Aircraft safety wire is a soft metal wire threaded through drilled fastener heads and twisted to a fixed anchor so that vibration cannot back a bolt, nut, or plug out in flight. It does not hold torque. It only stops a fastener that has begun to loosen from turning far enough to fall out. Under 14 CFR 43.13, the method you use has to come from the current manufacturer’s maintenance manual, and where the manual is silent, FAA Advisory Circular 43.13-1B is the accepted reference.1eCFR. 14 CFR Part 43 – Maintenance, Preventive Maintenance, Rebuilding, and Alteration When the two conflict, the manufacturer wins.
AC 43.13-1B itself is not law. It states outright that its contents “are not meant to bind the public in any way.”2Federal Aviation Administration. Advisory Circular 43.13-1B – Acceptable Methods, Techniques, and Practices—Aircraft Inspection and Repair What binds you is 14 CFR 43.13(a), which requires methods “acceptable to the Administrator.” Following the AC is one way to meet that standard.
Who Can Legally Do the Work
Safety wiring on a certificated aircraft is restricted work under 14 CFR 43.3. The people authorized to do it are holders of a mechanic certificate (A&P), repairman certificate holders, FAA-certified repair stations, and persons working under the direct, physically present supervision of a certificated mechanic.3eCFR. 14 CFR 43.3 – Persons Authorized to Perform Maintenance, Preventive Maintenance, Rebuilding, and Alterations A pilot-owner may perform the preventive maintenance items listed in 14 CFR Part 43, Appendix A, on aircraft they own and operate, but safety wiring a critical engine or structural fastener goes past that list and requires a certificated mechanic.
Choosing the Wire
Wire specifications come from the maintenance manual. The industry composition standard referenced across manufacturer manuals is NASM20995. Three diameters cover almost every job:
- 0.032-inch stainless steel handles most general aviation airframe and engine fasteners.
- 0.020-inch is for electrical connectors, small instrument screws, and light-duty hardware.
- 0.040-inch is used on heavy applications such as propeller mounting bolts and turnbuckles.
Corrosion-resistant (stainless) steel covers most airframe environments. Around exhaust components and other high-heat areas, Monel or Inconel is specified because stainless will degrade at those temperatures; Inconel handles the hottest work. Copper or brass in 0.020-inch has an opposite job: it secures fire extinguisher pins, emergency gear releases, and escape-hatch handles, where the operator must be able to break the wire by hand.
Where the manual does not name a diameter, use the largest wire that fits through the fastener hole. Thicker wire has more tensile strength and better vibration fatigue life.
Tools
Safety wire pliers are the specialized tool. They grip both strands, twist under controlled tension, and free a hand for routing. A pair with a locking jaw and pull-to-twist action is standard. You also need diagonal cutters for the pigtail and a wire gauge to verify diameter and twist count. AC 43.13-1B specifically warns against twisting wire ends off with pliers on the finished installation, because that mutilates the wire and can damage the surrounding parts. Cut, don’t twist off. The same rule applies when removing old wire: cut it.4Federal Aviation Administration. Advisory Circular 43.13-1B – Acceptable Methods, Techniques, and Practices—Aircraft Inspection and Repair
Torque First, Then Wire
Safety wire never substitutes for correct torque. The FAA’s guidance is direct: tighten the fastener to the specified torque first, then install the wire.5Federal Aviation Administration. Safety Wire Fact Sheet If the drilled hole doesn’t line up with the anchor point after torquing, you may tighten slightly past the nominal value to get alignment. Never back a fastener off to align the hole.
The One Rule That Governs Every Installation
The wire must be routed so that any attempt by the fastener to loosen pulls the wire in the tightening direction. The FAA calls this the “Righty-Tighty” test: picture the bolt trying to rotate counterclockwise, and confirm the wire resists by pulling clockwise.5Federal Aviation Administration. Safety Wire Fact Sheet Reversed direction of pull is the most common installation error, and an inspector who sees it will reject the entire run. Every bolt in a series has to pass this test on its own.
Two related routing rules: keep the run between fastener and anchor as short as practical, and anchor only to stationary points. That could be another safety-wired fastener, a dedicated anchor tab, or a drilled lug on the structure. The wire should be taut without being stressed to the point where normal vibration will fatigue it.
