Recycling of Aircraft: State of the Art in 2011

Recently, the end-of-service life for aging aircra and related parts has become a key subject in recycling industries worldwide. Over the next 20 years, approximately 12,000 aircra currently utilized for different purposeswill be at the end of service.us, reclaiming retired aircra by environmentally responsible methods while retaining some of the value becomes a signi�cant need. Recycling aircra components and using these in different applications will reduce the consumption of natural resources as well as land�ll allocations. Compared to the production of virgin materials, recycling aircra will also reduce air, water, and soil contaminations, as well as energy demand. In the present study, we have investigated the environmental bene�ts of recycling and reusing aircra components in the same or similar applications as low-energy input materials. During the aircra recycling, most of the aircra components can be recycled and reused aer reasonable modi�cations and investments.


Introduction
"Wastes [end-of-life material] from one industrial process can serve as the raw materials for another, thereby reducing the impact of industry on the environment" (Frosch and Gallopoulos, 1989) [1].is statement is one of the major motivations for the authors to focus on recycling aircra and related components.Recycling aircra is a series of activities: collecting recyclable materials and devices from aircra that would otherwise be considered waste and sorting and processing those useful materials into raw materials for future aircra and other industrial applications.Hundreds of recyclable materials are available in aging aircra, and this number continually increases, based on economic and technological developments in the reuse �eld.At the end of life (EoL), aircra are oen placed in aircra graveyards/parking places where these sit and degrade as the result of environmental in�uences, such as �V light, moisture, and oxygen/ozone.In most cases, the useful materials from aircra are high tech and should be valued for future production and materials conservation.e following section will focus on aircra recycling companies, aircra recycling methodologies, and related references.Environmental bene�ts associated with aircra recycling and rising market perception of regained products will be reviewed.
Generally, aircra are composed of a number of different materials and devices, including long and short carbon and glass �ber composites, wires, aluminum, titanium and steel alloys, foam, textiles and carpet, landing gears, �uids, electronic devices, engines, and other parts.Sometimes the complexity of materials and devices in aircra (e.g., military, business jet, and civilian) can reduce the recyclability rate; therefore, during the initial design, manufacturers should consider the EoL of aircra.

Recent Progress in Aircraft Recycling
Currently a number of aerospace alloys, including aluminum and magnesium-based alloys, as well as nickel, cobalt, and titanium-based alloys, have been produced and used for aircra manufacturing.Aluminum alloys are mainly copper, zinc, manganese, silicon, and magnesium.e two main classi�cations of aluminum-based alloys are cast and wrought, both of which are subdivided into heat-treatable and nonheat-treatable categories.Approximately, 80% of aluminum alloys are produced by the wrought process in the form of sheets and foils that are higher in strength and lower in density, which are desired by the aircra industry.Among the major aluminum alloys commonly used in aircra and other aerospace applications (helicopters and spacecra)-7075, 6061, 6063, 2024, and 5052-the 7075 aluminum alloy is most preferred by the aircra industry.e composition includes 5.1%-6.1% zinc, 2.1%-2.9%magnesium, 1.2%-2.0%copper, and less than 0.5% of silicon, iron, manganese, titanium, chromium, and other trace metals.ese alloys are extensively employed in aircra fuselages and other engineering structures and compounds in which light weight and corrosion-resistant materials are highly required.
Additionally, specially designed alloys make it possible for the aircra industry to produce high-strength products for jet engines and airframes where high temperature, pressure, and vibration are included during design and manufacturing.Also, stainless steel, nickel, copper, titanium, and these alloys are the major components of aircra alloys employed for engine blocks, providing high strength and the ability to perform at particularly high temperatures and pressure [2].
Bombardier became the �rst original equipment manufacturer to emphasize aircra dismantling operations and, in turn, obtained a dismantling certi�cation from the Aircra Fleet Recycling Association (AFRA) in 2010.e company successfully dismantled a CRJ100/200 regional jet, which was recognized by the AFRA as one of the best practices in the �eld.In August 2010, Bombardier Aircra Services of Charlotte, North Carolina, disassembled 10 CRJ100/200 regional jets for refurbishing and remarketing useable components for different aircra companies.ey recovered 1,500 reusable parts, including 300 line-replaceable units per jet [3].
