Every aircraft is an exercise in engineering compromise — designers must balance strength, weight, corrosion resistance, fatigue life, and cost for each structural component. The first major dividing line in aircraft metallurgy is whether a metal contains iron as its primary base element. Ferrous metals are iron-based alloys; non-ferrous metals are everything else. Understanding this distinction, and knowing which specific alloys belong to each family, is fundamental knowledge for any Aviation Maintenance Technician (AMT) — and it is heavily tested on the FAA General written exam.
This article walks through both categories in depth: their defining characteristics, the specific alloys you will encounter on real aircraft, how heat treatment and alloying affect their properties, and the practical maintenance implications of each. The goal is not just to pass the test, but to understand why a particular bolt is made of steel while the surrounding skin is aluminum — and what happens if you mix them incorrectly.
Ferrous Metals: Iron-Based Alloys
The word ferrous comes from the Latin ferrum, meaning iron. Any metal whose primary constituent is iron falls into this category. Pure iron itself is soft, heavy, and not particularly useful in aviation. Its value is unlocked by alloying — adding precise amounts of other elements, chiefly carbon, to create steel, or adding chromium, nickel, molybdenum, and other elements to create the alloy steels used throughout an airframe and powerplant.
Carbon Steel
Plain carbon steel is classified by its carbon content. Low-carbon steel (up to about 0.30% carbon) is relatively soft and ductile — it can be formed and welded easily but does not respond significantly to heat treatment for hardness. Medium-carbon steel (0.30%–0.60% carbon) offers a balance of strength and machinability, while high-carbon steel (above 0.60%) can be hardened significantly through heat treatment but becomes more brittle. In aircraft work, low-carbon steel is common for non-structural parts, sheet metal brackets, and welded tube fuselage structures where good weldability is prized.
Alloy Steels
Alloy steels add elements beyond carbon to enhance specific properties. The SAE/AISI four-digit numbering system identifies these alloys — the first two digits indicate the alloying elements, and the last two (or three) digits indicate the approximate carbon content in hundredths of a percent. For example, SAE 4130 steel (chromoly) contains about 0.30% carbon with chromium and molybdenum as the primary alloying agents. 4130 chromoly steel is extremely common in aircraft: it is used for engine mounts, landing gear components, control system hardware, and welded tube fuselage frames. It offers high strength-to-weight ratio, excellent weldability, and responds well to heat treatment. SAE 4340 provides even higher strength after heat treatment and is used in highly stressed parts such as crankshafts and heavy-duty landing gear.
Stainless Steel
Stainless steel is an alloy steel with a minimum of about 11–13% chromium content. The chromium reacts with oxygen to form a thin, stable chromium-oxide passive layer on the surface, which is self-repairing and dramatically improves corrosion resistance. 300-series stainless steels (such as 304 and 321) are austenitic — they cannot be hardened by heat treatment but are work-hardenable and offer outstanding corrosion resistance. They are used for exhaust systems, firewalls, and hardware in high-temperature or corrosive environments. 400-series stainless steels are martensitic or ferritic, can be heat-treated for higher hardness, and are common in ball bearings, valve seats, and cutting tools. 300-series austenitic stainless steels are essentially non-magnetic in their annealed state (though cold-working can induce slight magnetism), while 400-series martensitic and ferritic stainless steels are magnetic; a magnetic check can therefore help distinguish between the two families, though it is not definitive for identifying a specific alloy.
Non-Ferrous Metals: Beyond Iron
Non-ferrous metals contain no significant iron content. Their primary advantage in aviation is almost always a superior strength-to-weight ratio, frequently combined with inherent corrosion resistance. The major non-ferrous metals in aircraft construction are aluminum alloys, titanium alloys, magnesium alloys, and copper alloys. Each occupies a specific niche based on its properties.
Aluminum Alloys
Aluminum is the backbone of modern aircraft structures. Its density is roughly one-third that of steel, yet when properly alloyed and heat-treated, aluminum alloys deliver impressive structural strength. Pure aluminum is soft and highly corrosion-resistant due to a natural oxide layer, but it is too weak for structural use. Alloying elements — principally copper, manganese, silicon, magnesium, and zinc — create the families designated by a four-digit system similar to steel. The 2000-series alloys (chiefly 2024) use copper as the primary alloying element and are among the most common aircraft structural alloys; 2024-T3 is a standard skin and structural material offering high strength and good fatigue resistance. The 7000-series (especially 7075) use zinc as the primary alloying agent and achieve even higher strength, making them suitable for highly stressed components such as wing spars. A critical maintenance point: aluminum alloys suffer galvanic corrosion when in contact with dissimilar metals (particularly steel or copper) in the presence of an electrolyte. Correct use of isolation tape, sealants, and proper fastener materials is essential.
