Bjorn’s Corner: Aircraft Structures Part 12. Composite production.

Bjorn Fehrm's latest installment in Leeham News' aircraft structures series turns to the practical mechanics of composite production, focusing on hand layup—the oldest and still most widely used method for building composite aerostructures. The piece walks through the fundamentals: epoxy-impregnated carbon fiber or fiberglass fabric is laid by hand into a female mould, vacuum-bagged to compress out trapped air, and cured either at room temperature over several days or in an oven at temperatures up to 170°C for as long as 24 hours. Fehrm draws on a Diamond Aircraft facility example to illustrate how this process is used today for general aviation aircraft, gliders, and increasingly for eVTOL prototypes, where certifiable material systems like those under the NCAMP program allow new entrants to get airborne without developing bespoke, from-scratch material qualifications.

For working pilots, especially those flying or operating light GA aircraft, gliders, and emerging eVTOL platforms, this technical deep-dive underscores why airframe quality and provenance matter beyond the marketing language of "carbon fiber construction." The article makes clear that not all composite parts are created equal—resin-to-fiber ratio, porosity, cure temperature, and pressure all directly affect structural strength, and hand-laid parts using wet impregnation are inherently more variable than machine-controlled prepreg processes. Porosity above 1% introduces stress concentrations that can degrade fatigue life, a point that ties directly back into earlier installments of this series on notch sensitivity in composites. Pilots and maintenance personnel evaluating aircraft with hand-laid composite structures, or considering acquisition of light aircraft and eVTOL types built this way, should understand that manufacturing method is a first-order determinant of airframe integrity and long-term durability, not merely a cost or weight consideration.

The broader significance for the industry lies in the tradeoffs between production method, cost, and certification pathway. Hand layup remains attractive for low-volume GA and glider manufacturing and for eVTOL startups because it requires minimal capital equipment and can leverage pre-certified material systems, accelerating time to airworthiness approval. But as Fehrm notes, this comes at the expense of production rate and consistency compared to automated methods, autoclave curing, or precisely metered industrial prepreg systems—the same tension playing out across the eVTOL sector as companies balance speed-to-market against the rigorous structural substantiation regulators demand. The mention of autoclave curing using Hexcel's 8552 resin system, a material widely used in transport-category aircraft, signals where this series is headed: contrasting the artisanal, labor-intensive methods described here with the higher-volume, higher-consistency automated fiber placement and autoclave processes used by Boeing, Airbus, and major business jet OEMs for primary structure.

This series matters to the broader aviation community because composite usage continues to expand across all segments—from Part 23 GA aircraft and gliders to business jets and the nascent AAM/eVTOL sector—and understanding the production science behind these structures helps pilots, maintainers, and fleet operators better assess airworthiness directives, repair standards, and the real-world durability of composite airframes they fly and manage. As eVTOL certification programs accelerate under FAA and EASA scrutiny, the manufacturing method disclosed in a type certificate's means of compliance will increasingly matter to operators evaluating these aircraft for commercial service, making Fehrm's granular explanation of resin systems, cure cycles, and porosity control directly relevant to procurement and safety discussions happening across the industry today.