Composite materials have been used for a long time on aircraft now, up to the point that traditional materials such as aluminium alloy for aircraft structures are also being replaced with composites.

Within the aircraft cabin, besides the parts used for the flooring textile and the seats, composite materials are largely used for many other parts including sidewall and lining stowage compartments.

“For years, modern technologies have made it possible that even carbon fibre and glass fibre plastics can be used in the most complex geometries. Due to their lightweight, composite materials are very prominent in aircraft of the newest generation,” says Jörg Mäder, senior vice president, product innovation at Diehl Aviation.

“After the successful implementation of composite materials on the B787 and A350 structural parts (like the fuselage and wings), cabin parts manufactured with innovative materials and technologies are becoming very popular.

“The most common parts are seat shells, cabin walls and branding elements, which are currently made of plastic or glass-fibre composites,” says Stefano Capuano, project manager at ABC International.

According to Tony Morrin, technical director at AMS Composite Cylinders, as part of the constant drive towards weight savings, airlines are now looking to switch the aluminium and steel oxygen cylinders found in aircraft cabins, to lightweight carbon composite ones.

“These offer weight savings of around 50 per cent per unit. For a large passenger aircraft with a dozen cylinders on board, this provides a significant weight reduction,” he says.

Composite properties

In addition to being lightweight, composites are becoming increasingly common in aircraft cabins due to a set of very distinctive properties, including very high mechanical strength, a reduced need for subassemblies, lower production waste and significant room for production process improvements, according to Capuano.

“Composites, in general, are already widely used in the aircraft cabin: mainly in monuments (galleys and lavatories), stowage areas and overhead bins, but also to some extent in aircraft seats,” says Markus Höllerer, senior manager, corporate strategy at RECARO Aircraft Seating.

“The main advantage of using flat panel applications (or more complex geometrical shapes that can be molded) is the weight saving and the increased rigidity of the materials.”

Composites are stiffer than aluminium and this results in a high strength-to-weight ratio. “Additionally, versatile use and easy processing are special properties, and are therefore used more frequently in the cabin.

“In comparison to the outer structure of an aircraft, materials of inner parts are less susceptible to larger damage. In general, compared to many other components of the cabin, composite materials are designed to last the lifetime of an aircraft,” says Mäder.

A distinctive feature of carbon composite cylinders is their durability, according to Morrin. “Composite cylinders are designed to perform in the modern cabin environment and to stand up to regular movement and buffeting around in the overhead lockers.

“A key benefit of using carbon composite cylinders is that they can be filled to higher pressures (up to 300 bar) than their aluminium and stainless steel counterparts. This means that the cylinders can be smaller, saving valuable space inside the aircraft. Carbon composite cylinders can also be manufactured in customised sizes, to suit the space and layout requirements of each aircraft model,” he says.

“In addition, carbon composite gas cylinders are virtually maintenance-free. All they require is periodic testing every five years, the same way aluminium cylinders do. Today, there are non-limited life (NLL) carbon composite cylinders available on the market (prior to 2013, carbon composite cylinders were rated to a maximum working lifespan of 15 years).

“NLL cylinders can be used indefinitely, as long as they meet the periodic testing requirements every 5 years. In reality, NLL composite cylinders should outlast the working lifespan of the aircraft itself.”

“Our composite gas cylinders are accredited for use worldwide, holding a wide range of quality assurance accreditations, including: ISO 11119-2, UN-TPED Pi, DOT (US) and TC (Canada). They can be produced in customised sizes to suit the configuration of any aircraft cabin,” Morrin says.

“Moreover, fully wrapped carbon composite cylinders can feature integrated labelling, providing opportunities for airlines to add durable branding directly to the exterior of the cylinders during the initial manufacturing process.”

The main parameter considered in the design of aircraft interiors, and even more so when it comes to seats, is the optimisation of space.

“Optimising means allowing the occupant to have greater personal space, which translates into comfort and living space. Composites are materials that have a high aptitude for the shape, and therefore leave the designers a significant degree of freedom towards the choices of better shapes,” says Francesco Varriale, research and development engineering manager at Geven.

“Shape attitude, combined with the possibility of concentrating fibres, and therefore resistance only in the most critical areas, allows us to obtain very thin sections where necessary. If we think of a leaf we imagine it light and thin, and so is an aircraft seat today.

