The calculator
Runs in your browser. Inches or millimetres at the top.
The rules it applies
AC 43.13-1B paragraph 7-123b: “When using double-twist 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.”
For emergency equipment the AC is stricter: do not use stainless, monel, carbon steel or aluminium wire on emergency mechanisms such as exit handles, fire extinguishers or emergency gear releases; some require copper or brass safety wire, .020 inch only, so that it shears when it has to.
Figure 7-3a shows the twist by hand at 6 to 8 twists per inch. The calculator multiplies that by the distance between the parts, so a 1.5 in span takes 9 to 12 twists.
Worked example
Two drilled-head bolts 1.5 in apart, wire holes 0.051 in, double-twist.
- Hole 0.051 in is between .045 and .062 in, and the parts are under 2 in apart: .020 in is allowed; .032 in also fits.
- Twists = 1.5 × 6 to 1.5 × 8 = 9 to 12.
- Same bolts 2.5 in apart: the .020 in exception no longer applies, so .032 in minimum.
.020 in double strand, 9 to 12 twists. Move the bolts 2.5 in apart and it becomes .032 in.
Limits — when NOT to use this
- Manufacturer's data governs, including the wire material, and the number of parts that may be wired in series, which the AC leaves to the application (7-124a).
- Holes of .045 in or less have no figure in the AC.
- Seals on first-aid kits, extinguishers and oxygen regulators use copper (.020 in) or aluminium (.031 in) wire as called for in the technical orders. Do not confuse steel with aluminium wire.
- Torque first. Align the holes within the torque limits; never over- or under-torque to line them up (7-124).
Questions people actually ask
Can I use .020 in wire everywhere it fits?
No. On double-twist work the AC allows .020 in only for holes from .045 to .062 in with parts less than 2 in apart. Elsewhere it is .032 in minimum.
Do castle nuts with self-locking nuts need wire?
The AC says drilled boltheads and screws need not be safety wired if installed with self-locking nuts. Castle nuts are safetied with cotter pins or wire.
Sources
- FAA AC 43.13-1B CHG 1, Chapter 7, par. 7-38, 7-123 and 7-124, Figures 7-3 and 7-3a — wire diameters by hole and spacing, emergency-device wire, seals, 6 to 8 twists per inch, routing and pigtail rules, no reuse.