The temper designations that follow the alloy number (T3, T4, T6, etc.) describe the heat-treatment or work-hardening condition of the metal and directly affect its strength and ductility. An AMT must never assume that an aluminum part can be repaired with any available aluminum alloy — the specific alloy and temper must match the original specification.
Titanium Alloys
Titanium is remarkable: it has roughly the same strength as many steels but at about 60% of the weight, and its corrosion resistance is outstanding in most environments, including salt water. Titanium alloys are used in high-temperature areas near jet engines, in firewalls, in bulkheads of high-performance aircraft, and increasingly in fasteners. The most common aviation alloy is Ti-6Al-4V (6% aluminum, 4% vanadium). Titanium is difficult to machine, must not be worked with contaminated tools (contamination with steel particles can cause problems), and requires specialized welding procedures in an inert atmosphere. It is also considerably more expensive than aluminum or steel, which is why its use is targeted where the weight and temperature advantages justify the cost.
Magnesium Alloys
Magnesium is the lightest structural metal used in aircraft, with a density roughly two-thirds that of aluminum. It is used for gearbox housings, wheel castings, and some interior components. However, it has serious limitations: it is highly susceptible to corrosion and is highly flammable when ignited in chip or powder form. Maintenance technicians must never use steel tools to clean magnesium components, as steel particles can cause sparking. Magnesium fires must be fought with Class D dry powder extinguishers — water or CO2 will intensify the fire.
Copper and Its Alloys
Copper is used primarily in aircraft electrical systems due to its excellent electrical and thermal conductivity. Brass (copper-zinc) and bronze (copper-tin) appear in bushings, fittings, and fuel system components. Copper is heavy and not a structural material, but its conductivity and workability make it irreplaceable in wiring and certain fluid system components.
Why This Distinction Matters in Maintenance
The ferrous/non-ferrous distinction has direct, practical consequences every day in an aircraft shop. Corrosion control is different: ferrous metals are susceptible to rust (iron oxide) and require protective coatings, plating, or alloying to resist it, while most non-ferrous metals form protective oxide layers naturally but are vulnerable to galvanic and stress corrosion. Welding procedures differ: 4130 chromoly is gas- or TIG-welded with specific preheat requirements; aluminum requires separate procedures and filler alloys; magnesium requires inert-gas shielding and special filler. Heat treatment varies: steels are hardened by quenching and tempering; aluminum alloys are solution-treated and aged; magnesium alloys have their own aging treatments. An AMT who confuses these families risks creating a repair that looks good but fails structurally or corrodes prematurely.
Key Numbers and Rules
- SAE 4130 chromoly steel: ~0.30% carbon, primary alloying elements chromium and molybdenum; widely used for welded structures and high-stress fittings.
- 2024-T3 aluminum: 2000-series, copper-alloyed; standard aircraft skin and structural material; T3 temper means solution heat-treated and cold-worked.
- 7075-T6 aluminum: 7000-series, zinc-alloyed; higher strength than 2024, used for spars and heavily loaded structures.
- Stainless steel chromium content: minimum approximately 11–13% chromium to qualify as stainless.
- Ti-6Al-4V: the most common aircraft titanium alloy; 6% aluminum, 4% vanadium.
- Magnesium fire suppression: Class D dry powder only — never water or CO2.
- Galvanic corrosion: always isolate dissimilar metals (especially steel fasteners in aluminum structure) with proper coatings, sealants, or aluminum rivets.
Common Test Traps
- Confusing carbon content and heat treatability: Low-carbon steel cannot be significantly hardened by heat treatment. High-carbon steel can, but is more brittle. The FAA exam tests whether you know which carbon range responds to hardening.
- Misidentifying alloy series: 2024 is a copper-alloyed aluminum (2000-series), not zinc. 7075 is zinc-alloyed (7000-series). Questions often flip these to catch rote memorizers.
- Magnesium fire safety: The exam may offer water or CO2 as plausible-sounding answers for a magnesium fire — both are wrong and dangerous. Only Class D extinguishing agents are correct.
- Assuming stainless steel is non-magnetic: 300-series austenitic stainless is essentially non-magnetic, but 400-series martensitic/ferritic stainless is magnetic. Neither is the same as plain carbon steel, but ferromagnetism alone does not tell you whether you have stainless or plain steel.
- Titanium tool contamination: A common distractor question asks about grinding or cutting titanium — steel particles or certain lubricants can contaminate titanium and compromise its integrity. Always use dedicated, clean tooling.