Aerospace Wiring Tips: A Comprehensive Guide

 

Introduction to Aerospace Wiring

The electrical wiring interconnection system (EWIS) is a critical component in aircraft design and maintenance. Proper installation, handling, and maintenance of aerospace wiring can significantly impact aircraft safety, performance, and longevity. This comprehensive guide explores essential techniques, best practices, and industry standards for aerospace wiring, providing valuable insights for engineers, technicians, and aviation professionals.

Aircraft electrical systems have evolved tremendously over the decades, from simple circuits powering basic instrumentation to complex networks supporting fly-by-wire controls, sophisticated avionics, and integrated systems. With this evolution comes increased complexity in wiring requirements and heightened importance of proper wiring practices.

Regulatory Framework and Standards

FAA and EASA Regulations

The Federal Aviation Administration (FAA) and European Union Aviation Safety Agency (EASA) maintain strict regulations concerning aircraft wiring. These regulations are designed to ensure safety, reliability, and consistency across the aerospace industry.

Key FAA regulations include:

• 14 CFR Part 25.1701-1733: Specific requirements for transport category aircraft wiring
• AC 25.1701-1: Advisory circular on EWIS certification
• AC 43.13-1B: General guidance for acceptable methods and practices for aircraft inspection and repair

EASA regulations generally mirror FAA requirements with some regional variations, with primary guidance found in CS-25 (Certification Specifications for Large Aeroplanes).

Industry Standards

Beyond regulatory requirements, several industry standards govern aerospace wiring practices:

• AS50881: Aerospace standard for wiring aerospace vehicles
• MIL-STD-810: Environmental test methods for aerospace equipment
• ARINC specifications: Standards for avionics architecture and interconnections
• IPC/WHMA-A-620: Requirements and acceptance for cable and wire harness assemblies

These standards provide detailed specifications for wire selection, routing, bundling, termination, shielding, and testing, forming the foundation for all aerospace wiring activities.

Wire Selection for Aerospace Applications

Wire Types and Materials

Selecting the appropriate wire type is fundamental to creating safe and reliable aircraft electrical systems. Common aerospace wire types include:

Wire Type
Insulation Material
Temperature Rating
Key Applications
Advantages
MIL-W-22759
PTFE/Polytetrafluoroethylene
-65°C to +260°C
General purpose, high-temp areas
Excellent chemical resistance, low smoke
MIL-W-81381
Polyimide (Kapton)
-65°C to +200°C
Space-constrained applications
Lightweight, excellent abrasion resistance
EN3475
Crosslinked ETFE
-65°C to +150°C
European aircraft, general purpose
Good balance of properties
AS22759
Modified ETFE
-65°C to +150°C
General purpose
Good mechanical properties
NEMA HP3
PTFE/Fiberglass
Up to +260°C
Engine compartments, high-temp zones
Extreme temperature resistance

Conductor Materials

The conductor material significantly affects electrical performance, weight, and corrosion resistance:

• Copper: Standard choice offering excellent conductivity
• High-Strength Copper Alloys: Improved mechanical strength with slight conductivity reduction
• Copper-Clad Aluminum: Weight reduction (approximately 30% lighter than copper) with adequate conductivity
• Silver-Plated Copper: Enhanced conductivity and corrosion resistance for high-frequency applications
• Nickel-Plated Copper: Superior high-temperature performance for engine areas

Size Selection (AWG)

American Wire Gauge (AWG) sizing is critical for ensuring adequate current-carrying capacity while minimizing weight. Key considerations include:

AWG Size
Current Rating (Ambient 20°C)
Typical Applications
Weight Considerations
22-26 AWG
3-7 amps
Signal lines, low-current circuits
Minimal weight impact
18-20 AWG
8-11 amps
Secondary power distribution
Good balance of weight/capacity
14-16 AWG
15-22 amps
Primary branch circuits
Moderate weight impact
10-12 AWG
30-41 amps
Power feeders
Significant weight consideration
0-8 AWG
55-150 amps
Main power distribution
Major weight factor

Aircraft wiring design must balance minimum required ampacity with weight considerations. For aerospace applications, circuit protection devices typically require wire sizing at 150% of the maximum continuous current to account for voltage drops and thermal factors.

