Jim Banks, director of Aero-Composites Design reports.
From a systems engineering viewpoint, the interdependencies of systems including the airframe become more evident. It's often viewed that an airframe, once qualified and certified ‘fit to fly', is subject to little variation in its key features. This is not true and whilst change is often organic, it impacts the airframe and installed systems.
Obsolescence results from change; the major drivers affecting airframes are: legislation and industry practices; material supplies; customer requirements.
Replacing materials for better/less hazardous alternatives is an established practice. It may not just be the material causing concern: the manner in which it is produced or fabricated produces hazardous waste, high volumes of carbon, ozone depleting substances, high costs in terms of energy use, poor fly/buy ratios and high variability in the quality of parts produced.
Change drivers may be combinations of environmental issues, technologies deployed, manufacturing processes and changes in legislation and regulation together with improved industry practices. The situation is complex, often requiring specialist in-depth knowledge of material and manufacturing processes to become compliant, even with the law.
For metallics (and some non-metallics) the emphasis is upon removing heavy metals and hazardous chemicals used in processing. RoHS regulations such as EU directive 2002/95 ban the placing on the EU market of, ‘new electrical and electronic equipment containing more than the agreed levels of lead, cadmium, mercury, hexavalent chromium, PBB and PBDE flame retardants. All paints, sealants, coatings, plastics, rubbers, etc, used in airframe manufacture must pass fire, smoke and toxicity tests and as such my contain PBB and PBDE'.
Worldwide, airframe engineers share a problem with systems engineers. Aerospace is a relatively small volume user of materials and chemicals on the world stage. The industry is classified as ‘downstream users' but availability of chemicals is driven by legislation and market forces on the raw material producers.
By 2013, REACH is targeting the removal of hexavalent chrome from the EU supply chain. This is a core constituent of anodic anodising and chromate-based sealants. These industry standard protective treatments are key to controlling corrosion and, where they form part of the aircrafts structural earth return paths, maintaining electrical conductivity, connectivity and electromagnetic hazard protection for major aluminium structures.
Chromate-free sealants are available, but most aerospace products now in service were qualified using chromate based variants. Replacement will require requalification and some re-testing. Balancing the costs and delays incurred against the remaining life of some older products will require careful management.
Replacement treatments for anodising such as electroless nickel have been introduced on COTS equipment. However experience shows that electroless nickel plating is less robust with evidence of corrosion after approximately 12 months. This would not be a viable option for use on airframe parts. Furthermore nickel is a heavy metal and there must be questions about its longer-term availability before being affected by legislation.
The growth in composites for structural airframe components brought a sea change in volume airframe technologies. But this increased demand for composites brings with it other issues: those of world supply/demand, affects of green legislation upon the production of composite materials and composite components, and the recycling of waste.
The growth in world demand for composites comes from three sectors (see figure 1): 60% - industrial – low/medium modulus fibres (including automotive, marine, civil engineering, medical, wind energy); 20% - aerospace – high modulus; 20% - sports applications – low/medium modulus.
Figure 1: Civil aircraft carbon fibre demand
Total carbon fibre production in 2008 was about 30,000 tonnes and is predicted to rise to 70,000 tonnes/year in 2015. Indications are that this could rise to 300,000 tonnes/year by 2020, driven by demands from the industrial sector, increasing that sectors' market share and market influence.
Focusing on the aerospace sector; in 2009 the prediction was that by 2014 the sector needs just over 17,000 tonnes/year of high modulus fibre.
The predicted demand from four major civil projects (see figure 2) for the same period shows fly away demand of approximately 16,500 tonnes/year. The timeline may be modified due to fiscal issues but indications show these levels being reached within a two to three year window.
Figure 2: Sector use of carbon fibre in tonnes/year
Applying a fly/buy ratio of 60%, the actual material demand is nearer to 27,500 tonnes/year; significantly outstripping supply for four projects alone. By comparison a modern major military project with a production rate of 600 aircraft would have a ‘lifetime' buy for first time build of about 5,000 tonnes.
One obsolescence implication is that materials used in the design, test and qualification of a project may not be available for its lifetime with future build and in-service support (spares and repairs) needing vey robust long-term plans. Lifetime buys of fibre may be an option but lifetime buys of resin is not and the cost-effectiveness of lifetime buys is questionable.
Obsolescence and its wider context
In October 2003, Ted Dowling, technical manager, QinetiQ Consulting was quoted as saying: “A system is obsolete when it no longer fulfils a significant stakeholder need and cannot be made to do so in a cost-effective way.”
Composites could then be viewed as facing obsolescence in 5-10 years. Add that aerospace projects that don't use common fibre/resin combinations (especially on military projects), and it's feasible to see a long-term consolidation of specifications focusing in on supplying a few major civil programmes.
The process of producing carbon fibre materials from the Polyacrylonitrile precursor through carburisation into carbon fibre isn't efficient as it uses high levels of energy, hazardous chemicals and produces significant waste even before fly/buy ratios are considered.
Transforming Polyacrylonitrile into carbon fibre is at best 50% efficient and there is little reasonable potential to increase the worldwide supply of Polyacrylonitrile to meet the demand for raw carbon fibre at the 2020 predicted levels.
Taking a holistic view, the supply/demand figures for carbon fibre over the next 5-10 years are significantly imbalanced. Production methods are at odds with environmental precepts of reducing hazardous materials usage, energy demands and reliance upon oil-based products. Metallic technologies face similar issues. Whilst some aluminium small part production is moving towards alternative technologies, a solution to replace chromic acid anodising for large scale airframe aluminium products with a robust, economic and electrically conductive alternative, remains outstanding.
In conclusion, aerospace programmes, in service, in build, in design or at the concept phase now need to focus more firmly upon the potential obsolescence of the airframe due to material supply issues.
With thanks to Professor Andrew Walker, CEO National Composites Certification and Evaluation Facility, University of Manchester for assisting with the analysis of composite material supply and demand figures.
jim.banks@dogzone.myzen.co.uk
