Aircraft Wiring 102

Builders,

After todays Aircraft Wiring 101 story, a letter came in from California 750 builder David Josephson. I share it for several reasons;

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First, it is a view of what a builder can do if he wants to look at operating one level above the basic information I was sharing.  In all my conversations with David, what comes through is his unusual balance of technical-practical-detail application along with a very strong scientific understanding of the fundamentals of the question at hand.  When reading his take on a subject, it is a reminder that accessing this balance on any subject is what makes it interesting. While Tefzel and basic quality crimps and tools are what I use on planes, David’s notes are another step toward quality and away from the sub-airworthy.

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Second, The letter is a reminder to people who work outside Aviation or the tech world, that industry specs and standards mean something in our field. Many workplaces and topics have very subjective standards of performance or none at all; note that experts in aviation long ago put out the most detailed standards for things as small at how wire is plated. Working in aviation, I don’t know 2% of these standards, but critically, I know for almost every question, there is a correct tested answer, you just have to look for it.  Understand when a guy says “It will be alright” what his is actually doing is deciding to stop looking for the known way and proven path, and accept the item in question as it is. Conversely, a guy who works in aviation knows that when in doubt, keep looking until you find the proven standard. When you think like that, you have become an aviation professional, even if your paycheck has a different address.

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Third, The strength of the Corvair movement is the quality of the people we bring in as builders. Yes, I know the subject of Corvairs very well, but we have countless other people who know far more about engineering , electronics, flight, you name it. I like it this way, because it puts me in contact with people a lot smarter than me, and this expands my world and learning.  At Oshkosh a number of years ago, I was confronted by a man who leveled what he felt to be a damning charge against me; He wanted to publicly prove that I had changed my position on several technical points over the years. He was perplexed and disappointed by my response, where I told everyone present “Yes, I intentionally surrounded myself with smart people and then had the common sense to listen to them.”

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Oshkosh 2013: East meets West in the tent. Two of the sharpest minds in the Corvair movement belong to Ken Pavlou of Connecticut, at left above, and David Josephson of California on the right. Both are Zenith builders. Ken has been involved in numerous projects in support of the Corvair movement. David is a nationally known expert on acoustics, and is interested in extreme noise reduction in aircraft. Both of these men find the Corvair movement the right focal point for their efforts in aviation.

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____________________________

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"William,

I have a few comments on your article, and a resource to offer. Your
recipe is good, but if people want to drill a little deeper they can
understand a bit more if they want. A well designed simple airplane will
only have a few dollars of wire in it, there is no point in scrimping —
but it may be possible to buy NOS military wire and terminals and get
good quality for less.

1. The softness of the terminal insulation is only part of the picture.
Good crimp terminals are made with nylon insulating sleeves, which is
soft and transparent. More important, there is a bronze sleeve inside
that crimps around the wire insulation under the nylon to actually
provide strain relief. The terminals with vinyl sleeves lack this part
and no strain relief is actually provided, only insulation, because
there is no mechanical connection to the insulation of the wire. Good
crimp terminals are made by Thomas and Betts, AMP and Panduit (although
those companies also make cheap vinyl insulated terminals that have no
strain relief,) and are compliant with the former MS-25036 series, now
SAE AS 25036.

2. The crimp tool must be the one specified by the manufacturer of the
terminal or compliant with the relevant mil spec, such as the AMP tool
in your picture.

3. An even more secure approach is to use uninsulated crimp terminals,
soldered if you like, and heat shrink tubing, which if clear can include
a typed label inside. (But! Solder only after there is a secure crimp!)
The crimp tool for uninsulated terminals like the original T&B Sta-Kon
is completely different from that used with insulated terminals.