Wire Harness Design and Fabrication

Design Principles

Effective wire harness design begins with thorough planning and adherence to key principles:

1. Separation of Systems: Maintain physical separation between critical systems, power and signal wires, and potentially interfering circuits
2. Accessibility: Design harnesses to allow access for inspection and maintenance
3. Protection: Route wires to minimize exposure to damage, heat, chemicals, and interference
4. Strain Relief: Incorporate adequate strain relief at all termination points
5. Serviceability: Consider future maintenance needs in harness design

Harness Mapping and Documentation

Comprehensive documentation is essential for aerospace wire harnesses:

• Wire Lists: Detailed inventories of all wires, including circuit identification, wire type, gauge, and terminal connections
• Routing Diagrams: Graphical representation of harness paths throughout the aircraft
• Interconnection Diagrams: Documentation of all connection points and interfaces
• Digital Twins: Modern approach creating virtual representations of physical harnesses

Fabrication Techniques

Aerospace harness fabrication requires precision, attention to detail, and specialized techniques:

Cutting and Stripping

• Use calibrated tools with proper blade settings to avoid conductor nicking
• Maintain consistent strip lengths according to terminal specifications
• Inspect cut ends under magnification for damage or irregular cuts

Crimping Best Practices

Proper crimping is critical for reliable electrical connections:

Crimp Aspect
Best Practice
Common Errors
Verification Method
Tool Selection
Use calibrated tools specified for connector type
Using incorrect tool
Tool calibration records
Crimp Height
Follow manufacturer specifications
Over/under-crimping
Go/no-go gauge measurement
Wire Insertion
Full insertion with insulation positioned correctly
Insufficient insertion
Visual inspection
Pull Testing
Conduct pull tests to validate crimp strength
Skipping verification
Pull test to SAE-AS7928 standards
Inspection
Verify proper barrel deformation
Accepting damaged crimps
5-10x magnification inspection

Wire Marking and Identification

Clear identification is mandatory for aerospace wiring:

• Permanent Markers: Non-smearing, heat-resistant ink markers for direct marking
• Heat-Shrink Labels: Pre-printed or laser-marked identification sleeves
• Sleeve Markers: Slipped over wire before termination
• Laser Marking: Direct laser etching on wire insulation for permanent identification

Routing and Installation Techniques

Path Planning and Wire Separation

Proper wire routing prevents interference and damage:

1. Zoning: Divide aircraft into electromagnetic compatibility zones
2. Bundling Categories:
   • Category 1: Sensitive signals (voltage < 15V)
   • Category 2: Medium power (15-115V)
   • Category 3: High power (115V+)
   • Category 4: RF and antenna feeds
3. Minimum Separation Distances:

Category Combination
Minimum Separation
Notes
1 from 2
0.5 inches (12.7mm)
Can be reduced with shielding
1 from 3
2.0 inches (50.8mm)
Never route in same bundle
1 from 4
4.0 inches (101.6mm)
Maximum practical separation
2 from 3
0.5 inches (12.7mm)
Shielding recommended
2 from 4
2.0 inches (50.8mm)
Perpendicular crossings only
3 from 4
2.0 inches (50.8mm)
Perpendicular crossings only

Routing Constraints and Clearances

Maintain proper clearances to prevent chafing and damage:

• Moving Parts: Minimum 0.5 inches (12.7mm) clearance from moving components
• Hot Surfaces: Minimum 2.0 inches (50.8mm) from surfaces exceeding 150°C
• Fluid Lines: Minimum 0.5 inches (12.7mm) from fluid-carrying lines, route electrical below fluid lines
• Structure: Minimum 0.25 inches (6.35mm) from sharp edges or corners

Bend Radius Requirements

Proper bend radius prevents wire damage and insulation cracking:

