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Building harnesses on the bench vs in the plane

N546RV

Well Known Member
OK, so it's getting to be wiring harness time. I have my schematics and routing diagrams done, just a few more details to hammer out. I've read leok's excellent thread about nine hundred times, and I've always envisioned building beautiful harnesses on my workbench as seen in this photo of his:

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But the more I think through the mechanics of this, the less feasible it seems. That is, it doesn't seem possible to me to build something like that and then actually install it into the airplane. I'm thinking specifically of routing the portion of the harness that will go under the seat floors and back to the stuff behind the baggage area.

While that run should be fairly compact, and go through the conduit easily, once it's out of there it'll split off into several drops (ELT/xpdr/ADS-B receiver/rear seat accessory runs) before continuing to the tail. I feel like about the best I could do for this area would be to cut the assorted wires to length, temporarily bundle them into once nice trunk (but loosely to help them negotiate the conduit), and then split/lace/terminate them after going through the conduit.

But even that's only half the fun; I still have the drops for the wings and a few other items, which will be ahead of the conduit but behind the center section. Overall, it just seems unmanageable in my head to do anything but string the wires behind the center section one at a time, but that idea bothers me a bit too. On the other hand...seems like the wire count should be roughly the same, so maybe this just has to be the same as the aft avionics, where I'm just lacing and terminating the drops inside the plane.

Am I overthinking this? Anyone care to share stories of how they constructed their harnesses? Are you Team Build It On the Bench or Team Lace It Up Inside the Fuse?
 
I did a mix of both. My harness that connected all of the components under the dash was built on a board and installed in the aircraft ready to plug and play. You can look at my builders log link in my signature in the link below.
 
Take a look at a post by cadetstimpy: https://vansairforce.net/community/showthread.php?t=140138

He notes in item #2:

2) Terminations - Prefab vs In aircraft

Personal belief .... I believe routing, combing, anchoring and terminating harnesses in the aircraft is a much better than prefab bench work that gets placed into the aircraft. Don't get me wrong - some things such as control sticks and component disconnects should be done on the bench. If the majority of the termination is done in the aircraft - a natural service loop and serviceability will occur with fewer unnecessary disconnects. Placement of components, harness and assembly are naturally done better. Think about it...how you figure out to terminate and assemble in the aircraft is going to be very similar to how disassembly and maintenance will occur.

I'm been considering the same question. It seems logical to me to build a harness on the bench then install it but the reality of routing one big bulky harness at once doesn't seem feasible.
 
I did my Dynon wiring on the bench to ensure that I could power up the EFIS, the radio and transponder. The rest of the electrical wiring was completed on the plane. The downside of wiring the panel on the bench is some of my service loops are either too long or a bit on the short side. If I had to do it again, I will still wire the avionics on the bench because these modern EFIS have tons of tiny wires and tons of connectors. It was easier to crimping all of these tiny pins while being comfortable sitting at the bench and with good workbench lighting.
 
So the mechanics of this work just fine....

Following Leok's great thread too.

All the back shells that can be terminated will be. The connections that have to pass through small openings will have the pins installed and then heat shrink added to aid passing through the openings. Most of the connection will be behind the IP or SP, so those cane be terminated.

All my harness wires have been run and now I'm ready to terminate!

IMHO this is the way to make a harness come out professionally fo a beginner.

EDIT: I'll also add that I have a DB15 connectors at the aft fuselage, just behind the baggage compartment and at the wing roots, just inside the fuselage. So the main harness is not as bulky as one might think.
 

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The big wire shop at the depot facility I worked at made up wire bundles from a template, laid out similar to the one in the OP's photo. But, I believe before CAD/CAM was fully implemented, the templates were produced from original first-production wire bundles that were laid in by hand.

I'm planning on mounting the instrument panel in a mock up and prewiring the back-side with unterminated runs to all the remote peripheral components. That way it'll be easier to keep power and signal separate and terminate each peripheral connector as I go.

I've used this method twice now on panel upgrades for flying club aircraft. And, its worked pretty well.
 
