Guides on Aircraft Electrical Wire Material and Insulation Types 1

[kontiki99]

Anybody know of a good reference on Aircraft Electrical Wire Materials and Insulation Types?

I'm interested in finding out a bit more than DOD approved or not.

I just need to know more about the subject in general.


Thanks,

 

[Sparweb]

AC 43.13-1B, Chapter 11
Mil-W-22759 (or its sucessor AMS spec)
Mil-W-27500
SAE AS-5088

Advisory Circulars from the FAA free to download,
Mil specs from ASSIST, also free downloads.
SAE standards: cost an arm and a leg - basically reprints of the mil-specs and advisory anyway.


Steven Fahey, CET

 

[kontiki99]

Thanks SparWeb,

What I'm really looking for is probably best summed up in this sentence from AS50881, Wiring shall be of a type suitable for the application.

I'm fairly familiar with the various specs, it seems like I’ve picked through them for years. Ampacity and voltage drop are well covered in the 43-13 and 5088. I'm really looking for application notes or a selection guide for wire materials.

I recently saw a reference to Boeing BMS 13-60 Type 1 Class 1 wire used various places in our aircraft. It’s tin coated wire, with no identification on insulation type. Insulation may just be a don’t care for this particular application, it’s well protected.

But we needed to make a wire substitution and wanted to use something we already have set up in the inventory. We have a couple hundred wire types (Airbus, Boeing, Open Spec) listed in the computer already. The issue kicked off a fairly interesting exchange of ideas and stories around the coffee pot.

One of the contractors He mentioned that silver plated wire shouldn’t be used in fuel tanks because silver reacts with sulfurs in the fuels. He couldn’t remember which insulations were compatible with Skydrol.

Then we went through Kapton stories. So is Polytetrafluoroethylene/Polyimide (listed in AS22759) the dreaded Kapton?

When is PTFE insulation preferred over ETFE or TFE or FEP? When should I be using nickel plated copper vs tin or silver plated copper. AS50881 tells me that 22759/41 is ok in open air and 22759/11 ok in protected bundles.

I can assume one is mechanically tougher than the other, but it is abrasion, denting? I’ve seen engine wire bundles pulled apart for re-manufacture at overhaul. The very fine grit worked it’s way in between the wires and the constant vibration eroded large groves in the insulation. So in addition to heat considerations, should there be special abrasion resistance or extra thickness on engine wiring?

I just feel it’s time I moved my knowledge up a notch on this front.

 

[eng3000]

"So is Polytetrafluoroethylene/Polyimide ... the dreaded Kapton?"

Polyimide is the generic chemical description for substances which include Kapton (a trademark name). The most problematic Kapton wiring is just Kapton wound over the conductor with or without a paint overcoat. Polytetrafluoroethylene/Polyimide or 'TK' is a composite insuation with the named constituents. TKT (Teflon/Kapton/Teflon) is reputed to offer among the best mix of properties as an airframe wire and it's important to remember that Kapton has some desireable properties which can be usefully exploited in a composite construction.

For engines you'll likely need something else possibly a filled Teflon with a glass reinforcement or layer. Sorry, but I'm unable to refer you to a spec or guide right now.

Until quite recently the regulators position on wiring was famously 'wire is wire'. That has changed now and manufacturers are required to select materials and plan for maintenance and inspection over the lifetime of the aircraft.

 

[kontiki99]

eng3000

Do you know if they solved the moisture problem with Polytetrafluoroethylene/Polyimide.

Searching around this morning, I'm now uncertain if older Kapton wire insulation simply absorbed water or if the moisture was drawn in through cracks in the insulation.

When an aircraft descends, moisture on connectors, fittings and jackets tends to be forced in by the pressure change.

 

[eng3000]

I would suggest you contact a manufacturer of TKT (or other kapton wire) re your specific queries, e.g.:

http://www.carlisleit.com/v2/productDetail.asp?catid=1&serid=10&productID=20

(formerly Tensolite)

Depending on your application and the specific installation, performance and regulatory requirements related thereto, you might be needing a more robust construction, e.g. SWAMP wire (Severe Wind And Moisture Prone).

I believe that some operators are currently replacing Kapton and possibly TK with TKT. Can you tell us a bit more about your application/needs?

 

[kontiki99]

eng3000,

Right now, a lot of my work is standardizing avionics systems in older airplanes (lately TCAS and Mode S Transponder systems). Where I’m in a project based environment, I could be conceivably asked to address a modification or repair to any aircraft electrical system.

Sometimes projects involve supporting repair shops and line maintenance too.

As I stated in my original post, I'm trying to find a guide that addresses wire materials selection in a practical way.

Maybe it's time for me to take a shot at writing a practical avionics electrical guide for airline engineers. There is a lot a specialized tribal based expertise that can take years to pull together in a useful way.

Every project seems to contribute to my general model for dong things. There is very little training out there that you can just attend and apply to this environment.

It’s always a matter of absorb what’s offered and try to fit it into the framework of technical knowledge, policy and regulations as I understand them.

Like everything, it's constantly changing with changes in regulations, government policies and technology.

 

[Sparweb]

Going back to AC 43.13, I can see a lot of info about wire jackets, such as ease of marking and which types give better results for which labelling methods, etc.

In my experience, M22759/16 and /34 are most popular as single conductors. I get to see old helicopters and fixed-wing aircraft, too, and some have been stripped of all wire at one point. The airframe might be 50 years old but the wire bundles are bright white Tefzel.

