thumb|upright=1.35|Safety wiring of two pairs of drilled bolt-heads wired one-to-the-other
A safety wire or locking-wire is a type of positive locking device that prevents fasteners from falling out due to vibration and other forces. The presence of safety wiring may also serve to indicate that the fasteners have been properly tightened.
Safety wire is available in a variety of gauges and materials, depending on the application. In aircraft and racing applications, stainless steel wire is used, such as in diameter. Typically, the wire is threaded through a hole drilled into a fastener or part, then twisted and anchored to a second fastener or part, then twisted again.
Application
Principle
There are a few techniques for different applications. The word safetying is universally used in the aircraft industry. Briefly, safetying is defined as: "Securing by various means any nut, bolt, turnbuckle etc., on the aircraft so that vibration will not cause it to loosen during operation." These practices are not a means of obtaining or maintaining torque, rather a safety device to prevent the disengagement of screws, nuts, bolts, snap rings, oil caps, drain cocks, valves, and parts. The wire itself maintains tension and remains in place by being twisted around itself and attached to the fastener to be secured on one end and an anchor point (which could be another fastener) on the other end. Since safety wire is made of a malleable alloy, it retains its shape after being bent, rather than springing back to its original shape. This property allows it to remain locked around an object, such as when it is passed through a small hole on a fastener, looped back upon itself and then twisted. The same process is then repeated around the anchor point, which could be another fastener. Since it remains twisted instead of unraveling, it acts as a fixed loop and will not back out without considerable force (greater than the stresses which it is intended to counter) being applied.
thumb|A moused shackle
Mousing (pronounced ) is the application of a molly or safety wire, called mousing wire in this use, to secure a threaded clevis pin to a shackle. This is done by passing a couple of turns of mousing wire through the reach-hole provided for this purpose in the unthreaded end of the clevis pin and around the body of the shackle's hoop. Alternatively, some threaded shackles are provided with a hole through the threaded end of the pin beyond where it emerges from the threaded hole. A cotter pin or a couple of loops of mousing wire through this hole serves the same purpose and secures the shackle in a closed position. Nylon zip-ties are also commonly used in applications where the shackle must be secured but easy removal is required.
Use
A safety wire is used to ensure proper security for a fastener. The wire needed is long enough to reach from a fixed location to a hole in the removable fastener, such as a pin — a clevis fastener, sometimes a linchpin or hitch-pin through a clevis yoke for instance — and the wire pulled back upon itself, parallel to its other end, then twisted, a single end inserted through a fastener, and twisted again, possibly then anchored to a second fastener or other part, then twisted once again, having excess slack pulled relatively taut to be secure. The two ends of the wire-loop thus formed are joined by twisting them together with a tool, using enough twists to be secure, then released from the twisting tool. The removable fastener — possibly a nut, wing nut, turnbuckle, a bolt or a pin similar to a bolt — having a hole through a part of it that will remain accessible when it is fastened in place will be secured with the wire passing through it. When finished, any excess length of wire would be cut off with a pair of wire cutters, such as pliers that may, also, be the twisting tool. If the fastener part to be secured does not come with a hole for the safety wire, one may need to be drilled. MS33540, and FAA AC 43.13-1B identifies only 6 to 8 twists per inch (about 6 to 4 mm pitch). 43.13-1B has no other reference to twists per inch either by hand twisting with special tools. Aviation Mechanic Handbook identifies different twists per inch as at 8–14 twists per inch (~3-2 mm pitch), at 6–11 twists per inch (~4 to 2 mm pitch), and at 4–8 twists per inch (~6-3 mm pitch). The safety wire should be threaded through the object fastener such that it creates tension in the opposite direction of the fastener's removal. For example, if a standard automotive bolt in the U.S. is being secured, then the safety wire when installed should put tension on the bolt in a clockwise direction, since that is the direction that the bolt turns to tighten.
When drilling a fastener, the choice of where to drill it depends on the type of fastener and to what it will be wired. The alternative to drilling holes in fasteners is to use safety wire tabs (see Safety wire tabs section below), or to purchase pre-drilled fasteners.
AC43.13-1B, par. 7-124 f. page 7-21 specifies "Safety wire ends must be bent under and inward toward the part to avoid sharp or projecting ends, which might present a safety hazard." The first picture in this article does not conform to proper techniques and practices and ought to be replaced with a better example.
Witness wire
A more simplistic application of safety wire, more commonly referred to as witness wiring, is the use of light gauge, single strand, copper wire to provide positive visual confirmation of the security or closure of specific equipment within the aerospace industry. Common applications include the security of safety equipment such as fire extinguishers, and safety equipment bags, but also as an assurance that critical system switch covers remain in place, such as those associated with the application of fire suppression, or ejection systems. This application of witness wires is widely varied, and may cover a broad range of types of equipment and numerous situations.
Witness wires also serve the purpose of providing a rapid method for ensuring critical safety equipment or systems have not been used or tampered with since their last repair, reset, or inspection, and also that the container of such equipment has not been inadvertently opened, disturbed or tampered with, therefore providing confidence in their readiness for use. In a similar manner, critical system switch covers are protected from inadvertent activation, through the application of witness wire.
The gauge of copper wire utilized in this application is such that the wire security can be overcome with minimal breaking force by hand, without damage to the equipment or persons, and once broken, remains affixed to the equipment without the introduction of foreign object damage (FOD).
Typically the wire is threaded through existing holes in the associated equipment, using a single strand loop, and a single crossover, such that the closure is secured without impedance to the normal functioning of the equipment. The single crossover provides the appropriate friction such that the wire cannot fall completely free of the equipment when broken. The loose ends of the strand may be twisted in a pigtail fashion, or crimped with a lead seal, securing both strands as close to the closure as practical. In each case, the loose ends of the strands should be tucked neatly away from inadvertent impact.
Equipment
thumb|upright|right|A one-pound spool of stainless steel safety wire
Safety wire
Safety wire is commonly in diameter, but diameters are also available. It is usually made of stainless steel, but is also available in monel and inconel alloys for high temperature applications and copper for break-away applications.