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Safety Wire ... It Can Save Your Life

By Jennifer Caron
Source: FAA Safety Briefing Nov/Dec 2020

aircraft maintenance

Loose hardware or components have led to accidents, many of them fatal. Properly secure aircraft components with safety wire, ensure that hardware locking mechanisms are correctly installed on your aircraft, and check them often to confirm they are taut and ready for flight.

Fasteners, Wires, and Fast Facts

Safety wiring, or positive wire locking, is a type of locking device used to secure (“safety”) cap screws, nuts, bolt heads, and turnbuckle barrels, which cannot be safetied by any other practical means. Safety wire is necessary in areas where vibration could loosen a bolt. Any tendency of the hardware to loosen is counteracted by the tightening of the wire. Used properly, it will lock so that the wire remains taut and prevents further movement.

Keep it Locked

While there are several methods of safetying aircraft parts, two other basic methods include cotter pins and self-locking nuts.

  • Cotter pins: used for securing bolts, screws, nuts, and pins on aircraft and engine controls, landing gear, and tailwheel assemblies, or any other point where a turning or actu-ating movement takes place. They should not be re-used.
  • Self-locking nuts: a type of nut that resists loosening when exposed to vibrations and torque, used in applications where they will not be removed often. Replace them when they’re no longer maintaining minimum prevailing torque, since repeated removal and installation will cause the self-locking nut to lose its locking feature.

Tips to Keep Things Tight:

  • If there is a hole in a bolt ... it likely requires a cotter pin or safety wire in it. Be sure to ask.

hole in a bolt

  • When installing or inspecting safety wire, ask yourself, does it pass the “Righty-Tighty” test (i.e., ensure that the safety wire is applied so that it is in tension in the tightening direction — the direction that would cause the bolt to “Tighty”).

bolt tightening

  • Safety wire should be tight and maintain a light tension when secured. You should notice about six to eight twists per inch with a good safety wire job. 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. Always remove all old safety wire before installing new.
  • When inspecting fiber or nylon locknuts, make sure the bolt or stud extends at least the full round or chamfer through the nut. Flat end bolts, studs, or screws should extend at least 1⁄32 inch (or 1 and 1/2 threads) through the nut.
  • Castle nuts require a cotter pin or safety wire to lock them down. Turnbuckles should either have safety clips or safety wire.

If you’re doing owner-performed maintenance, make sure that you know what you’re doing, and get a second set of eyes to look at your work after you’re done. If you have any doubts, ask your mechanic, get some advice from your Experimental Aircraft Association (EAA) Chapter, or talk to your local FAA Flight Standards District Office inspectors — they are more than willing to help.

There are many other parts that require safety wire or other means of locking. FAA Advisory Circular AC 43.13-1B outlines the various locking methods and the proper safety wiring procedures.

Jennifer Caron is FAA Safety Briefing’s copy editor and quality assurance lead. She is a certified technical writer-editor in the FAA’s Flight Standards Service.

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