What is the maximum length of the wire when you are Safetying?

Airplanes undergo a lot of stress. From takeoff to landing and the times in between, there’s stress everywhere. Luckily, there are safety measures in place to ensure an airplane can do its job flight after flight. One such safety measure is small but mighty: safety wire. Also known as locking wire, it is an integral part of aviation maintenance and performance. But what is it and what does it do?

What is Safety Wire?

Safety wire, as the name suggests is wire. However, there’s a bit more to it than that. Safety wire is used to secure screws, nuts, bolts, studs, and more. These are things that cannot be safely secured by any other practical means. When used properly, the wire will prevent these fasteners from moving. Safety wire is a necessary precaution in areas of the aircraft that undergo a lot of vibration that could potentially cause the loosening of fasteners.

Safety wire is threaded through a hole drilled into the fastener. It is then twisted and anchored to either a second faster or another part, and then twisted again. These twists in the wire are what make it so secure. If done properly, any loosening of the fastener will be counteracted by the tightening of the wire. One way to make sure it’s been installed properly is to see if the twists pass the “righty tighty” test. Does the installed safety wire cause the bolt to tighten, or “tighty”? Then it’s been installed correctly!

However, you must be careful not to over-twist the safety wire. This can make it break more easily when additional stress is added to it. You also don’t want your safety wire to be under twisted either. In general, you want about 6 to 8 twists per inch.

The Different Types of Safety Wire

Because there are different mechanisms that need to be secured, there are different types of safety wire. The most common sizes for safety wire are 0.032 and 0.041 inches in diameter. However, thicker wire will be needed to hold larger screws in place. The most common material for safety wire is stainless steel. Stainless steel is used when you don’t want what it is securing to move.

However, there are times when you do want safety wire to be able to shear or break. For example, you’ll want copper or brass wire when securing emergency equipment such as switch handles, emergency exists, and fire extinguishers. This is because the successful operation of this emergency equipment is dependent on the safety wire being able to break. Copper and brass safety wire, while also more malleable than stainless steel, is usually only 0.015 to 0.020 inches in diameter.  

Safety wire should always be new upon each application since it is single use and disposable. If, upon inspection, you notice safety wire that is loose or missing, be sure to have it replaced as soon as possible.

Precautions to Keep in Mind Before Wiring

Before installing safety wire, you want to make sure that whatever it is securing has been properly assembled first. Safety wire is a failsafe against the vibration and loosening of fasteners, not a replacement for tightening those fasteners.

Make sure you are always using twisting pliers. These special pliers will allow you to grip the two loose ends of a piece of safety wire and twist it properly. However, keep in mind over-twisting will weaken the wire.

General Safety Wire Locking Rules

It is important to remember these rules the next time you are installing safety wire:

• Only use the safety wire once and make sure it isn’t damaged before installation.
• Safety wire should not be chafing or rubbing against any other parts. This can cause it to weaken or even break.
• The length of wire between two fasteners should be as short as possible.
• Fasteners with left-hand and right-hand threads need to be identified before the installation of safety wire.
• The double wire method is the most common and recommended, however the single wire method can be used in hard-to-reach areas.
• The maximum length of a wire should be no more than 23.62 inches, or 600mm.
• Safety wire should be used to secure no more than 3 items together.

Whether you’re doing your own maintenance or maintenance for someone else, it is always a good idea to get a second pair of eyes to look over your work. Installing safety wire is something that aviation maintenance technicians must practice until it becomes second nature. If you’d like to learn more about being an aviation maintenance technician and the different types of maintenance performed on aircraft, contact NCI today!

Safety wire locking is the method to lock the nut in place after tightening with the bolt using a lock wire. A piece of stainless steel wire passes through a hole in the nut and gives a lock wire load to the nut and other ends tighten to the body. So the nut will not get loose itself.

Safety wire locking is the most positive and satisfactory method of safetying cap screws, studs, nuts, bolt heads, and turnbuckle barrels, which cannot be safely tied by any other practical means. It is a method of wiring together two or more units in such a manner that any tendency of one to loosen is counteracted by the tightening of the wire.

What is Safety Wire

Safety wire is a wire used to prevent bolts, screws, nuts, and other types of fasteners from loosening. 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.

Lock Wire Material Specification

NASM20995(..)

• CRES – C
• MONEL – NC
• INCONEL – N
• Copper – CY

Safetying Emergency Equipment

For safetying emergency equipment, always use copper or brass wire.

Never use stainless steel, monel, carbon steel, or aluminum alloy safety lock 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 emergency devices for the safety of flight require copper or brass safety wire (0.015 to .020 inch diameter only), where the successful emergency operation of the equipment is dependent on shearing or breaking of the safety wire.

Particular care should be used to ensure that safetying does not prevent emergency operations.

Safety wire locking method

There are two methods of safety wiring –

1. Double wire method
2. Single wire method

The double wire method that is most commonly used.

Single wire method used on screws, bolts, and/or nuts in a closely-spaced or closed-geometrical pattern such as a triangle, square, rectangle, or circle. The single wire method may also be used on parts in electrical systems and in places that are difficult to reach.

When using the double wire twisting method of safety wiring, .032 inch minimum diameter wire should be used on parts that have a hole diameter larger than .045 inch. Safety wire of .020 inch diameter (double strand) may be used on parts having a nominal hole diameter between .045 and .062 inch with a spacing between parts of less than 2 inches.

