6.1 EWIS Principles, Wire Types & Loom Routing Practices

6.1 EWIS Principles, Wire Types & Loom Routing Practices

Approved-Data Control

The figures and hardware examples in this section illustrate principles. For an actual aircraft or component, current approved maintenance data, product instructions, organisation procedures, and applicable law control the material, limit, interval, sequence, tooling, PPE, and acceptance decision.

Aircraft electrical wiring was historically treated as passive commercial airframe hardware—installed during manufacture and largely ignored unless a specific malfunction occurred. However, a series of fatal transport-category aircraft accidents revealed that aging, damaged, or contaminated electrical wiring represents a severe threat to aircraft airworthiness. In response, international regulatory authorities transformed wiring maintenance through the establishment of the Electrical Wiring Interconnection System (EWIS) regulatory framework. For certifying aircraft maintenance engineers under EASA Part-66 Module 7, mastering EWIS principles, conductor metallurgy, insulation behavior, and physical routing standards is vital for flight safety.

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Regulatory Genesis: EASA CS-25 Subpart H & AMC 20-21

The fundamental transition in aviation wiring philosophy was prompted by comprehensive accident investigations, most notably:

• TWA Flight 800 (1996): A Boeing 747-100 suffered an in-flight explosion of the center wing fuel tank. The investigation concluded that the most likely ignition source was an electrical fault outside the fuel tank that allowed high voltage from a lighting or power circuit to cross over into low-voltage Fuel Quantity Indicating System (FQIS) wiring, initiating a spark inside the tank.
• Swissair Flight 111 (1998): A McDonnell Douglas MD-11 crashed following an uncontrollable in-flight fire above the cockpit ceiling. Arcing on aromatic polyimide (Kapton) wire insulation ignited nearby metallized polyethyleneteraphthalate (MPET) thermal-acoustic insulation blankets, spreading catastrophic fire and smoke through flight-critical cockpit systems.

These disasters led the FAA Aging Transport Systems Rulemaking Advisory Committee (ATSRAC) and European regulators to overhaul certification codes. EASA introduced CS-25 Subpart H, officially codifying Electrical Wiring Interconnection System (EWIS).

Scope and Definition of EWIS

Under CS-25.1701, an EWIS is defined as any wire, wiring device, or combination of these, including termination devices, installed in any area of the airplane for the purpose of transmitting electrical energy, signals, or data between two or more intended termination points. The scope encompasses:

1. Bare and insulated electrical wires and cables (including coaxial and data bus cables).
2. Multi-pin electrical connectors, backshells, and associated environmental sealing grommets.
3. Terminal blocks, busbars, junction boxes, and grounding/bonding studs.
4. Clamps, standoffs, raceways, conduits, and protective flexible sleeving.
5. Splices, crimp contacts, and pressure lugs.

EWIS elevates wiring from passive hardware to a certified aircraft system with the same regulatory stature as hydraulic, pneumatic, or propulsion systems. Under AMC 20-21 (Acceptable Means of Compliance for EWIS), maintenance programs must incorporate Enhanced Zonal Analysis Procedures (EZAP). EZAP establishes systematic zonal inspections focusing on contamination (lint, hydraulic mist, de-icing fluid, metal shavings), chafing against structure, clamp degradation, and strict "clean-as-you-go" housekeeping rules during maintenance.

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Aircraft Conductor Materials, Plating & Stranding

Aircraft electrical conductors must provide high electrical conductivity, mechanical ductility, low weight, and extreme resistance to cyclic fatigue induced by aerodynamic vibration and engine oscillation.

Copper vs. Aluminium Conductors

• Electrolytic Tough Pitch (ETP) Copper: The primary conductor material across modern aircraft. Copper provides superior electrical conductivity, exceptional tensile fatigue strength, and ductility. Solid-core copper wire is strictly prohibited in aircraft airframes; all aircraft wires utilize stranded conductors (typically 19 strands for smaller gauges up to AWG 12; 37 to 133+ strands for larger power cables). Multi-strand construction provides the flexibility needed to withstand continuous airframe vibration without work hardening and fatigue fracture.
• Aluminium Conductors: Aluminium offers approximately 68% weight savings compared to copper for an equivalent electrical length, but exhibits only 61% of copper's electrical conductivity. Aluminium suffers from severe metallurgical limitations: rapid work hardening, susceptibility to "creep" or cold flow (gradual mechanical relaxation under sustained clamp pressure), and aggressive galvanic corrosion when contacting dissimilar metals. In aviation, aluminium conductors are strictly restricted to large-gauge feeder cables (AWG 6 and larger), such as primary AC generator and DC bus distribution lines. They require specialized crimping techniques and approved anti-oxidation inhibiting pastes.

