Safety Wire Install A and P license

Installing safety wire on hardware is second nature for most A&P licensed mechanics.        The purpose of purpose of a safety or lockwiring is to ensure that the hardware will not come loose during operation.  AC43.13 defines safeting as;

“Securing by various means any nut,bolt turnbuckle etc., on the aircraft so that vibration will not cause it to loosen during operation.”

Safetying is a general term that covers,

  • Safety wife or lock wire
  • Cotter Pins
  • Self locking nuts
  • Locking tab washers


I will focus on safety wiring and cotter pins for this article since it is probably the most common safety installed on aircraft by mechanics.   Safety wire comes in 3 basic sizes.  .032 which is most common, .040 heavy-duty stuff and .020 for those intricate jobs.  There is also copper safety wire which is used to show whether or not a safety device has been actuated.  For instance, some guarded switches have copper safety wire which can be broken easily.  The broken wire indicates that the switch may have been actuated.

Unless called out specifically by the maintenance manual, use the wife that takes up approx 75% of the hole in the bolt or screw.

The double twist method is the most common use of safety wire for most mechanics.  Here are some examples;

A and P license Safety
Tight twists make a good safety
A and P license Guide
BK117 Mast
A and P license lockwire AC43.13
AC43.13 chapter 7 – click to enlarge
A and P license lockwire picture
Avoid gaps in the twists – click to enlarge

Key things to remember while installing a safety.

  • The safety must be installed so that it has the tendency to tighten the bolt.  We call this being “positive”.  A “negative” safety has the tendency to loosen the bolt which is bad.
  • Try to create 6-8 twists per inch.
  • Keep the distances short as possible, if proper alignment cannot be achieved, washers may be added or get another bolt.  It is rare but it happens sometimes.
  • Make a nice pig tail at the end, curve it downward so that you don’t cut your hand next time you’re in the area.



Split Pins or Cotter Pins, You decide

Cotter pin installation should also be neat and orderly.  My favorite type of installation is pictured here.

A and P license cotter pin
Nice and Neat


A and P license cotter pin

Sexy cotter pin

This method is preferred by me because it holds the cotter pin tight.  There is no looseness like sometimes you get with the typical installation.  There is also very little chance of cutting your hand on the end of the cotter pin also.  The end of the split pin gets folded back into a castellation.

A and P license Cotter Pin installation aircraft mechanic

Both ends get folded back and the tips placed between the castellations.

Final Thoughts

Safeties provide redundancies to aircraft hardware to keep them from vibrating loose.  Pride should always be taken while installing safeties.  These safeties are normally on the exterior or the components and therefore in plain view.     If the safety looks like crap, even if it is installed to the “standard” then it reflects poorly on the mechanic installing it.   I know there are many safeties that are near impossible to make look good and I am not talking about those situations here.  If a pilot or another mechanic looks at the poor safety on the outside, they will probably think that the rest of the job was also poorly done.    You can overhaul an engine perfectly but if the safeties on the exterior look shabby, people will start to question the workmanship.   We as aircraft mechanics take pride in what we do because we enjoy doing it.  That is what creates a safer environment  for everyone.

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