Working on the jet engines in the hangar at our Air Force technical school, one job came up again and again: tying parts together with steel wire.
We took two strands, ran them between two parts, and twisted them together, the way two ropes look when you twist them into one. Run from one part to the other, the twisted wire made a kind of S. At the end we used another tool to cut it, so it was not left too long.
We did this a lot, and it mattered. It is called safety wire, or lockwire.
Why a bolt undoes itself
A tightened bolt is held by friction in its threads and under its head. On a bench that is enough.
An engine is not a bench. It vibrates the whole time it runs, and every small shake lets the joint slip a tiny amount. Enough cycles of that and a fastener can work itself loose. On a jet engine a loose part is not an annoyance, and nobody can reach in during flight to tighten it.
So the wire is there as an extra measure, on top of the bolt being tight: whatever the vibration does, the parts stay where they were put.
The direction is the whole idea
The wire is not just tied on. It is routed so that it pulls each fastener in its tightening direction. If a bolt starts to turn loose, it has to pull against the wire, and the wire pulls it back.
That is why the path looks like an S between the two parts rather than a straight line. Each end wraps the way that tightens. Run it the other way and the wire does the opposite of its job: it helps the bolt undo.
Twisting the two strands together makes the wire stiff and strong enough to hold that tension, and it keeps the run neat between the parts.
Cutting it short
The last step is the one that looks least important: cutting off the end with a separate tool so the twisted tail is short. A long, sharp end of steel wire catches on gloves and hands, and in an engine it can catch on things that should not be touched.
It is a small, repetitive job. Done a few hundred times, it is also the clearest lesson I had in how machines fail: not all at once, but by small movements nobody was watching, until something designed in advance refuses to let them add up.