Jim, wire you insulate?
What Cyndi generally asks me when I come in at 2300 after “working on the airplane” late into the evening with a few of my beer-buddies. Groan.
Seriously, the first recorded two-way conversation air-ground was in 1912 by the RAF using the newfangled Morse code. The “key” in the aircraft resembled a reverse clothespin with thumbtacks as contacts. History does not record the type of wire or insulation but I think I’m on safe ground to report the wire wasn’t Tefzel.
As a matter of fact, the first reliable sources I could find in the library reported in 1939 that “rubber” was used as the insulation. And, as we found out early in the electrics game, although silver was proven to be the “best” conductor known to science, copper was the second and overwhelming choice known to people wiring automobiles, homes and (much later) aircraft.
Later experiments came to the conclusion that braided cotton could be substituted for the much heavier rubber, but both cotton and rubber suffered from a fatal disease called fire. Both of them burned quite nicely once ignited.
Enter a 1940s invention called nylon. Nylon was the DuPont miracle invention that made plastic toothbrushes, water glasses and sheer covering for young ladies’ legs called “nylons.”
Later experiments in nylon fabrication in the early 1940s showed that extruded nylon could be used to cover cotton wiring, and thus the flaming cotton with a nylon sheath (called DCC) was the wire of choice for the tens of thousands of bombers, fighters, ships, tanks and other war machines that used DCC wire in their construction. Nylon didn’t burn, but it did melt.
(The war machines consumed most of the nylon production of the country, much to the consternation of the young ladies of the era who couldn’t find nylons for their lower extremities at any price. Young soldiers, being the remarkable folks they are, could every now and again score a pair of nylons, with which I’m sure they impressed their ladies.)
But I digress. DCC was the standard for civilian aircraft until the 1960s when a plethora of these new plastics came aboard, vinyl being prime. And amazingly enough 78 rpm ceramic music records of the era gave way to the vinyl LP 33 rpm records.
In this era of the ’60s, several versions of vinyl were attempted, but all of them failed in one way or another until some remarkable chemist mixed a mixture of several versions of vinyl and then treated them with common table salt. They called it poly (the Greek word for many) vinyl chloride (common table salt is sodium chloride) and came up with polyvinyl chloride, soon shortened to PVC.
PVC soon became all the rage for wires—home, automotive, electronic instruments and airplanes. It wasn’t perfect, but it was better than anything else on the market at the time. It was difficult to strip; if you didn’t do it just right you got mostly insulation but a long wispy strip of PVC that you had to cut with wire cutters. After a while in an aviation environment that could get fairly warm and uncomfortably cold, it could develop cracks that slowly developed into chunks of the plastic falling off of the wire. Worse, if exposed to flame, it “gooped up”—that is, it melted into a large sticky mess of insulation that left some of the inside metal wires exposed to touch each other that exacerbated the problem into a bigger fire and so on.
Try as I might to research the genius who invented today’s version of PVC, I failed. My sense is that it was an internal corporate effort and that the actual inventor (or group of inventors) may well never be known. At any rate, some brilliant group of inventors decided to run some PVC through a radioactive beam and, lo and behold, an irradiated PVC came out the other end. It didn’t goop. It didn’t coagulate. It did char but the char didn’t come off the wire in chunks, but stuck together.
The bottom line after all this line of invention is a popular aviation wire of today, MIL-W-16878, also called UL-1061 (Underwriter’s Labs) in the “real world.”
So, along comes the space race, and we found that if we chemically jammed a few fluorine atoms into the PVC plastic, we got polytetrafluoroethylene, which the inventor folks at Dow Chemical called Teflon. Having worked with Teflon wire in my time in the space labs, I can attest to the fact that it was the witch of the bunch to strip, but you couldn’t ding it with a hammer or burn it with a torch.
As with all good inventions with a bad habit, jamming a few more atoms hither and yon turned Teflon into a well-behaved ethylene-fluorocarbon polymer that Dow nicknamed Tefzel (MIL-W-22759). It strips decently, is very impervious to flame and generally is a well-behaved competitor to I-PVC for general aircraft wiring.
Tefzel or I-PVC, you still have to handle interconnects between wires other than with large expensive “twist lock” connectors. Especially with homebuilt aircraft, the capability of being able to remove one piece of electrical equipment and install another without completely rewiring the aircraft is a necessity. Born out of the requirement for a relatively simple way of changing avionics around during WW-II and thereafter, the lowly terminal strip has evolved from a large clunky hunk of pressed paper, rivnuts and such to today’s Bakelite and inlaid nuts in all screw sizes from #2 to huge. Crimp terminals also have evolved to a rather stylish combination of plated copper terminals with a PVC insulator.
So much for the history lesson. Over the years, I’ve worked out a few solutions to common wiring problems—meaning the physical wiring, not which pin is the push-to-talk. Here are a few of them.
Trick #1. Crimp terminals stop at #22 wire, but #24 is common for very low power and signal wires in the aircraft. Trying to crimp a #24 wire into a #22 terminal is a recipe for failure unless you use an old-timer’s trick. Strip the #24 wire to the length of the insulator. Tin the wire. Bend the tinned wire back over the wire insulation. Now spiral the tinned wire over the wire insulation, insert the wire into the terminal until the wire is just visible at the end of the insulation. Now crimp the spiraled wire into the terminal.
Trick #2. Trying to connect two crimped terminals to one terminal strip screw. Flip one of the terminals over so that the ring part of one terminal lies flat against the ring terminal of the other terminal. Now put the screw through this assembly and fasten.
Trick #3. Trying to connect a lot of wires to the same circuit. Generally, this involves trying to connect a lot of wires to the +12 power supply and also to ground. Daisy chain the +12 wires to adjoining terminals on one side of the terminal strip and then connect the equipment, one or two wires at a time, to the opposite side of the strip. Same trick for all of the ground wires.
So much for nearly all I know about wiring. Let me surprise you with my next column, and until then, stay tuned…
