Wire size by use
The AC sets wire size by the diameter of the hole in the part and the spacing, for the double-twist method, and leaves single-wire to “the largest size wire that the hole will accommodate.”
| Use | Wire | Source |
|---|---|---|
| Double-twist, part with a hole larger than 0.045 in | 0.032 in minimum | AC 43.13-1B 7-123b. Handbook p. 2-55: most common is stainless or Monel, 0.032–0.040 in. |
| Double-twist, hole 0.045 to 0.062 in, parts less than 2 in apart | 0.020 in, double strand | AC 7-123b |
| Single-wire method | The largest wire the hole will accept | AC 7-123b |
| Hole of 0.045 in or less | No size given | Not in AC 7-123 or the Handbook |
| Emergency mechanisms that must shear (switch guards, exit handle covers, fire extinguishers, emergency gear releases) | Copper or brass, 0.020 in only | AC 7-123. No stainless, Monel, carbon steel or aluminum alloy wire here. |
| Seals on first-aid kits, portable fire extinguishers, emergency valves, oxygen regulators | Copper 0.020 in or aluminum 0.031 in, or as the technical order says | AC 7-123b. Handbook p. 2-55: the seal must break by hand without tools. |
Diameters in inches. “Double strand” is the AC's wording for the 0.020 in wire used in the double-twist method.
The two methods
| Method | Where it is used | Examples in AC Figure 7-3 |
|---|---|---|
| Double-twist | The most common method. Studs, cable turnbuckles, flight controls, engine accessory bolts (Handbook p. 2-55). Two wires twisted together between the parts. | Single fastener; screw heads; bolt heads; castle nuts |
| Single-wire | Screws, bolts or nuts in a closely spaced or closed geometric pattern: a triangle, square, rectangle or circle. Also parts in electrical systems and places that are hard to reach. | Small screws in a closed pattern; external snapring |
AC 43.13-1B par. 7-123a and Figure 7-3. The Handbook calls the double-twist method “double-wrap” in the Figure 2-91 caption.
Twists between the parts
Figure 7-3a shows hand twisting at 6 to 8 twists per inch. The Handbook (p. 2-55) says the same: “six to eight twists per inch of wire.” Multiply by the length of the twisted section.
| Distance between parts | Twists at 6–8 per inch |
|---|---|
| 1/2 in | 3–4 |
| 1 in | 6–8 |
| 1½ in | 9–12 |
| 2 in | 12–16 |
| 3 in | 18–24 |
| 4 in | 24–32 |
Computed from 6 to 8 twists per inch; the span is the length of the twisted section between the two parts.
Turnbuckle safety wire (AC 43.13-1B Table 7-8)
For turnbuckles and swaged terminals the AC gives wire size by cable size and wrap. Double-wrap is the better method; the single-wrap methods “are acceptable, but are not the equal of the double-wrap methods” (7-181). Wrap each wire at least four turns around the shank, with no more than three threads of the terminal exposed.
| Cable size | Wrap | Wire diameter | Wire material, annealed |
|---|---|---|---|
| 1/16 | Single | 0.040 in | Copper, brass ¹ |
| 3/32 | Single | 0.040 in | Copper, brass ¹ |
| 1/8 | Single | 0.040 in | Stainless steel, Monel, “K” Monel |
| 1/8 | Double | 0.040 in | Copper, brass ¹ |
| 1/8 | Single | 0.057 in min. | Copper, brass ¹ |
| 5/32 and greater | Double | 0.040 in | Stainless steel, Monel, “K” Monel ¹ |
| 5/32 and greater | Single | 0.057 in min. | Stainless steel, Monel, “K” Monel ¹ |
| 5/32 and greater | Double | 0.0512 in | Copper, brass |
¹ The AC's footnote: galvanized or tinned steel, or soft iron wires are also acceptable. The last row is printed “0.0512”; the page has no footnote 2, so the final digit may be a stray mark (0.051 in). Check the manual. Par. 7-182b: smaller wire sizes tend to crack when twisted.
Wire materials named in the sources
| Material | Where the sources use it | Where it is excluded |
|---|---|---|
| Soft brass or steel wire | Cylinder studs, control cable turnbuckles, engine accessory attaching bolts (AC 7-122a) | Not stated |
| Turnbuckle lock wire | Carbon steel, corrosion-resistant steel, Inconel, Monel or aluminum alloy (AC 7-182a); Table 7-8 lists copper, brass, stainless, Monel and “K” Monel | Table 7-8 has no aluminum row although 7-182a names aluminum alloy; follow the table for size |
| Stainless steel or Monel, 0.032–0.040 in | The usual double-twist wire (Handbook p. 2-55) | Not on emergency mechanisms (AC 7-123) |
| Carbon steel | Named only in the exclusion | Not on emergency mechanisms (AC 7-123) |
| Copper, 0.020 in | Seals, and emergency devices that need shear wire (AC 7-123, 7-123c) | — |
| Brass, 0.020 in | Emergency devices that need shear wire (AC 7-123) | — |
| Aluminum, 0.031 in | Seals only (AC 7-123b; Handbook p. 2-55) | Not on emergency mechanisms. “Care should be taken not to confuse steel with aluminum wire.” |
The AC and the Handbook name these materials but do not list wire specifications in the sections used here. Check the manufacturer's maintenance manual for the wire specification.
