Wire Substitution for Repairs and Maintenance EWIS manufacturers are required to perform rigorous qualification testing of wires. The original aircraft manufacturer (OAM) may have special concerns regarding shielding and insulation for certain wiring that performs critical functions, or wiring chosen based on a set of unique circumstances. It is important to review the aircraft maintenance manual or contact the OAM when wire replacement is required. EWIS Routing
In general, EWIS should be routed and positioned to avoid chafing against aircraft structure or other components, to eliminate or minimize use as a handhold or support, to minimize exposure to damage by maintenance crews or shifting cargo, and to avoid exposure to corrosive fluids. Extra wire length should be supplied to allow for at least two re-terminations. EWIS components must be protected in wheel wells and other areas where they may be exposed to damage from impact of rocks, ice, mud, etc. Where practical, EWIS should be routed above fluid lines. Wires and cables routed within 6 inches of any flammable liquid, fuel, or oxygen line should be closely clamped and rigidly supported. The compression clamps should be spaced so that if there is a wire break, the broken wire will not contact hydraulic lines, oxygen lines, pneumatic lines, or other equipment whose subsequent failure caused by arcing could cause further damage. For all types of wire breakouts—“Y,” “T,” and complex multi-branch—there should be sufficient slack in the breakout wires to avoid strain. Care should be taken when plastic tie wraps are used so that the tie wrap head does not cause chafing damage to the wire bundle at the breakout junction. The EWIS design should preclude wire bundles from contacting the aircraft structure, using stand-offs to maintain clearance. Employing tape or protective tubing as an alternative to stand-offs should be avoided. Clamping and cable ties
Clamps and cable ties must be constructed of appropriate materials for their installation environment. Clamps must be properly sized for their wire bundles, snug enough to prevent free movement and chafing, and not used where their failure could result in interference with crucial aircraft controls or movable equipment. Clamps must be installed with their attachment hardware positioned above them so they are unlikely to rotate as the result of wire bundle weight or wire bundle chafing. Wire bundles need to be routed perpendicular to clamps. Appropriate slack needs to be maintained between clamps to protect the wires from stress while keeping the bundle free from contacting the structure. Also, sufficient slack should be left between the last clamp and the termination or electrical equipment to prevent The minimum radii 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. The bend radius for delicate thermocouple wire is 20 times the diameter, and for RF cables (e.g. coaxial and triaxial) is no less than 6 times the outside diameter of the cable. Unused Wires and Excess Wire strain at the terminal. Wire Bend Radii
Ensure 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. Coil and stow methods are often used to secure excess length of a wire bundle or to secure unconnected spare bundles. The wire bundle must be secured to prevent excessive movement or contact with other equipment that could damage the EWIS. Coil and stow in medium and high vibration areas requires additional tie straps, sleeving, and support.
Qwik Connect • April 2019
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