Carbon Fiber Remanufacturing (CFR), a company established in 1997 in Whitewater Kansas, specializes in the recycling of carbon �ber of composites.is company generally recycles scrap carbon �bers obtained from the manufacturing process and reuses about 30% of these in further manufacturing processes to make new composites.CFR utilizes detangling, cutting, and hammer milling processes to remanufacture carbon �ber scraps for different product applications.e recycled carbon �bers are used as secondary raw materials without any mechanical property losses, so the remanufactured carbon �bers are nearly the same as the virgin carbon �bers.ese can be cut into speci�ed lengths (0.6 cm-7.5 cm) and integrated into nonwoven rolled cloth products and compounds for preferred speci�cations.ese can be added into �ber-reinforced plastics and polymers to make composites and also blended with glass and Kevlar �bers in order to develop new products.Recycled carbon �bers can be used in structural, insulation (thermal and acoustic), and �ltration (air and liquid) applications.CFR provides many recycled materials to local and national aircra industries, as well as appliances, agriculture, construction, automotive, waste water �ltration, trucks, textiles, military equipment, outdoor products, power generation, and sporting goods [4].
Recent studies show that the AFRAs series of best management practice guides have become the industry standard for dismantling and recycling aircra [5].Since 2008, 16 companies have been accredited by AFRA worldwide, 10 of which are located in the US and the remaining in Europe [6].Since these companies were certi�ed by AFRA, they have experience in how to dismantle and disassemble aircra efficiently.ey offer a reliable source of high-quality aer market airframe spares to global customers, primarily including avionics, electronics, engines, rotables, landing gear, interior decorations, and other �ight control parts [6].us, AFRA is the major accreditation process and provides the aircra and recycling industries with all necessary procedures and tools in order to maximize environmental efficiency and economic pro�t during aircra disassembly.is increases the value of the recyclable aircra at EoL by developing new technologies and approaches that utilize higher-valued materials and devices from aircra and other closely related industries.
AFRA and Boeing also intend to reduce the amount of aircra manufacturing waste transferred to the land�ll by 25% by 2012.AFRA is the global organization dedicated to environmentally responsible management of airplanes during the EoL service and also to the practice of continual life-cycle improvement.According to the AFRA, the quality of recycled composite materials needs to be improved, and new applications and markets for inside and outside the aviation sector need to be de�ned.Recycling also makes excellent business sense, because the market desires are satis�ed at lower costs [5].
Processing for the advanced management of EoL for aircra (PAMELA) is a program for aircra dismantling that is supported by the European Commission's "LIFE" initiative.is dismantling project mainly focuses on subjects, including management, recycling, and reduction of land�lls.In this project, the Airbus Company dismantled a 24-year-old airplane (A300B4) into four categories: structure, cockpit/cabin/cargo, systems, and power plant.e main idea behind the project was to increase the amount and quality of materials from retired aircra.Purity was not the main issue in this project, but the quantity and amount of recovered materials (e.g., mostly nonferrous materials) were considered major advantages.Unlike current processes, dismantling was more efficient and recovered up to 70%-80% of the scrap by weight for reuse.According to Airbus, recovered scrap metals are reasonably pure, so that the recovered aluminium alloys and other parts can be reused directly in the aerospace industry.