“The optimal shape allows us to reach optimal pressure distribution and therefore comfort, while the reduced sections allow us to optimise space, which results in increased living space. Additionally, composites feature excellent properties in relation to fatigue life and corrosion, meaning reduced scheduled maintenance and therefore longer durability.”

Composite challenges

Despite composite materials having enjoyed a long momentum, metallic materials will not be discarded forever. Varriale explains that metallic materials resist compression loads very well, but react badly to tension loads.

“Conversely, composite materials behave very well with tension loads and less well with compression loads. The ideal choice, therefore, will not be to completely remove traditional materials, but rather to design using hybrid metal and composite structures that allow an optimisation of the final performance. These ‘hybrid’ structures will find some application everywhere within the aircraft cabin,” he says.

It is notoriously difficult to understand whether a composite part is damaged or how seriously its structure is affected.

“Unlike on aluminium, damage may not always be detected by visual inspections, but it might cause severe damage to the inner structure which goes undetected by untrained personnel,” says Capuano.

“For this reason, all personnel who are responsible for the inspection and maintenance of composites must undergo special training on how to safely detect damage on the inside and outside of these materials. Most OEMs require specific training or certification to perform repairs on their products.”

Capuano believes that some of today’s biggest challenges with composites include the non-standardisation of processes among OEMs, which should be tackled through the collaboration among OEMs, operators, suppliers and service providers with ad hoc seminars, forums and committees.

Another challenge concerns the trade-off between composite repairs and brand new options. “Sometimes it is still more convenient to manufacture from scratch rather than repair a damaged composite part. Suppliers are working hard to lower the repair cost base through technological innovation,” he says.

A particular challenge with the repairs of composite materials concerns the fire resistance property. Resin is used for adherence of composite materials and has exact requirements of a certain quantity to maintain the light weight of the parts in combination with fire requirements.

“For repairs, the specialist staff must again use resin, which firstly makes the part a little heavier and deteriorates the fire property,” says Mäder. “At present, however, professionals can easily carry out repairs according to specifications and manuals.

“For instance, a repair of a tear in a duct has become easier due to precise specifications on the extent to which a part can be repaired or whether it must be completely replaced in order to prevent the fire property-related challenges.”

Varriale is particularly supportive of the practice of strictly following the maintenance manuals. “A few years ago composites were a very little known matter, but today they are within everyone’s reach.

“The approach that is needed is to draw from accumulated experience and best practice, and accept that to treat composite materials comprehensively you have to be very rigid in the application of procedures. It is certainly not the case to turn to these technologies with an ‘artisanal’ attitude,” he says.

“Repair of panels is straightforward nowadays and described in the maintenance manuals,” says Höllerer. The exchange of complete panels, however, is difficult within the aircraft cabin.

Therefore, the main challenge is to develop design solutions offering composite panels that are easily exchangeable, he adds.

Brand differentiation

Composites are also providing airlines with valuable opportunities related to cabin branding strategies. “As long as composites are manufactured through moulds and autoclave tools, design organisations can adopt different shapes and patterns in the most innovative ways,” says Capuano.

“As a matter of fact, during AIX 2018 in Hamburg, we showcased various products and samples manufactured with different composite materials. These are called Carbon Fiber, Eco-Fiber, Alu-Fiber and Hybrid Fiber. In parallel, we are currently working on branding projects with products composed of composite materials for more than 50 per cent. We see these figures set to become higher in the upcoming years.”

Usually airlines select classical geometrical patterns in different colours and geometries when it comes to pursuing branding opportunities.

“In the future we will offer more options via digital printing technologies. Complete sidewalls can be printed with full-faced patterns or pictures without visible joints. The corporate design of the airlines can be adapted with close consultation, and it can adapt to other cabin elements. Modern materials and the intelligent integration of various control elements, all combine to turn one single cabin part into a sophisticated system component,” says Mäder.

“Branding is an important aspect for our customers and we see the need for differentiation beyond just seats. Unique differentiation opportunities exist when going for customised 3D curvatures on seats and monuments. Large flat composite panels (like on front row monuments or partitions) offer the opportunity to fit specific airline branding panels,” says Höllerer.

“The comfort and living space optimisation enabled by composites are huge advantages for airlines. Moreover, the freedom of form allows to reach architectural solutions that also improve the latent quality of cabin product. When we enter a well-designed cabin, we must experience the wow effect as we sit down,” concludes Varriale.

Editor’s Note: The post was originally published in October 2018