The Double-Twist Method
Double-twist is the default for bolts, nuts, and drain plugs. Two strands twisted together give redundancy: if one strand fatigues, the other still holds the fastener.
Thread the wire through the drilled hole in the first fastener head. Pull roughly equal lengths through so you have two strands to work with, and cross them so that the routing to the anchor already respects the direction of pull. Grip both strands with the safety wire pliers close to the fastener, keep steady light tension on the wire, and twist. For 0.032-inch wire, aim for six to eight twists per inch, and keep the spacing even along the whole run. Uneven twists create weak spots that break first under vibration, and over-twisting produces a brittle section that can look neat and still crack after a few flight hours.
Route the twisted pair to the next fastener or the anchor point, threading a single strand through the next hole, then twist the two strands again to the following anchor. When you wire a row of bolts, each hole has to be oriented so the wire arriving at it and the wire leaving it both resist that bolt’s loosening rotation. The last bolt anchors to a fixed point or loops back on itself.
When Single-Wire Is Acceptable
A single untwisted strand is used only where double-twist is physically impractical or where the maintenance manual specifically calls for it. Typical single-wire jobs are small screws on electrical connectors, closely spaced fastener patterns that won’t accept a twisted pair without interference, and tight-access areas on instruments and avionics. Because there’s no redundant strand, use the largest wire that fits the hole. Single-wire is generally not acceptable on primary structural fasteners or critical engine components.
Finishing the Pigtail
The twisted end past the last anchor is the pigtail. AC 43.13-1B calls for four to six complete twists after the final loop, producing a pigtail roughly 1/2 to 5/8 inch long.4Federal Aviation Administration. Advisory Circular 43.13-1B – Acceptable Methods, Techniques, and Practices—Aircraft Inspection and Repair Cut it with diagonal cutters, then bend the stub inward toward the part so it cannot snag a hand, a rag, or a wire bundle. A pigtail left sticking straight out is both a laceration hazard and an automatic rejection at inspection.
What Inspectors Look For
Every installation is inspected before the aircraft returns to service, and a single deficiency means the whole run gets replaced. The acceptance points:
- Tautness. No visible slack, no significant movement under finger pressure.
- Direction of pull, checked at every fastener in a series.
- Uniform twist spacing, without bunching, gaps, or over-twisted sections.
- Wire condition. No nicks, kinks, or visible damage anywhere along the run.
- Pigtail cut cleanly, correct length, bent toward the part.
- Routing clear of sharp edges, moving parts, and high-temperature surfaces the wire isn’t rated for.
The most frequent rejections in practice are reversed direction of pull, crossed wires at the fastener hole, over-twisted brittle sections, and unbent pigtails.
Two Situations With Their Own Rules
Turnbuckles on flight control cables are not wired the same way as bolts. AC 43.13-1B has separate double-wrap and single-wrap procedures for them, and Table 7-8 specifies wire diameters and materials by cable size; TSO-C21 clip-type locking devices are an accepted alternative.4Federal Aviation Administration. Advisory Circular 43.13-1B – Acceptable Methods, Techniques, and Practices—Aircraft Inspection and Repair If you’re working on a turnbuckle, work from that section, not from the bolt procedure above.
Safety cable systems such as DMC’s Safe-T-Cable are FAA-approved alternatives to twisted wire, using a pre-formed cable and crimped ferrules instead of hand-twisted strands. Approval is not universal. The component maintenance manual has to authorize the substitution; where the manual calls for traditional safety wire, cable cannot be swapped in without an approved engineering disposition.
What Happens When It’s Wrong
A loose fastener on an exhaust system, a flight control, or an engine accessory can cause an in-flight failure, and the FAA treats maintenance violations accordingly. Consequences range from informal corrective action to certificate suspension or revocation for the mechanic, repair station, or operator involved. The FAA’s sanction guidance prescribes certificate suspensions from 30 to 180 days depending on severity, and civil penalties from $5,000 to $20,000 per violation for individuals acting as airmen; business penalties run higher. Intentional or reckless violations receive no leniency.6Federal Aviation Administration. FAA Order 2150.3C – FAA Compliance and Enforcement Program Safety wiring is one of the simpler tasks on the airplane. The penalty for doing it badly is out of all proportion to the time it takes to do it right.