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Thread: Avionics Wiring Harness: A Tutorial

  1. #1
    Senior Member Eric Page's Avatar
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    Default Avionics Wiring Harness: A Tutorial

    In the posts that follow, I’ll show the fabrication and assembly of a short interconnect harness for the Dynon SkyView network. Most other avionics systems will use similar harnesses that can be fabricated in the same way. Hopefully this will help to demystify the wiring of a modern glass panel.

    I’ll cover the topic in four parts:

    • In Part 1, I’ll go through the tools required for the job, and I’ll provide links for examples that I’ve found to work well.
    • In Part 2, I’ll list the parts and materials required to build a reliable harness, again with links.
    • In Part 3, I'll show the wiring and connector pinout diagrams.
    • Finally, in Part 4 I'll demonstrate the process to fabricate a harness from start to finish.

    Just because all of this made sense to me when I wrote it, doesn’t mean it will make sense to anyone else, so feel free to ask for an English translation!
    Last edited by Eric Page; 07-17-2021 at 09:46 PM.
    Eric Page
    Building: Kitfox 5 Safari | Rotax 912iS | Dynon HDX
    Member: EAA Lifetime, AOPA, ALPA
    ATP: AMEL | Comm: ASEL, Glider | ATCS: CTO
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    Senior Member Eric Page's Avatar
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    Default Part 1: Tools

    I would consider the following to be the minimum required tools to fabricate a high quality, reliable harness. Like any other part of your airplane build, using good quality tools will pay dividends in reduced frustration and increased quality of workmanship.

    1. WIRE CUTTER

    Any decent quality wire cutter will be fine, as long as it makes a clean, square cut and doesn’t crush the wire. Small electronics flush cutters like the Hakko CHP-170 (Hakko / Amazon) are ideal. These are also handy for cleanly clipping cable ties and lacing cord in tight places.

    Hakko_CHP-170.jpg

    2. WIRE STRIPPER

    The key here is to use a stripper that cleanly cuts the insulation from the wire but doesn’t nick any of the wire strands. The wire typically used in aircraft (see Part 2: Materials) has insulation that can be difficult to strip cleanly with a poor stripping tool. My recommendation is the Ideal Industries Stripmaster.

    Ideal_Stripmaster.jpg

    Look for model 45-092 (10-22AWG) or 45-093 (14-22AWG). Replacement blade sets are available to strip wire sizes from 8 to 30AWG in various combinations, and the blades are quick and easy to swap. Used tools are often available on eBay at good prices, perhaps needing a blade set to work like new for the wire sizes you want.

    Tool P/N Blade Set P/N AWG Crimped
    45-090 L-4419 8, 10, 12
    45-091 L-4420 10, 12, 14, 16, 18
    45-092 L-4421 10, 12, 14, 16, 18, 20, 22
    45-093 L-4422 14, 16, 18, 20, 22
    45-097 L-4994 16, 18, 20, 22
    45-098 L-5361 20, 22, 24, 26, 28, 30
    45-099 L-8763 8, 10, 16, 18
    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.

    D-Sub_Crimp_Tool.jpg

    4. D-SUB PIN EXTRACTOR

    Inserting a pin or socket in the wrong connector cavity isn’t matter of if, it’s a matter of when and how often. This is one place where the cheap tool will drive you into a purple-faced rage. Avoid, at all costs, buying the tiny red and white plastic tool with the metal probes at each end. The metal in those things is thin and flimsy so it will break after a few uses, and it’s very easy to stab your hand with the other end while using it.


    Bad_Extractor_Tool.jpg
    Avoid this tool!

    Instead, I would strongly recommend buying the Harting Electronics 09990000368 D-Sub Removal & Insertion Tool (Digi-Key / Mouser / eBay). The business end of this tool is made of stouter material, it has a handle big enough for human hands, and the retractable probe cover will protect the tool between uses. Even at $30, this little tool is a bargain. It can still be bent, so don’t go full gorilla on it.

    Harting_09990000368.jpg
    This is the tool you want.

    5. HEAT GUN

    Any model will do, and you’ll finally get to use that strange curved attachment that came with it.

    6. LABEL PRINTER

    You can get a decent electronic label printer at any office supply store or online for not much money, so there’s no reason not to clearly label your wires and harnesses. If you use printable heat shrink tubing, you can label individual wires directly, and you can use standard label tape to mark harness connector backshells. Virtually all of the label printers on the market have refill cartridges available with either label tape or heat shrink.
    Last edited by Eric Page; 07-17-2021 at 09:48 PM.
    Eric Page
    Building: Kitfox 5 Safari | Rotax 912iS | Dynon HDX
    Member: EAA Lifetime, AOPA, ALPA
    ATP: AMEL | Comm: ASEL, Glider | ATCS: CTO
    Map of Landings

  3. #3
    Senior Member Eric Page's Avatar
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    Default Part 2: Parts & Materials

    1. WIRE

    The usual choice for aircraft wiring is MIL-W-22759/16 (often abbreviated as M22759/16). It’s a multi-strand tinned copper wire coated with Ethylene Tetrafluoroethylene (Tefzel) insulation. The basic part number is followed by two “dash numbers.” The first indicates wire gauge and the second, insulation color.

