Installation and inspection of Aircraft wires and cables

 K.Lacey Wires and Cables t.+44 (0)1903 771336

Wires and cables should be inspected for adequacy of support, and general condition throughout.

For personal safety, and to avoid the possibility of fire, turn off all electrical power prior to starting an inspection of the aircraft electrical system or performing maintenance.

Wires and cables, airframe cables, are supported by clamps, grommets, or other devices at intervals of not more than 24 inches, except when contained in troughs, ducts, or conduits. The supporting devices should be of a suitable type, with the wires and cables held securely in place without damage to the insulation.

Metal stand-offs must be used to maintain clearance between wires and structure. Employing tape or tubing is not acceptable as an alternative to stand-offs for maintaining clearance.

Phenolic blocks, plastic liners, or rubber grommets are installed in holes, bulkheads floors, or structural members where it is impossible to install off-angle wiring clamps to maintain wiring separation. In such cases, additional protection in the form of plastic or indicating tape may be used.

Wires and cables in junction boxes. - panels and bundles are properly supported and laced to provide proper grouping and routing.

Clamp retaining screws are properly secured so that the movement of wires and cables is restricted to the span between the points of support and not on soldered or mechanical connections at terminal posts or connectors.

Wire and cables are properly supported and bound so that there is no interference with other wires, cables, and equipment.

Wires and cables are adequately supported to prevent excessive movement in areas of high vibration.

Insulating tubing is secured by tying, tie straps or with cable clamps.

Continuous lacing is only used in panels and junction boxes where this practice is optional. When lacing is installed in this manner, outside junction boxes should be removed and replaced with individual loops.

Do not use tapes (such as friction or plastic tape) which, will dry out in service, produce chemical reactions with wire or cable insulation, or absorb moisture.

Insulating tubing must be kept at a minimum and must be used to protect wire and cable from abrasion, chafing, exposure to fluid, and other conditions which could affect the cable insulation. However, the use of insulating tubing for support of wires and cable in lieu of stand-offs is prohibited.

Do not use moisture-absorbent material as "fill" for clamps or adapters.

Ensure that wires and cables are not tied or fastened together in conduit or insulating tubing.

Ensure cable supports do not restrict the wires or cables in such a manner as to interfere with operation of equipment shock mounts.

Do not use tape, tie straps, or cord for primary support.

Make sure that drain holes are present in drip loops or in the lowest portion of tubing placed over the wiring.

Ensure that wires and cables are routed in such a manner that chafing will not occur against the airframe or other components.

Ensure that wires and cables are positioned in such a manner that they are not likely to be used as handholds or as support for personal belongings and equipment.

Ensure that wires and cables are routed, in so far as practicable, so that they are not exposed to damage by personnel moving within the aircraft, storage or shifting of cargo.

Ensure that wires and cables are routed so that there is not a possibility of damage from battery electrolytes or other corrosive fluids.

Ensure that wires and cables are adequately protected in wheel wells and other areas where they may be exposed to damage from impact of rocks, ice, mud, etc.

Where practical, route electrical wires and cables above fluid lines and provide a 6 inch separation from any flammable liquid,fuel_ or oxygen line, fuel tank wall, or other low voltage wiring that enters a fuel tank and requires electrical isolation to prevent an ignition hazard. Where 6 inch spacing cannot practically be provided, a minimum of 2 inches must be maintained between wiring and such lines. related equipment, fuel tank walls and low voltage wiring that enters a fuel tank. Such wiring should be closely clamped and rigidly supported and tied at intervals such that contact between such lines, related equipment, fuel tank walls or other wires, would not occur, assuming a broken wire and a missing wire tie or clamp.

Ensure that a trap or drip loop is provided to prevent fluids or condensed moisture from running into wires and cables dressed downward to a connector, terminal block, panel, or junction box.

Wires and cables installed in bilges and other locations where fluids may be trapped are routed as far from the lowest point as possible or otherwise provided with a moisture-proof covering.

High Temperature Fire Resistant Cable

This should always be used where required.

Separate wires from high-temperature equipment, such as resistors, exhaust stacks, heating ducts, etc., to prevent insulation breakdown. Insulate wires that must run through hot areas with a high-temperature insulation material such as fiberglass or PTFE.. Many standard coaxial cables have inadequate insulation, so proper defence standard cable should be used, high temperature fire resistant cable.

The minimum radius of bends in wire groups or bundles must not be less than 10 times the outside diameter of the largest wire or cable. except that at the terminal strips where wires break out at terminations or reverse direction in a bundle. Where the wire is suitably supported, the radius may be 3 times the diameter of the wire or cable. Where it is not practical to install wiring or cables within the radius requirements. the bend should be enclosed in insulating tubing. The radius for thermocouple wire should be done in accordance with the manufacturer's recommendation and should be sufficient to avoid excess losses or damage to the cable.

Ensure that RF cables, e.g., coaxial and triaxial cables are bent at a radius of no less than 6 times the outside diameter of the cable.

Ensure that unused wires are individually dead-ended, tied into a bundle, and secured to a permanent structure. Each wire should have strands cut even with the insulation and a pre-insulated closed end connector or a 1-inch piece of insulating tubing placed over the wire with its end folded back and tied.

Ensure that wires and cables, that are attached to assemblies where relative movement occurs (such as at hinges and rotating pieces; particularly doors, control sticks, control wheels, columns, and flight control surfaces), are installed or protected in such a manner as to prevent deterioration of the wires and cables caused by the relative movement of the assembled parts.

Ensure that wires and electrical cables are separated from mechanical control cables. In no instance should wire be able to come closer than 1/2 inch to such controls when light hand pressure is applied to wires or controls. In cases where clearance is less than this, adequate support must be provided to prevent chafing.

Ensure that wires and cables are provided with enough slack to permit ease of maintenance, prevent mechanical strain on the wires, cables, junctions, and supports.

Ensure that all wires and cables are identified properly at intervals of not more than 15 inches. Coaxial cables are identified at both equipment ends.

Aircraft wiring replacement

Wiring must be replaced with equivalent wire, Airbus ASN-E spec cable with Airbus ASN-E spec, Eurofighter JN 1026 spec cable with Eurofighter JN 1026 spec, when found to have any of the following defects:

a. Wiring that has been subjected to chafing or fraying, that has been severely damaged, or that primary insulation is suspected of being penetrated.

b. Wiring on which the outer insulation is brittle to the point that slight flexing causes it to crack.

c. Wiring having weather-cracked outer insulation.

d. Wiring that is known to have been exposed to electrolyte or on which the insulation appears to be, or is suspected of being, in an initial stage of deterioration due to the effects of electrolyte.

e. Check wiring that shows evidence of overheating (even if only to a minor degree) for the cause of the overheating.

f. Wiring on which the insulation has become saturated with engine oil, hydraulic fluid, or another lubricant.

g. Wiring that bears evidence of having been crushed or severely kinked.

h. Shielded wiring on which the metallic shield is frayed and/or corroded. Cleaning agents or preservatives should not be used to minimize the effects of corrosion or deterioration of wire shields.

i. Wiring showing evidence of breaks, cracks, dirt, or moisture in the plastic sleeves placed over wire splices or terminal lugs.

j. Sections of wire in which splices occur at less than 10-foot intervals, unless specifically authorized, due to parallel connections, locations, or inaccessibility.

k. When replacing wiring or coaxial cables, identify them properly at both equipment and power source ends.