In addition to economic returns, recycling has environmental bene�ts, as well.For example, aluminium manufacturing is an energy-intensive process due to the electrolysis step (or Bayer process).However, when aluminium is directly recovered and reused, it reduces the initial energy by 90% [7], which in turn also reduces raw material consumption.e dismantling process consists mainly of three steps de�ned by PAMELA.e �rst step is decommissioning, which includes cleaning, draining of tanks, and various safety procedures.e second step is disassembling, which consists of equipment and parts removal from the body of the aircra.e third step is the �nal draining of the systems, removal of hazardous materials, and deconstruction of the aircra.According to Airbus, some of the components can be reused if the conditions of the parts are still in good shape, such as engines and engine parts, the auxiliary power unit, landing gear, avionics, system equipment, and movable parts and devices.Many other good-quality materials and devices can be directly recycled and reused as provided below: (i) some components, such as �uids (fuel, oil, and hydraulic �uid), security and safety equipment, batteries, avionics, and tires (requiring regulatory recycling); (ii) aluminum, titanium, and nickel alloys; hot-rolled and corrosion-resistant steel alloys; wiring; harnesses; thermoplastics; foams; textiles; carpeting; papers/tissues with special recycling techniques; (iii) cabin and cargo lining, wastes, and various other parts that are not recovered and usually go to land�lls and aircra graveyards; (iv) all composite parts of the aircra, including the fuselage and interior parts could be potentially utilized in other industries.
(v) the entire aircra used for demonstrations and exhibitions in shows and museums for raising public attention for aviation.
Figure 1 shows the aircra life cycle as well as a reverse supply chain of an airplane.It was reported that the initial weight of an average airplane is 106 tons, and, aer three steps in the dismantling process, about 85% of the airplane materials could be recovered and the remaining 15% put in land�lls [7].e 85% of the recycled parts are either used, as is, in the same �eld, or modi�ed for other applications.Further studies are needed for the unrecoverable materials and parts of the aircra.It was recommended that the remaining 15% of the components and materials such as interiors could be used in other aircras [8][9][10].Airbus reported that approximately €250-€300 could be spent per ton of dangerous aircra parts during storage, such as with asbestos and hexavalent chromium (chromium VI) of the aircra skin.Lately, Airbus has been persistently working on this subject to further decrease the high costs of the land�ll allocations [8,9].
P3 Aviation is the United Kingdom-based supplier of rotating components (e.g., engines, actuators, turbines, generators, and alternators), aiming to take advantage of the growing market for dismantling and recycling aircra.is company supervises the removal process from aircra during the deregistering, dismantling, and selling of parts at the highest values.e company has already dismantled seven aircra in 2010 and successfully turned aircra scraps and cabin parts into cash.According to the company, the interior part of the aircra generally goes to the land�ll, but they have sold entire interiors of Boeing 737-400 airplanes.e company also sells diverse aircra parts for use as office furniture and for decoration purposes.e fuselage has been utilized for desk partitions, �aps and stabilizers for boardroom tables, and engine inlet cowls as reception desks in bars and restaurants [10].
With the aim of recycling plastics and composite materials from aircra, Allred and Salas (2005) investigated a lowtemperature catalytic tertiary recycling process.ey claim that the catalytic conversion process could transform all types of plastics (e.g., rubber, thermosets, and thermoplastics) into valuable hydrocarbon products and fuels.Catalytic conversion, a closed-loop process without access to the environment, is nonpolluting because of the rapid conversion times.Other experiments on the utilization of used plastic blast media, hazardous waste streams, and composite scraps and parts involve a low-temperature catalytic conversion process.It was stated that using this conversion process for recycled plastics and other hydrocarbon-associated materials could reduce hazardous substances by a factor of �ve.Inorganic parts, such as heavy metals and other oxides, could be remelted to eliminate toxicity and used in aircra.Imide, polyester, epoxy, and other engineered thermoplastics and composite matrices could be converted into low-molecularweight hydrocarbons to produce valuable �bers for reuse in the fabrication of additional composite materials for aircra and other industries.Economic analysis illustrates that a recycling plant based on this method will have a return on investment of one to two years.An interrelated technology illustrated a signi�cant amount (100 ton�day) of recycled tires used, con�rming that there is a high possibility of implementation with a large-scale tertiary recycling of plastics and composites [11].