    ColorCodes.png

    There are a few additional color codes that are fairly uncommon in the wild: P = pink, T = tan, D = dark (i.e. D6 = dark blue), L = light (i.e. L6 = light blue). Striping is indicated by two color numbers: -90 would be white with black stripe. Twisted pairs are sometimes called out by separating the conductor colors with a forward slash: -95/96 would be a twisted pair composed of white with green stripe and white with blue stripe.

    It’s perfectly acceptable to use one color – usually white – for everything, but if you do this be sure to do a good job labeling your wires, twisting the correct pairs and keeping the pinout matched from end to end in harnesses.

    If buying wire in 100’ increments in standard colors, Skygeek has good prices.

    For shorter lengths, or for custom color striping, ProWire USA is a great source. They have a 10’ minimum for standard colors and they make custom striped wire at very reasonable prices with a 100’ minimum.

    Also be sure to check the Bargain Bin at American Wire & Cable (click “MIL-Spec Wire & Cable” in the menu on the left, then “M22759”).

    2. D-SUB CONNECTOR BODIES

    This is one item for which Aircraft Spruce is very competitive on price. They sell good quality AMP connector bodies cheaper than any place I’ve found. Common sizes hold 9, 15, 25, 37 or 50 pins. Your avionics installation manuals will describe the sizes you need. Pay attention to connector gender, which is determined by contact type, not connector body. The female connector uses sockets and the male uses pins, but the female connector fits inside the male.

    D9M_D9F_Connectors.jpg
    Top: male. Bottom: female.

    Links to connector bodies at Aircraft Spruce:
    9-Pin Male Female
    15-Pin Male Female
    25-Pin Male Female
    37-Pin Male Female
    SteinAir stocks similar connectors, including 50-pin and high density types, as well as contacts, backshells and cinch nuts.

    Be careful not to buy high density connectors or contacts unless that’s really what you need.

    3. D-SUB CONTACTS (PINS & SOCKETS)

    As I mentioned in Part 1, I recommend using turned, not stamped-and-formed contacts. The turned parts are more expensive, but they’re much easier to use, and to get consistently good results with. A cheap, generic 4-way indent tool will crimp turned pins and sockets just fine. Finding a tool with good dies for stamped-and-formed contacts is a challenge at reasonable prices, and getting consistently good results with them takes skill and practice.

    Turned pins for male connectors – P/N: M39029/64-369 or 205089-1
    Turned sockets for female connectors – P/N: M39029/63-368 or 205090-1

    Both are available from any electronics parts distributor, or from...

    B&C Specialty Products (pins / sockets)
    SteinAir (pins / sockets)
    Aircraft Spruce (pins / sockets)

    There’s also an eBay seller offering bags of 100 at very attractive prices (pins / sockets).

    4. D-SUB BACKSHELLS

    This is the plastic or metal case that holds the connector body and provides strain relief for the wires. You can spend a small fortune on fancy metal backshells, but I wouldn’t bother. The Dynon factory uses inexpensive plastic parts (Assmann WSW “A-FT” series). You’ll need one for each d-sub connector you install.

    SteinAir sells similar parts at similarly low cost.

    5. D-SUB CINCH NUTS

    The Assmann WSW “A-FT” series backshells come with thumbscrews to hold the (usually female) connector in place. Those work great in cases where your harness will connect to an avionics box that has a (usually male) connector with cinch nuts already installed. However, if you need to make an extension harness with both male and female ends, you’ll need a pair of cinch nuts for the male end (in place of the thumbscrews), so that a female connector’s thumbscrews can clamp the connectors together.

    You’ll need so few of these that there’s little point in shopping around. Aircraft Spruce sells them in a complete set, with washers and #4-40 hex nuts to install them on the connector body, for $1.35/set. If you prefer, SteinAir sells connector bodies with cinch nuts already installed.

    Note that you may have to clip away part of the molded internals of the backshell to install a connector with cinch nuts attached.

    6. HEATSHRINK TUBING

    The good stuff is so cheap, there’s no reason to use anything else. I use Qualtek "Q5-3X" series, which has a 3:1 shrink ratio (meaning it shrinks to 1/3 of its original diameter) and is both adhesive-lined and flame retardant. Digi-Key carries it in several useful diameters, in 4’ lengths.

    I find that the 0.354” inside diameter Q5-3X tubing is a good fit over expandable sleeve on a 9-wire Dynon SkyView network harness.

