Composite materials in modern aircraft — from carbon fibre to repair

Where older aircraft were largely built from aluminium, an increasingly large part of new aircraft today are made from composite materials — typically carbon-fibre-reinforced plastic (CFRP) and fibreglass, often combined with a honeycomb core in sandwich constructions. The Airbus A350 and Boeing 787 are both built with around half the structural weight in composite, and the development means that as an aircraft maintenance technician you must master a different set of inspection and repair techniques than those that apply to metal.

A composite material consists of fibres — carbon fibre or fibreglass — laid in layers and bonded together by a resin matrix that cures into a stiff light and very strong structure. In areas that need extra stiffness without extra weight such as flaps rudders and floor panels a sandwich construction with a honeycomb core material between two thin composite layers is often used.

• 01Lower weight results in lower fuel consumption
• 02No corrosion in the traditional sense, but moisture uptake must be monitored
• 03Fewer parts, because complex shapes can be cast in one piece
• 04Good resistance to fatigue from repeated load cycles

Composites' strength is at the same time their challenge in inspection. An impact — from a trolley, a dropped toolbox or shot — can cause internal delamination between the layers without visible denting on the surface. It is called 'barely visible impact damage' (BVID), and that is precisely why composite structures are inspected with methods other than the naked eye.

• 01Cosmetic damage on the outside of the protective layer can often be repaired with a resin coat (fill and finish)
• 02Structural damage requires an approved repair method from the SRM, e.g. a cut-out and patched repair
• 03Repairs are cured either in an autoclave under pressure and heat (workshop repair) or with vacuum bag and heat blankets directly on the plane (field repair)
• 04Any repair must be documented with category and whether it is temporary or permanent

Sanding dust from carbon fibre is conductive and can cause electrical short circuits if it gets into electronics, while being skin and respiratory irritant and must be collected with point extraction and filtered masks. Curing resins can cause allergic reactions on repeated skin contact, so gloves and proper ventilation are a fixed part of the work — not an option.

The transition from metal to composite has changed what an aircraft technician must be able to do from memory. Where a metal structure often warns of its damage clearly — a dent, a scratch, a discolouration — a composite structure can look completely intact even though it has lost much of its strength inside. This requires that you not only learn to perform a specific test method, but also learn to think about when suspicion of hidden damage should prompt your attention in the first place — for example after a hard landing, a collision with a jetway or simply long-term exposure to UV light and moisture.

At the same time the composite's wide application means you rarely work with only one material type on a modern aircraft. Most maintenance tasks require you to navigate between metal composite and combinations of the two — for example where a composite panel is bolted to a metal frame. Here knowledge of both materials' strengths weaknesses and repair methods becomes crucial so you can assess damage correctly and choose the right repair path from the SRM.