Best Practices for Wire Harness Assembly in Aerospace Applications

Understanding Wire Harnesses in Aerospace

Also known as a wiring loom or cable assembly, a wire harness consists of electrical cables with connectors or terminals on the ends. The purpose of these wires is to transmit power or electrical signals from one system to the other in the connected network.

Wiring harnesses essentially perform 3 functions in aircraft.

• Transmission: Helps to transmit power, data, and signals between instruments, systems, and electrical devices
• Regulation: Assists in regulating critical control systems, such as engines, wings/fins, and landing gear
• Simplify Maintenance: Makes it easier to identify faults and repair aircraft

Key Requirements for Aerospace Wire Harnesses

Whether it’s planes or rockets, aerospace vehicles are subjected to the extremes and their wire harnesses must withstand these conditions to provide reliability and safety to the people on board. Some of the requirements for these harnesses include:

• Weight reduction to enable the vehicle to carry more payload
• Durability to prevent catastrophic failure
• Compactness to leave more space for the payload
• Resistance to extreme temperatures, corrosion, and other external conditions
• Insulation and grounding to prevent shorting and electrical failures
• Shielding to prevent internal and external noise interference

Best Practices for Wire Harness Assembly

Material Selection

Keeping in mind the requirements listed above, these aerospace wire harness solutions should be made using specially selected materials for the different sections.

• Core: The core of the wire harness assembly is the conductive part, which is usually made using copper. But considering airplanes and large rockets have miles of harnesses, they can be quite heavy. To keep the weight to a minimum, some harnesses have an aluminum core.
• Insulation: Nylon is commonly used as the sheath in aerospace wire harness assemblies because it is tough, flexible, and abrasion-resistant. But fluoropolymers like PTFE are better because they are highly resistant to most chemicals, can withstand higher temperatures, are non-hygroscopic, wear-resistant, and provide excellent insulation.
• Other Components: Aerospace wire harnesses also need connectors, tie wraps, heat shrink tubing, string ties, locks, terminals, protective sleeves, mounting hardware, etc.

Design Considerations

Once the materials have been selected, the harness should be designed while considering these factors.

• Space Optimization: Compact designs maximize space efficiency, using as little real estate as possible inside the plane.
• Flexibility: The gauge or thickness of the wire should enable maximum flexing for that specific section in the vehicle without breaking.
• Ease of Maintenance: The harnesses must be well designed to allow for quick and easy diagnosis/troubleshooting and simple replacement of connectors and other parts.
• High Compatibility: Since the aerospace industry is advancing quickly, wiring harnesses should be designed with high compatibility in mind. Wiring harnesses should be able to cater to higher data transmission speeds and have the bandwidth to handle more data as passengers demand faster internet in the future.
• Redundancy: Wire harness failures in aerospace applications can cause extreme losses in human life, property, or both. Therefore, it is critical to have redundancy connections that keep the electrical signals flowing in case of cuts or wear.

To achieve all these considerations, it is necessary to use CAD software to create an efficient layout before assembling the wires physically.

Assembly Techniques

The best assembly techniques practices for aerospace wire assemblies include:

• Organizing wires neatly while avoiding stress points like sharp bends
• Shielding to prevent noise
• Cable management through bundling or spacing out to prevent crosstalk or overheating. Small bundles can also work to avoid overheating.
• Using grommets and other strain reliefs to protect the wires from mechanical stress, vibrations, and environmental factors
• Connecting cables using terminals that have insulators
• Using the right methods to attach the wires to the connectors and terminals (soldering and crimping)

It is important to maintain high-quality connections between the wire strands and the terminals or connectors to ensure reliable electrical signal transfer, so we only crimp or solder the joints and shield them if necessary to prevent noise interference.

Here’s an overview of the wire harness assembly process from start to finish.

Quality Control and Testing

Two of the most common tests that should be carried out are continuity and insulation resistance.

Using a wiring analyzer, we check if every wire connects in line as per the specific design. This is done at 0.5 amps for about 0.2 seconds at a constant voltage. On the other hand, insulation resistance or megohm testing checks for short circuiting to reveal unwanted connections between two wires or when connectors have inadequate material to separate the two contacts. The resistance should be higher than 100 megaohms to pass the test.

At WellPCB, we employ rigorous testing procedures and abide by various industry-standard certifications, such as AS9100, ISO 461-1:2003, DO-160, AS50881, and MIL-STD-202 (for military aerospace vehicles).

Compliance with Aerospace Standards

The key aerospace standards and regulations to comply with as a wire harness assembler include the following.

• AS9100: This standard defines the guidelines for implementing a quality management system for space, military, and aviation organizations. It is based on ISO 9001 and the current one (version D), implementing product safety, counterfeit part prevention, and configuration management.
• DO-160: Published by the RTCA, this international standard defines the environmental test conditions and procedures for avionics equipment to ensure avionics (including harnesses) function correctly in the EMI and environmental conditions around the craft.

Other notable ones we comply with include IPC/WHMA-A-620, which defines the requirements and acceptance for cable and wire harness assemblies, and ISO/TC 20/SC 1 standards, which specify aerospace electrical requirements.