Wire Type
Minimum Static Bend Radius
Minimum Dynamic Bend Radius
Unshielded < 12 AWG
10 × wire diameter
15 × wire diameter
Unshielded ≥ 12 AWG
8 × wire diameter
12 × wire diameter
Shielded, any size
12 × wire diameter
18 × wire diameter
Coaxial
Per manufacturer spec (typically 10× diameter)
Per manufacturer spec (typically 15× diameter)

Clamping and Support

Secure wire harnesses properly to prevent movement and chafing:

• Maximum Distances Between Supports:
  • Horizontal runs: 24 inches (610mm) maximum
  • Vertical runs: 30 inches (762mm) maximum
  • Near connectors: Support within 6 inches (152mm)
• Clamp Selection:
  • Use cushioned clamps with appropriate size
  • Avoid metal-to-wire contact
  • Select clamps rated for environmental conditions
  • Ensure proper tightening torque

Shielding and EMI Protection

Electromagnetic Interference Fundamentals

Aircraft electrical systems must function reliably in challenging electromagnetic environments. Understanding EMI sources is essential:

• Internal Sources: Aircraft generators, motors, switching power supplies, digital equipment
• External Sources: Radar systems, radio transmitters, lightning, high-intensity radiated fields (HIRF)
• Coupling Mechanisms:
  • Conducted (through physical connections)
  • Radiated (through space)
  • Capacitive (electric field coupling)
  • Inductive (magnetic field coupling)

Shielding Techniques

Effective shielding is critical for protecting sensitive circuits:

Shielding Type
Protection Level
Weight Impact
Applications
Installation Notes
Foil Shield
Good HF protection
Minimal
Signal wires, avionics
Requires drain wire
Braid Shield
Good overall
Moderate
General purpose
85-95% coverage recommended
Double Braid
Excellent
Significant
Critical systems
Separate termination possible
Combination
Superior
Highest
Flight-critical, high EMI areas
Complex termination

Shield Termination Methods

Proper shield termination is as important as the shield itself:

1. 360° Termination: Provides best high-frequency performance
   • Use backshell adapters with shield rings
   • Ensure metal-to-metal contact around entire circumference
2. Pigtail Termination: Used when 360° termination isn't possible
   • Keep as short as possible (< 1 inch/25.4mm)
   • Terminate to designated shield pins or ground points
3. Floating Shield: Used for low-frequency applications or ground loop prevention
   • Terminate at one end only
   • Insulate unterminated end

Grounding Practices

Proper grounding creates a low-impedance path for fault currents and shields:

• Single-Point Grounding: Prevents ground loops in sensitive circuits
• Multi-Point Grounding: Required for high-frequency shields (>1 MHz)
• Ground Plane Connection: Direct connection to metallic structure for lowest impedance
• Chassis Grounding: Bonding equipment cases to aircraft structure

Connector Selection and Installation

Connector Types for Aerospace

Aerospace connectors must withstand extreme environments while maintaining reliable connections:

Connector Type
Key Features
Common Applications
Environmental Rating
MIL-DTL-5015
Threaded coupling, robust
Power distribution
-55°C to +125°C
MIL-DTL-38999
High vibration resistance, quick coupling
Critical systems
-65°C to +200°C
MIL-DTL-26482
Moderate cost, good reliability
General purpose
-55°C to +125°C
ARINC 600
High-density rectangular
Avionics racks
-55°C to +85°C
MIL-DTL-83723
Firewall rated
Engine systems
-65°C to +200°C

Contact Selection

Choosing appropriate contacts ensures reliable connections:

• Contact Plating:
  • Gold: Best conductivity, corrosion resistance (required for low-level signals)
  • Silver: Good conductivity, adequate for power circuits
  • Nickel: Durable for high-insertion cycle applications
• Contact Retention: Verify proper retention force per specification
• Contact Size: Match to wire gauge and current requirements

Installation Best Practices

Proper connector installation is critical for long-term reliability:

