Avionics Wiring Harness: A Tutorial

[Eric Page]

1. ADJUST CRIMPING TOOL

First, we need to adjust the crimping tool so that contacts sit at the correct depth and are crimped in the right place. Insert a pin or socket, with its striped wire cavity facing out, into the little hole in the flat face of the tool. On the opposite side of the tool, turn the knurled collar to adjust the contact stop until the end of the pin/socket is flush with the flat face of the tool. If you wish, you can apply a dab of super glue to keep the collar from moving.

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2. DETERMINE HARNESS LENGTH

You can do this with a piece of wire or string, or for very short cables, a calibrated eyeball. Be sure that your test wire/string follows the same path your harness will follow, and accounts for cable ties or lacing cord securing it to the aircraft structure.

Consider adding a few inches to allow for the vagaries of routing, then add a bit more to allow for shrinkage when the wires are twisted. You’ll be much happier if a harness is a bit longer than necessary than if it’s 3” too short!

3. CUT WIRES

Pull your wires off the spools and cut them to length. In a Dynon SkyView network harness, the orange wire (contact #5) isn’t twisted, so it doesn’t need that extra length. It can be cut to match the other wires after they’re twisted, then be prepared in the same way and added to the first connector before completing step 8.

In this example I’m making a very short cable to daisy-chain components that are close to one another, so my wires will only be about a foot long.

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4. PREPARE WIRE ENDS

At one end of each wire, strip about 0.16” of insulation (you’ll get an eye for this after doing it a few times). Test your strip length by inserting the wire into the cavity of a pin or socket. It should bottom out before the insulation touches the contact, and the wire strands should be visible in the tiny witness hole.

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5. CRIMP CONTACTS

Using your crimping tool, crimp a pin/socket on the prepared end of each wire. Insert the contact into the tool, then insert the wire into the contact’s wire cavity. While holding the wire and contact in place with one hand, fully cycle the tool with the other.

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6. POPULATE CONNECTOR

Insert each crimped contact into the connector body in the location corresponding to its wire color in the diagram. Push the contact in until you feel or hear it click. An insertion and removal tool can be used for this if you wish, but pushing on the wire is usually sufficient to seat the contact. While inserting, try not to sharply bend the wire where it exits the contact. Once seated, give each wire a tug to be sure it’s secure.

If you make a mistake and put a wire in the wrong location, use the insertion and removal tool to take it out. Lay the wire into the metal channel of the tool, then push-and-wiggle the channel into the contact cavity in the connector body. You’ll feel the tool bottom out against the contact, then push-and-wiggle some more and it will move a bit further, unlatching the contact. Pull on the wire to withdraw the contact and tool together.

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7. TWIST WIRE PAIRS

A. First, make a simple twist gauge. Cut a piece of cardboard to 4” in length and make five marks on it, 1” apart. By comparing these marks to the twisted pair, and matching one twist per mark, it will indicate when you reach 12 twists per foot. This over-twist to 12 per foot is required due to wire spring-back in order to achieve the 8-10 twists per foot specified by Dynon.

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Another technique is to calculate the number of turns required based on length (i.e. 7 ft 6 in harness x 12 twists per foot = 90 twists). Put a tape marker on the drill chuck in step C, below, and count rotations until you reach the required number.

B. Without distorting its metal shell, hold the connector body in a bench vise with the wires trailing out to one side. Separate one pair of wires to be twisted and pull them out horizontally from the connector.

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C. Insert the loose ends of the wires into a drill chuck and tighten the chuck to hold the wires firmly. At low speed, run the drill motor to twist the wires.

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D. Hold the drill in one hand and the cardboard twist gauge in the other. Once you reach 12 twists per inch, hold the wires firmly while you release them from the drill chuck; if a long twisted pair gets away from you, you’ll have a rat’s nest to unwind. Slowly release your grip on the wires, allowing them to untwist a bit without pulling toward the connector, until they’re relaxed.

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Repeat this process for each of the other pairs that need to be twisted.

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8. DRESS HARNESS

NOTE: Add the orange wire at this point if you’re making a Dynon SkyView network cable.

If you’re using cable ties or lacing cord, apply them periodically along the length of the harness to contain the wires.

If you’re using expandable sleeve, cut a piece long enough to fit between the connectors, with about 1/2–3/4” of gap between the end of the sleeve and the connector body at each end.

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Slide the sleeve over the wires to contain the harness, then slide two ~2” long pieces of heat shrink tubing over the sleeve. At the end with the connector attached, use your heat gun to shrink one piece of heat shrink tubing over the wires and the loose end of the sleeve.