Failure to adhere to these standards will lock you out of the target market because they are the baseline that assures customers of quality custom aerospace wire harness assembly. 

Ensuring Safety and Reliability

The three ways of ensuring safety and reliability in wire harness assemblies include:

Minimizing Electrical Interference

The most effective way of minimizing EMI is through shielding, where conductors reflect the noise or conduct it to the connected ground, keeping it from the sensitive signal lines. But this can make the harness costly. So, it is also worth looking at efficient routing or braiding/twisting (like in Ethernet cables) to ensure the harnesses don’t experience EMI either from internal or external sources.

However, airplanes are subjected to a lot of EMI, including from lighting, so shielding cannot be avoided. Using metalized, lightweight fiber materials like Kevlar can produce shields that are up to 75% lighter than copper, which is significant considering the length of wires in aircraft.

Other important strategies include using filters, impedance and power level matching/balancing, and implementing good shield interconnects and termination.

Redundancy and Fail-Safes

Redundancy and fail-safes in aerospace wire harness solutions ensure the connected systems continue to exchange data and receive power if the primary connection channel fails. These backup channels are important for critical components like landing gears, avionics, rudder, flaps, aileron, and elevator adjustments to prevent catastrophic failure.

Environmental Considerations

To ensure maximum reliability and eliminate the need to activate redundancy or failsafe channels, aerospace wire harness assemblies must be tough enough to withstand the extreme conditions thrown at them in flight. These include extreme temperatures, vibrations, and moisture.

Materials like polyurethane and neoprene can make harnesses waterproof when wrapped as outer jackets, while rubber grommets introduce vibration resistance.

Other solutions include heat shrink tubing, waterproof connectors, silicone sealants, nylon or polyester protective sleeves, and conformal coatings. Cable management and careful routing can also help to avoid areas exposed to these extremes.

Advanced Technologies in Aerospace Wire Harness Assembly

Automation and Robotics

Due to the intricate nature of assembling wire harnesses, most of the work requires manual assembly. However, we’ve automated some processes to hasten the production time, reduce human errors, and maintain consistency. These include:

• Cutting wires to the exact length
• Crimping terminals to wires
• Soldering wire ends for sealing
• Twisting wires
• Partially inserting wires (with crimped terminals) into connectors.

Smart Wire Harnesses

Troubleshooting errors in well-optimized wire harnesses is relatively easy, but it can be even easier if the network of wires features sensors and monitoring equipment. Smart wire harnesses have protective functions built-in, such as over-temperature and short-circuit protection with reporting. These keep track of the performance figures and report if the values go below the threshold to prompt maintenance.

3D Printing and Additive Manufacturing

Using custom wire harness components, such as connectors and mounting hardware, can be costly but 3D printing can make these unique parts feasible to use because it eliminates tooling costs. Companies like Airbus use 3D printing to make wiring harness connectors for their aircraft. They design lightweight connectors with complex geometries that provide the required structural integrity and electrical performance.

Overcoming Common Challenges in Aerospace Wire Harness Assembly

Assembling aerospace wire harnesses has its challenges, and here’s how we overcome the typical ones.

Managing Complexity

Since these harnesses are hundreds of miles long and with multiple branches, they are complex to manufacture. We use these strategies to manage this complexity.

• Modularity: We break down the entire aerospace wire harness design into smaller parts that provide a set of requirements or functions.
• Standardization: Wire harness standardization also reduces complexity because it streamlines production (including the supply chain), simplifies maintenance, and makes the designs reusable across different aircraft platforms.
• Performing Requirements Analysis: We consider factors like the aircraft’s configuration, electrical loads, system interfaces, and environmental conditions to define all requirements first, then manage complexity using trade-offs and focused design decisions.
• DFM and DFS: Designing these wire harnesses with manufacturability in mind streamlines the production process by ensuring the parts are readily available to reduce the assembly time. On the other hand, design for serviceability simplifies the installation and maintenance processes by incorporating features like connectors with quick-release mechanisms.
• 3D Routing and Simulation: These techniques make it easy to visualize the entire harness digitally to verify installation feasibility, optimize routing, and spot potential issues.

Ensuring Traceability

Traceability is important in aerospace wire harness assembly because it helps to track the flow of items in the production line to make it easy to fix issues early on. This results in fewer recalls, better product quality, and customer satisfaction.

An effective way to ensure traceability is configuration management, which involves maintaining precise and up-to-date data of the production documents. These include the configuration control procedures, bill of materials, wiring diagrams, and schematics.

Addressing Cost Constraints

The primary factors that determine the cost of a wire harness assembly are the raw materials, connectors, and ease of manufacturing.

• Raw Materials: These include the conductors, insulators, jackets, and shielding materials. Using low-grade materials is unacceptable, but the highest-grade raw materials come at a premium. S,o the best way to manage costs is by assessing the requirements and picking the raw materials that provide the required performance. 
• Connectors: Connectors are part of the raw materials. Using unique, specially produced connectors is a sure way to drive up the cost, so we use standard, high-quality connectors instead.
• Ease of Manufacturing: We try to keep manufacturing costs as low as possible by doing extensive DFM checks, which optimize materials and tolerances while also reducing the aircraft wire harness fabrication time.