1. Preparation:
   • Inspect connector for damage
   • Verify pin assignment and orientation
   • Clean contact surfaces with approved solvent
2. Assembly:
   • Insert contacts until positive lock engages
   • Verify with insertion/removal tool
   • Install backshell with proper torque
   • Apply strain relief to cable
3. Testing:
   • Conduct continuity testing of all pins
   • Perform insulation resistance testing
   • Verify proper connector mating force
4. Environmental Protection:
   • Apply connector sealing plugs to unused positions
   • Install environmental backshells in exposed areas
   • Use protective caps when unmated

Special Considerations for Aircraft Zones

Engine and High-Temperature Areas

Engine compartments present extreme challenges for wiring systems:

• Temperature Rating: Use wire rated for sustained temperatures above 200°C
• Fire Protection: Install fire sleeves over critical harnesses
• Vibration Protection: Use extra support points and vibration-damping clamps
• Fluid Resistance: Select insulation materials resistant to fuel, oil, and hydraulic fluids

Fuel Areas and Intrinsically Safe Wiring

Wiring in fuel areas requires special considerations:

• Intrinsic Safety: Design circuits with energy limitation to prevent ignition
• Isolation: Use barriers and isolation transformers
• Shielding: Complete shield coverage with proper termination
• Conduit: Use sealed metallic conduit for critical runs

High-Moisture and Exposed Areas

For wiring in wheel wells, exterior areas, and high-moisture zones:

• Environmental Sealing: Use fully sealed connectors with appropriate IP rating
• Moisture Barriers: Apply heat-shrink boots with adhesive liners
• Corrosion Prevention: Use nickel-plated components and protective coatings
• Drip Loops: Incorporate drip loops to direct moisture away from connections

Lightning Strike Protection

Aircraft wiring systems must withstand lightning effects:

• Bonding: Ensure low-impedance bonding across structures (< 2.5 milliohms)
• Transient Protection: Install surge protectors on vulnerable circuits
• Shielding: Use overall shields on critical harnesses
• Routing: Keep wiring away from likely strike points and entry/exit paths

Testing and Verification

Continuity and Insulation Testing

Comprehensive testing ensures wiring system integrity:

Test Type
Test Parameters
Acceptance Criteria
Equipment Requirements
Continuity
< 0.5 ohms
Pass if resistance below threshold
Calibrated multimeter or continuity tester
Insulation Resistance
500V DC for low voltage, 1000V DC for high voltage
> 100 megohms
Insulation tester (Megger)
Dielectric Withstand
1500V AC or 2× rated voltage + 1000V
No breakdown or flashover
High-potential tester
VSWR (coaxial)
Frequency-dependent
< 1.5:1 typical
Vector network analyzer

Specialized Testing Techniques

Advanced testing ensures complete system verification:

1. Time Domain Reflectometry (TDR):
   • Locates impedance discontinuities and faults
   • Essential for long cable runs and coaxial systems
2. Aircraft Power-On Testing:
   • Conduct progressive power application
   • Monitor for proper system function and interactions
   • Verify absence of EMI issues
3. Environmental Testing:
   • Altitude simulation
   • Temperature cycling
   • Vibration testing
   • Humidity exposure

Test Documentation and Reporting

Comprehensive test records are mandatory for aerospace applications:

• Test Procedures: Detailed step-by-step instructions
• Test Reports: Documentation of all results with pass/fail criteria
• Non-Conformance Reports: Documentation of failures and corrective actions
• Traceability: Link tests to specific requirements and aircraft areas

Maintenance and Troubleshooting

Inspection Techniques

Regular inspection maintains wiring system integrity:

• Visual Inspection: Look for abrasion, heat damage, contamination, and improper routing
• Tactile Inspection: Feel for brittleness, roughness, or unusual stiffness
• Detailed Inspection: Use borescopes for hidden areas and magnification for close examination

Common Failure Modes

Understanding failure mechanisms aids effective troubleshooting:

