Every aircraft electrical system depends on two fundamental categories of material: conductors, which allow electrons to flow freely, and insulators, which resist or block that flow. For an Aviation Maintenance Technician (AMT), knowing which materials belong to each category — and why specific materials are chosen for specific jobs — is not merely an academic exercise. Wiring errors, improper insulation choices, or misunderstood conductor ratings can lead to fires, equipment failures, and catastrophic accidents. This article walks through the types, properties, ratings, and real-world applications of both conductors and insulators as they appear in certificated aircraft.
The movement of electric charge through a material depends on how tightly electrons are bound to their atoms. In conductive materials, outer-shell (valence) electrons are loosely held and can drift from atom to atom under the influence of an electric field. In insulating materials, those electrons are tightly bound and resist movement. Between these extremes lies a third category, semiconductors, whose conductivity falls between the two and which are fundamental to solid-state electronics — but the primary focus in aircraft wiring is on true conductors and insulators.
Electrical Conductors Used in Aircraft
A conductor's ability to carry current is described by its resistivity — the inherent resistance of a unit volume of the material. Lower resistivity means better conductivity. The most common conductors found in aircraft electrical systems include silver, copper, gold, and aluminum, ranked in that order by conductivity — though copper and aluminum are by far the most widely used in aircraft wiring itself, while silver and gold are reserved for specialized contact and plating applications. Each has practical trade-offs that determine where it is used.
Copper
Copper is the dominant conductor in aircraft electrical wiring because it offers an excellent combination of high conductivity (second only to silver among common metals), mechanical strength, flexibility, and ease of soldering and terminating. Aircraft-grade copper wire is typically stranded — composed of many fine individual strands twisted together — rather than solid. Stranded wire is far more resistant to fatigue cracking caused by vibration, which is a constant presence in the aviation environment. The more strands per gauge, the more flexible the wire. FAA guidance specifies that wire should be selected not only for its current-carrying capacity but also for its ability to survive the mechanical stresses of routing, clamping, and repeated flexing.
Aluminum
Aluminum's conductivity is commonly cited as roughly 61 percent of copper's (on an equal cross-sectional area basis), meaning an aluminum wire must be larger in cross-section to carry the same current as a copper wire. Despite this disadvantage, aluminum's significantly lower density makes it attractive for large, heavy-current runs — such as main bus feeders and battery cables — where weight savings outweigh the penalty of a larger wire gauge. A critical concern with aluminum wiring is oxidation: aluminum naturally forms an oxide layer that is highly resistive, unlike copper oxide which is far less of a problem. Aluminum terminations require special anti-oxidant compound applied at connections, and only connectors and terminals rated for aluminum conductors should be used. Galvanic corrosion is also a concern wherever aluminum contacts dissimilar metals.
Silver and Gold
Silver has the highest electrical conductivity of any element, but its high cost limits use to specialized components such as high-frequency RF connectors, switch contacts, and relay points where low contact resistance is critical. Gold, while less conductive than both silver and copper, does not oxidize under normal conditions, making it ideal for low-current connector contacts where even a thin oxide layer would cause unacceptable resistance. Gold plating on connector pins in avionics is common for this reason.
Wire Gauge and Current Ratings
Aircraft wire is sized using the American Wire Gauge (AWG) system, where a lower gauge number indicates a larger diameter wire capable of carrying more current. For example, 4 AWG wire is much larger and carries more current than 22 AWG wire. Wire selection must account for two separate limits: the current-carrying capacity (ampacity) determined by how much heat the wire can safely dissipate, and the voltage drop over the length of the run. Long wire runs in large aircraft can develop significant voltage drop even at safe current levels, so wire size is sometimes increased beyond the ampacity minimum purely to limit voltage drop to acceptable levels. The FAA's Aviation Maintenance Handbook and aircraft wiring standards (such as those referenced in AC 43.13-1B) provide current-carrying capacity tables that also factor in the number of wires bundled together, because bundled wires shed heat less efficiently than isolated ones.
Electrical Insulators Used in Aircraft
An insulator surrounds the conductive wire to prevent unintended current paths, protect against short circuits, and shield personnel from shock. Aircraft insulation must survive a demanding environment: wide temperature swings, hydraulic fluid, fuel, oil, UV radiation, vibration, abrasion, and moisture. Different insulating materials are engineered to handle different combinations of these stresses.
Thermoplastic Insulation
Thermoplastic materials soften when heated and re-harden when cooled. Common examples include polyvinyl chloride (PVC) and polyethylene. These materials are inexpensive and easy to process but have a relatively low continuous-temperature rating (often 60–80 °C) and can become brittle at low temperatures. They are generally not preferred for primary aircraft wiring where temperature extremes are expected, but may appear in protected areas of light general aviation aircraft.
