Can lightweighting aircraft aid the industry’s recovery?

Peter Zuber, Senior Manager, Composites and Lightweighting at SABIC, discusses how the right materials could help get the world moving again.

Research from the International Air Transport Association (IATA) has predicted that $84.3bn is set to be wiped off airline balance sheets due to COVID-19¹ and the existence of some airlines are under threat.

As such, these companies must take proactive, and creative, measures to help adapt and overcome the current situation. One way to save the operators money is to reduce the aircraft weight for greater fuel savings, whilst still maintaining its structural integrity.

Cruising into the 21st century

Although more and more aircraft are being built using advanced materials technology (such as carbon fiber composites) for the exterior structure, the shift from traditional materials to lighter weight modern materials on the aircraft interior is not happening fast enough, and the emergence of COVID-19 and subsequent drop in R&D funding has exasperated the problem.

Manufacturers have historically used metals like aluminum for the exterior and the internal structural parts that form the aircraft skeleton. More recently, advanced commercial aircraft have been engineered with carbon fiber reinforced thermoset materials to replace the heavier metal structures. These areas of the aircraft require these materials because of the high loads and harsh environments they must endure.

The interior of the aircraft consists mostly of composite sandwich panels that are typically based on thermoset phenolic skins, reinforced with interwoven glass fiber and a honeycomb core constructed from heat-resistant aramid fibers. The issue with these traditional materials is that they add a significant amount of weight to the aircraft, which ultimately requires more fuel for the aircraft to get off the ground and maintain cruising altitude.

Another drawback to the use of thermoset plastics is in the manufacturing process. Whilst these materials provide the required rigidity, they cannot be remolded after their initial forming, meaning decommissioned aircraft components cannot be recycled for use in new applications. Although initially more expensive, switching to thermoplastics allow manufacturers to more efficiently mold parts and also to recycle parts without any significant reduction in physical properties or structural weaknesses, saving money in the long run.

SABIC’s material portfolio offers solutions whereby much of the cabin’s interior thermoset or metal components can be replaced with thermoplastics or thermoplastic compounds. By using a range of modern manufacturing techniques like injection molding or extrusion, the aircraft’s weight is reduced, maximising fuel and cost efficiencies while providing end-of-life recycling options.

Aircraft manufacturers are already using SABIC solutions on passenger service units, window frames, table trays, armrests, electrical connectors, ventilation systems, panels, and seat cladding. Additionally, concepts have been developed for composite panels for sidewalls and ceilings, while hybrid molding is being investigated to reduce the weight of armrests and structural seatbacks.

All of these materials can be widely recycled, and remolded, to build new components when needed, helping to save costs on components, improving sustainability and reducing the carbon footprint. All of which are important traits in today’s sustainably-focused world.

This has the potential to be a game-changer for the aviation sector, which is constantly in the public eye thanks to the 915 million tonnes of carbon dioxide (CO₂) emissions² it produces annually.

Helping airline suppliers reach new heights

The use of thermoplastic materials in aircraft interiors can help to reduce the weight of aircraft but these materials must also meet stringent standards for flame, smoke, toxicity and heat release.

To help meet these specifications, SABIC developed aerospace grades of its ULTEM and LEXAN FST resins. These are amorphous thermoplastic polyetherimide (PEI) and polycarbonate copolymer resins respectively which meet all rigorous aerospace industry requirements, including the FAR25.853 and OSU standards.

SABIC aerospace grades can be used as the basis to explore new methods of manufacturing such as thermoplastic composites. For example, to produce a honeycomb core, the resins are extruded into hollow tubes that are cut to length and joined using a thermo-bonding process for adhesion. For the laminate skin, a glass fiber or carbon fiber fabric is incorporated into an ULTEM resin matrix. After the sandwich structure is assembled, it can be thermoformed into two- and three-dimensional shapes, and directly painted with no prior preparation.

By using SABIC resins, manufacturers can produce a honeycomb core product that meets aircraft and transportation industry specifications for passenger safety. As the ULTEM resins are inherently flame retardant, they can avoid the use of halogenated additives which are regulated by the European Union and other countries as hazardous materials.

Unlike some other amorphous thermoplastics, these resins retain strength and resist stress cracking when exposed to automotive and aircraft fluids, aliphatic hydrocarbons, alcohols, acids, and weak aqueous solutions. The resin also provides exceptional stiffness at a very low weight, contributing to improved fuel savings for the operator.

Developing new materials to lighten the load

Now, whilst it’s true that lightweight composites are already being adopted by industry heavyweights like Boeing, Airbus, and Bombardier. work must be done to increase their adoption across other areas of the growing aerospace market. Research by the Aerospace Research Institute (ATI)³ explored some of the perceived barriers to lightweight composite adoption and finance was identified as the biggest roadblock. Companies were dissuaded by factors such as the cost of materials, R&D investment and access to capital equipment.

These may be legitimate concerns but neglecting the use of lightweight composites is arguably short-sighted. By focusing on an innovation cycle for composites technologies, the industry can create a scalable, cost-effective, composite manufacturing system and harness the materials’ performance benefits.

The aerospace sector is typically regarded as an early adopter of new and innovative advanced manufacturing methods. This is largely because the sector faces unique challenges due to the extreme environmental conditions experienced by aircraft, wherein the component materials must be strong, lightweight, and highly resistant to fluctuating temperatures and corrosion.

These challenges have driven the industry to develop a range of advanced manufacturing solutions. Additive manufacturing, often called 3D printing, was widely adopted and is often used to make lattice structures which can help reduce weight and dissipate heat. Furthermore, given the extremely high safety standards rightly expected of aerospace firms, high precision is essential. As such, the industry adopted laser beam welding to replace traditional welding techniques to maximise precision and repeatability. But this isn’t the procedure’s biggest benefit. Laser beam welding transfers virtually no heat to the affected zone, meaning there are no heat fractures or potential failure points.

Charting a course for other industries

The race for early adoption of advanced manufacturing methods is a trait which should inspire other safety-critical industries to follow suit. Sectors like oil and gas, offshore renewables and maritime shipping should explore similar modern engineering systems to experience their associated benefits. Using 3D printing techniques to produce working prototypes, for example, would enable the industries to cheaply stress test key equipment and components against specific variables to see how they perform before manufacturing at scale.

However, as with any safety-critical work, particular effort must be made to ensure the developments meet and surpass, required industry standards. SABIC, for example, commits a significant portion of time to work with trade bodies and classification societies, such as the Federal Aviation Administration (FAA) for aircraft applications, to test and qualify its solutions for deployment.

Looking to the future, as more aircraft manufacturers widely adopt the use of composite materials to help reduce the overall weight of their airframes, they will be able to leverage the associated cost and efficiency gains, whilst the environmental benefits will help them keep pace with their increasingly eco-conscious customers. By truly implementing lightweight procedures and materials, the aerospace sector can bounce back from the crippling effect of COVID-19.

1 https://www.iata.org/en/pressroom/pr/2020-06-09-01/

2 https://www.atag.org/facts-figures.html

3 https://www.ati.org.uk/media/lw4f212o/insight_9-composites_amended-2018-09-20.pdf