Safety wire or lockwire is a type of positive locking device that prevents fasteners from loosening or 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.
The word safetying is a term 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.”
Wire, either soft brass or steel is used on cylinder studs, control cable turnbuckles, and engine accessory attaching bolts.
Do not use stainless steel, monel, carbon steel, or aluminum alloy safety wire to secure emergency mechanisms such as switch handles, guards covering handles used on exits, fire extinguishers, emergency gear releases, or other emergency equipment. Some existing structural equipment or safety-of-flight emergency devices require copper or brass safety wire (.020 inch diameter only). Where successful emergency operation of this equipment is dependent on shearing or breaking of the safety wire, particular care should be used to ensure that safetying does not prevent emergency operation.
Copper wire (.020 inch diameter), aluminum wire (.031 inch diameter), or other similar wire called for in specific technical manuals, should be used as seals on equipment such as first-aid kits, portable fire extinguishers, emergency valves, or oxygen regulators.
When installing safety wire always consult the maintenance manual. Be sure to use the size and material wire specified in the appropriate maintenance manual.
Safety wire twisting can be done with a pair of duckbill pliers or one of the reversible safety wire twisting tools makes the job much faster and more uniform. There is a small problem using pliers to install safety wire and that is nicks and damage to the wire or pulling it too tight. For this reason some airlines such as must Japanese airlines require all safety wire to be install using the hand
Safety wire must never be overstressed. Safety wire will break under vibrations if twisted too tightly. Safety wire must be pulled taut when being twisted, and maintain a light tension when secured. Safety wire will break under vibrations if twisted too tightly. Often you will see this on small General Aviation aircraft oil filters where the safety wire had cut into the oil filters before breaking.
Be sure that the twists are tight (taut) and even, and the twisted wire between the fasteners is taut but not too tight. The recommended number of twists per inch depend upon the diameter of the wire. In AC 43.13-1B figure 7-3a shows 6 to 8 twists per inch by hand.
However, 6 to 8 twists per inch by hand may not be appropriate of all safety wire. The recommended number twists per inch depends upon the diameter of the wire according to Aviation Mechanic Handbook, third edition by Dale Crane as follows:
Wire diameter Twists per inch
0.0200 - 0.025 8 – 14
0.032 – 0.041 6 – 11
0.051 – 0.060 4 -9
I have researched several FAA Handbooks and manuals such as FAA-8083-30 General chapter 5 that talks about safety wire, no mention of twists per inch. Only AC 43.13-1B figure 7.3a indicates 6 to 8 twists per inch.
FIGURE 7-3a. Wire twisting by hand.
I know from experience installing propeller bolt safety wire it may not be possible to obtain 6 twist per inch between the propeller bolts and the 4 twists per inch would seem to be more appropriate in this installation using 0.051 wire.
For those that will be taking the FAA written test and oral and practical tests the correct answer is 6 to 8 twists per inch in accordance with AC 43.13-1B.
Beside mechanics being allowed to install safety wire Part 43 Appendix A (c)(5) Preventive maintenance allows pilots to replace defective safety wiring or cotter keys provided it does not involve complex assembly operations. What this means is a pilot legally cannot cut safety wire. However, if the safety wire was defective they the pilot could legally replace it, such as when they change the oil filters during an oil change.
So as the old saying goes the safety wire must have been defective at the oil change allowing the owner cut the safety wire for a new filer to be installed and properly safety wired in place of course with a record entry as preventive maintenance.