Failure Mode
Visual Indicators
Contributing Factors
Prevention Strategies
Chafing
Worn insulation, exposed conductors
Vibration, improper clearance
Additional protection, secure clamping
Heat Damage
Discoloration, brittleness
Proximity to heat sources, overcurrent
Proper routing, adequate wire sizing
Corrosion
Green/white deposits, tarnishing
Moisture exposure, dissimilar metals
Environmental protection, proper materials
Arc Tracking
Carbonized paths on insulation
Insulation damage, contamination
Clean installation, proper circuit protection
Cold Solder Joints
Dull, grainy appearance
Insufficient heat, movement during cooling
Proper technique, inspection

Repair Techniques

When repair is permitted, follow these guidelines:

1. Splice Repairs:
   • Use aerospace-approved splice devices
   • Apply appropriate heat shrink with adhesive liner
   • Document all repairs in aircraft records
2. Terminal Replacement:
   • Use identical terminals and proper crimping tools
   • Verify terminal compatibility with wire type and size
   • Conduct pull tests after installation
3. Connector Repair:
   • Replace damaged pins with identical part numbers
   • Use manufacturer-approved extraction/insertion tools
   • Test continuity after repair

Preventive Maintenance

Proactive maintenance extends wiring system life:

• Scheduled Inspections: Develop zone-based inspection intervals
• Cleaning: Remove contaminants with approved solvents
• Protective Treatment: Apply approved corrosion inhibitors and protectants
• Documentation: Maintain detailed records of all maintenance activities

Aging Aircraft Wiring Concerns

Degradation Mechanisms

Aircraft wiring systems face numerous aging challenges:

• Insulation Cracking: Caused by thermal cycling, vibration, and chemical exposure
• Insulation Embrittlement: Results from thermal aging and UV exposure
• Conductor Fatigue: Due to vibration and repeated flexing
• Connector Contact Degradation: Fretting corrosion, oxidation, and relaxation of contact force

Detection Methods

Advanced techniques help identify aging wiring issues:

1. Non-Destructive Testing:
   • Infrared thermography for hot spots
   • Ultrasonic inspection for insulation changes
   • Corona detection cameras
2. Electrical Parameter Testing:
   • Impedance changes
   • Insulation resistance trending
   • Partial discharge measurements

Mitigation Strategies

Addressing aging wiring requires systematic approaches:

• Enhanced Zonal Inspections: Targeted inspections of high-risk areas
• Wire System Replacement Programs: Scheduled replacement of vulnerable segments
• Protective Treatments: Application of specialized coatings and treatments
• Environment Control: Reducing exposure to moisture, chemicals, and temperature extremes

New Technologies and Future Trends

Advanced Wiring Materials

Emerging materials offer improved performance:

• High-Temperature Composites: New polymer blends with 300°C+ capability
• Carbon Nanotube Conductors: Potential for dramatic weight reduction
• Self-Healing Insulation: Materials that automatically repair minor damage
• Lightweight Alloy Conductors: Advanced aluminum alloys with improved conductivity

Wireless and Data Bus Technologies

Reducing wire count through alternative technologies:

• Wireless Sensors: Elimination of wiring for non-critical monitoring points
• Power Line Communication: Using power wires for data transmission
• Optical Data Buses: High-bandwidth optical fibers replacing copper data lines
• Integrated Vehicle Health Monitoring: Embedded sensors monitoring wire condition

Smart Connector Systems

Next-generation connection technology:

• Built-in Test Capability: Self-diagnosis of connection quality
• Active Monitoring: Real-time analysis of electrical parameters
• Predictive Maintenance: Early warning of impending failures
• Digital Twin Integration: Connector status feeding into aircraft digital models

Best Practices Summary

Design Phase

Implement these critical design practices:

1. Circuit Protection: Properly sized circuit breakers and fuses matched to wire capacity
2. Redundancy: Separation of redundant systems and appropriate wire routing
3. Accessibility: Design for maintainability and inspection
4. Documentation: Complete and accurate wiring diagrams and harness details