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9. INSTALL BACKSHELL

Put the small cable grip saddle into the backshell cavity opposite the screw holes. Lay the populated connector body into the backshell with the connector’s metal edge just behind the lip of the backshell and the heat shrink tubing laying over the cable grip saddle. Put the thumbscrews into the backshell with their retaining ridge between the molded retainers in the backshell and their threaded ends poking through the ears of the connector body.

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While keeping the thumbscrews and cable grip saddle in place, close the backshell’s lid and squeeze until it clips closed. Using the two small screws, tighten the cable grip saddle to retain the heat shrink and prevent any strain on the wires or crimped contacts.

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10. TERMINATE OPPOSITE END

If the wires at the unfinished end are slightly uneven in length, trim them to match the shortest wire, then install the contacts, connector body, expandable sleeve, heat shrink tubing and backshell by repeating steps 4, 5, 6 and 9. Be careful to maintain twist on the twisted pairs, and to match the wire locations end-to-end. Don’t worry if the expandable sleeve unravels a bit while you’re working; it will be captured under the heat shrink tubing, never to be seen again.

11. LABEL HARNESS

Using your label printer, make labels and apply them to the connector backshells to identify function.

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Congratulations – you now have a well made, professional looking harness!

[airlina]

Eric , fantastic tutorial - I know this had to take some of your precious build time to prepare and photograph and post, but guys preparing for this part of their builds will really appreciate your time and effort. Generous members like you make this a great forum . Thanks for sharing Bruce N199CL

[bumsteer]

Excellent tutorial Eric!! Thank you for taking the time and sharing your expertise.

Rick

[alexM]

Wow, awesome tutorial.

Can you explain when/why twisting is needed? My understanding is quite murky on that one. Say for example the wires going to the wing tip strobes. Do yo twist those? Do you keep twisted wires away from non twisted wires (say, magnetometer)?

[Eric Page]

Thanks, everyone; I'm glad it's well received. On the 'net you never know when someone will shoot you full of holes!

Can you explain when/why twisting is needed? My understanding is quite murky on that one. Say for example the wires going to the wing tip strobes. Do yo twist those? Do you keep non twisted wires away from non twisted wires (say, magnetometer)?

Keep in mind that I'm not an electrical engineer (or any other kind, for that matter). I'm a bit murky as well, but here goes...

As I understand it, twisting helps to mitigate the effects of electromagnetic interference. It places the two wires the same average distance from any source of interference, which means the noise induced in the pair will be the same on both wires (called "common mode"). That's easier for a receiving circuit to filter out because it's looking for a difference between the pair, and any noise that's common to both isn't a difference, so it isn't detected.

Apparently it also reduces the pair's ability to transmit interference as well, but I can't begin to explain that.

Computer network cables specify different twist rates (twists per meter) for each pair in the cable, so that the same wires in each pair don't lie next to each other on every twist (eliminates "crosstalk"). This begs the question why Dynon doesn't specify different twist rates for the pairs in their cables. Perhaps the two communication pairs are redundant/duplicates carrying the same data, so interference between them isn't a concern? I suspect there's a fairly steady current draw to a SkyView system over short time scales, so twisting the two pairs of power wires is a belt-and-suspenders kind of thing that could probably be eliminated at no cost to function.

Anyway, I would say to just follow each manufacturer's recommendation on wiring their particular appliance. I did try to keep all other wires in my wings away from the magnetometer cable. All of my lighting wires (landing/taxi and wingtip NPS) are just 2 and 4-conductor unshielded cables. I did pass them all through the rear spar, which should provide some shielding effect, while the magnetometer cable is outside the spar.

Honestly, if it weren't made of solid core 26AWG wire and PVC insulation, I'd probably just use the spool of Cat6 Ethernet cable left over from wiring my house for all the SkyView network cables in my plane, and save myself all this twisting shenanigans.

The best things you can do to prevent problems for yourself are to be sure your antenna cables are well made, with high quality connectors that are properly installed (so there's no RF leakage that might effect other electronics), and be careful to closely follow MGL's guidance on shielding and grounding when wiring audio circuits. Isolate headphone and microphone jacks from metal structure, be sure that shielded audio cables have their shields grounded at only one end (usually at the intercom/radio where they start), and tie audio grounds/shields solidly to the place specified in the installation manual.

