Bjorn’s Corner: Aircraft Structures Part 13. Dry fiber infusion.

Bjorn Fehrm's latest installment in Leeham News' aircraft structures series turns to dry fiber infusion, the second of the two dominant manual composite layup methods used in commercial aircraft manufacturing today. Unlike wet layup or pre-impregnated (pre-preg) fabric methods covered in the prior installment, dry fiber infusion involves laying dry, unimpregnated fiber fabric into a mold and only introducing low-viscosity epoxy resin later, drawn through the fabric stack by vacuum pressure. Fehrm walks through the process step by step—mold preparation, fabric tacking, application of release and flow fabrics, vacuum bagging, leak-checking, and controlled resin propagation—drawing on an Easy Composites Ltd. instructional video as a proxy for techniques he has personally observed at full industrial scale. The technical payoff, he notes, is a composite structure with fewer air pockets and voids than wet pre-preg layup typically achieves, provided the resin flow front is carefully managed so the last point wetted coincides with the vacuum outflow port.

The article's centerpiece is its explanation of how Bombardier—now Airbus—applies dry fiber infusion to produce the A220's wingbox spars and covers at the Belfast facility. This is a case study in how a manual, labor-intensive layup technique scales to primary structure on a certified commercial airliner. Fabric cutting is NC-automated, but the actual layup of cover panels and integration of stringer fabric into dedicated stringer molds remains manual work, followed by autoclave curing under heat and pressure to produce a single-piece, integrated cover-and-stringer assembly with void content below 1%. That figure is notable: it underscores why Bombardier invested in proprietary, patented techniques for stringer placement, since void content directly affects fatigue life, damage tolerance, and ultimately certification margins for wing primary structure.

For working pilots, this level of manufacturing detail may seem far removed from the flight deck, but it has direct bearing on aircraft performance, weight, maintenance philosophy, and dispatch reliability. The A220 wingbox is a load-bearing, life-limited structure; the quality of its composite construction affects everything from fuel burn (via weight savings versus aluminum) to inspection intervals and repair procedures that maintenance crews and MCC personnel must understand when composite damage is reported. Pilots flying the A220, and increasingly other composite-intensive types across Airbus and Boeing fleets, benefit from understanding that these structures are not monolithic metal parts but engineered laminates with specific cure histories, void tolerances, and repair limitations that differ fundamentally from riveted aluminum skin.

More broadly, this piece fits into an industry-wide trend toward greater composite content in primary and secondary structures, from the A220 and 787 to business jets like the Falcon 6X and various Gulfstream models. As manufacturers push for lighter, more fuel-efficient airframes to meet both economic and emissions targets, the manufacturing science behind composites—wet layup, dry infusion, and the automated methods Fehrm promises to cover next—becomes increasingly relevant to flight operations, MRO planning, and even insurance and residual-value assessments. Understanding these processes gives pilots and operators better insight into why certain composite repairs require OEM-specified facilities, why lead times for structural repairs can be lengthy, and why manufacturers guard techniques like Bombardier's stringer-placement patent so closely: these methods are foundational to airframe integrity and directly influence long-term fleet reliability and maintenance costs.