Increasing the use of composite materials in cabin products has many clear benefits, with the JPA Design/Williams Airtek seat targeting a weight reduction of some ten percent versus comparative products. But in the same way that airframes using composite materials as primary structure elements are fundamentally different to aircraft with those elements made from metals, there are certification challenges to these new components within the cabin environment.
“Composite components should be designed and substantiated as composites, not just replacing metallic design using composite materials,” explains Nigel Smith, managing director and head of design at SWS Aircraft Certification, which is also part of the Airtek team. “Airworthiness certification of composite aircraft structures, parts and appliances has to deal with the continual change in certification requirements as composite materials replace metal as main structural components.”
Fundamentally, Smith says, “Certification has to take into account the design, manufacturing, maintenance aspects and operation of composite products and the operational challenges that will be faced by an increasingly large fleet of aircraft with composite structures as they age.”
One key change from previous materials is that composite materials can be designed and manufactured so that different sections of the same component have different properties and perform differently. For example, for carbon-fibre reinforced materials, changing the direction, length or type of fibre in a particular section means it may be stronger, more damage tolerant, more pliable, and so on.
The regulations are, perhaps unsurprisingly, complex, with SWS citing key areas requiring more extensive substantiation during the certification process as the US 14 CFR and EASA CS25 sections on:
- Material and Process Control [§25.603, .605]
- Design Values [Ref. §25.613]
- Proof of Structure (including Damage Tolerance*) [Ref. §25.307, .561, .562, .571*]
- Environmental & Protection [Ref. §25.609, RTCA DO-160]
- Fire Properties [§25.853 & Appendix F]
- Function & Installation [§25.1309]
- Continued Airworthiness [§25.1529 & Appendix H]
On the flammability and toxicity side, Smith says, “There is no difference as far as the regulations are concerned, but due to the nature of the variations in material and processes of the composite, the base materials often require a larger quantity of tests. Toxicity is not currently a certification requirement unless you choose to comply with the Airbus additional requirements specified in ABD0031.”
Final trim and finish material combinations also require testing — with even slight design changes and any supplier shift requiring repeat testing — and any item falling into the category of ‘large panel’ requires smoke density and heat-release testing. For composites, this is more relevant to the resin systems in which the fibres are bound than the structural fibres themselves.
In terms of that structure, the modular monocoque design is crucial, explains Francesco De Cola, senior engineer for mechanical test and development at Williams Advanced Engineering. That modularity has similarities to the Formula 1 cars with which the Williams name is readily associated.
The Airtek design balances the drive towards a lower number of components per seat with the benefits of independent, swappable and lightweight subassemblies: a sort of ‘PaxEx pitstop’, if you will, where specific replacements are quick and weigh in under the point on the scale at which a second technician is required to swap out a part.
One key change is that the primary load-bearing structure for the seat is the composite plinth via which it is attached to the rest of the aircraft. This structure is attached to the specific seat track system used on individual aircraft types in a novel way.
“The seat-track attachment strategy used for Airtek is innovative for both number of feet used per seat and foot design,” De Cola says, noting that “we cannot give too many details at this stage. What I can say is that the feet design, which is under development, allows for a reduction of the deformation that gets transferred to the composite seat plinth during the seat dynamic testing, whilst satisfying the allowable interface loads.”
It’s all certainly a challenge: getting the interface between composite structures and the non-composite-based rest of the aircraft will be complex here, as it has been at the airframe level. But it’s one that’s both necessary and, it seems, possible.
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Featured image credited to JPA