Aerospace manufacturing consumes a vast amount and array of parts and materials, and utilizes a broad supply chain. This document provides information on processes commonly used in aerospace manufacturing and the supply chain, along with typical materials and resource inputs and waste outputs of this industry.
Aside from final assembly of the aircraft, aerospace manufacturing has common capabilities to many other sectors, such as machining, and metal surface prep and finishing (painting, plating, coating, etc.), printed circuit board fabrication, and composites fabrication and layup.
Some of these processes may occur at the aerospace manufacturing facility itself, or may be outsourced to suppliers.
Additional information on composites, metal fabrication and finishing may be found in the following topic hubs:
Aircraft assembly requires the coordination of thousands of parts coming together to form one large final product. Assembly landing gear assembly, windshield and window installation, wire harness assembly, wing structure assembly and installation to the fuselage, engine hanging/dropping/installation, final painting, internal assembly - e.g., seats, carpet, windows, doors, electrical instrumentation systems, and much more.
Some of the common fabrication processes used in final assembly are described below.
Fusion welding is performed with a metal arc in the presence of an inert gas which prevents the oxidation of the metals to be welded. An alternating or direct current, depending on the type and thickness of the metal, is typically applied through an electrode.
Resistance welding requires: a primary electrical circuit from a transformer; a secondary circuit and electrodes to conduct the current to the desired spot; a mechanical system to hold the components and apply force; and control equipment to measure duration and magnitude of the electrical current. This process is used to create seam welds, which are leak-proof and pressure-tight.
Electron Beam Welding
Electron beam welding is achieved by concentrating a beam of high velocity electrons onto the surfaces to be joined.
Riveted joints are usually used in sheet metal parts where the rivets take a shearing load.
Sealants are applied throughout the aerospace vehicle primarily to seal out moisture and contaminants. This helps prevent corrosion, particularly on faying (i.e., closely or tightly fitting) surfaces, inside holes and slots, and around installed fasteners. Sealants are also used to seal fuel tanks and pressurized components. They are applied using tubes, spatulas, brushes, rollers or spray guns. Sealants are often stored frozen and thawed before use, and many are two-component mixtures t hat cure after mixing.
Adhesive bonding involves joining together two or more metal or nonmetal components. This process is typically performed when the joints being formed are essential to the structural integrity of the aerospace vehicle or component. Bonding surfaces are typically roughened mechanically or etched chemically to provide increased surface area for bonding and then treated chemically to provide a stable corrosion-resistant oxide layer.
Nonstructural adhesives are used to bond materials that are not critical to the structural integrity of the aerospace vehicle or component, such as carpeting, gaskets around windows or to non-structurally joined components. These adhesives are applied using tubes, brushes, and spray guns.
A wide variety of tests are performed by the aerospace industry to verify that parts meet manufacturing specifications. For detection of defects in metal components and bonds, many different kinds of penetrants, fluids, dyes, and etchants can be applied to the surface of metal parts. Non-destructive testing methods may also be used.
Inspection and Flight Testing
Typically prior to installation of the interior, and final aircraft painting, a rigorous series of flight test procedures and flight tests are conducted.
Interior /Final Paint
When the aircraft passes all flight tests and inspections, the interiors are installed and airplanes are painted.
Repair operations generally include all conversions, overhauls, maintenance programs, major damage repairs, and minor equipment repairs. Typical maintenance and repair operations include the following:
The Aerospace Industries Association estimates that there are 15,000 to 30,000 different materials used during manufacturing and as components of the final assembled craft, many of which may be potentially toxic, highly volatile, flammable, contain chlorofluorocarbons, and/or contribute to global warming. Significant quantities of water and energy are consumed as well.
