection.

AC 43.13-1B paragraph 11-117 sets the minimum bend radii:

The AC also warns against sharp bends in wire marked by hot stamping, because stamped insulation can crack.

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Adel Cushion Clamps (MS21919) Installation Practices

The standard hardware for securing aircraft electrical harnesses to airframe structure is the MS21919 cushion clamp (commercially known as the Adel clamp). An Adel clamp consists of a formed metal band (aluminum alloy or corrosion-resistant steel) lined with an elastomer cushion.

MS21919 ADEL CUSHION CLAMP:

                 Mounting Hardware (above clamp if practicable)
                        |
                     +--v--+
                     | (O) | <--- Clamp Tabs (Mating Flush Metal-to-Metal)
                     +-+-+-+
                      /   \
       Metal Band -> /     \ <- Elastomer Cushion (Chloroprene or Fluorosilicone)
                    |   O   |
                     \     / <--- Wire Bundle Held Snugly (No axial sliding)
                      \___/

Cushion Materials

AC 43.13-1B paragraph 11-146 requires clamps and other primary supports to be made of materials compatible with their environment: temperature, fluid exposure, ultraviolet (UV) light, and the mechanical load of the bundle. Clamps come with different cushion materials (for example, a general-purpose chloroprene cushion, or fluid- and heat-resistant cushions for hot or fluid-exposed areas). Select the cushion the clamp specification and the aircraft data call for in that zone.

Critical Installation Rules for Adel Clamps

• Correct Sizing: Clamps must give a snug fit without pinching wires and must not let the bundle move through the clamp when a slight axial pull is applied. Clamps on RF cables must fit without crushing, but may let the cable slide under a light axial pull. The AC allows one or more turns of electrical tape around the bundle when needed to achieve this fit, but adhesive tape is not acceptable as a clamping means.
• Do Not Force the Tabs: Do not use the mounting screw to pull widely separated clamp tabs together; that pinches wires between the cushion edges. Choose the correct clamp size instead.
• Hardware Position: Install clamps with their attachment hardware above the clamp wherever practicable, so the clamp is unlikely to rotate under bundle weight or chafing (Figure 11-11). Rest the back of the clamp against a structural member when practical.
• Clamp Spacing: Space clamps at intervals not exceeding 24 inches. Ties may be used between clamps, but they do not substitute for adequate clamping.
• Where Plastic Is Not Allowed: Plastic clamps or cable ties must not be used where their failure could cause interference with movable controls, contact with movable equipment, or chafing of essential or unprotected wiring, and not on vertical runs where slack could migrate.

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Bulkhead Feed-Throughs and Chafing Protection

Whenever a wiring harness penetrates an airframe bulkhead, frame, or rib cutout, it encounters sharp sheet-metal edges. Under continuous vibration, contact with a sheet-metal edge can wear through wire insulation.

Chafing Protection Hardware

1. Synthetic Rubber Grommets (MS35489): Installed in circular pass-through holes. AC 43.13-1B paragraph 11-146d calls for a grommet whenever a clamped bundle has less than 3/8 inch of clearance to the bulkhead cutout. The grommet may be cut at a 45-degree angle for installation if it is cemented in place with the slot at the top of the cutout.
2. Nylon Caterpillar Grommets (MS21266 / NAS557): Flexible, serrated nylon edging fitted around the inner perimeter of non-circular lightening holes. It must be bonded permanently to the sheet metal using approved adhesive (e.g., Pliobond or nitrile cement).
3. Abrasion Sleeving (Convoluted Tubing & Nomex Braid): Where harnesses pass close to structural flanges or landing gear linkages, bundles must be enclosed in flexible convoluted fluoropolymer tubing or braided aramid (Nomex) sleeving.

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Service Loops and Maintenance Slack

Wiring harnesses must never be installed taut between airframe tie points. A tight wire transmits vibration directly to connector pins, causes pin pull-out, and prevents maintenance.

• Slack at Terminations (paragraph 11-118): Wiring at terminal lugs or connectors should have enough slack to allow two reterminations without replacing wires. That slack is in addition to the drip loop and any allowance for movable equipment.
• Sag Between Supports: Bundles should normally not deflect more than 1/2 inch between support points (Figure 11-9a), unless there is no way for the bundle to touch a surface that could abrade it.
• Service-Loop Harnesses (paragraphs 11-135 to 11-139): Instrument-panel and console units that must be pulled out to reach their connectors are wired through service loops. Components should extend far enough to rotate and unlock the connector, usually 3 to 6 inches. Ties go on service-loop harnesses at 4- to 6-inch intervals, and the loop keeps a bend radius of at least 3 times its diameter.
• Drip Loops: In unpressurized areas, wheel wells, and external wing roots, wiring entering an electrical connector or LRU must dip downward below the connector level before rising into the receptacle. This drip loop ensures that condensation and liquid runoff flow to the low point and drip off, rather than tracking along the bundle into the connector backshell.

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Securing Bundles: MS3367 Cable Ties vs. Waxed Lacing Cord

PROPER CABLE TIE FLUSH CUT:

   CORRECT (Flush-Cut Tool)          INCORRECT (Diagonal Cutters)
        +--------+                       +--------+
        |        |                       |   /\   | <--- RAZOR-SHARP
   =====|  HEAD  |=====             =====|  /  \  |=====      DAGGER
        |        |                       | /    \ |
        +--------+                       +--------+
     Smooth flush cut                 Protruding angled tail
     (Zero injury hazard)             (Causes severe lacerations)

MS3367 Nylon Cable Ties (Zip Ties)

Modern aircraft assembly widely utilizes MS3367 (MIL-S-23190) self-locking nylon cable ties. However, improper installation creates major hazards:

• Installation Tooling: Install ties with the tie manufacturer's tensioning tool, which pulls to a set tension and cuts the tail flush with the head.
• Sharp Tails: Do not trim ties by hand with diagonal cutters. A hand-cut tail leaves a sharp, angled edge that can cut hands and chafe nearby wiring.
• Where Straps Are Restricted: AC 43.13-1B paragraph 11-158 allows MS17821 or MS17822 plastic straps only where the temperature does not exceed 120°C. Straps may not be used in SWAMP areas such as wheel wells, near wing flaps, or at wing folds, or in high-vibration areas where a failed strap would let wiring move against damaging parts. They also may not be used where they are exposed to UV light, unless the straps are resistant to it.

Waxed Nylon Lacing Cord (A-A-52080 / MIL-T-43435)

Traditional waxed braided nylon or Nomex lacing tape remains the preferred method for wire harness fabrication on layout boards:

• Lacing (paragraph 11-158a): Lace wire groups or bundles inside junction boxes and other enclosures. Use single-cord lacing on bundles 1 inch or less in diameter, starting with a clove hitch secured by a double-looped overhand knot. Use double-cord lacing on bundles 1 inch or larger, starting with a bowline on a bight.
• Ties (paragraph 11-158b): Use group or bundle ties where supports are more than 12 inches apart. A tie is a clove hitch around the bundle secured by a square knot.
• Aerospace Advantages: Flat braided tape distributes radial clamping pressure over a wide surface area, eliminating insulation pinching. Lacing cord leaves no sharp tails.