WellPCB’s Expertise in Aerospace Wire Harness Solutions

Proprietary Technologies and Methodologies

With over 10 years of experience in this industry, we have developed unique technologies and proprietary assembly methodologies that are specifically suited for aerospace applications to make the process fast while upholding quality. 

All our manufacturing plants feature state-of-the-art equipment, which includes bench-top crimping machines and robots for wire processing, among many others. These innovations place us ahead of the competition in terms of precision, tolerance, and quality.

Customization Capabilities

WellPCB excels in providing customers with customized wire harness solutions from design to specifications and material choice. We work closely with our aerospace clients to develop cables that are resistant to oil, heat, water, abrasion, and other hazardous conditions during flight when connecting avionics and control systems.

Implementing Best Practices: A Step-by-Step Guide

Step 1: Planning and Design

This is the most important part because it determines if the harness will provide the required performance and reliability or not. The steps we follow include:

• Gathering requirements for project scope analysis
• Creating the harness schematic
• Selecting the most appropriate, non-obsolete components

Step 2: Material Procurement

Once we’ve determined the required raw materials and quantities, we procure them from our trusted suppliers who we’ve meticulously selected based on quality, which is particularly important for aerospace applications.

Step 3: Assembly and Manufacturing

If the wiring harness only requires pre-made parts, we do the assembly process, which involves:

• Wire preparation
• Terminal to wire crimping/soldering
• Inserting wires to connectors
• Backbone wire harness assembly
• Secondary wire branching
• Protective covering/sheathing
• Final assembly

But if you need custom components, such as connectors, we fabricate them using 3D printing or any other relevant method.

While doing these assembly and manufacturing steps, we adhere to the best practices, such as neat organization of the wires to avoid stress points, shielding, using strain reliefs, etc.

Step 4: Quality Assurance and Testing

The complete harness must undergo thorough testing to ensure it meets all the required performance standards and client specifications. At a minimum, we do continuity and insulation resistance testing. Each connection in the harness must be tested individually to ensure there are no shorts, miswirings, or open circuits.

Another test we carry out is dynamic stress testing, which involves simulating the operating environment in the aircraft (exposure to moisture, chemicals, and oil) to ensure longevity and reliability.

Step 5: Final Inspection and Delivery

The final step is visual inspection, where the appearance, insulation, connector crimping, and labeling must be spot on to pass the test and move to delivery.

Conclusion

The best practices for aerospace wire harness assembly start from the basics, which is material selection, to reduce weight and provide resistance to external factors. Design best practices like space optimization, flexibility, and redundancy are also critical, and so are the assembly techniques.

But the most critical best practice that determines if the harness is aerospace-worthy is the quality control aspect. WellPCB places keen emphasis on implementing these industry standards, and we combine these with our internal quality control measures to ensure aircraft running on our harnesses are safe and reliable. 

Contact us for personalized consultation or to start a custom aerospace wire harness assembly project.

Frequently Asked Questions (FAQ)

What are the best practices for wire harness assembly in aerospace applications?

The best practices for wire harness assembly in aerospace applications include:

• Material Selection: Using lightweight, durable, and high-performance materials for the core, insulator, connectors, and other components
• Design Considerations: Factors like space optimization, maximizing compatibility, redundancy, flexibility, and ease of maintenance
• Assembly Techniques: Using strain relief, shielding, cable bundling, and wire organization to avoid stress points.
• Quality Control and Testing: Continuity, insulation resistance, and dynamic stress testing
• Compliance with Aerospace Industry Standards: These include AS9100, DO-160, MIL-STD-202, AS50881, etc.

How does WellPCB ensure compliance with aerospace wire harness standards?

WellPCB ensures compliance by training its staff regularly on the relevant aerospace wire harness standards. Additionally, we employ precision engineering to ensure each harness strictly adheres to these standards during manufacturing and assembly, after which the complete assemblies undergo quality control processes and rigorous testing to ensure they deliver the guaranteed performance.

What materials are recommended for aerospace wire harness fabrication?

The common conductive materials are copper and aluminum, while nylon and fluoropolymers provide the best insulation properties. Aluminum is preferred for its lightweight properties, which is critical in aircraft because it doesn’t eat into the payload capacity as much. On the insulation side, both nylon and fluoropolymers are resistant to most chemicals, but fluoropolymers are not hygroscopic and provide better electrical insulation.

What are the common challenges in assembling wire harnesses for aircraft?

The typical challenges in assembling wire harnesses for aircraft include:

• Managing the complexity of the miles of wires
• Ensuring traceability during and after manufacturing
• Addressing cost constraints without lowering harness quality

How can custom wire harness solutions improve aircraft performance and reliability?

Custom wire harnesses are better than off-the-shelf solutions because these are designed to suit specific aircraft applications. Therefore, you can expect better performance, quality, reliability, and flexibility to meet your needs. These harnesses are also cheaper because you won’t need additional components, such as adapters and wiring, to meet your specific needs.