📖 The lesson
Cable types and construction
- A wire is a single insulated conductor. A cable is two or more separately insulated conductors in one jacket, or a shielded/coaxial wire. A wire bundle (harness) is a group of wires tied together.
- Conductors are stranded (for flexibility and fatigue resistance) annealed copper, or high-strength copper alloy for small gauges. Copper is plated against oxidation: tin (to about 150 °C), silver (to about 200 °C) and nickel (to about 260 °C).
- Aluminium wire saves weight in large sizes. Sizes smaller than 8 AWG are discouraged, and it is not used on engine-mounted equipment, in severe vibration or corrosive areas, for frequently disconnected circuits or for runs under about 3 ft. Use terminals designed for aluminium.
- Wire size uses the American Wire Gauge (AWG): the smaller the number, the larger the wire. Size is chosen for current-carrying capacity (heating) and voltage drop. Wires smaller than 20 AWG need extra support.
Insulation
| Material | Notes |
|---|---|
| PVC | Older aircraft; gives toxic, dense smoke when burnt — no longer used in new designs. |
| Polyimide (Kapton) | Thin, light, tough and heat-resistant, but stiff, prone to cracking and to wet and dry arc tracking (carbonised insulation becomes conductive). |
| ETFE (Tefzel) | Tough, good general-purpose airframe wire (about 150 °C). |
| PTFE (Teflon) and composites (PTFE/polyimide) | High temperature (about 200–260 °C), chemical resistance; used in engine areas. |
- High-temperature / fire-resistant cable — nickel-plated conductors with glass-fibre, PTFE or ceramic insulation — is used for fire-detection and engine wiring. Thermocouple cable uses matched alloys (chromel/alumel).
- Shielded cable has a metal braid or foil around the insulated conductors to protect against electromagnetic interference (EMI). The shield is terminated (grounded) as the manual specifies — often at one end only for audio circuits.
- Coaxial cable has a centre conductor, dielectric, braided outer conductor and jacket. It is used for RF/antenna signals and is specified by characteristic impedance (50 Ω or 75 Ω). Avoid sharp bends, kinks and crushing, which change the impedance.
Identification and crimping
- Wires are marked with circuit, wire number and size codes at intervals of not more than 15 in, and within 3 in of each end. Marking is by UV laser (preferred), ink-jet or hot-stamp (which can damage thin insulation), or by marked sleeves.
- Stripping: use a precision stripper with the correct die, perpendicular to the wire; no nicked or broken strands (critical with aluminium).
- Crimping: choose the terminal for the wire size and stud size. Pre-insulated terminals are colour-coded — red 22–18 AWG, blue 16–14, yellow 12–10. Use the matching ratchet crimp tool, which will not release until the full crimp cycle is complete. Check by inspection and pull test. Tools are calibrated and checked with go/no-go gauges.
- No more than four terminals per stud (or as limited by the manual). Splices are crimped, staggered in a bundle, and not placed in bends or inside connectors. Solder sleeves are used for shield terminations.
Connectors
- A connector pair is a plug and a receptacle. Contacts are pins (male) and sockets (female). The live (supply) side carries sockets, so no exposed pin is energised when disconnected.
- Coding: keyways, keys and insert rotation (polarisation) prevent mismating of adjacent connectors.
- Contacts are usually crimped and then pushed into the insert with an insertion tool; an extraction tool releases the retention clip. Unused contact positions are filled with sealing plugs.
- Coupling: threaded, bayonet, or push-pull (rack-and-panel ARINC connectors for avionics boxes). Environmental (sealed) connectors have rubber grommets and interfacial seals; potting compound seals the rear where required. Backshells give strain relief and shield termination.
- Common families: MIL-DTL-5015, MIL-DTL-26482, MIL-DTL-38999 (high-density circular), ARINC 404/600 rack connectors, D-subminiature, and terminal blocks/modules.
- Inspect for bent or pushed-back pins, corrosion, moisture, damaged seals and loose backshells. Use contact cleaner — never abrasives on plated contacts.
Installation and protection
- Bend radius: not less than 10 × the OD of the largest wire or cable in the bundle (3 × where well supported at terminals). Coax has its own larger minimum.
- Support: clamps or other supports at intervals of not more than 24 in, unless in conduit or ducts. Clamps are cushioned, and the bundle must not chafe on structure.
- Slack: about 1/2 in deflection between supports under normal hand pressure; enough slack at connectors for removal and for movement.
- Drip loops keep fluid from running into connectors. Route wires above fluid lines, separate them from fuel, oxygen and hydraulic lines, and keep at least the specified clearance from controls and hot areas. Separate redundant circuits and high-power from sensitive wiring.
- Protect bundles with grommets through structure, conduit in exposed areas, and fire-resistant sleeving in fire zones. Lacing and ties secure bundles; tie spacing follows the manual.
- EWIS (Electrical Wiring Interconnection System) inspection: look for chafing, cracking, contamination, heat damage and loose connections; keep wiring clean — dust and fluids promote arc tracking.