[mike92104]

I'd like to add that Aircraft Tool Supply has many of the electrical tools you may need (and many others) for very reasonable prices.

https://aircraft-tool.com/shop/searc...ORY=Electrical

[alexM]

3. D-SUB CRIMPER

You can spend many hundreds of dollars on tooling to crimp d-sub pins and sockets, but it’s definitely not necessary. Note that we’re going to be using turned contacts, not stamped-and-formed (I’ll explain why in Part 2). This means you want the 4-way indent type of crimper with a rounded head, not the type with replaceable dies in jaws that hinge open. I use a clone of the Greenlee or Paladin PA1460 that I got on Amazon for $27, and it works perfectly.

This is super useful information. I currently have a Daniels AFM8 on my ebay watch list. I used one previously when I installed the COM and TXP equipment in my Citabria and the T-6 (under supervision of course). It is also mentioned in the aircraft electrical book I bought, so I thought it was required equipment. That Daniels AFM8 is $250-ish used, and it doesn't come with the "positioner" which is an easy $55-80 each. With the Daniels tool you don't adjust anything, it just works flawlessly.

Now with your recommendation I would say that for 1/10 the cost I can fiddle around adjusting for a moment or two.

[napierm]

The problem I have with these crimpers is that there isn't a calibrated adjustment for the depth of crimp. If they are adjustable at all then you could use set of pin gauges (go/no go) to set them. The crimp depth is set for the size of terminal as well as the wire gauge.


"3. D-SUB CRIMPER

You can spend many hundreds of dollars on tooling to crimp d-sub pins and sockets, but it’s definitely not necessary. Note that we’re going to be using turned contacts, not stamped-and-formed (I’ll explain why in Part 2). This means you want the 4-way indent type of crimper with a rounded head, not the type with replaceable dies in jaws that hinge open. I use a clone of the Greenlee or Paladin PA1460 that I got on Amazon for $27, and it works perfectly."

This is super useful information. I currently have a Daniels AFM8 on my ebay watch list. I used one previously when I installed the COM and TXP equipment in my Citabria and the T-6 (under supervision of course). It is also mentioned in the aircraft electrical book I bought, so I thought it was required equipment. That Daniels AFM8 is $250-ish used, and it doesn't come with the "positioner" which is an easy $55-80 each. With the Daniels tool you don't adjust anything, it just works flawlessly.

Now with your recommendation I would say that for 1/10 the cost I can fiddle around adjusting for a moment or two.

[MikeFairbanks]

I spent 12 years in the military as an avionics tech... went through aviation soldering and connector repair school, and for the last 25 years have worked in the satellite communications industry, where I had to get my NASA soldering certification.

First I want to respond to the comments that didn't like the exposed wire on the crimped pins. When I went to school for this if you brought up work that had the insulation right up against the pin you failed. I was taught that there should be a slight gap there... if you can push the insulation all the way up against the pin or cup, then you can't really be certain that the wire completely fills the pin or cup.

Second, regarding butt splices. I like the hermetic sealing splices with the solder ring in them. What you have to be careful of with these though is that there is not enough solder in that ring for bare wires... you need to tin your wires before using them. The ring is just to bond the tinned wires together. If you use them with bare wires it will probably stick them together, but the electrical connection will be questionable, and occasionally you'll be able to pull them apart. We used these on F/A-18's in the military, so if you use them right there is nothing wrong with them.

http://tinyurl.com/23uuv8s6 here's an example

[napierm]

Thanks, everyone; I'm glad it's well received. On the 'net you never know when someone will shoot you full of holes!


Keep in mind that I'm not an electrical engineer (or any other kind, for that matter). I'm a bit murky as well, but here goes...

As I understand it, twisting helps to mitigate the effects of electromagnetic interference. It places the two wires the same average distance from any source of interference, which means the noise induced in the pair will be the same on both wires (called "common mode"). That's easier for a receiving circuit to filter out because it's looking for a difference between the pair, and any noise that's common to both isn't a difference, so it isn't detected.

Apparently it also reduces the pair's ability to transmit interference as well, but I can't begin to explain that.

I am an EE so I'll give it a go. The reason to twist the wire with its ground is to average out the induced voltage. Any stray AC magnetic field will induce a voltage in a loop of wire. Make the wires very close together and minimize the size of the loop. Twist the wires and they spend 1/4 the time facing the field and 1/4 away. Averages out to 0. And 1/2 the time they are at right angles to the field so the induced voltage is also 0. Note this is most effective for low frequency. At high frequency (RF, ignition impulse noise) the field can couple onto the wire pair like an antenna. The solution is a shield that is terminated only at one end: at the power ground of the instrument. Thus the shielded twisted pair.

Similar situation with transmitted EMI. The generated magnetic field follows the wire (right hand rule) and gets bigger with the size of the loop. Minimize the loop size. Twist the wires and the magnetic field goes + and -. Averages out to 0.

Hope this helps.