Just me

I tried to do my wiring on a mockup but gave up after a very short time. Without a CAD design for everything it seemed nearly impossible to get the lengths right.
I ended up mounting the sub panel on the bench and wiring in situ. I will now add the instrument panel and finish the wiring between the subpanel and instrument panel. Lastly i will install the subpanel and instrument panel in the plane and do final terminations. There are just too many computer connections to do this in the plane, and not enough access to add the harness that is built on a bench. JMHO
 
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Well, today I pulled some ribbon through the plane and marked up my preexisting harness line drawing with lengths for individual runs and branches. In keeping with the idea that this just isn't going to be perfect on the bench, I chose to instead just add some padding to my branch lengths, and whatever I construct on the bench will probably be largely complete in terms of wires being present, but there will be no terminations whatsoever. Once I get the thing weaseled into the plane I intend to finalize all the branches, trim to final length, and terminate in place.

Stand by to find out how much I hate my life in a week or two.

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There is an option, and that creating a harness that is relatively easily removable from the aircraft. In that situation, you will still need to lay on your back to get all the wiring run lengths (assuming you don't want massive service loops) but the bulk of the wire stripping, crimping etc. can occur on the bench.

For me the key was two large D-sub plugs that allowed the avionics harness to connect to the fuselage. Everything went through this, including the shielded grounds for the individual CAN wires (I would use a solder sleeve and feed into a pin of the d-sub).

There were also d-sub connectors for the ground wires and power wires, which made removing the harness super simple compared to pulling off heaps of spade connectors. Even for things like audio jacks I would have the small JR servo connectors (they work great and are dirt cheap when you can find them).

Am I adding a failure mode? Yep, but D-sub connectors are super reliable, hence the reason we use them in aircraft. The biggest risk is having an avionics system that is difficult to maintain, then you never want to make changes in the future even though you know you probably should. Not having to spend nearly as much time on your back and being able to do quality work on your bench makes it well worth it to me.

Tom.
RV-7
 
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Team Lace It Up Inside the Fuse

When I helped build an RV-10 I/we found it easy enough to pull wires through one at a time to where they needed to go. We just left lots of loose cable ties everywhere as guides to pull through. As soon as we'd zip one tight thinking we were done there we'd add another wire or three, so we went through a few!
Most were ultimately replaced with clamps as we finalised each run.

That included leaving the required amount of feet of wire (plus a bit more!) coiled up at the wing root for things like the strobes, lights and pitot heat. When we moved to the airport and bolted the wings on, it took just minutes to pull through all of the wires through conduit and drop them out where they needed to be terminated. That saved having a wing root plug/socket/maintenance point. We left a foot of slack there just in case we might ever have to take a wing off (god forbid!).

I can see the merit in Tom's methods above, but I didn't see the -10 panel as being a single drop-in unit. It was simple enough to run one wire at a time that I'd do it the same way again.
[We also decided against using primer on the tailwheel on the -10...:D]




I came across this pic a few years ago and I can't cite the source. Much respect to the folks who had to install and maintain this!
RVs are not this complicated!
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Big aerospace always uses layout boards, but then they are making many aircraft at a time. If you know the lengths of all the spurs why not, but you probably don't so its much more difficult to get it right on a layout board. Accepting there is likely to be some changes when the harness meets the airframe it is certainly a good starting point.
 
I did the fuse harness for my RV14 on a board in the house. It was convenient and tidy. But I had the benefit of the excellent drawing that showed all the proper lengths for everything and it was somewhat cookbook. For the 8 I don't have that sort of great drawing, so I'll be doing the fuse and wing wiring on the fly. For the panel on the 14 I would make up one wire that was long enough, add for service, and then wrote down that length for that connection between boxes. Every time I needed a connection for that pair I would use that length. I used tie wraps for loose organization, cutting them off if the bundle got too big and just using another at that point; throwaway tools. At the end I laced it all in place which was a little awkward sometimes but I'm pretty good at knots and tight spaces.

Came out great. So if you know all the dimensions go for it. But it can be done beautifully on the plane too.
 
Update: I don't hate my life yet, but so far everything's still on the table. I did a tape layout sorta like in leok's thread, and for the past couple days I've been stringing wires. To be honest, I'm really enjoying this work; it's satisfying in general, and all the grunt work I put into detailed schematics, layouts, and such means that this part is almost dirt simple.