If you're involved with modern airliners (and it sounds like you are) then references like AC43.13 are just the starting point, and you'll have to be mindful of the Boeing/Airbus/whoever's specific practices. I haven't personally had to do this: is there a Boeing Standard Wiring Practices Manual? I know there's one from several other manufactuers that I regularly deal with, why not Boeing?

Ask an avionics tech if there are types easier to strip, bend, pull through conduit, or mark than others. Then go ask his manager which are cheaper... the answer they have in common will get used on the shop floor.


Steven Fahey, CET

 

[eng3000]

Your first port of call for what you describe (civil aircraft) will be the aircraft manuals, specifically the wiring practices and hook up lists. There you should find the original wiring type specification for almost all wires on the aircraft wiring diagrams, excluding 'furnished' wires.

The responsibility for wire selection starts with the manufacturer (Type Certifcate or TC holder); any replacements need to be per the aircraft manufacturer's specification or their 'approved' alternatives. If the aircraft has been modified, it's down to the STC holder.

The use of 'higher specification' wire in place of the original or approved alternative type is a fairly common practice , but beyond the above you really get no 'free hand' in choosing wire types whatever local practice may be. If you're the TC or STC holder it's different of course, but one of the pillars of aircraft airworthiness is adherance to the TC and STC specs.

If you're designing as a prospective STC holder, make your specification, pick a trusted wire distributor and work with them to select and procure suitable wiring; the range available is quite finite and you shouldn't have difficulty choosing one given a good spec write up. If there's any doubt, go for the higher spec and/or look at protecting the wires somehow and check with the manufacturer. It can also be helpful to speak informally with an experienced installation/maintenance engineer for the relevant aircraft type, but remember they're not directly concerned with TC/STC issues.

*

Thinking about your post, asking for a list of wire types for corresponding applications is really putting the cart before the horse from an engineering standpoint. You need to define all requirements for the performance of a wire in a particular environment and application including fault and failure conditions before selecting a suitable wire type.

To list specific application(s) for a given wire type is sticking one's neck out a bit given the inevitable uncertainties of actual aircraft installations and configurations, (even TC and STC holders very often get it wrong, I can tell you from experience,) and we have only to look at the number of 'new imporved' wires which have come and gone in recent decades to see why it's maybe not so surprising that such a set of rules might have limited trustworthiness and current life.

 

[kontiki99]

eng3000

I basically agree with you, perhaps I have situations and priorities organized a little differently. This is how I see the problem.

I believe OEM wiring standard practices are provided to comply with continuous airworthiness requirements. That document is not a design handbook, it’s repair data. I’m not aware of any Boeing or Airbus design reference that’s available to customers or that’s public knowledge. They safeguard their IP very well.

If a conductor on a large modern commercial airplane breaks, through the use of the Wiring Diagram Manuals and the Standard Practices Manual, Maintenance people can obtain all the approved materials and tools and apply the detailed fabrication processes to repair or replace that wire and restore the system to the FAA approved standard.

I view the 4313 as applicable for repairs only when OEM data does not exist (depending on the scope of the repair). I have seen many FAA Designated Engineering Representatives (DERs) point to the 4313 either as a standard practice or as a design reference, I believe the design references for non OEM modifications probably should be specifications, design handbooks and engineering reference material. I believe approval by similarity is still recognized and is valid when applied properly. I’m not a DER so I don’t keep up so well.

Now, under some Parts of FARs applicable for what’s called General Aviation, the A&P can make certain substitutions during a repair. It’s been a long time since I reviewed those regulations and the limitations on that practice, but I recall being taught that in A&P school (back in the late 70s).

In Part 121 environments, maintenance generally is not allowed to make any substitutions unless those substitutions are already described in an approved document.

The Engineering department in a 121 environment usually has the authority to approve FAA Minor Alterations. Usually Part 121 organizations have Engineering Departments and that is one of the roles they fill. When maintenance does not have the specified materials to make the repair, depending on the circumstances they may contact the Engineering department to get approval to make the substitution.

So here we are, Boeing used tin coated copper wire rated to 150C with don’t care insulation (BMS 13-60, Type 1 Class 1). It’s AOG for the repair, you have silver coated copper wire and nickel coated copper wire rated to 200C with PTFE insulation in stock. Which do you choose?

I believe effective engineering requires a mix of two types of knowledge, empirical and analytical. We know about the problems with Kapton wire that nobody foresaw; after all, it has excellent properties, because it was tried and there is a documented record of major problems. Engineering learns a lot from the sentinel events that alert us to problems. (You don’t ever want to be a passenger when a sentinel event is about to occur.)

Selection of any material should certainly be based on application requirements. The wire manufacturer may or may not know anything about anything about the conditions around a submerged optical fuel level sensor in the center bay of an XYZ aircraft. The OEM may have had to issue corrective SBs to replace all the wiring harnesses in a similar application on some obsolete aircraft fleet 15 years ago, so somebody there may remember (but not necessarily).

In my mind, the question is, what does the airline engineer use as a guide. I think that a practical guide should lean on the obvious design information and specifications and at the same time be a repository of empirical (experienced based) information that may not be obvious to someone that hasn’t been keeping up with what’s happened in that specific niche in the industry over the last 20 years. The 4313 goes a long way toward doing that for GA, but it’s not really approved data for anything.

 

[johnSchwaner]

Try this link. It has some good history on wire insulation and wire conductor coatings.

http://www.tsb.gc.ca/eng/rapports-reports/aviation/1998/a98h0003/01report/01factual/rep1_06_10.asp