From Aluminium to Carbon Fibre: How Airliner Fuselages Evolved

Run your hand along the outside of a Boeing 737 and you’ll feel rows of rivet heads, thousands of them, holding aluminium sheets to a metal skeleton. Do the same on a Boeing 787 and the skin is almost seamless. The 787’s fuselage isn’t a patchwork of metal sheets at all. It’s made of carbon fibre, wound and baked into giant one-piece tubes.

That change, from riveted aluminium to carbon-fibre composites, is the biggest shift in how airliners are built since metal replaced wood a century ago. To understand why it happened, and why it isn’t a simple upgrade, we need to start with what an aircraft structure actually has to do.

What a fuselage has to survive

An airliner fuselage is really a pressure vessel, a sealed tube that holds air at a higher pressure than the thin air outside. At cruising altitude, the cabin is kept at a pressure similar to being on a mountain about 2,400m (8,000ft) up, while outside it’s far thinner. That pressure difference pushes outward on every square centimetre of the skin.

Every flight, the fuselage is inflated on the way up and deflated on the way down, like a balloon pumped up and let down thousands of times. On top of that come bending loads from the wings and tail, turbulence, landing impacts and temperature swings from about -55°C at cruise to well above freezing on the ground. The material has to be light (every kilogram costs fuel), strong, stiff, and above all able to survive repeated stress without cracking. That last property is called fatigue resistance, and it turns out to be the key to the whole story.

Wood, fabric and the first metal airliners

The first airliners were built like big kites: wooden frames, wire bracing and fabric stretched tight and painted with a stiffening varnish called dope. They were light and easy to repair, but wood rots, swells with moisture and varies from tree to tree, all bad news for an aircraft expected to fly daily for years.

Metal arrived in the 1920s. Germany’s Junkers and America’s Ford built airliners skinned in corrugated metal, the ridges stiffening thin sheets the way corrugated cardboard stiffens paper. You can still see those ridges in the photo below.

The aluminium age

The breakthrough of the 1930s was stressed-skin construction: smooth aluminium-alloy skins that carried part of the load themselves, riveted to a framework of hoops (frames) and lengthwise stiffeners (stringers). The Douglas DC-3 made the approach famous, and for the next 70 years almost every airliner, from the 707 to the 747 to the A320, was built this way.

Aluminium alloys were the perfect material for the job. They’re light for a metal, strong, easy to shape, well understood, and when they do crack, they usually crack slowly and visibly enough for inspectors to find it. Engineers built a century of knowledge around them.

The Comet and the lesson of fatigue

The world’s first jet airliner, the de Havilland Comet, entered service in 1952, flying higher and faster than anything before, which meant a bigger pressure difference across its skin on every flight. In January and April 1954, two Comets broke up in flight. To find out why, investigators placed an entire Comet fuselage in a huge water tank and pumped the pressure up and down again and again, simulating thousands of flights.

The fuselage eventually cracked, at a corner of a window opening. Repeated pressurisation had concentrated stress at sharp corners until the metal fatigued. The findings changed aircraft design for good: rounded window shapes, much more rigorous fatigue testing, and structures designed so that a crack in one place can’t quickly spread through the whole aircraft. Every airliner since, metal or composite, is built on those lessons.

What carbon fibre actually is

Carbon-fibre composite, often shortened to CFRP (carbon-fibre-reinforced plastic), is two materials working as a team. The fibres are threads of carbon, each thinner than a human hair, which are extremely strong and stiff when pulled along their length. On their own they’re floppy, like cotton thread. So they’re set in a resin, usually an epoxy, which glues them together, holds their shape and spreads the load between them.

Here’s the clever part. Unlike metal, which is equally strong in every direction, carbon fibre is strongest along the fibres. So engineers lay up many thin layers (plies), each with its fibres pointing in a chosen direction, putting strength exactly where the loads are and saving weight everywhere else. The layers are then cured under heat and pressure, typically in an autoclave, a giant pressurised oven, until the resin sets rock-hard.

Composites creep in, then take over

Airlines didn’t jump straight to carbon-fibre fuselages. Composites first appeared on parts where a failure wouldn’t bring down the aircraft: fairings, floor panels, control surfaces. The first big step for a primary structure, a part the aircraft can’t fly without, came with the carbon-fibre vertical tail fin of the Airbus A310-300, which flew in 1985 and saved more than 250kg. Boeing’s 777 followed with composite tail surfaces in the 1990s, and the A380 used composites for about a quarter of its structure, plus a metal-and-glass-fibre laminate called GLARE in parts of its upper fuselage.

