Safety Wire Techniques
It's one of those things that every maintenance-minded operator should know how to do. In fact, it's the cornerstone of maintenance procedures as diverse as brake relining and oil-filter changing. But many non-mechanics are fearful of tying safety wires, because they don't know how or because they think they're not allowed to do it. AVweb is here to help.
You may not know how to tie a safety wire, or you may be convinced you can't do it unless you're an A&P mechanic. Legally speaking, you're on solid ground: The current FAR Part 43 (Appendix A) specifically allows pilots to replace defective safety-wire without apparent limitation (although we urge you not to touch turnbuckles or control-system items without supervision). But you can't just start wiring. The rules say you have to use practices acceptable to the Administrator: The rule is FAR 43.13(a); and the practices are outlined in AC 43.13-1B.
Section Seven of AC 43.13-1B (Safetying) covers all the aspects of general safety wiring practices, including several diagrams of more and more elaborate safety wiring applications. Section Eight (Control Cables and Turnbuckles) contains extensive additional safety wiring practices dedicated to turnbuckles.
AC 43.13-1B provides a wealth of data in so many fields of aviation that it should be considered a "must-have" book for anyone contemplating maintenance at any level. The price from the government is out of sight (unless you download all the PDF files here), but from commercial vendors like Aircraft Spruce & Specialty it's only $18.95 plus shipping.
Aircraft Spruce & Specialty's catalog has a number of other excellent aviation books, as well as information on just about any other area of interest to aircraft owners, including conversion tables. The catalog is free to U.S. residents, and you can order it online. It also is a must-have reference item for any owner/operator.
For most safety wiring you will need a specialized set of pliers and a spool of 0.32-inch stainless-steel safety wire. Fortunately, pliers are not expensive for the more basic models, in the neighborhood of $23. For the automatic, return-type models, plan on a better quality Milbar brand for $52.
For most owner needs the nine-inch model is fine, with six-inch and other specialized models also available for especially hard-to-reach spots. Aircraft Spruce and Specialty has a substantial pliers and wire selection in their catalog -- and at very competitive prices, I might add.
To do simple wiring tasks such as oil filters, just practice ahead of time at home using a vise to hold practice objects to get the basic feel of making twists with six to eight turns per inch. If you make more twists in the wire, then the wire work hardens and becomes very brittle, which makes it too easy to snap broken. Let the pliers do all the twisting work.
Remember to safety the wire from the oil filter to a tie point that will move in unison with the oil filter so the wire will not snap under engine movement or vibration. For example, if the oil filter is on the engine, you would not tie it to the firewall.
For single-point wiring such as an oil filter or oil screen plug, anyone can do it with a bit of practice. Multi-point connections with one continuous wire take substantially more practice. Don't be too proud to ask an A&P or other experienced person for some hands-on pointers.
Lastly, remember to make a "pigtail" at the end of the wire, because an exposed wire end acts like a knife to lacerate hands and arms later reaching into the engine compartment.
General Wiring Procedures
Procedures for installing safety wire include the following (with quotations from AC 43.13):
- Safety wire must be new upon each application. (This seems fairly obvious until you catch yourself trying to redo a short run using the same wire.)
- All safety wires "must be tight after installation, but not under such tension that normal handling or vibration will break the wire."
- Twists should be "tight and even, and the wire between nuts as taut as possible without over-twisting." (Six to eight turns per inch.)
- A "pigtail" of 1/4- to 1/2-inch (three to six twists) should be made at the end of the wire run. This pigtail "must be bent back or under to prevent it from becoming a snag."
- When castle nuts are safetied, "tighten the nut to the low side of the selected torque range, unless otherwise specified, and if necessary, continue tightening until a slot aligns with the hole."
- The wire must be applied "so that all pull exerted by the wire tends to tighten the nut."
- The wire should always "be installed and twisted so that the loop around the (bolt) head stays down and does not tend to come up causing a slack loop."
In addition to these mostly common-sense rules, you're expected to use the proper material (and, unless you're a fool, you'll use the right tools, too). Safety wire comes in various types, for various jobs, and it's important not to mix up what is called for, especially in terms of wire type. Some types are designed to be broken open by hand, such as in emergency handles on windows and doors.
Probably the two kinds you'll see and use the most are Type 304 stainless steel and Monel, the latter being appropriate for use in or around exhaust systems or hot engine parts. The most common sizes are 0.032-inch and 0.041-inch, although you'll sometimes see the ultra-skinny 0.020-inch kind on delicate jobs.
Stainless steel, Monel, and brass are the materials of choice simply because they don't corrode. Safety wire can also be found in aluminum and copper. (Safety wire wouldn't be much good if it rusted through.) Regardless of which type you buy, the base metal will always be fully annealed (dead soft), to guard against brittleness. (One reason you never reuse old wire is that it work-hardens when it gets twisted, and further hardens in service as it undergoes vibration and stretching.)
For most jobs you'll want Type 304 stainless steel wire (MS 20995 or equivalent), and you'll want the largest size wire that will fit through the bolt hole, unless the manual calls for something different, so always check the manual.
The common lengths for pliers are six, nine and 10 inches. The nine and 10-inch versions are the most popular, with the short lengths used for those really tough to get at spots.
Do The Twist
If you've never wired anything before with a pair of professional twisters, here's a quick walkthrough:
First, identify the point(s) you'll be connecting with wire, and guesstimate the length of wire required. Remember that a double-stranded run requires a piece of wire equal to twice the work length, plus about 15 or 20 percent for take-up (the wire "shrinks" as it's twisted), plus an adequate margin for a pigtail and gripping with the pliers.
Second, with the to-be-wired item temporarily out of the way, loop the untwisted wire through the anchor point (for example, the base pad, if you're doing an oil filter) and bring half of the wire through the anchor. Try to minimize bending or flexing of the wire. Any working hardens (embrittles) the metal.
Next, install the oil filter (or bolt(s), etc.) to be wired, and torque them as required. Look and see how much distance there is from the anchor point to the tie-point on the work itself.
Pull the wire with your fingers until your fingertips are opposite the intended tie point; keep going another 15 percent or so (for take-up), then, at the "plus-15" point, grab both strands of wire with your twisting pliers. Lock the pliers by squeezing them and sliding the locking collar into place.
The idea now is to twist the length of wire -- exercising the pull-to-twist feature of the pliers -- to give the desired six to eight turns per inch, with all turns ending just short of the intended (distal) tie point on the work. That is to say, when you've finished twisting the wire run, you should end up with a wire 'Y' whose nexus barely reaches the tie point. If you go too far, you'll have to start over, so watch it.
To finish the job, you want to be able to thread one arm of the 'Y' through the work, continue the other arm around the bolt-head (if that's what you've got), mate up the two arms again, and twist them tightly together. (Relocate your pliers as necessary to accomplish this.)
It's important to continue twisting well past the final anchor or loop point in this fashion, so that the bolt head is firmly gripped by the wire. Generally, a pigtail of half an inch is sufficient. (Be careful cutting off the excess: Wire shrapnel is extremely hazardous to unprotected skin and eyes.)
Using your pliers as encouragement, carefully bend the pigtail around the hex (or back and out of the way if no hex is present). Avoid grazing your arm against the sharp pigtail.
Now stop and examine your work. Is there any slack? Did you remember to make the run go in a direction that tends to tighten the bolt? Snip and discard the wire if there's any doubt.
Don't feel bad if you end up having to redo the run more than once to get it just right; the important thing isn't to get it perfect on the first try, after all. The important thing is that you do eventually get it right regardless how many tries it takes.
For more about maintaining general aviation aircraft, check out AVweb's Maintenance section.