Conductor Plating & Temperature Limits

Bare copper oxidizes rapidly above 100°C, leading to high electrical resistance, localized heating, and termination failure. Consequently, copper strands must be electroplated with protective metals:

Conductor Plating Continuous Temperature Hierarchy:
[Tinned Copper: 150°C] ----> [Silver-Plated: 200°C] ----> [Nickel-Plated: 260°C]

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Insulation Materials & Arc-Tracking Hazards

Aircraft wire insulation must withstand wide temperature swings (-65°C to +260°C), mechanical abrasion, exposure to aggressive fluids (Skydrol phosphate-ester hydraulic fluid, jet fuel, turbine lubricating oils, de-icing solutions), and electrical stress while remaining lightweight and fire-resistant.

The Aromatic Polyimide (Kapton) Problem

Introduced in the 1970s, extruded aromatic polyimide (commercially known as Kapton) offered high dielectric strength and thin, lightweight construction. However, decades of service exposed two catastrophic degradation modes:

1. Hydrolysis and Radial Cracking: In humid, warm environments, Kapton undergoes chemical degradation (hydrolysis), becoming brittle and developing circumferential radial cracks under mechanical bending or vibration.
2. Arc Tracking (Dry and Wet): If an electrical arc occurs (triggered by mechanical chafing against airframe structure or conductive fluid contamination), aromatic polyimide undergoes pyrolysis. Instead of melting or extinguishing, the polyimide decomposes into conductive elemental carbon. This carbonized track forms a dead short circuit between adjacent conductors. Because the resistance of the carbon track is relatively low but sustains current below thermal circuit breaker trip ratings, an explosive, self-propagating electrical arc cascades along the harness bundle ("flashover"). Temperatures exceed 1,000°C, vaporizing adjacent copper wires, burning through aluminium bulkheads, and puncturing hydraulic and fuel lines.

Due to arc tracking, pure Kapton is prohibited on new aircraft designs. When encountered on legacy fleets, technicians must never use plastic zip ties that cinch into the insulation, must enforce generous bend radii, and must replace degraded sections with modern composite insulations.

Modern Aircraft Wire Insulation Types

• PTFE (Polytetrafluoroethylene / Teflon): Rated to 260°C. Highly inert chemically, impervious to Skydrol, and immune to arc tracking. However, it exhibits "cold flow" (tendency to thin out and deform under sustained mechanical clamp pressure) and poor mechanical abrasion resistance.
• ETFE (Ethylene Tetrafluoroethylene / Tefzel) & Cross-Linked ETFE (XL-ETFE): Conforming to specifications such as MIL-W-22759 (AS22759). The standard workhorse of modern aerospace wiring. Cross-linking via electron-beam irradiation enhances cut-through resistance, mechanical toughness, and thermal stability (rated from 150°C to 200°C).
• Composite Hybrid Insulations (e.g., BMS 13-60, EN 2267): Modern high-performance wiring utilizes multi-layer tape-wrapped construction combining a thin polyimide core (for dielectric strength) sandwiched between inner and outer layers of PTFE. This traps the polyimide, shielding it from moisture and eliminating carbon arc tracking while retaining light weight.

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American Wire Gauge (AWG) Sizing System

Aircraft wiring follows the American Wire Gauge (AWG) standard. AWG is an inverse geometric sizing system: as the gauge number increases, the wire diameter and cross-sectional area decrease.

