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FIT TO FLY? F
AIRFRAME OBSOLESCENCE: Obsolescence changes everything: Major drivers affecting airframes are legislation, industry practices, material supplies and customer requirements
Heightened recognition of aerospace obsolescence issues has understandably concentrated upon individual system issues, down to piece parts, but does this risk not seeing the wood for the trees? Jim Banks, director of Aero-Composites Design reports.
rom 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 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
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