Safety Wire Size Selection Guide for Aircraft Mechanics

Safety wiring is one of the fundamental skills of an aircraft mechanic — and one that every inspector looks at closely. The wrong wire size, wrong direction, or sloppy technique can mean a rejected sign-off and a redo. This guide covers how to choose the right safety wire size for any job, the correct installation technique, and the rules from AC 43.13-1B that govern safety wiring on certificated aircraft.

Why Safety Wiring Matters

Vibration is everywhere in aviation. Engines, rotors, props, and airflow all create constant vibration that will work a fastener loose over time if nothing prevents it. Safety wire creates a mechanical link between fasteners so that if one starts to back out, the wire immediately goes taut and prevents further rotation.

A properly safety-wired fastener cluster is one of the most reliable locking methods in aviation — it doesn't degrade with heat, doesn't lose preload, and gives a clear visual indication if tampered with.

Safety Wire Sizes: Which One to Use

.020" Wire — Light Applications

The lightest common safety wire. Used for small screws, cowling fasteners, instrumentation fittings, and low-vibration areas where fastener size limits you to a thin wire. Common on small general aviation aircraft and interior hardware. Not suitable for engine or hydraulic applications.

.025" Wire — Medium Applications

A step up from .020", used for medium-sized bolts and screws in moderate applications. Found on light aircraft accessory hardware and some avionics mounting. Good general purpose wire for GA maintenance where .032" would be too large for the hole.

.032" Wire — The Standard (Use This Unless Told Otherwise)

This is the most important safety wire size to know. MS20995C32 (.032" stainless steel) is the default safety wire for virtually all commercial aviation turbine engine maintenance. When your AMM says "safety wire the fitting" without specifying a size, you use .032". It fits the standard safety wire holes drilled in AN/MS hardware and is strong enough for most engine, hydraulic, and structural applications.

.041" Wire — Heavy Duty Applications

The heaviest standard safety wire. Specified for high-vibration zones, large fasteners, helicopter main rotor hardware, and high-temperature areas where thinner wire could fatigue and break. Requires more force to twist and is harder to work with. Only use .041" when the AMM specifically calls for it or when the fastener size requires it.

Safety Wire Materials

The most common material is 304 stainless steel, which covers the vast majority of maintenance tasks. Specialty materials are used for specific environments:

• Stainless steel (MS20995C) — standard for almost all work
• Inconel (MS20995N) — hot section turbine bolts and high-temperature zones above 1400°F
• Monel (MS20995M) — corrosive environments and some fuel system fittings
• Copper (MS20995Cu) — soft fittings that could be damaged by harder wire

Installation Rules Per AC 43.13-1B

• Twist rate: 6–8 twists per inch is standard. Over-twisting weakens the wire — it should look tight and uniform, not kinked
• Direction: The wire must be routed so that the natural loosening rotation of the fastener puts the wire in tension — not slack
• Cluster limit: No more than 3 fasteners per wire run
• Maximum span: 24 inches between wire holes in a cluster
• Pigtail: Minimum 1/2" to 1" — cut cleanly and fold back against the wire so there are no sharp ends
• Tension: Wire must be taut — slack safety wire provides zero locking function
• Single vs. double wire: Double-wire method is preferred for most applications; single-wire may be used on closely spaced or small fasteners
• Never reuse safety wire — always cut and install new wire

The Most Common Safety Wiring Mistakes

• Wrong direction — wire routed so that loosening the fastener slackens the wire instead of tensioning it. This is the most critical mistake.
• Over-twisted — the wire looks kinked and will fatigue-crack quickly under vibration
• Under-twisted — looks sloppy and won't hold under vibration
• Sharp pigtail — unfolded pigtail ends are a FOD risk and a hand injury waiting to happen
• Too much slack — the wire has to be taut to work
• Advancing the fastener to line up a hole — never advance a fastener past its torque spec to reach a safety wire hole. Back it off slightly if needed (within AMM limits), or use the next fastener in the cluster

Quick Reference: Which Wire to Use