Aviation Maintenance Safety Methods


The process of securing all of the nuts, bolts, screws and fastners on an aircraft is known as safetying. The proper use and method of safetying helps ensure that the aircraft hardware does not become loose from vibration forces. It is necessary that you become familiar with the various methods and means to secure this hardware.

Depending on the part or hardware, used in a given installation, will determine the method of safetying those parts to ensure their security. The most common methods of safetying items is through the use of cotter pins, safety wire, self-locking nuts and special fastners, lock washers, snap rings and safety clips.

Safety Wire

The utilization of safety wire is the most positive and accepted method of securing hardwarewhich cannot be secured by any other method. Safety wire is used to secure two or more hardware pieces in such a way that any tendency of one of the pieces to lossen will have a reciprical effect (of tightening) on the other piece of hardware.

The most common sizes of safety wire are .020, .032, .040 inch diameter. The wire is made from extruded stainless steel which provides adequate strength and workability and is compatible with a wide variety of materials from a corrosive standpoint. Safety wire is also made from copper for use on emergency levers and switch covers. This method secures the control from inadvertant activation and yet allows the operator to break the safety when the employment is needed. Copper safety wire should be replaced periodically because the copper may work harden or age harden over time. Another spacialty type of safety wire is made from alumel and is found in aircraft with turbine engine installations and is sometimes used as the resistance wire for the engine tempurature indicating system. Aluminum and zinc plated steel wire should not be used in aviation applications.

Safety wire techniques vary depending on the item being secured, however, what does not change is the direction of pull. All safeties with safety wire are tied in such a way so as to pull the hardware in a tighening direction.

To see the various acceptable methods of employing safety wire, please refer to publication AC43.13-1.

The two most common methods of employing safety wire is through either the single wrap or double wrap method.

The single wrap method when used on bolst or screws is simply feeding one end of the wire through the hole or holes in the hardware and continuing the process all the way around the component, ensuring a positive direction of pull between adjacent fastners. The safety is completed by securing the two ends by double wrapping, cutting off the excess wire and bending the end in (to protect people from injury).

The double wrap method is a method of twisting the wire, esstantially braiding the wire, to sceure two or more components. Cut a length of wire of the appropriate size and pass through one of the holes in the hardware or component till you reach the approximate mid-point of the wire. Pay attention to the direction of pull for the fastners so you can stratagize how the wire be routed between the two points. With our wire at the mid point, we will have a left and right hand length of wire in front of us. Put a slight amount of tension on the wire so that it is not slack and cross the left over the right at approximately a 45 degree angle. Congratulations- you just completed one twist. Continue the process of crossing left over right until you reach the hole on the adjacent fastner or component. Pass the nearest wire through the hole and then apply tension, as you did before, and hold. Apply tension to the opposite length, which we will say is now in our left hand, and pass around the perimeter of the fastner head to where the two ends meet at the outlet of the hole. Now this time cross right over left to secure the two lengths. If doing a safety in pairs we can secure, cut annd secure the end of the wire, if we have a three-way safety just continue right over left to the next point- pass the wire through and around and then switch back to left over right to scure the tail (pigtail) of the wire.

The alternating method of twisting the wire helps secure the wire closely to the hardware so there is little or no slock in the wire. If the wire is too slack it can pop over the head of the fastner and will not be effective in securing your hardware.

Note: The brief description just given is for hardware with right hand threads.

Oil caps, valves, draincocks, electrical connectors can all be secured using the double wrap method of security.

Turnbuckels can and must be secured either with safety wire or clips. If using safety wire please refer to AC 43.13-1 for reference and very clear pictures of how to secure turnbuckles with their various end fittings. Safetying turnbuckles is not difficult but will take some practice, if you would rather slide in a couple of clips they are part number NAS 651.

One more point on turnbuckles and then we will move on. There is some very specific guidelines the FAA anad most other aviation regulatory agencies have established for safetying control cables, be sure to use the proper data when selecting the appropriate size diameter and material type of safety wire before you start.

Here are some general guidelines when installing or inspecting safety wire installation.

a. A pigtail of three to six twists or 1/4 to 1/2 inch should be made to secure the end of the wiring. The pigtail must be bent back or under to prevent it from becoming a snag or injuring yourself or someone else.

b. Safety wire can never be reused. When cut off a new piece must be put in its place.

c. It is not acceptable to unwrap a safety and attempt to wrap in the opposite direction. If the wrap is wrong, loose or ill fitting, remove the wire and give it another attempt intil it's correct.

d. When castellated nuts are to be secured with safety wire, use the low side of the torque range (unless otherwise specified) and turn until the next slot aligns with the hole.

e. The wire must be applied so that the pull is in the direction so as to tighten the fastner.

f. Twists should be tight and even, and the wire between the fastners as taut as possible without overtwisting.

g. All safety wire must be tight after installation, but not under such tension that normal handling or vibration will break the wire.

h. Safety wire should always be installed and twisted so that the loop around the head stays down and does not tend to come up over the bolt head, causing a slack safety wire installation.

i. The wire should be of a diameter to fill approximately 2/3 of the hole diameter unless otherwise called out.

Cotter Pin Safetying

The FAA recognizes one preferred method of installing cotter pins which we will cover here. There are some optional methods when space does not allow for the preferred method and those optional methods can be found in AC43.13-1.

First and foremost the cotter pin must be of the correct size. If not called out, a good rule of thumb is the 2/3rds of the hole diameter but in any case the cotter pin should fit neatly into the hole, with very little side play.

If you look closely at a cotter pin you will notice it has a long leg and a shorter leg. The long leg is intended to go over the top of the bolt or fastener and the short leg along-side the nut.

1. Insert the cotter pin with the long leg on top.

2. Seat the bulbed end of the cotter pin securely into the nut and bolt.

3. Using duck-billed or needle nosed pliers, grab hold of the upper leg and in one fluid motion bend to a vertical position.

4. Using side-angle cutters (dikes- to most of us) trim the short leg so as not to cut into the structure when bent.

5. Bend the short leg down vertically along-side the nut.

6. Using your dikes trim the remaining long leg so it will lay 1/2 to 3/4 across the end of the protruding bolt or fastener.

7. Bend the remaining leg over top of the bolt or fastnener ensuring it is flush so as not to become a snag.

Here are some general rules that can help you in installing and inspecting your cotter pin installation.

a. The prong leg bent down should not rest against the surface structure or washer.

b. Never reuse a removed cotter pin, use a new cotter pin at each installation.

c. If you mess up on the installation, remove the cotter pin and give it another go until it is installed correctly.

d. The prong leg bent over the bolt end should not extend beyond the bolt diameter. This can cause a snag and injure you or someone else.

e. All prongs should be bent over a reasonable radius. Sharp-angled bends can cause breakage. Lightly tapping with a light mallet can assist you in bending the prong legs on larger diameter cotter pins.

Other Safety Devices

Items such as safety clips ( other than those used on turnbuckles) and special fastners are pretty straight foreward as to how they secure the item properly but be sure to consult the manufacturers data and installation instructions to prevent damage to the fastener or adjoining component or structure.