Bjorn’s Corner: Aircraft Structures Part 8. Composite Fibers.

Bjorn Fehrm's latest installment in Leeham News' aircraft structures series moves from foundational composite theory into the comparative engineering trade-offs among fiber types—glass, aramid (Kevlar), and carbon—that define how modern airliners are built. The piece quantifies strength, stiffness, and toughness using standard SI metrics (MPa, GPa) rather than the imperial ksi units common in older US engineering references, a deliberate choice that underscores how thoroughly international the composite supply chain and design community have become. The core technical revelation is the toughness-versus-strength trade-off: carbon fiber offers superior absolute and specific strength along with high stiffness (Young's Modulus), but this comes at the cost of brittleness—carbon fractures instantly once its tensile limit is exceeded, with no plastic deformation to absorb impact energy. Kevlar 29, by contrast, is the weakest fiber in absolute terms but excels in specific strength and toughness, explaining its continued use in fan containment structures designed to survive a blade-out event.

For working pilots, particularly those flying the 787 and A350, this material science translates directly into how their aircraft behave under real-world damage scenarios—bird strikes, hail, ramp equipment contact, and ground handling incidents. The article's breakdown of where Boeing and Airbus deploy each material is instructive: carbon fiber dominates high-load primary structure (wingbox spars, covers), while tougher, more impact-resistant materials—fiberglass, aluminum alloy, or carbon sandwich construction—are reserved for leading edges, fairings, radomes, and mechanism covers where impact resistance matters more than raw strength-to-weight. Notably, Airbus opted to keep the A350 cockpit structure in aluminum alloy rather than composite specifically because that section must reliably withstand hail and bird strikes without catastrophic brittle failure. Pilots who fly these aircraft benefit from understanding why certain zones show different damage tolerance and repair protocols than others—a composite fuselage skin puncture from a catering truck is a fundamentally different structural event than a similar impact on a legacy aluminum aircraft, with different inspection thresholds and repair pathways dictated by these same material properties.

The article also surfaces a program-management dimension with real operational history: the low toughness of carbon fiber reinforced polymer (CFRP) was reportedly a primary driver of development delays on both the 787 and A350. Engineers discovered that fuselage skin thicknesses optimized purely for aerodynamic and flight loads were too thin to survive routine ramp rash and ground service impacts—a problem that didn't exist in the same way with legacy aluminum fuselages, which have more forgiving ductile deformation characteristics. Solving this required additional reinforcement layers, revised stringer and frame designs, and supplementary layers for lightning-strike electrical dissipation, adding both weight and schedule risk. This history is directly relevant to maintenance and operations personnel today, since it explains the more conservative damage-tolerance philosophy and stricter ground-damage reporting culture that airlines have had to adopt around composite-fuselage aircraft.

More broadly, this series illuminates a trend reshaping the entire industry: the shift from isotropic metallic structures—uniformly strong in all directions and comparatively forgiving—to anisotropic composite structures that are meticulously tailored, layer by layer and fiber orientation by fiber orientation, for the specific loads a given zone will encounter. This is not merely an academic distinction. It means the next generation of pilots, maintenance technicians, and even accident investigators must think about airframe integrity in fundamentally different terms than those trained on Boeing 737NGs or Airbus A320ceos. As composite usage expands into narrowbody replacements, business jets, and eVTOL airframes, understanding why a wingbox is carbon, a radome is fiberglass, and a cockpit nose section might revert to aluminum becomes increasingly relevant to flight crews assessing damage reports, dispatch reliability, and the real-world resilience of the aircraft they fly.