Rules that apply to every job
Tight and taut. The wire must be tight after installation and pulled taut while twisted, then left under light tension. “Safety wire will break under vibrations if twisted too tightly” (par. 7-124c, 7-124e).
Pull in the direction that tightens the part. The wire must be installed so it works against the tendency of the part to loosen (7-124d). Figure 7-3 shows it for right-hand threads and says to route it the opposite way for left-hand threads. Where practicable put the wire around the head and keep the loop close to the contour of the unit (7-124h).
Ends. Bend ends under and in toward the part so there is no sharp or sticking-out end (7-124f). When cutting off, leave 4 to 6 complete turns (1/2 to 5/8 in) after the loop (7-124g(2)). Never twist the ends off with pliers.
Torque first, then align. Check the units are correctly torqued and the wire holes line up. Never overtorque or loosen a part to get a hole to line up. If it cannot be done within the torque limits, try another bolt (7-124g(1)). Use the torque chart when the manual gives no value.
In series. Closely grouped bolts, nuts or screws can be wired in series; the number that may be wired together depends on the application (7-124a). Drilled bolt heads and screws installed with self-locking nuts need not be wired (7-124b).
Not across flow. Wire inside a duct or tube must not cross or block a flow passage if another routing will do (7-124g).
Once only. Do not reuse safety wire or cotter pins (7-38). Pliers must not nick, kink or mutilate the wire; grasp the ends (7-124g(2)).
Turnbuckles. Use Table 7-8 for wire size and the double-wrap method where possible (AC 7-180 to 7-182; Handbook p. 2-45 also prefers double-wrap). Clip-locking turnbuckles are secured with the clip (7-184), not wire.
Worked example — two bolt heads, 1½ in apart
Two engine accessory bolts with drilled heads, the holes 0.064 in, the bolts 1½ in apart. The sizes are an example, not a table figure.
- Hole diameter 0.064 in is larger than 0.045 in, so the double-twist method with 0.032 in minimum wire applies (7-123b).
- Twisted length 1½ in at 6–8 twists per inch = 9 to 12 twists.
- Wire goes around the head and is twisted in the direction that tightens the part; for right-hand threads as in Figure 7-3.
- At the second bolt, twist off leaving 4 to 6 complete turns (1/2 to 5/8 in) after the loop; bend the end under and toward the part.
0.032 in double-twist wire, 9 to 12 twists between the bolts, 4 to 6 turns on the pigtail. The manual for the engine can say otherwise.
Questions people actually ask
What size safety wire should I use?
0.032 in minimum for the double-twist method on parts with a hole larger than 0.045 in. 0.020 in double strand may be used for holes of 0.045 to 0.062 in when parts are less than 2 in apart. For single-wire use the largest size the hole accepts (AC 7-123b).
How many twists per inch?
6 to 8 (AC Figure 7-3a and Handbook p. 2-55).
Can I reuse safety wire?
No. AC 7-38 says not to reuse cotter pins or safety wire.
Which wire for a fire extinguisher or emergency handle?
Where success depends on the wire shearing, the AC calls for copper or brass 0.020 in. Seals on portable fire extinguishers and similar equipment use copper 0.020 in or aluminum 0.031 in. The Handbook says the seal must break by hand without tools.
Double-twist or single-wire?
Double-twist is the most common. Single-wire is for screws, bolts or nuts in closely spaced closed patterns, electrical parts and hard-to-reach places (AC 7-123a).
Limits — when NOT to use this chart
This chart collects what AC 43.13-1B Chapter 7 and the Airframe Handbook state. It does not replace the maintenance manual.
- The aircraft's manual governs, including wire material, size and the number of parts that may be wired in series.
- Emergency equipment has its own wire. Copper or brass 0.020 in only, so that it shears. Do not use stainless, Monel, carbon steel or aluminum alloy wire there.
- No size is given for holes of 0.045 in or less. Use the manufacturer's data.
- Cotter pins and self-locking nuts are separate means of safetying with their own rules (AC 7-122, 7-127).
- Turnbuckle wrapping is only summarised here (size table and turn counts). The step-by-step wrap is in AC 43.13-1B 7-180 to 7-182 and Figures 7-26 and 7-27.
Sources
- FAA AC 43.13-1B CHG 1, Chapter 7, par. 7-38, 7-122 to 7-126, 7-181, 7-182, Figures 7-3 and 7-3a, Table 7-8 (pp. 7-19 to 7-21, 7-43 to 7-45) — wire diameters by hole and spacing, the two methods, emergency and seal wire, 6 to 8 twists per inch, the safety wiring rules, end length, no reuse; turnbuckle wire sizes (Table 7-8) and turn counts.
- FAA-H-8083-31, AMT Handbook — Airframe, Volume 1, Chapter 2, pp. 2-45, 2-54 and 2-55, Figures 2-91 to 2-93 — stainless or Monel 0.032–0.040 in as the common double-twist wire, seal wire, six to eight twists per inch, the double-wrap method for turnbuckles.