The knob is pulled and voilà — the safety wires are twisted
One of the puzzles that faced the 23 students in the May 1971 airframe- and-powerplant technician class at Northrop-Rice Aviation Institute of Technology in Inglewood, California, was a simple wooden box. Inside were two lengths of primary control cable connected together with a turnbuckle.
Every student did battle with the box. For the few who had experience with the poke, wrap, crossover, and wrap steps required to weave two strands of safety wire around an aircraft turnbuckle (sometimes called a turn barrel), completion of the exercise didn't present a problem. For others, the steps involved in applying a double-wrap safety to the turnbuckle in the box were frustrating and time consuming — the task had to be accomplished entirely by feel since the only access to the turnbuckle was through left- and right-hand holes in the sides of the box.
Aircraft owners who take on safety-wiring — occasionally referred to as lock-wiring — tasks on their airplanes usually have the advantage of being able to see what they're doing. Safety wiring is a critical part of maintenance safety. With that as a mandate, let's look at some safety-wiring basics.
What do I need to safety?
"Lock or safety all bolts and/or nuts, except self-locking nuts. Do not reuse cotter pins or safety wire," reads paragraph 38 in chapter 7 of an advisory circular on maintenance. Eleven pages later at paragraph 122, an eight-page section devoted to safety wiring starts. In this section, 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." Farther on, there's an additional five-page section on turnbuckle safetying methods.
The paragraphs referred to are in Advisory Circular (AC) 43.13-1B/2B, dated September 1998. The title is "Acceptable Methods, Techniques, and Practices — Aircraft Inspection and Repair." This advisory circular is a book of acceptable aircraft maintenance guidelines and is a must have for every maintenance-minded airplane owner. Copies are for sale at almost all aviation supply stores or online ( www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/ 99C827DB9BAAC81B86256B4500596C4E?OpenDocument).
You — as a certificated pilot — can install safety wire on an airplane you own or operate, provided the airplane is not operated under Part 121, 129, or 135. The regulations say so in a list of tasks that are approved under preventive maintenance in Appendix A of Part 43 of the federal aviation regulations — it's time to clarify this 43 business because it's easy to get confused. The advisory circular is AC 43.13-1B/2B, but in the previous sentence "Part 43" refers to the regulation that is titled "Maintenance, Preventive Maintenance, Rebuilding, and Alteration." Part 43 is regulatory while AC 43 is advisory.
Appendix A of Part 43 contains a list of preventive-maintenance items that certificated pilots legally can perform. One of the items in that appendix says, "Replacing defective safety wire or cotter keys." Another owner-approved task listed therein is "cleaning or replacing fuel and oil strainers or filter elements." These tasks definitely require the owner to install new safety wire. So let's learn how to do it right.
The three sizes of wire
At one time brass wire was used to safety items on airplanes, and this wire — as well as Monel and "K" Monel wires — is still listed in a table in the turnbuckle-safetying section of AC 43. Mainly because of the extra expense — brass wire is three times more expensive than same-size stainless wire — and standard shop practices, commercial maintenance shops don't use brass wire anymore. However, it's still available for those who are interested in authenticity with antique or historic aircraft. Modern safety wire is made of soft stainless steel and typically comes in three sizes — 0.020 (20-thousandths of an inch) safeties electrical plugs; 0.040 or 0.041 (most often referred to as 40-thousandths) almost exclusively safeties propeller mounting bolts and turnbuckles; and 0.032 (32-thousandths) safeties virtually everything else.
One other type of wire may be seen on airplanes — soft copper wire (0.020-inch diameter only). Commonly called breakaway wire, it is used to prevent vibration problems with equipment that may be needed during abnormal operations. Breakaway wire is used for tasks such as securing fire-extinguisher safety pins, emergency landing-gear releases, escape-hatch actuating handles, and switch covers — it is always intended to be broken if the need arises.
Virtually all of the safetying on general aviation airplanes is accomplished with 32-thousandths wire. Like almost everything meant for installation on certified airplanes, safety wire is also manufactured to quality standards — look for either Type 302/304 or MS20995 as an assurance of aircraft- quality wire. All three sizes of stainless-steel safety wire are usually sold in one-pound cans or rolls — prices range from a low of about $6 to a high of $10, depending on where it's purchased.
Basics of safety wiring
Safety wire is not intended to take the place of proper installation of fasteners. Always ensure that fasteners are tightened to the proper torque first, and then install the safety wire. The tension of the safety wire always should tend to tighten the bolt, nut, or fastener.