Geometric Characteristics

• For every decrease of 3 gauge sizes, the cross-sectional area approximately doubles (e.g., AWG 17 has twice the copper area of AWG 20).
• For every decrease of 6 gauge sizes, the conductor diameter doubles.
• Aircraft wiring ranges from AWG 24 (common for low-current signal and data lines) to AWG 0000 (4/0) for main battery and heavy starter-generator feeders.

Minimum Mechanical Sizing Restrictions

To prevent mechanical breakage caused by airframe vibration or maintenance handling:

• Wires smaller than AWG 20 must NOT be installed as individual, unsupported single wires.
• Small wires (AWG 22 and AWG 24) are permitted ONLY when enclosed in protective multi-wire harness bundles, routed inside conduits, or provided with continuous mechanical support.
• Wires smaller than AWG 24 (such as AWG 26) are restricted to specialized data bus lines or encapsulated avionic LRU internal racks.

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Harness Bundling, Lacing & Conduits

Individual aircraft wires are grouped into looms and harnesses to facilitate installation, provide collective mechanical strength, and prevent chafing.

Lacing Tape vs. Plastic Cable Ties

• Braided Lacing Tape (MIL-T-43435): Flat braided nylon tape (for low-temperature areas) or Kevlar/Nomex tape (for high-temperature/engine zones) is the preferred method for binding aircraft harnesses. Flat lacing tape distributes bearing pressure across the bundle, preventing insulation cut-through.
• Lacing Knots: Harness lacing must initiate with a clove hitch secured by a square knot (reef knot). Intermediate ties along the bundle consist of running lock stitches or half hitches spaced at 2 to 3 inches (50 to 75 mm) intervals. The lacing run terminates with a final clove hitch locked with a square knot.
• Plastic Cable Ties (Zip Ties): Permitted in specific, non-vibrating airframe zones only if authorized by the Aircraft Maintenance Manual (AMM). Cable ties must be installed using a calibrated tensioning gun that cuts the strap flush. Cutting cable ties with standard diagonal side-cutters leaves sharp, razor-like plastic burrs that slice technicians' hands and chafe adjacent wiring. Cable ties are strictly prohibited in high-vibration zones, engine nacelles, and unpressurized landing gear bays.

Typical Wire Harness Lacing Sequence:
[Clove Hitch + Square Knot] === 2-3" === [Lock Stitch] === 2-3" === [Lock Stitch] === [Clove Hitch + Square Knot]

Conduits and Protective Sleeving

Where wiring runs through hazardous or exposed areas (wheel wells, wing leading edges, cargo holds), bundles must be protected inside conduits:

• Metallic Conduits: Aluminium alloy or flexible brass conduit providing crush resistance and electromagnetic shielding. Conduits must feature drain holes at their lowest geometric points to vent trapped moisture and condensation.
• Non-Metallic Conduits & Sleeving: Expandable braided sleeving (Nomex, PPS) or convoluted Teflon tubing to protect against mechanical chafing.

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Clamping Practices, Spacing & Slack Standards

Harnesses are supported along airframe structure using rubber-cushioned loop clamps conforming to MS21919 (or equivalent ASG standards).

Cushion Material Compatibility

Clamps feature a metallic band (aluminium or stainless steel) fitted with an elastomeric cushion. Technicians must match the cushion material to the installation zone:

• Neoprene (Chloroprene): General airframe and pressurized cabin areas; resistant to ozone and general weathering; operating limit up to 100°C.
• Fluorosilicone / Viton: Engine pylons, wheel wells, and wing roots; impervious to Skydrol hydraulic fluid, jet fuel, and synthetic oils; rated up to 200°C.
• Silicone: High-temperature engine nacelle areas; rated up to 260°C; limited tear resistance.

Clamp Spacing & Harness Sag

• Clamp Spacing: Clamps must support harnesses at intervals not exceeding 10 to 15 inches (250 to 380 mm) along straight airframe runs. In high-vibration zones, spacing must be reduced to 6 to 8 inches (150 to 200 mm).
• Harness Sag: Wires must never be stretched tight like guitar strings. A controlled amount of slack prevents mechanical strain on connector pins, allows for structural airframe flexing, and facilitates maintenance. The maximum allowable sag between adjacent support clamps is 0.5 inches (12 mm) under light finger deflection. Clamps must grip the harness firmly without pinching or distorting wire insulation.