The leap came with the Boeing 787, which entered service in 2011. About half of it by weight is composite, including the whole fuselage and the wings. Each fuselage section is built as a single carbon-fibre barrel instead of being riveted together from many aluminium panels. Airbus answered with the A350, at about 53% composite. Airbus chose large carbon-fibre panels fastened to a frame rather than one-piece barrels, a different route to a similar result.

The benefits, in depth

• Weight. Composites are much lighter than aluminium for the same strength in the directions that matter. A lighter airframe burns less fuel on every flight for its whole working life. The 787’s overall fuel saving of around 20% compared with the aircraft it replaced comes from the lighter structure combined with new-generation engines.
• Fatigue resistance. Composites cope far better with repeated loading than aluminium. That means fewer fatigue inspections and the ability to run a higher pressure difference across the skin.
• No corrosion. Aluminium corrodes, especially when it’s damp. Composites don’t, which cuts maintenance and allows something passengers can feel: more humidity in the cabin.
• A more comfortable cabin. Because the fuselage can take a higher pressure difference, the 787 holds its cabin at the equivalent of about 6,000ft instead of the usual 8,000ft. Combined with the higher humidity, many passengers report feeling less tired and dried out after a long flight. Read more in our cabin pressurisation explainer.
• Bigger windows. The 787’s windows are noticeably larger than older jets’, partly because the composite structure tolerates bigger openings.
• Fewer parts. Building whole barrels or huge panels at once removes thousands of fasteners and joints, each a potential site for cracks and leaks.

The downsides, honestly

• Hidden damage. When aluminium is hit, it dents, and you can see it. Carbon fibre can bounce back from an impact, such as a baggage vehicle bump, looking fine on the surface while the layers have separated inside. Engineers call this barely visible impact damage. Finding it needs ultrasound and other non-destructive inspection tools, plus new training for maintenance crews.
• Repairs are specialist work. Fixing composite damage means grinding out the damaged layers, building up new ones and curing them under controlled heat. It’s slower and needs more specialist skill than riveting on an aluminium patch.
• Lightning protection. Aluminium carries lightning’s current around the outside of the aircraft naturally. Carbon fibre conducts much less well, so composite aircraft need a fine copper or bronze mesh built into the skin. See what happens when lightning strikes a plane.
• Galvanic corrosion. Carbon fibre and aluminium touching each other in the presence of moisture can make the aluminium corrode. That’s why composite aircraft use titanium for many fasteners and fittings, and one reason the 787 is about 15% titanium.
• Cost and production. Carbon fibre is expensive, autoclaves are huge investments, and building a new kind of airliner from scratch is hard. The 787 programme suffered long delays and production problems early on as Boeing and its global suppliers learned to build it at scale.
• Recycling. Melting down an old aluminium airliner is routine. Most aircraft composites use thermoset resins that can’t be melted, so recovering the fibres is harder and costlier, although the methods are improving.

Where metal still wins

Composites haven’t replaced metal everywhere, and probably never will. Titanium stays in the hottest and most heavily loaded places, around engines, in engine mounts and in landing-gear parts, because it’s strong, heat-resistant and doesn’t react badly with carbon fibre. Steel is still used where sheer strength in a small space matters, such as landing gear. And aluminium remains the main material of the world’s most numerous airliners, the 737 and A320 families, because it’s cheaper and faster to build in huge numbers, and for short flights the weight saving matters less.

What today’s aircraft use

• Boeing 787 and Airbus A350: composite fuselage and wings, about half the structure by weight.
• Boeing 777X: an aluminium fuselage but the world’s largest composite wing, built in a dedicated composite wing centre.
• Airbus A220: composite wings and tail on an aluminium-lithium fuselage.
• Airbus A380: about 25% composites, plus GLARE in the upper fuselage.
• Boeing 737 and Airbus A320 families: mostly aluminium, with composites in tails, flight-control surfaces and fairings.

What comes next

The next challenge is speed. Today’s best-selling jets are built at rates of dozens per month, far faster than the big, slow autoclave processes used for widebodies. Manufacturers are working on thermoplastic composites, which can be softened with heat and welded rather than bolted, and on curing methods that skip the autoclave. If those work at scale, the next generation of single-aisle jets could be the first mass-produced composite airliners.

Sources & Further Reading

• Boeing sets pace for composite usage in large civil aircraft — CompositesWorld ↗
• Aviation’s material evolution — Airbus ↗
• Designs that changed the way aircraft are built — Aviation Week ↗
• 787 Dreamliner by design — Boeing ↗
• Safe operations with composite aircraft — Airbus Safety First ↗
• Comet’s Tale — Smithsonian Magazine ↗