Installation Phase

Follow these key installation guidelines:

1. Cleanliness: Maintain clean work environment and protect exposed connectors
2. Protection: Install chafe protection at potential wear points
3. Support: Provide adequate clamping with appropriate materials
4. Verification: Test all connections before closing access panels

Maintenance Phase

Adhere to these maintenance principles:

1. Training: Ensure technicians are properly trained on aerospace wiring practices
2. Tools: Use calibrated, aircraft-specific tools for all wire work
3. Documentation: Record all repairs and modifications
4. Inspection: Perform regular visual and electrical checks

Frequently Asked Questions

What are the most critical factors in aerospace wire selection?

When selecting wire for aerospace applications, the most critical factors include temperature rating, weight, chemical resistance, and electrical performance. Temperature rating must exceed the maximum expected environmental temperature plus a safety margin of at least 30°C. Weight considerations become particularly important for large wire bundles, where material selection can significantly impact aircraft payload capacity. Chemical resistance must be matched to the installation environment, with special attention to fuel, hydraulic fluid, and cleaning solvent exposure. Electrical performance, including insulation resistance, voltage rating, and signal integrity characteristics, must be appropriate for the specific circuit application.

How do EMI protection requirements differ between civilian and military aircraft?

Military aircraft typically require more robust EMI protection than civilian aircraft due to more extreme operational environments and higher reliability requirements. Military specifications often mandate higher levels of shielding effectiveness, particularly against intentional jammers and weapons systems. Military aircraft wiring must also withstand higher levels of HIRF (High-Intensity Radiated Fields) exposure. Additionally, military systems frequently require TEMPEST protection against compromising emanations that could reveal sensitive information. Civilian aircraft focus primarily on protecting against internal EMI sources and standard external sources like commercial radio transmitters, while ensuring compliance with less stringent regulatory requirements.

What special considerations apply to wiring in composite aircraft structures?

Composite aircraft structures present unique challenges for electrical wiring systems. Unlike metal structures, composites provide limited electrical conductivity for current return paths and EMI shielding. This necessitates dedicated ground return wires and enhanced shielding for sensitive circuits. Lightning protection requires special attention, with additional conductive paths and transient protection devices. Mounting provisions must be specially designed, as traditional metal clamps and self-tapping screws are often incompatible with composite materials. Temperature management is also critical, as composites typically provide less effective heat dissipation than aluminum structures, potentially requiring derating of wire current capacity or additional thermal protection.

How often should aerospace wiring systems be inspected?

Aerospace wiring inspection intervals depend on aircraft type, age, and operating environment. For commercial transport aircraft, general visual inspections should be conducted during regular maintenance checks (typically A-checks or equivalent), with more detailed inspections during heavy maintenance (C-checks or D-checks). Critical areas such as wheel wells, engine compartments, and areas with known moisture exposure should receive more frequent inspection. Aging aircraft (typically over 15 years) benefit from enhanced inspection programs targeting known degradation mechanisms. The inspection program should be adjusted based on service history and findings, with increased frequency when issues are discovered. All inspections should follow structured protocols with specific focus areas based on the aircraft's maintenance history.

What are the best practices for wire harness repairs in the field?

Field repairs of aerospace wire harnesses must follow approved maintenance manual procedures and use only authorized parts and materials. Best practices include:

1. Complete documentation of the damage before repair
2. Selection of repair method based on wire type and location
3. Use of aircraft-approved splice devices compatible with the original wire
4. Application of proper environmental protection (heat shrink with adhesive liner)
5. Testing of the repair (continuity and insulation resistance minimum)
6. Documentation of all repairs in aircraft records

Repairs in critical systems may require engineering approval before implementation. Always use calibrated tools designed for aerospace applications, and ensure technicians are properly trained and certified for the specific repair type. Field repairs should restore the harness to a condition functionally equivalent to the original installation, with particular attention to maintaining proper wire separation, EMI protection, and environmental sealing.