I'm hoping to have all the wires laid in by this weekend, at which point I'll get to figure out how to bundle everything so I can try to coax it into the fuselage. If there's a point where this will become a lot less fun, that'll probably be it...

But for now...still fun!

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I remember walking by a layout board in the BAE factory when I was doing my Avro type in Woodford. I thought...that doesn't look fun.

Maybe when you get to own fly the product it's different.
 
Somewhere between 2005 and 2010 I was in a factory in Rockford, IL, working on some computer systems. They showed me a very large bunch of racks, all folded together, that had all of the wire runs for the Boeing 787. Don't really remember too much, I only got a short look at it and absolutely no pictures, but it was kind of neat. Figured the wire runs would look somewhat like an airplane, you know, a fuselage run, perpendicular wing runs, etc. but these were in metal frames on wheels that looked like they would open up and all of the wire lengths and gauges were run on these.
 
Great idea ,, father in-law retired from Ford was in quality control so he did many plant visits , remember a plant that did wiring harnesses he showed me pictures of they had this exact process , great idea thanks for sharing the photos ,,
 
This is a lot of stinkin' wire. But it's getting pretty close to being complete, at least in terms of wire being laid down. I've worked my way "inward" to the "borders" of the panel harness itself. Next up will be adding power/ground runs to the panel components, getting the EMS-FWF runs, and finally all the inter-panel type stuff.

I think I have a fighting chance at having it all laid out tomorrow. Then comes the real fun, 1) QCing the branches to make sure I didn't miss anything, 2) figuring out what I do and don't want to permanently lace on the table, 3) figuring out how to bundle the stuff I don't want to lace, and 4) maneuvering this strange python-like contraption into the actual aircraft.

Oh by the way, I'm still having fun. Things haven't been as cut-and-dry the past day or two, but the number of decisions I've had to make have been minimal. Another endorsement for doing lots of planning up front.

Anyway...hey, wire photos!

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When I lived in Atlanta, there was one big interchange (I-285 and I-85) that was locally known as Spaghetti Junction. I suspect there are probably other cities that have something similar, maybe with the same name. Anyway, here's the Spaghetti Junction of my harness. Can't wait to see how fat this bundle is going to be when it's laced up:

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How'd you decide on wire colors? I see red, white and black there. I assumed green or black would be for grounds, but you haven't run those yet. Just wondering about how you thought about it.
 
How'd you decide on wire colors? I see red, white and black there. I assumed green or black would be for grounds, but you haven't run those yet. Just wondering about how you thought about it.

You're on the right track, I may just have been unclear about the current state. Early on I chose to use red for power runs, black for grounds, and white for everything else. Sort of a compromise between "everything's white" and a full-fledges color-coding scheme.

The one slight exception I made to this was for the Skyview network cables I'm fabricating; I did decide to go with green and blue for the twisted data pairs there, just to maintain some sort of consistency with the factory color coding.

You're seeing red and black in there because I've already done a bunch of power/ground runs; the only ones remaining are those to actual panel components (Skyview displays, com radio, etc).
 
Whew. Got all the wire laid in as of today (with the exception of two small parts, for which I'll get materials in tomorrow). Which meant it was time to start dong some cable lacing. Not quite sure how much of this I'll do on the bench vs. in the plane, but this particular branch will be in the gear tower, so it's pretty much now or never.

It sure is satisfying starting to turn those piles of wire into nice bundles.

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Wiring basis of design

Prefab’g look likes a good way to go, especially for upgrading a panel to minimize downtime, but a solid plan is critical. Other than Van’s site, I haven't found much by the way of Garmin 3GX schematic layouts to use as a basis of design. I'm curious if most are planning out the wiring themselves or farming out the schematic?

Best,

Greg
RV-7A
 
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I'm doing it myself, one cable at a time. Where the device manufacturer specifies wire colors, I'm following that plan. I label the cable bundle at each end.

Dave
RV-3B, now setting up the cockpit
 
G3X Wiring Schematics

Prefab’g look likes a good way to go, especially for upgrading a panel to minimize downtime, but a solid plan is critical. Other than Van’s site, I haven't found much by the way of Garmin 3GX schematic layouts to use as a basis of design. I'm curious if most are planning out the wiring themselves or farming out the schematic?