Thermoset Insulation
Thermoset materials are cross-linked polymers that cure into a permanent shape and do not soften with heat. Neoprene and cross-linked polyethylene (XLPE) are examples. They offer better temperature resistance and mechanical toughness than thermoplastics. Neoprene jacketing is often found on flexible cable assemblies.
High-Performance Fluoropolymer Insulation
Fluoropolymer insulations are among the most widely used aircraft-grade wire insulations, alongside polyimide (Kapton) and polyimide/fluoropolymer hybrid constructions, which are also extremely common in transport-category aircraft. Polytetrafluoroethylene (PTFE), sold under the trade name Teflon, and related materials such as ETFE (Tefzel) and PVDF (Kynar) are standard in modern aircraft. These materials offer exceptional temperature resistance (PTFE is rated to 260 °C continuous), chemical resistance to virtually all aviation fluids, low dielectric constant for excellent electrical isolation, and good abrasion resistance. Wires with PTFE insulation are lighter and thinner than many alternatives at equivalent voltage ratings, which matters greatly in weight-sensitive aerospace applications.
Kapton (Polyimide)
Polyimide film insulation, commonly known by the brand name Kapton, offers outstanding thermal stability and is extremely thin and lightweight. However, Kapton has a well-documented vulnerability: when damaged mechanically — nicked, chafed, or kinked — it can arc at surprisingly low voltages. The FAA and industry groups have issued guidance warning maintainers to handle Kapton-insulated wire with extra care, avoid tight bending radii, and inspect it thoroughly because damage is not always visually obvious. Understanding this property is a safety-critical point for AMTs working on older transport-category aircraft where Kapton wiring is common.
Wire Jacketing and Shielding
Many aircraft wires have an outer jacket over the insulation for additional mechanical protection, and some carry a conductive shield — a braided or spiral-wrapped metallic layer — to contain or exclude electromagnetic interference (EMI). The shield is typically grounded at one end to drain induced signals to structure ground. When stripping shielded wire, the AMT must be careful to maintain insulation integrity between the center conductor and the shield, and to make a proper low-resistance shield termination.
Why Material Choice Matters
Selecting the wrong conductor or insulator can have serious consequences. Undersized wire overheats, potentially igniting adjacent materials and causing an in-flight fire. Insulation that degrades from chemical exposure can create arcing paths between wires or from wire to structure, tripping breakers at best and causing fires at worst. Aluminum wiring with improperly torqued or un-treated connections will develop high-resistance joints that generate heat and ultimately fail. The FAA requires that replacement wire be of the same or equivalent type and gauge as the original, and any deviation must be approved through the aircraft's maintenance data or an FAA-approved alternate means.
Key Numbers and Rules
- AWG sizing: Lower gauge number = larger wire = higher ampacity. Wire gauge must satisfy both ampacity and voltage-drop requirements.
- Bundling derating: Wires bundled together must be derated (their ampacity reduced) because bundled wires cannot shed heat as efficiently as individual wires routed in free air.
- PTFE/Teflon: Rated to approximately 260 °C continuous — the highest of common aircraft insulations.
- Aluminum oxidation: Requires anti-oxidant compound at terminals; galvanic corrosion risk with dissimilar metals.
- Kapton vulnerability: Mechanically fragile; arcing risk if nicked or kinked even under modest voltage.
- Stranded vs. solid: Aircraft wiring is almost universally stranded for vibration and fatigue resistance; solid wire is generally not used in airframe wiring applications.
- Shielding: Conductive braided shields are grounded (usually at one end only) to reduce EMI; improper grounding can make shielding ineffective or introduce ground loops.
Common Test Traps
- AWG direction: Many students assume a higher AWG number means a larger wire. The opposite is true — AWG 4 is much larger than AWG 22. Expect the FAA test to exploit this directly.
- Aluminum vs. copper terminals: You cannot substitute standard copper-rated terminals on aluminum wire. The test may present a scenario where a technician swaps terminal types — this is incorrect and potentially dangerous.
- Bundling and derating: A wire that is adequate for a circuit run in free air may be undersized once bundled with other wires. Students often overlook bundling factors when sizing wire.
- Kapton wire handling: The test may ask about the special precaution required with polyimide insulation. The correct answer involves careful handling to prevent mechanical damage and thorough visual inspection, because internal arc damage may not be visible externally.
- Silver oxidation myth: Students sometimes confuse gold and silver in connector applications. Silver does oxidize over time and its oxide is somewhat conductive, but gold is preferred in avionics connectors precisely because it does not oxidize, maintaining consistently low contact resistance.