    7. EXPANDABLE SLEEVING (OPTIONAL)

    This stuff isn’t strictly necessary, but it makes for a professional looking harness. If you prefer not to use expandable sleeve, then wire bundles can be dressed with nylon cable ties or lacing cord. For expandable sleeve, I use Techflex Clean Cut FR. 1/4” diameter sleeve costs ~$0.53/ft in 100’ rolls. It will easily expand (like a Chinese finger puzzle) to cover wire bundles up to about double its nominal diameter, so I don’t bother with the larger, more expensive sizes. The main advantages in using expandable sleeve are that the harness remains more flexible than if it’s dressed with cable ties or lacing cord, and it’s less likely to get caught on something while it’s being fished through the aircraft.
    Last edited by Eric Page; 07-17-2021 at 09:54 PM.
    Eric Page
    Building: Kitfox 5 Safari | Rotax 912iS | Dynon HDX
    Member: EAA Lifetime, AOPA, ALPA
    ATP: AMEL | Comm: ASEL, Glider | ATCS: CTO
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    Senior Member Eric Page's Avatar
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    Default Part 3: Wire Tables & Pinout Diagrams

    Below are the example diagrams and tables that I’ll use to build a harness in Part 4. They show the pinout, wire colors and twist callout for a Dynon SkyView network cable.

    NOTE: There is an error in the connector diagram. Dynon’s avionics have male connectors for SkyView network connections, so harnesses need female connectors. In the diagram below, Dynon refers to the connector as a D9M (male), when it should be D9F (female). The contact locations shown are correct for a female connector.

    SkyViewPinout.JPG

    Pay careful attention to the note below the connector diagram, which indicates that it’s a view looking at the contact insertion side of the connector, not the mating side. These location numbers are molded into the plastic lining of the connector body on both sides, but magnification is needed to see them.

    D9F_Rear_Mag.jpg

    I’ll be using wires matching the Dynon color scheme in Part 4, but as I said in Part 2, using all one color is fine as long as you’re careful to match pin numbers at each end and twist the correct wire pairs.

    Speaking of twist, here are the tables showing which wires do what in the SkyView network, and how to twist them within a harness.

    SkyViewTwist1.jpg
    SkyViewTwist2.jpg

    Finally, I’ll offer another reminder about pin mirroring between male and female connectors. In order for the connectors to mate with the correct wires connected, the male and female parts have their contacts in mirrored locations. It’s very easy to forget which type you’re fabricating, so pay careful attention and consider keeping separate diagrams in front of you depending on which type you’re working with at the moment.

    D9_Male_Pinout_Rear.gif
    Male D-Sub Pinout (Crimped Pins)
    Contact Insertion Side View

    D9_Female_Pinout_Rear.gif
    Female D-Sub Pinout (Crimped Sockets)
    Contact Insertion Side View
    Eric Page
    Building: Kitfox 5 Safari | Rotax 912iS | Dynon HDX
    Member: EAA Lifetime, AOPA, ALPA
    ATP: AMEL | Comm: ASEL, Glider | ATCS: CTO
    Map of Landings

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    Senior Member Eric Page's Avatar
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    Default Part 4: Harness Fabrication

    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.

    D-Sub_Pin.jpg IMG_1661.JPG

    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.

    IMG_1659.jpg

    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.

    IMG_1660.jpg

    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.

    D-Sub_Crimped.jpg

    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.

    IMG_1663.jpg IMG_1664.jpg

    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.

    IMG_1668.jpg

    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.

    IMG_1670.jpg

    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.

    IMG_1671.jpg

    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.

    IMG_1672.jpg

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

    IMG_1673.jpg

    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.

    IMG_1674.jpg

    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.

    IMG_1675.jpg IMG_1677.jpg
    IMG_1678.jpg

    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.

    IMG_1679.jpg IMG_1680.jpg

    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.

    IMG_1681.jpg

    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.

    IMG_1683.jpg

    Congratulations – you now have a well made, professional looking harness!
    Eric Page
    Building: Kitfox 5 Safari | Rotax 912iS | Dynon HDX
    Member: EAA Lifetime, AOPA, ALPA
    ATP: AMEL | Comm: ASEL, Glider | ATCS: CTO
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    Default Re: Avionics Wiring Harness: A Tutorial

    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

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    Default Re: Avionics Wiring Harness: A Tutorial

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

    Rick

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    Default Re: Avionics Wiring Harness: A Tutorial

    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)?
    Last edited by alexM; 07-18-2021 at 09:49 AM.
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    Default Re: Avionics Wiring Harness: A Tutorial

    That had to take a lot of time, Eric. It should be really helpful for many builders… great job!
    John Evens
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    Senior Member Eric Page's Avatar
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    Default Re: Avionics Wiring Harness: A Tutorial

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

    Quote Originally Posted by alexM View Post
    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.
    Eric Page
    Building: Kitfox 5 Safari | Rotax 912iS | Dynon HDX
    Member: EAA Lifetime, AOPA, ALPA
    ATP: AMEL | Comm: ASEL, Glider | ATCS: CTO
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