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Critical Segregation Rules & Drip Loops

Segregation prevents a failure in one aircraft system from propagating into another.

Clearance from Fluid Lines

Under EASA CS-25.1707 and AMC 20-21, electrical wiring must be physically segregated from plumbing carrying flammable fluids (fuel, hydraulic fluid, engine oil) and oxygen lines:

1. Standard Minimum Separation: Maintain a minimum clearance of 2.0 inches (50 mm) between electrical wiring and fluid/oxygen lines.
2. Reduced Separation with Positive Mechanical Barriers: If structural space constraints make 2.0 inches impossible, clearance may be reduced to an absolute minimum of 0.5 inches (12 mm), provided the wiring and fluid line are rigidly clamped to prevent any relative deflection, and an approved non-conductive, fire-resistant physical barrier or sleeve is installed between them.
3. Approved routing relationship: Position, separation, drip protection, and support relative to fluid lines must match the approved EWIS installation data. A universal “always above” rule does not cover every certified design. This mandatory rule ensures that any leaking fluid, dripping condensation, or line rupture drains downward away from the electrical harness, preventing flammable liquids from wetting connectors or pooling on energized looms.

+-------------------------------------------------------------------------+
|                   EWIS CRITICAL SEGREGATION GEOMETRY                    |
|                                                                         |
|    [ELECTRICAL WIRING HARNESS]   ====> MUST BE ON TOP                   |
|                 |                                                       |
|                 |  Minimum Clearance: >= 2.0 inches (50 mm)             |
|                 v  (>= 0.5" / 12 mm only with rigid clamps & barrier)   |
|                                                                         |
|    [FLUID PLUMBING LINE]         ====> MUST BE ON BOTTOM                |
|    (Fuel, Hydraulic, Oxygen)           (Gravity drains leaks away)      |
+-------------------------------------------------------------------------+

Drip Loops

Where an electrical harness approaches a connector, terminal block, or bulkhead penetration, a drip loop must be formed. The harness routes downward below the termination level and curves back upward into the fitting. Any moisture condensing along the harness or fluid splashing onto the wires migrates to the lowest apex of the loop and drips harmlessly to the airframe bilge, rather than running down into the connector backshell.

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Minimum Bend Radii Rules

Bending a wire or bundle too sharply damages the outer jacket, overstresses the insulation dielectric, and causes stranded copper conductors to separate or buckle on the inner radius.

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Realistic Maintenance Scenario & Common Exam Traps

Realistic Maintenance Scenario

During an extensive C-check inspection in the hydraulic service bay of an Airbus A320, a technician discovers that an auxiliary hydraulic pump control harness has chafed against an aluminium hydraulic return line. The neoprene clamp cushion had deteriorated from Skydrol exposure, and the harness was routed directly underneath the hydraulic line with only 10 mm of clearance.

To correct the defect according to EWIS standards, the technician removes the damaged wiring section, installs fresh XL-ETFE wire conforming to AS22759, and replaces the perished clamp with a fluorosilicone MS21919 clamp. Crucially, the engineer reroutes the harness onto an upper standoff bracket, positioning the electrical loom 60 mm (2.4 inches) above the hydraulic line, and forms a pronounced drip loop before terminating into the pump pressure switch connector.

Common Exam Traps

• Trap 1: Believing wiring can run below a fluid line if clearance exceeds 2 inches. Regardless of clearance distance, EWIS standards mandate that wiring must run above fluid plumbing whenever feasible. If an unforeseen vertical leak occurs, gravity will cause spraying or dripping fluid to impact wiring placed underneath.
• Trap 2: Assuming AWG 24 wire can be routed as a single unbundled lead to save weight. In aircraft maintenance, unsupported single wires smaller than AWG 20 are strictly forbidden. AWG 22 and 24 must always be integrated within multi-wire bundles or enclosed in protective tubing.
• Trap 3: Trimming plastic zip ties with standard wire cutters. Trimming cable ties with diagonal cutters leaves razor-sharp edges that chafe wiring under vibration and present severe cut hazards to technicians. AMM-approved flush-cutting tools must be used.