As far as detailed schematics, they are all in the G3X installation manual, downloadable on the Garmin site. Every schematic needed to design and draw out an individual installation is in there.

At 900+ pages, it seems imposing at first. But it quickly becomes digestible, and if a builder is planning and self-installing the avionics, there really is no option other than spending quality time with the installation manual, at least in my opinion.

If what is meant by “schematic layouts” is a standardized generic drawing which shows exact wire lengths and layout of the harness, then that probably does not exist except in a professional shop that has standardized their installation. Otherwise, all of that is unique to an individual project and will vary depending on builder choice and placement of components and individual design.

Whether to design and draw out the system yourself, or let a professional shop do it depends on individual interests, talents, time available, and finances.

I planned out my dual Garmin G3x system with typical LRU’s, am doing the drawings, and am in the process of installing it on my RV-7 project. Although very time consuming, I have found the process fairly straightforward, especially after attending the AEA “Avionics Installation for Experimental Aircraft” course a while back, and obtaining the correct tools.

Doing it in the plane while everything is exposed before closing up the top skins, or doing it on the bench will yield equally good results if standard acceptable workmanship practices are followed. It depends entirely on individual builder preference.

Either way it is one wire at a time after spending significant time yourself in the installation manual and planning. Or paying a professional shop $$ to design and draw it out, and then spending significant time yourself studying the pro's drawing and the installation manual to install it yourself. Or paying a professional shop $$$$$$ to design, draw and install it, which also means wait several months to get a slot in an avionics shop, and then additional time for them to complete the work.
 
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Prefab’g look likes a good way to go, especially for upgrading a panel to minimize downtime, but a solid plan is critical. Other than Van’s site, I haven't found much by the way of Garmin 3GX schematic layouts to use as a basis of design. I'm curious if most are planning out the wiring themselves or farming out the schematic?

Best,

Greg
RV-7A

I did all the planning myself. Drawing up the schematics was one of those "idle time" projects starting, well, about two years ago. It was a really good way to spend my morning/evening bus ride to/from work.

I started off by reading the Dynon Skyview install manual from cover to cover. I just wanted to sort of abstractly digest the big picture of how the system would go together. What surprised me was how, after reading through the manual, I felt a lot less intimidated by the wiring. Instead of massive bundles of magic leads, now I was seeing power/ground pairs, serial data pairs, contact inputs, and so forth.

Once I'd determined what components were needed I started putting together schematics. This was a highly iterative process - I don't know that there's a single detail in these things that hasn't been revised at least once. For me, good software was a key to making this work. I tried out an assortment of options before settling on KiCad.
 
And now, for a general update, since this has sort of turned into "as the soldering iron burns" or something like that.

Originally my plan had been to get all the wires laid out on the bench, lace up whatever could be reasonable done there, and put the whole thing in the fuselage before actually doing any terminations. The more I thought about it, though, the more that seemed unnecessary. Everything aft of the spar would definitely need to wait, since the harness needs to get fed through the spar penetrations and a conduit run under the seat floors, but when it came to the busy area behind the panel...why subject myself to all that work in a tight space?

So instead I decided to first "test fit" the harness in the fuselage. I fed it down through the gear tower as it'll be routed long-term, but just passed it over the spar and conduit - those are going to be one-time tasks. All I wanted to determine was that my rough branches for the behind-the-panel stuff were of reasonable lengths, and that everything laid out roughly the way I wanted. Then I pulled the whole shebang right back out, plopped it on the table again, and got to lacing, followed by adding my first couple of actual plugs:

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Basic plan now is to add all the D-sub terminations for the panel components, and probably lace up the harness all the way to where it goes through the spar. Once all that's done, then I'll get to fish this thing through the fuselage for good.
 
Piecing it together

Prefab’g look likes a good way to go, especially for upgrading a panel to minimize downtime, but a solid plan is critical. Other than Van’s site, I haven't found much by the way of Garmin 3GX schematic layouts to use as a basis of design. I'm curious if most are planning out the wiring themselves or farming out the schematic?

