Composite Materials in Aircraft Construction

What Are Composite Materials?

A composite material combines two or more constituent materials to achieve properties neither could provide alone. In aviation, the term almost always refers to fiber-reinforced polymers: high-strength fibers (carbon, glass, or aramid) embedded in a polymer matrix (typically epoxy resin). The fibers carry tension and compression loads while the resin binds them, transfers loads between fibers, and protects against environmental damage.

Composites differ fundamentally from metals in that their properties are directional. A sheet of aluminum has the same strength in all directions (isotropic). A composite laminate can be engineered with fibers oriented precisely where strength is needed — stronger in tension along the length of a fuselage panel than perpendicular to it, for example. This tailored anisotropy is one of composites' great advantages: designers can put material strength exactly where loads demand it.

CFRP Explained

Carbon Fiber Reinforced Polymer (CFRP) is the dominant aerospace composite. Carbon fiber is made by oxidizing and then pyrolyzing polyacrylonitrile (PAN) precursor fibers in a carefully controlled process, aligning the carbon atoms into graphite-like planes that give extraordinary stiffness and strength. Aerospace-grade carbon fiber has a tensile strength of 3,500–7,000 MPa — roughly five times stronger than structural steel at about 20 percent of the weight.

Raw carbon fiber is supplied as prepreg (pre-impregnated) tape or fabric: fibers already coated with partially cured epoxy resin. Prepreg is stored frozen to halt curing until the manufacturer is ready. It is cut to shape, laid up in precisely oriented layers (plies), and cured in an autoclave — a large pressure vessel that applies heat (175–180°C) and pressure (5–7 bar) to cure the resin and consolidate the laminate without voids.

Manufacturing Process

Modern composite aircraft parts are made through several processes depending on part size and complexity:

• Hand layup / automated tape laying (ATL): Prepreg plies are laid onto a mold, either by hand for complex shapes or by ATL machines for flat or gently curved panels. The Boeing 787 fuselage barrels are made by winding prepreg tape around a mandrel up to 20 meters in diameter.
• Resin Transfer Molding (RTM): Dry fiber preforms are placed in a closed mold and resin injected under pressure. Used for complex shapes like rib flanges and brackets.
• Out-of-autoclave (OoA): Newer processes use vacuum bag curing at lower temperatures without an autoclave. Suitable for large structures where autoclave costs are prohibitive; the Airbus A400M cargo door uses this technique.

Boeing 787 vs. Airbus A350 Approach

The Boeing 787 was the first commercial aircraft with over 50 percent composite structure by weight. Boeing's most distinctive choice was the one-piece fuselage barrel: each section of the fuselage is wound as a single tube, eliminating the longitudinal skin lap joints used in all previous metal fuselages. The result is fewer parts (thousands fewer fasteners per barrel) and a pressure vessel that is naturally leak-tight. The 787's composite primary structure includes the fuselage, wings, empennage, and floor beams.

The Airbus A350 also achieves about 53 percent composites by weight but uses a slightly different approach: the fuselage panels are flat composite panels joined by composite frames and stringers, similar to the structural concept of metal aircraft but in composite. Airbus argues this is more repairable and better understood by maintenance organizations. Both aircraft use composites in the wing box, with CFRP spars and skins providing a structure lighter and stiffer than aluminum equivalents.

Benefits

• Weight reduction: The 787 is roughly 20 percent lighter than an equivalent aluminum aircraft of the same size, directly translating to fuel savings.
• Fatigue and corrosion resistance: Unlike aluminum, CFRP does not corrode or suffer metal fatigue in the traditional sense. This allows higher cabin pressure (lower cabin altitude, better for passengers) and extends service life.
• Higher cabin humidity: Aluminum fuselages must be kept very dry to prevent corrosion. The 787's composite fuselage allows cabin humidity of 16 percent vs. 3–4 percent on aluminum aircraft.
• Design flexibility: Complex curves and integrated structures that would require many metal parts can be made as single composite pieces.

Challenges

Composites are not a panacea. Impact damage is harder to detect: dents that would be obvious in aluminum can leave CFRP with internal delamination invisible to the naked eye, requiring ultrasonic or thermographic inspection. Repair is more complex and costly — repairing a 787 fuselage after bird strike or lightning damage requires skilled technicians and specialized equipment unavailable at smaller airports.

Manufacturing scale-up proved more difficult than Boeing anticipated: the 787 was over three years late largely due to supply chain and composite manufacturing challenges. End-of-life recycling of composite aircraft remains an unsolved problem, unlike aluminum which has an established recycling industry.