When using the single wire method, the largest size wire that the hole will accommodate should be used. Copper wire (.020 inch diameter), aluminum wire (.031 inch diameter), or other similar wire called for in specific technical orders, should be used as seals on equipment such as first-aid kits, portable fire extinguishers, emergency valves, or oxygen regulators.

Importance of safety wire locking on aircraft

Safety wire locking can save your life and helps in aviation safety.

The lockwire is installed to put tension on the wire when the parts become loose.

Loose hardware or components have led to accidents, many of them fatal. Safety wire is necessary in areas where a bolt could loosen during vibration. Used properly, it will lock so that the wire remains taut and prevents further movement. Think of it as wearing a belt with your suspenders.

Safety wire is not intended to take the place of the proper installation of fasteners. Always make sure that the fasteners or components are tightened to the proper torque first, then install the safety wire.

Safety wire must never be overstressed. It 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 ends must be bent under and inward toward the part to avoid sharp or projecting ends, which might present a safety hazard.

General Safety Wire Locking Rules

When using the safety wire method of safetying, the following general rules should be followed:

• Use lock wire only one time and make sure that it is not damaged.
• When you install lock wire, make sure that there will not be chafing or fatigue. This is to prevent damage to the lock wire or adjacent parts. The causes of chafing or damage are vibrations or increased tension on the wire to prevent the loosening of the locked items.
• The length of the wire between the attachment points must be as short as possible.
• When you safetying with lock wire in the fuel tank areas, make sure that there is no protrusion of the wire ends:
  • For the single wire method, you must twist the wire end with protrusion back to the wire.
  • For the double wire method, you must bend the wire ends with protrusion back to the wire or the part that you lock (e.g. screw/nut).
• The line of approach of the lock wire must be approximately tangential to the parts that you will lock.
• When you install the lock wire, the strand that goes through the hole will tighten the locked item.
• It is important to identify items with left-hand and right-hand threads carefully before start wire locking.
• When you use an anchor lug, the wiring must start at the lug and the lug must align with the wire. The wire must not cause radial movement of the lug.
• The recommended method is the “double wire twisting method“.
• The maximum distance between fasteners should be 100 mm (3.94 in.).
• If the distance is more than 100 mm (3.94 in.) – use one more attachment point.
• The maximum length of the wire should be 600 mm (23.62 in.).
• You can safetying a maximum of 3 items together with a lock wire.

Precautions Before Wire Locking

• Make sure that all lock wire holes are drilled.
• Make sure that the assembly was torqued to the requirements of the related task.
• Use SPECIAL TWISTING PLIERS, to twist the lock wire clockwise at a maximum angle of 60° and Do not put too much tension on the wire.
• Keep the lock wire tight during the twisting, until the twisted part comes very near to the hole in the next unit.
• The direction of the wrap and the twisted strands must be such that the loop around the part comes below the strands that go out of the hole. Thus the loop will stay down and will not become loose.
• Twist the strands in the direction that will cause the lower lock wire to always be pulled down.
• A pigtail of 1⁄4 to 1⁄2 inch (three to six twists) should be made at the end of the wiring. Some aircraft manufacturer recommends that the length of the twisted part (pigtail) must be between 10 mm to 15 mm.
• If necessary, cut and discard the unwanted length of lock wire and you must collect the cut-off material to prevent damage to the aircraft.
• Bend the pigtail around the fastener head.
• When you safetying with lock wire in the fuel tank areas, make sure that you bend the wire ends with protrusion along the wires. You must also make sure that the wire ends stay near to the wire or the part that you lock.
• Make sure that there are no signs of damage (nicks, kinks) on the lock wire.

Use these types of wires for lock wire

• Use monel, inconel, or corrosion-resistant lock wire in high-temperature areas.
• Use copper wire, 0.015 in. (0.381 mm) in diameter, on emergency equipment only.
• Use Clad 5056 aluminum alloy wire for lock wire that touches magnesium to prevent galvanic corrosion.

NOTE: Use copper wire where a seal is necessary on emergency equipment to show no person has used the equipment. Emergency equipment includes portable fire extinguishers, first aid kits, emergency valves, and oxygen regulators.

Safety wire twister

A safety wire twister is a tool that allows the user to grip the two loose ends of a piece of safety wire in order to twist the safety wire.

Twist the wire with a wire twister as follows –

• Grip the wire in the jaws of the wire twister and slide the outer sleeve down with your thumb to lock the handles or lock the spring-loaded pin.
• Pull the knob, and the spiral rod spins and twists the wire.
• Squeeze handles together to release wire.

When using wire twisters, and the wire extends 3 inches beyond the jaws of the twisters, loosely wrap the wire around the pliers to prevent whipping and possible personal injury. Excessive twisting of the wire will weaken the wire.

Diameter of lock wire vs number of twists

• Less than 0.019 inch dia – 11 to 14 twist per inch
• 0.019 to 0.026 inch dia – 9 to 12 twist per inch
• 0.023 to 0.042 inch dia – 7 to 10 twist per inch
• 0.043 to 0.065 inch dia – 5 to 8 twist per inch
• More than 0.065 inch dia – 4 to 7 twist per inch