Best,

Greg
RV-7A

I pieced my schematics together from the G3X install manual. It is all in there, plus the other separate install manuals for the transponders and radios. Garmin has done a very nice job, but since everyone has a slightly different mix of components, it takes a few schematics married together to get the whole system.

I only wish Garmin had a "check desk" to verify the schematics are right before wiring.

After 4 months of drawing the schematics and then wiring the plane, I figure I will be ready to produce smoke in a month.
 
Good thread... For a small fee that is very worth it, SteinAir will produce a schematic of the pin to pin connections for everything in your plane or panel and save you the hassle of sorting through the G3X data. When I got my schematic from them I made a few small pencil changes from the manual but solely for some personal preferences; their work was excellent. I did all the wiring myself but with a laid out cross-checked plan.
 
Combine the two

As I was travelling in the NE last year being restricted to the hotel rooms due to COVID, I did a lot of planning virtually in CAD.
I stole the the idea of putting the subpanel structure in a jig from KJBSouth, so I could work comfortable on it from below.
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I laid out the cable routes in CAD and plan to have the front part of the harness removeable from below.

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The cyan colored items are CAN bus nodes. I'm just about the set it out in practice....
 
As I was travelling in the NE last year being restricted to the hotel rooms due to COVID, I did a lot of planning virtually in CAD.
I stole the the idea of putting the subpanel structure in a jig from KJBSouth, so I could work comfortable on it from below.
I laid out the cable routes in CAD and plan to have the front part of the harness removable from below.

.

One thing - the GSU 25 location is only .025" and might be buzzy. You might want to check into that. Braces etc, even a .025 angle stiffener can make a huge difference.

Also, you have removed part of the ribs from the firewall to the panel. They stiffen the sub panel. Since it is in 3 pieces, an angle all along the bottom (longeron to longeron) would greatly stiffen that panel to vertical motion, thus restoring some structure.
 
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One thing - the GSU 25 location is only .025" and might be buzzy. You might want to check into that. Braces etc, even a .025 angle stiffener can make a huge difference.

Also, you have removed part of the ribs from the firewall to the panel. They stiffen the sub panel. Since it is in 3 pieces, an angle all along the bottom (longeron to longeron) would greatly stiffen that panel to vertical motion, thus restoring some structure.

Trying my best not to hi-jack a harness thread and failing......

De-buzzing is a very good idea. I was wondering about making the left outboard more stiff. The bottom part is also connected to the panel via added avionics trays (in green). For now it's more about position for harness development. I need to research other builders stiffening solutions when using that location. Maybe an angle diagonally behind the GSU will do the trick unless it just concentrates the vibration into the GSU..... I get the impression that after on the GDU itself the selected location is a common solution and putting it behind the subpanel complicates future pitot-static testing.

I want to clarify that I have not removed any material from the tip-up subpanel and firewall substructure. This is how Van's designed the tip-up sub-structure. I have added some additional angles to support the Garmin LRUs (in green). One slight modification is that the non-structural rear portions of the ribs (in yellow) are moved towards the center to support the panel and radio stack. Maybe you have the slider configuration in mind?

I will do some more lightening, but it will wait until the harness support points are determined.

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I suppose I haven't really been keeping this updated. The harness is now in the plane. Routing it through the spar center section and conduit run to the tail wasn't too bad. It was necessary to temporarily remove a few wire runs, however - basically anything that was local to the behind-the-spar area and didn't need to feed through. This was only a few branches, and not a big deal really.

The one thing I did run up against was that even after removing those runs, the harness trunk was just slightly too big to go through a single 3/4" snap bushing. I considered asking the mothership about enlarging the hole, but in the end just decided to split the harness, since there are two holes in fairly close proximity.

Overall things are going well. I was really anxious that I'd put this in the plane and find out I underestimated branch lengths, but so far everything has matched up very nicely.

Sorta halfway-done behind-the-panel area. Still needs some cleanup:

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Completed center section passthrough to conduit run, plus branch for the Flyleds controller and a branch going across to the other side of the cockpit:

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Cross-cockpit branch, also showing the branch to serve the rear stick:

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I'm still considering this to be fun work. The only real crack in that opinion came when I was trying to get the harness fed through the center section, which necessitated some introductory-level homebuilder yoga:

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