Usually two fasteners are safetied together using the double-twist method. This consists of looping the wire through a drilled hole in the head of fastener A, pulling the two wire ends toward the next fastener (fastener B), and with the two wire ends spread apart approximately 60 to 80 degrees, beginning the first twist as close to the head of fastener A as possible. The two ends are then twisted continually until the twisted section occupies the space between fastener A and fastener B. The twisted section of the wires should have six to eight twists per inch. At fastener B one strand of the wire is inserted through a hole in the head of the fastener while the second strand is wrapped around the fastener head in such a way that it meets the through-head end. Then some tension is put on the two ends as they are given five or six twists. The excess wire is then cut off, leaving a pigtail of about one-half inch long. This tail must be tucked under and looped back toward the bolt. The effort should create a smooth, evenly twisted, taut wire connection that keeps the heads of both fasteners from rotating.
Failure to properly tuck and loop safety-wire pigtails is dastardly and is so serious that even the thought of working on or around poor safety wiring has prompted more than a few mechanics to suddenly decide it's time to pay a long-promised visit to Aunt Tilly rather than face a week of bloodletting. Experienced pilots bear the marks of their vocation with squinty eyes while longtime mechanics can be identified by the fine network of safety-wire scars on their hands and arms.
The oil-filter safety
Aircraft owners may change the engine oil and oil filters under the preventive-maintenance tasks. Let's use a filter change as lesson one in Safety Wiring 101. But first, a safety announcement. Used oil contains lead and acids that can be absorbed through the skin. Always wear hand protection when handling used oil.
Oil should be at operating temperature when draining. Ideally, this should be done after the engine has been flown for at least one-half hour. In a previous article (see " Airframe & Powerplant: The Unfiltered Truth," May Pilot) there were a couple of hints given to reduce the amount of oil that spills out of spin-on filters when they are removed. An alert AOPA member wrote in with one more — surround the filter with a one-quart plastic bag before starting to unscrew it off the mounting pad — if the bag is carefully held in place, oil will flow down into it instead of the engine compartment. The few extra minutes spent will eliminate a messy pool of oil that has to be cleaned up later.
After the new filter has been installed, with the correct gasket lubrication and seating torque, then the safetying can begin.
The needed tools
Successful safetying can be accomplished with a pair of diagonal-cutting pliers, called dikes. Used in at least half of general aviation maintenance tasks, all mechanics have at least one pair of dikes — experienced techs have two. The old hands have the pair they use and then they have their "loaner" pair, which, customarily, is completely worn out. It's easier to hand the loaner to the borrower — who can be an enthusiastic but toolless (clueless?) owner or another employee — than it is to explain why the good pair is never loaned. It's OK if the borrower thinks the tech's tools are worn out — perhaps he will be prompted to buy his own pair — at least the tech won't have to go buy a new pair when the borrower ruins the cutting edge by trying to cut through a stainless-steel hinge pin with the tech's favorite electric wire-cutting pair.
So the first tool needed is a pair of dikes. Too small a pair won't work for safetying because the small handles won't provide enough leverage to comfortably cut the wire. Too big of a pair will slow down the job because of access requirements. A good all-around pair of dikes is the 6-inch size. They're available at every hardware store in the country or can be ordered from aviation supply houses. Don't scrimp on the cost of your dikes — spend as if your mental health depends on getting the best pair you can afford.
The other tool needed is a pair of safety-wire pliers, which are specifically designed for cutting and twisting safety wire. They have three features that help to create a workmanlike safety-wire job — the clamping jaws are cutaway at a 30-degree angle, which permits a straight line pull on the two wire ends; there's a provision to lock the jaws onto the wire ends prior to twisting; and the pliers rotate around a coarsely threaded shaft that is attached to a single knob at the back of the pliers. After the wires are clamped in the jaws, the knob is pulled and voilà — the wires are twisted. Economy safety-wire pliers are available for $15 to $25. The really good ones cost about $75 and will give good service for decades.
Righty tighty, lefty loosey
Pilots have their mnemonics for kick-starting memorized procedures. A mechanic's favorite is "righty tighty, lefty loosey." It works for many tasks, and one of these is determining how to run safety wire.
One end of the wire loop is secured to another fastener or some other stable structure — most oil-filter safety wires will be threaded through a hole near the filter that's been drilled by the engine manufacturer in the engine accessory case. The next step is to determine which safety-wire hole in the end of the filter will best keep tension on the filter. After determining which hole is best for securing the filter, start with a couple of quick twists on the wire that has been threaded through the hole in the accessory case. Then pull the two wires to the selected filter hole, and clamp the safety-wire pliers on the wire at that point. Pull the knob on the end of the pliers until the wire is twisted at a rate of six to eight twists per inch. Within a few seconds you — and your new safety-wire pliers — will have created an evenly twisted pair of wires between the starting point and the filter.
Then all that's left is to put one of the two wires through the filter hole — there's a right wire and a wrong wire, and experimenting will show which wire gives the best-looking result — tucking the remaining wire into a twist on the other side of the filter hole and twisting the wire for a couple of inches while maintaining tension on the wires. Cut off the end and tuck and curl that end.
Safety-wiring skills are learned. Don't be afraid to cut off a bad job and start all over again — there's a lot of satisfaction in the creation of a workmanlike safety-wire job.
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