Wire Harness Construction – By Ryan Evans
Building any type of wire harness, small or large, correctly is a critical job. Whether the harness is for a single vhf radio, or for an entire aircraft, attention to details is paramount. In more than 19 years’ experience, it is unfortunate but true that nothing surprises us anymore. We have seen wire harnesses supplied from large distributors where the pins just fell off the wires, and recently we were asked to look at finishing an air frame rewire that had been started by another shop. At our initial inspection, it was quickly noted that the wire codes were not according to the maintenance manual; someone had decided to go their own way. The result for the customer was that everything had to be removed and installed all over again. Over the years we have completed numerous wiring modifications, stc’s and complete air frame rewiring on more than a dozen Beavers, Bell 206s, Cessnas and home built air frames.
We are going to share some pitfalls to avoid in wire harness construction, steps to plan for future maintenance, and look at what you can do to ensure longevity for your wiring. It all begins at the drawing board with the design. Typically wiring and connections are straight forward, unless you are getting into complex Automatic Flight Control Systems (afcs) or complex multi-audio controller systems with relays and options.
If you are wiring a home built, or even designing the installation of an audio panel and radio, the key is to keep it simple. When it comes to wiring, ac43-13 is a great resource. If required, it can help to determine size of wire needed for safe operation, along with mounting guidelines and many other best practices. Aircraft specific wiring with a Mil-Spec should always be used. It has proper fire ratings and chafe protection for use in aircraft and will contribute to the durability of your wiring. We have seen a plastic tie wrap wear through wire and create a short, so the placement and installation of tie wraps is something to watch.
Also, it is possible to have too much slack, or not enough slack in a wire harness. Ask, “what will this bundle come into contact with?” Move it in all directions to visualize wear points. Once a wiring project is started, the next key is to ensure the project remains relatively clean and wires are not all tangled up. Sometimes you end up with a big rat’s nest of wire going in all different directions and all wrapped up in each other. This is where you need to take the time and sort it out as it will alleviate headaches in the future and for the rest of the installation. Arrange the wires into groups, so if from a central point you have a bundle going up, to the left and down, sort the wires out. You may have to lay the bundles in such a way that they are out of the way while you route and secure a bundle going down, for example. Once that is completed, then work on the bundle that goes up from that point of separation.
Another key is to keep the wires from twisting around each other too much. Try to comb out the bundle so that the wires are all running parallel to each other. Not only does it look good, but it helps with troubleshooting or repairs in the future, the replacement of equipment or the addition of systems. It makes the entire process go smoothly. A secret for combing out wires is to use a tie wrap. Attach the tie wrap to the bundle and tighten it just to the point where it is tight and firmly on the bundle, but not locked down completely. You can then slide this tie wrap along the bundle and install another tie wrap behind it. The tie wrap that is tight but can slide will naturally sort the wires out.
Terminating wires is the next critical area. Crimps can be too hard and cut wires, or as a wire moves with vibration it will slowly break off. Using proper crimping tools ensures that the crimp is done correctly. It is a good practice to use a highlighter to indicate when a wire is terminated and highlight the wire on the wiring diagram if it is run. This way, at an easy glance you can see that specific wires have been installed. It also gives a quick indication if a wire has been terminated yet or not.
After wiring is routed, secured, chafe protection placed in appropriate areas, the next step is to perform a ring out of the system. It is best to use a different coloured highlighter for the ring out than for anything else. Once again, highlight the entire wire, from one termination to the other. On some drawings there can be numerous wires going all over the place. This helps you to keep track of what you have accomplished, what has checked out as ok, and what needs to be checked. After a ring out, the next step will be to perform power checks. This is always a good step as if you have a power or ground in the wrong place, it can become very costly very fast. This inspection can be completed relatively quickly as you are only focused on the power and ground connections. With radios removed and main components disconnected, turn aircraft power on. Go around checking for power and ground at the proper places. Once completed, turn power off. You are now ready to connect your avionics, components and turn power on. Then the next phase begins: programming, which can be quite complex with modern avionics systems…