Aircraft wiring harnesses  

Wire bundles are everywhere and critical in an aircraft. Shielding contributes significantly towards an aircraft’s continued airworthiness by maintaining electromagnetic protection over the lifetime of an aircraft. Aircraft aging and exposure to environmental conditions affects aircraft wiring harness shielding. The quality of the electrical bonds between the cable shield, backshells, connectors, and metallic structures can deteriorate with time.

Aircraft wiring harnesses are 24-inch-long wire bundles with end connectors and connector backshells, termination boxes and bulkhead receptacles. Wire bundles can consist of a combination of unshielded wires and shielded wires, secured along their length with plastic tie wraps. This is a standard wire bundle configuration.

Temperature, altitude, vibration, salt spray and humidity can all cause mechanical degradation of the aircraft wiring shielding.

Electrical Wire Interconnect Systems

As an aircraft ages and the integrity of the Electrical Wire Interconnect Systems (EWIS) becomes compromised, it can lead to aircraft system malfunctions and in some cases, arcing, which can result in more significant damage. When there is a breach in the insulating material, contact with grounded structure or foreign fluids can result in electrical arcing. Polyimide insulation is susceptible to environmental deterioration due to hydrolysis that can reduce the polymer’s molecular weight.

Attacks on wiring insulation can come in many types: Extreme heat and cold temperature swings can accelerate hardening (or softening), humidity and chemical spills or leaks can create brittleness, mechanical strains can fatigue the wire conductor creating a gap that can arc and generate heat. Over voltage can cut holes in electrical insulation leading to flashover. Mechanical damage that occurs during the wire installation, often in the form of metal chips lodged in wire bundles, can go undetected for years. Tight bend radiuses can chafe the insulation and also fatigue the conductor.

Most electrical problems can be described in two simple terms, short or open. If a wire breaks cleanly, the circuit is broken and the result is an inoperative component. With a short circuit, the load is bypassed and the surge in current should trip circuit breakers.

The most dangerous problem with aircraft wiring occurs when there's a breakdown of the wire's insulator and current jumps outside of the conductor, creating a corona discharge or spark. Insulation does not have to completely disappear for this to happen. If the insulative properties weaken through the effects of age, ozone, ultraviolet and chemical attack, the insulator can actually become a resistor and generate heat. Heat just needs a fuel source and oxygen to create fire.

Most wiring harnesses that are replaced are located mid fuselage, aft fuselage, engine bay, Flap bay, and wheel wells. Cabin reconfiguration changes and conversions often attempt to leave wiring that is tucked neatly out of sight and out of mind untouched.

Every large modern passenger jet contains some 150 miles of wiring, like hidden blood vessels that carry power to thousands of functions.  For years aircraft contained a particular aircraft wire insulated with a plastic material known as ‘Kapton’.

Although flame resistant and tough, the Kapton insulation can explode

like a firework if it’s chafed and rubs against metal.  This is called ‘dry arcing’.  Salt water or solvent can penetrate the insulation and cause the same problem.  This is called ‘wet arcing’. 

Most wires, when they short-circuit, simply spark and blow a fuse.  But with Kapton insulation, the spark explodes and, worst case scenario, sets light to the plastic insulation.  Inside the confines of the plane, this can quickly spread into a lethal weapon of toxic fumes and flames. Pilots resetting circuit breakers can only increase the chances of aircraft fire.

For exactly these reasons, Kapton insulated wiring has been largely replaced. The L-1011, known in Britain as the Tri-Star, was built by the Lockheed Company in the late 60s and 70s and was one of the world’s earlier more comfortable jumbos.  Several still fly with Kapton insulated wiring.