It is reported that the recycling of cured composite materials used for aircra manufacturing is a difficult process because of the complexity of the composite structures [12].ese materials are produced using thermosetting epoxy matrices that form an intimate connection with the surfaces of �bers, metals, and coatings.Recent techniques developed to recycle thermoset composites were recently reviewed [12].Because of the structural concerns of these recycled �bers, the materials may not regain the original values of reinforcing �bers in order to be directly used in the fuselage structures of aircra.
e Boeing and Alenia Aeronautica companies founded Italy�s �rst aircra composite recycling facility of materials for future manufacturing.is facility processes an average of 1,102 tons of composite scrap yearly and creates roughly 75 jobs in the regional economy.Boeing also cooperated with Milled Carbon Limited to establish a pilot industrial plant for processing cured and uncured composite parts on a continuous basis in order to extract high-quality carbon �bers.e recycled material is likely to be utilized for noncritical structures of aircra, such as galleys, interior linings, seat parts, and tools that produce stronger and lighter-weight materials in the same industry [13].
Most of the wires used in aircra are conductive metals (e.g., copper, silver, and aluminum) of various sizes and shapes, covered by plastic insulators.Recently, �ber optic cables have been used in aircra manufacturing, as well.Among the wires and cables, copper is the most commonly used for electrical wiring in aircra.It was reported that the Boeing Dream Liner 787 has about 60 miles of cables, while Boeing 777 has about 100 miles of cables (Boeing, 2008 [13]).Once the metal wires are properly removed from aircra, recycling is easier.Larger-scale wires are shredded and then granulated into smaller particles, so that separators (e.g., gravity, electrical, and optical) can remove the metals from the plastic parts.For instance, an eddy-current separator is one of the frequently utilized electrical separators in which granulated copper wires are fed into the separator to remove copper particles from the plastic particles.Both of these materials can be used in aircra aer the melting and reprocessing steps.For very thin copper wires, the recycling process becomes quite complex.In this case, burning the plastic part of the wires can be the main option for metal recovery, and the heat produced during the burning process can be used in the same industry as an energy source.Depending on the size and complexity of the wires and recycling methods, the recovery rate of wires and cables can vary from 50% to 90%, or even more [14].Yi et al. (2008) offered an algorithm on the disassembly strategy of mechanical parts in aircra that relies on the disassembly wave concept [14].is method is relatively simple and also efficient to search and develop.Furthermore, the method offers an optimal and selectable disassembly sequence of mechanical components with lower numbers of computing and processing steps.is is considered to be a very signi�cant process for maintaining, recycling, and discarding aircra parts.e authors conducted a study of wave propagation for the selectable disassembly analysis.ey claimed that the wave propagation method provides an optimal sequence to disassemble the selected components with the wave that propagates from the selected component to a set of boundary components.e authors also discussed geometric algorithms for both single-dependent and multiple-dependent components.e major drawback of the study was the identi�cation of an optimal sequence used for the selected items with the minimal component removals and examining a division of components from the assembly [15].

Marketability of Recycled Aircraft Materials
e major components that can be recycled from aircra include wires, electronics, aluminum and other alloys, stainless steel, other organic and inorganic compounds, and carbon and glass �bers [16].Recycling offers economic and environmental bene�ts because of less energy consumption, labor, and emissions (solid, liquid, and air).Figure 2 shows aluminum sheets that were recycled from aircra and used for aluminum tile products.e energy consumption of the recycling process is 5% of that required for �rst-generation aluminum production.Service scrap and manufacturing scrap can be recycled and reused to reduce the consumption of natural resources and mining operations worldwide [17].Another direct application (described in an unreviewed blog) of the recycled product is in iPad cases, which are made of a solid block of recycled aircra aluminum [18].
Carbon �bers recycled from a military aircra (F18) were aligned and integrated into a compression molding unit to make new composites.Materials Innovation Technologies (MIT), located in Fletcher, North Carolina, fabricated automotive parts from recycled �bers using chopping, mixing, and molding processes.Chopped composite parts from the F18 were delivered to both Milled Carbon and Adherent Companies.ese �ne composites were successfully injection molded to produce different components for testing and evaluation.Test results con�rmed that the mechanical strength of the injection molded parts closely matched the shelf virgin carbon �ber-�lled compounds.Figure 3 shows a  Corvette C6 fender well component molded from recycled F18 carbon �ber, as a demonstration pro�ect.As can be seen, the Corvette component (Figure 3(a)), produced from recycled carbon �ber (Figure 3(b)), is about 20% lighter than the �berglass components, even without any engineering processes.e stiffness of the new composites was also signi�cantly enhanced by adding the recycled �bers [19].e recycled and remolded products are currently being utilized in the car.