Material inputs used during production and/or in the final aircraft include metals, solvents, paints and coatings, chemicals, plastics, composites, rubbers, foams, lubricants, glass, electronics, and textiles. Metals used in manufacturing include steel, aluminum, titanium, many specialty alloys, and metals used as blasting abrasive. Materials relating to the manufacture of electronics (for electrical controls, panels, and switches) are another input. Many of these materials and items are specified by Federal Aviation Administration (FAA) guidelines. Chemicals range from (but are not limited to) solvents, thinners, degreasers, penetrants, fluids, dyes, adhesives, degreasers, plating chemicals, cutting fluids, flux, abrasive metal polishers, rust inhibitors, and etchants including ferric chloride in printing board etching.
Acids are commonly used for etchants, pickling, and passivation of metallic surfaces. Etching typically utilizes sodium hydroxide for aluminum, nitric acid and hydrofluoric acid for titanium, dilute sulfuric acid for magnesium, and a mixture of nitric acid and hydrofluoric acid for stainless steel. Anodizing processes typically use sulfuric, or boric acid, sulfuric-boric, or chromic acids. Various acids are also used in metal passivation, which is the spontaneous formation of a hard non-reactive surface film that inhibits further corrosion. Phosphoric acid is used as a rust preventative.
Electroplating may use heavy metal-based electrodes, such as lead and cadmiun ion-selective electrodes. Plating solutions contain metals specified by customers and/or the function of the plated substrate, and may contain cadmium, chromium, copper, nickel, zinc, gold, silver or other metals. Cyanides may also be used in electroplating solutions and some stripping and cleaning solutions.
Due to the size and weight of aircraft components, materials handling equipment such as conveyors, cranes, scaffolding and ladders are necessary, along with an assortment of industrial vehicles such as forklifts, flatbeds, carts, aerial work platforms, and booms and lifts.
Relying on this huge assortment of equipment, materials, and chemicals, the industry and their supply chain, is faced with environmental releases or outputs from operations.
Air Emissions and Releases
Air emissions, primarily volatile organic compounds (VOCs), result mainly from the sealing, painting, depainting, bonding, finishing application processes including material storage, mixing, applications, drying, and cleaning. These emissions are composed mainly of organic solvents which are used as carriers for the paint or sealant and as chemical coating removers. Most aerospace coatings are solvent-based, which contain a mixture of organic solvents, many of which are VOC's. The most common VOC solvents used in coatings are trichloroethylene, 1,1,1-trichloroethane, toluene, xylene, methyl ethyl ketone, and methyl isobutyl ketone. The most common VOC solvent used for coating removal is methylene chloride.
Due to the significant energy consumed by aerospace manufacturing facility, and the resulting greenhouse gas emissions, energy and carbon inventories are a growing concern for global warming impacts.
Solid and Hazardous Wastes
The nature of wastes, solid vs. hazardous, depend upon the specific cleaning application, manufacturing operation, and whether hazardous or non-hazardous materials are used. Table 1 lists illustrate some of the waste streams.
Table 1 - Wastes Generated During Aerospace Manufacturing Operations
|Surface cleaning||Solvents and degreasers|
|Depainting||Spent blast media
Paint removers/strippers, removed paint, and solvent sludges
Paint and solvent sludges
|Surface prep (e.g., profiling)||Spent blast media
Spent solvents and degreasers
|Painting and Coating||
|Metal Fabrication and Working||
Spent or contaminated metalworking fluids such as those used in plating, etching, or machining
|Printed Circuit Board Fabrication||Etchants
Photo developing solutions
Equipment or vessel cleaning solvents or solutions
Equipment maintenance fluids such as lubricants and hydraulic fluids
|Final Assembly/Other||Waste containers, packaging, spent equipment, or raw materials
Spent or expired chemicals used in bonding, sealing, adhesion, and laboratory wastes
Wastewater is generated in almost every stage of the manufacturing process. Metalworking fluids, used in machining and shaping metal parts, and parts or metal cleaners are a common source of wastewater contamination. Metalworking fluids can be petroleum-based, oil-water emulsions, or synthetic emulsions that are applied to either the tool or the metal being tooled to facilitate the shaping operation. Spent cooling waters can also be contaminated with metalworking fluids.
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