Usable life of avionics wire
This has to be a seriously obscure topic 
Most avionics work is done with Raychem 55 or similar wire
I have noticed that a lot of the stuff is very hard to solder, evidently due to corrosion. This isn’t just on the ends; you can cut the wire a metre from the end, strip it, and it still won’t solder. So whatever causes the corrosion gets in through the insulation.
Apparently this is a well known issue; avionics people I know have pieces of the stuff which they say are old. But they keep odd bits for odd jobs because the stuff is so expensive, and some of the styles cannot be bought by the metre and the standard size reels (300ft or so) tie up a huge amount of money.
I realise most avionics installers crimp rather than solder but I can’t see a crimped joint being affected less by the wire being oxidised. All that happens is that the installer won’t notice the wire is dud because it will crimp just the same.
There is a lot of old stock around, but I’ve bought “brand new” wire from proper stockists such as RS (example – data sheet which suggests Raychem 55 now comes from Tyco) and immediately upon receipt it turned out to be just the same.
I wonder whether this is a wider issue which explains some intermittent issues. Crimping in particular does rely on a gas-tight joint being created. With soldering, if it doesn’t wet properly then you can see it isn’t wetting properly, but with crimping there is no feedback.
I had a neighbour who worked for Raychem; he told me that with time, the insulation generates hydrochloric acid which might be the source of the probblem. I was also told that this process takes around 25 years to begin. At the time I was operating an aircraft with wiring that was 35 years old.
That’s really interesting, and makes sense. Except I think it is a lot less than 25 years…
There are two different types of this wire around. There is the ETFE insulated stuff (which I think is the original Type 55 and is -55C to +200C) and there is the Polyolefin stuff (which is cheaper and is -65C to +150C).
Looking at “aviation” suppliers, you get e.g. this “CAA approved cable” (which probably means exactly nothing – what exactly does the CAA “approve” on a wire?) which cross-references to the old Type 55 again. This is ETFE again but is -65C to +150C.
Here is an “airframe wire”, also Type 55, -65C to +150C but ETFE, dual wall as normal for avionics work.
It looks like the two types are sold interchangeably, and maybe a lot of the “aviation supplier” wire is Polyolefin, not ETFE – looking at the temp range.
Doing a bit of googling suggests that it is ETFE that might release the wire-degrading acid, and not the cheaper stuff.
Looking at “aviation” suppliers, you get e.g. this “CAA approved cable” (which probably means exactly nothing – what exactly does the CAA “approve” on a wire?)
Most installation manuals nowadays call out for MIL 22759 for single core and MIL 27500 for multi core shielded wiring.
A lot of “commercial” hook-up wire will generate smoke and contribute to fire. That is not allowed for aviation.
Within the mil specs there are different conductors, for example silver plated copper and nickel plated copper.
Also some older wire is no longer acceptable, such as MIL 5086 wire, as it produces toxics fumes when on fire.
The wire I am talking about is the proper avionics wire. The Raychem Type 55 data sheet cross-references to MIL 22759.
MIL 22759 is the same as Raychem Type 55, so this takes us back to the same stuff which “goes off” after maybe 10 years.
One would not use normal (PVC) wire for avionics – its max temp is about +80C. It’s rubbish. I don’t use it for anything serious; not even for test equipment.
I don’t either, but some do… An example is the Sigtronics SPA-400 with poor quality wire.
I don’t agree that you could say MIL 22759 is Raychem 55, as that seems to indicate that all MIL 22759 is Raychem 55, which is not true.
Raychem 55 cables meet MIL 22759 requirements. There are different manufactures, who make wiring that also meets MIL 22759 and that is NOT equal to Raychem 55.
You could have PVDF, PTFE, FEP, TFE isolation for example, this could be from resin or could also be extruded. As conductor you can have copper, tinned copper, silver plated copper or nickel plated copper which could all meet MIL 22759. Note that nickel plated copper wire doesn’t have to be solderable, while being a MIL 22759 cable.
Therefor it could be that this solder problems only occur only with manufacture A, while B doesn’t have any corrosion problems. Solder problems could also be caused by the plating of the copper wire’s. I have seen this issues just a few times. It might be only with one type cable, or only with one manufacturer.
I agree… I wonder if there is any data on this issue? Raychem cable from from TE Connectivity which often appears under the same heading at Tyco. The TE data sheets mention Raychem under their name so I guess TE bought Raychem.
However, thus far, I have not seen any of the common “white avionics” cable which does not have this problem after some years.
Incidentally is there a requirement for double wall insulation – particularly for the type of double wall where the inner wall is a contrasting colour so mechanical damage can be readily seen? I don’t see a requirement for this in MIL 22759 and only some Type 55 wire has a double wall. Very little new wire has the two-colour wall.
I don’t know about any data. When working on large aircraft I know quite some dual isolation wiring was used. They also had seperate stripper tools for dual isolation and single isolation. In general aviation I haven’t seen much single core dual isolated wiring to be honest. The problems I had was with older wiring (the wiring that hasn’t got smooth isolation surface).
The dual isolation wiring isn’t that same as that wire, and at that company I worked back then, didn’t allow soldering, so wouldn’t have found out then either. Besides most wiring was installed as new wiring.
Most manuals just call out MIL 22759, and a cauge. It doesn’t state which variant to be used. I normally use 22759/16, and I use colored wires as well. Most installers only use the standard white coloring.
As I haven’t seen solderability problem it is like that it is manufacturer / variant depended I guess.
“Cheap” GA aviation type 22759 cables are much lower quality than compared to Raychem 44 55 series…
I have noticed (in rare occasions) that the “GA cable” type may melt in some strange way when using a soldering gun (solder sleeves) also the insulation specification is not exact.. some shippments are ok and others not ie hard to strip. But we still use it beacuse it’s overal performance is acceptable – but if I would do an installation on a space craft Id go for something else.
I found this good document from NASA detailing the 27500…MIL spec..why go for less..?
from aircraft spruce…MIL-C-27500 & replaces MIL-C-7078/3. It incorporates MIL-W-22759/18 basic wire and features a Tefzel insulation, a shield of tin- plated copper braid and a jacket of Tefzel. It is temperature rated to 150°C & voltage rated at 600 volts. Top quality wire to military specs. 22-16 gauges sold in 10 ft increments only.
http://nepp.nasa.gov/docuploads/7A417E00-8C53-41FA-9E56670435AE573C/MIL-C-27500.pdf