Lastly, an aircra engine can comprise up to 80% of the total value of the aircra, depending on the applications for which it will be used.According to Mark Gregory of Air Salvage International (ASI), the remaining part of the plane can be sold as spares for approximately $350,000 [20].Engines (Figure 3(c)) are the �rst items to be taken out of an aircra at the recycling facility.ese are �rst checked and then investigated for further use in another airplane.Most of these high-tech materials (e.g., nickel, cobalt, and chromium alloys) with very high mechanical strength and high temperature and corrosion resistance are placed in aircra engines, so the recycling value of these materials is considerably higher than other aircra parts [20].

Environmental Impacts of Aircraft Recycling
By reducing the amount of energy used by the aircra industry, recycling reduces greenhouse gas (GHG) emissions and minimizes global climate change.Additional bene�ts of recycling would be a reduction in emissions from incinerators, and lesser materials to �ll the land�lls.Table 1 shows the energy savings of different recycled aircra materials in the US.For example, copper and aluminum recycling from scrap has 85% and 95% less energy requirement, respectively [21].Utilizing secondary raw materials reduces the use of natural resources.e third column of Table 1 shows the energy savings of recycling metals and related materials.Compared to the concentrating that occurs in mineral processing facilities and the smelting that goes on T 1: Energy savings of recycled materials in comparison to virgin production and percentage of new metals produced by using recovered metals (source: Lund, 2000 [21], and BMRA, 2011 [22]). in metallurgical plants, recycling needs only about 10% of the total investment, increases employment, facilitates the sustainment of a practical manufacturing base, and eradicates land�ll waste [22,23].Damgaard et al. (2009) assessed greenhouse gas emissions associated with the recycling of metals from aer consumer waste in the frame of waste management.A material recovery facility (MRF) was used for the sorting of the recovered metal.According to the authors, GHG emissions are derived from three sources: indirect upstream emissions, direct activities at the MRF, and indirect downstream processes needed in reprocessing the metal scrap.Energy savings result from the avoided production of virgin metal.e global warming factor (GWF) of the upstream activities and the MRF as GHG emissions were 12.8-52.6kg CO 2 /ton recovered aluminum and 400-1020 kg CO 2 /ton recovered steel.Reprocessing is associated with a large saving as the result of avoided virgin production of aluminum and steel.e authors de�ned a net downstream savings of 5,040-19,340 kg CO 2 /ton of the treated aluminum and 560-2360 kg CO 2 /ton of the treated steel.e authors concluded that recovery of the scrap metal mainly relies on the technology and method chosen during the recycling processes, which make the comparison a little more difficult.Energy usage during the recovery and the avoidance of energy used in primary production are also important issues [24].

Recycled materials
Other studies have con�rmed that regained �bers from recycling serve as feasible replacements for new �bers within many high-end industrial manufacturing processes and also offer a noteworthy savings of money and carbon dioxide emissions.Recycled carbon �ber can be made at about 70% of the cost with 98% less energy than manufacturing virgin chopped �ber.It is estimated that commercial �et T 2: Evaluation of virgin and recycled carbon �ber (source: Carberry, 2011 [19]).manufacturing will create up to two million pounds of carbon �ber scrap in 2014.Recycling and substituting carbon �ber for virgin �ber in manufacturing and applications would save enough electricity to power 175,000 typical homes in a year [16,17].
According to Jim Stike, CEO of the recycling �rm Materials Innovation Technologies, using recycled carbon �ber not only avoids the waste of virgin carbon �ber being sent to land�lls aer the �rst use but also produces new parts because carbon can maintain a signi�cant portion of the virgin properties even aer a second reclamation.As well, the recycling process for the �bers reduces energy costs.Based on Boeing�s estimates, carbon �ber can be recycled at approximately 70 percent of the cost to produce virgin �ber (Table 2) ($8/lb-$12/lb versus $15/lb-$30/lb), using about 5 percent of the electricity required (1.3-4.5 kWH/lb versus 25-75 kWH/lb).In addition, MIT is willing to capitalize on the potential high quality of the recycled chopped carbon �ber to efficiently create intermediate-modulus materials that can be used instead of virgin aerospace and industrialgrade products [25].Table 3 shows the environmental impact (environmental load unit, ELU) of virgin carbon �bers on energy consumption during the manufacturing process [26].In order to produce 1 kg of virgin carbon �ber, 400 MJ of total electrical energy (equivalent to oil) is required.Avoiding higher energy in the conventional virgin processes will reduce gas emissions because the energy is mainly from fossil fuels, so recycling makes a substantial improvement on the environment [26].