4. The wire doesn’t have to be Tefzel (crosslinked ETFE), but it does
have to be aircraft wire. There are three criteria: the strands must be
individually plated, not bare copper or batch tinned, the stranding must
be fine enough to provide good flexibility, and the insulation must be
rugged enough not to deform when clamped, rubbed or mildly abraded.
Generally people use Tefzel because it can get hotter and not smoke
versus PVC, but the fumes from burning Tefzel are worse than from
burning PVC. Tefzel wire compliant with MIL-W-22759 is the best
compromise but is expensive. It comes in many colors and is stamped with
the mil spec number and gauge. PVC is okay but it must have a nylon or
fiberglass jacket, typically compliant with MIL-W-5086 or the later
MIL-W-16878. There are also good aircraft wire made to Boeing
specification. Teflon (PTFE) is okay inside equipment, but is softer and
more easily damaged. Teflon also more expensive than Tefzel because it
is usually silver plated — and in many cases the silver plating on
surplus Teflon wire has tarnished so badly you can’t use it.

5. There is a good stock of surplus aircraft wire, $8 a pound for mil
spec vinyl and $15 a pound for fluorocarbon (when in doubt they charge
you the vinyl price,) at Apex Electronics in southern Calif.,
www.apexelectronic.com. Joe has come back from retirement and will
usually find what you want, better if you go there. They also have
multiconductor milspec cable. They have mostly 16 gauge and smaller, I
recently bought spools of 16 and 20 gauge and even at $15 a pound it’s
1/4 the current market price, and you have a chance to get it in colors
if you are lucky. It is worth a visit, it’s one of the remaining
aviation surplus stores on the west coast. Take a flashlight and good
glasses, the lighting inside isn’t so great.

6. The Delphi Weather Pack connectors are great, *if* you buy into the
whole system. The parts are cheap, but you need to have the correct
sealing glands and the correct crimp tool or the reliability of the
system is lost. If you really need to be able to disconnect things
quickly, fine. Frankly I prefer to have fewer connectors and am willing
to spend a little more time unscrewing terminals.

7. NO BARREL SPLICES. You cannot inspect the crimp of a barrel splice,
so you have no way of knowing it’s secure. If you need to splice a wire,
use two knife blade connectors like AMP 32446, in a length of vinyl
tubing tied with nylon twine.- David”

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About William Wynne
I have been continuously building, testing and flying Corvair engines since 1989. Information, parts and components that we developed and tested are now flying on several hundred Corvair powered aircraft. I earned a Bachelor of Science in Professional Aeronautics and an A&P license from Embry-Riddle Aeronautical University, and have a proven 20 year track record of effectively teaching homebuilders how to create and fly their own Corvair powered planes. Much of this is chronicled at www.FlyCorvair.com and in more than 50 magazine articles.

3 Responses to Aircraft Wiring 102

  1. Tom Graziano says:

    William,

    Chapter 11 in AC 43.13-1B has the information one needs when it comes to aircraft wiring. Builders would be exhibiting poor judgement to not reference this chapter when it comes to wiring their aircraft. (There are several pertinent and informative Advisory Circulars too.)
    One thing many – way too many – people do not understand is that soldering is not the preferred method of splicing wires/attaching wires to terminals. Crimping is. If I’m not mistaken, the FAA doesn’t even mention soldering wires in Chapter 11. Rule of thumb in aviation is that crimping should be considered the mechanical means of attachment; solder just to ensure current flow (i.e. to maintain a constant electrical value).
    Do NOT use KAPTON wire! Kapton wire has been implicated in numerous fatal inflight aircraft fires. I have first hand experience with this stuff causing inflight emergencies. The cost of wire is a drop in the bucket when it comes to your aircraft. Buy the good stuff and use the proper tools and connectors!

  2. joe says:

    I’m seeing connectors with heat shrink material surrounding the crimp. This appears new is it a good choice.
    Joe

  3. Tom Graziano says:

    Joe,

    Those are called environmental connectors (a.k.a. harsh environment connectors). They’re not really new. They are very good connectors, but they’re a bit expensive and not needed in many applications.. AC 43.13-1B covers these types of connectors and their application.

    Tom

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