Locking Wire

Wire-locking (or Safetying as it is known in the USA) is the commonest form of locking in use throughout the aircraft industry. The wire is usually made of corrosion-resistant steel or heat-resistant nickel alloy. Fine copper wire is also used for some special locking operations.

The wire is normally classified by its diameter in increments of ‘Standard Wire Gauge’ (SWG) or ‘American Wire Gauge’ (AWG). The most usual gauge used is 22 SWG (or its American equivalent), although great care must be taken to check the correct wire gauge for each particular application.

Wire-locking is a positive method of securing items such as bolts, pipe unions, turnbuckles and nuts. Components designed to be wire-locked have holes in the appropriate positions to enable the lock wire to pass through.

When installing the wire, it should not span a distance of more than 75 mm without being supported. The wire is also positioned so that the item being locked will be restrained from turning in a loosening direction.

There should be approximately eight turns to every 25.4 mm length of wire and no length of more than 9.5 mm should be left untwisted. The angle of pull or approach (refer to figure below) should be not less than 45^o to the rotational axis.

When the wire has been passed through the last hole, the wire must be pulled tight and the twisting continued for at least 12-13 mm. The wire is then cut and the end doubled under, to prevent personnel getting snagged or badly cut.

FIGURE

Some forms of wire-locking are done with a single strand of the specified wire, especially in cases of where a complete ring or similar formation of nuts is found (refer to figure below). The wire is passed in sequence through the holes in their respective nuts and bolts (or screws) until the wire ends meet. Again the wire must be threaded, so that any tendency of a nut or bolt to attempt to slacken off, will add tension to the wire.

FIGURE

Use of Locking Wire with Turnbuckles

As with any threaded fastener, turnbuckles must be locked to prevent them from coming loose and jeopardising the control runs they are connecting.

There are a number of different types of wire-locking used on turnbuckles and the AMM must be consulted to find which method is specified. Methods used include the single wrap and single wrap spiral as well as the double wrap and double wrap spiral.

The single wrap and single wrap spiral use a single strand of the appropriate wire that passes through the hole in the centre of the turnbuckle, finishing up wrapped around each end. The single wrap spiral also uses a single piece of wire that is spiralled around the turnbuckle barrel and passed through the centre hole twice.

Two pieces of wire are used in the double wrap method, which are basically two single wraps, one in each direction. A double wrap spiral consists of two single wrap spirals, again one in each direction.

Use of Locking Wire with Locking Tabs

When locking tabs are used, they should be installed in such a way that the tabs and the wire are in complete alignment (refer to figure below). Whenever possible, the closed end of the wire should be in the tab and the twisted end at the component to be locked, although the exact method may be found in the AMM.

FIGURE

Thin Copper Wire

Thin copper wire is used to hold some switches and levers in a ‘set’ position, thus preventing the accidental operation of those switches which control certain critical systems such as emergency circuits.

When the switch is required to be operated, then a deliberate movement is made, which will break the copper wire and permit movement of the switch.

A secondary purpose of copper wire is as an indicator or ‘witness’, where a broken wire indicates that the switch or control has been operated. This method is employed on systems where it is necessary to know when a system has been operated (such as in a Fire Protection system).