Aircra recycling activities in the world promote development of communities and social interactions between communities.Other social impacts consist mainly of an increased lifespan made possible through a cleaner environment, safer working conditions for employees and employers; increased citizen interest in seeking employment or volunteer work in recycling, and improved scienti�c, cultural, and other activities nationally and internationally.Recently, social as well as educational impacts of recycling have been gaining much attention globally.

Limitation and Future Research
Recycling provides a number of different bene�ts to the environment and economy.For use as interior materials of aircra, recycling competes with the option of disposal [5].Although the AFRA emphasizes this issue, several companies are already reusing aircra interior parts for furniture, decoration, and art components.One of the dismantling programs is based on the A300-B4 airplane, which contains 4% composites, 4% titanium, 12% steel, 77% aluminum, and 3% other materials, so this speci�c model contains a high ratio of aluminum (77%), while later models contain a high ratio of composites.us, a better composite recycling process for higher composite-containing airplanes should be investigated for each airplane.e most efficient recycling methods need to be de�ned for composite materials of various sizes and shapes and marketability should be sought in detail [27].
For more than four decades, thousands of obsolete aircra have been sitting in the Southwest desert; however, the demand for recycled aluminum is still increasing.e abandoned aircra have great potential as a source of valuable metals and �bers for several industrial applications.Nevertheless, cost-effective recycling of aircra alloys is difficult due to the fact that these characteristically have quite high levels of alloying elements, such as zinc (7xxx series) and copper (2xxx series), and low levels of minor elements, in order to optimize fracture toughness and other mechanical and corrosion properties.Changing the structure and types of aircra materials makes the recycling process more complex and challenging [28].
According to Das (2009) feasible aluminum recovery is 90% or greater for the older aircra.Aluminum alloys contain high amounts of zinc and copper, which makes the recycling process more complicated than the lesser-alloyed aluminum and other alloys used in other applications.Speci�c recycling trials are needed to make aircra recycling more economic and efficient [29].From a market point of view, the recycling of glass �ber is not practical because of its low cost (< $1/lb) and a sufficient supply of virgin glass �ber available to the market.However, recycled carbon �bers have a higher demand in the market because of the lower cost and similar physical properties compared to the virgin �bers, so it is economically more feasible for many companies [30].Lastly, the design stage is very important due to the cost of raw materials and recyclability of products.Designing a product in a reliable and recyclable way should be investigated in detail for each aircra produced.

Conclusion
Recycling aircra parts offers numerous environmental ben-e�ts, including reduction of water, soil, and air pollution, as well as land�lls.Some of the materials (composites and alloys) in aircra are costly to produce, so regaining these kinds of materials at a reasonable price is an environmentally responsible approach is of great interest to recycling and aircra industries.Aluminum is a high-demand material for most industries, so recovering this material with less effort has gained the considerable interest of many industries.Recycling of a material requires less energy than that of producing virgin material and also reduces gas emissions (e.g., CO 2 , CO, NO  , and SO 2 ) and other emissions.A direct recycling (or as is recycling) methodology can be used for some aircra components, such as engines and electronics.Also, using aircra components for furniture and decoration requires less energy because it necessitates only some basic processes such as cutting, reshaping, sanding, and painting, in order for these to be turned into sophisticated furniture for many consumers.

F 1 :
Life cycle of aircra and reverse supply chain for future use as efficient and low-cost materials (adapted from PAMELA-Life Project Led by Airbus) (Airbus France S.A.S., 2008[7]).