History and future trends
When Wilbur and Orville Wright made their first successful flight in 1903, aircraft manufacturing was a craft practised in the small shops of experimenters and adventurers. The small but dramatic contributions made by military aircraft during the First World War helped to take manufacturing out of the workshop and into mass production. Second-generation aircraft helped post-war operators to make inroads into the commercial sphere, particularly as carriers of mail and express cargo. Airliners, however, remained unpressurized, poorly heated and unable to fly above the weather. Despite these drawbacks, passenger travel increased by 600% from 1936 to 1941, but was still a luxury that relatively few experienced. The dramatic advances in aeronautical technology and the concomitant use of air power during the Second World War fostered the explosive growth of aircraft manufacturing capacity that survived the war in the United States, the United Kingdom and the Soviet Union. Since the Second World War, tactical and strategic missiles, reconnaissance and navigational satellites and piloted aircraft have taken on ever greater military significance. Satellite communication, geo-monitoring and weather-tracking technology have become of increasing commercial importance. The introduction of turbojet-powered civilian aircraft in the late 1950s made air travel faster and more comfortable and began a dramatic growth in commercial air travel. By 1993 over 1.25 trillion passenger miles were flown worldwide annually. This figure is projected to nearly triple by 2013.
Employment in aerospace industries is highly cyclical. Direct aerospace employment in the European Union, North America and Japan peaked at 1,770,000 in 1989 before decreasing to 1,300,000 in 1995, with much of the employment loss occurring in the United States and the United Kingdom. The large aerospace industry in the Confederation of Independent States has been significantly disrupted subsequent to the break-up of the Soviet Union. Small but rapidly growing manufacturing capability exists in India and China. Manufacture of intercontinental and space missiles and long-range bombers has been largely restricted to the United States and the former Soviet Union, with France having developed commercial space launch capabilities. Shorter-range strategic missiles, tactical missiles and bombers, commercial rockets and fighter aircraft are more widely manufactured. Large commercial aircraft (those with 100 or greater seat capacity) are built by, or in cooperation with, manufacturers based in the United States and Europe. The manufacture of regional aircraft (less than 100 seat capacity) and business jets is more dispersed. The manufacture of aircraft for private pilots, based primarily in the United States, decreased from nearly 18,000 aircraft in 1978 to fewer than 1,000 in 1992 before rebounding.
Employment is divided in roughly equal measures among the manufacture of military aircraft, commercial aircraft, missiles and space vehicles and related equipment. Within individual enterprises, engineering, manufacturing and administrative positions each account for approximately one-third of the employed population. Males account for about 80% of the aerospace engineering and production workforce, with the overwhelming majority of highly skilled craftspeople, engineers and production managers being male.
The markedly different needs and practices of governmental and civilian customers typically result in the segmentation of aerospace manufacturers into defense and commercial companies, or divisions of larger corporations. Airframes, engines (also called powerplants) and avionics (electronic navigational, communication and flight control equipment) are generally supplied by separate manufacturers. Engines and avionics each may account for one-quarter of the final cost of an airliner. Aerospace manufacturing requires the design, fabrication and assembly, inspection and testing of a vast array of components. Manufacturers have formed interconnected arrays of subcontractors and external and internal suppliers of components to meet their needs. Economic, technological, marketing and political demands have led to an increasing globalization of the manufacture of aircraft components and sub-assemblies.
Manufacturing Materials, Facilities and Processes
Airframes were originally made from wood and fabric, and then evolved to metal structural components. Aluminium alloys have been widely used due to their strength and light weight. Alloys of beryllium, titanium and magnesium are also used, particularly in high-performance aircraft. Advanced composite materials (arrays of fibre embedded in plastic matrices) are a family of strong and durable replacements for metallic components. Composite materials offer equal or greater strength, lower weight and greater heat resistance than currently used metals and have the additional advantage in military aircraft of significantly reducing the radar profile of the airframe. Epoxy resin systems are the most commonly used composites in aerospace, representing about 65% of materials used. Polyimide resin systems are used where high temperature resistance is required. Other resin systems used include phenolics, polyesters and silicones. Aliphatic amines are often used as curing agents. Supporting fibres include graphite, Kevlar and fibreglass. Stabilizers, catalysts, accelerators, antioxidants and plasticizers act as accessories to produce a desired consistency. Additional resin systems include saturated and unsaturated polyesters, polyurethanes and vinyl, acrylic, urea and fluorine-containing polymers.
Primer, lacquer and enamel paints protect vulnerable surfaces from extreme temperatures and corrosive conditions. The most common primer paint is composed of synthetic resins pigmented with zinc chromate and extended pigment. It dries very rapidly, improves adhesion of top coats and prevents corrosion of aluminium, steel and their alloys. Enamels and lacquers are applied to primed surfaces as exterior protective coatings and finishes and for colour purposes. Aircraft enamels are made of drying oils, natural and synthetic resins, pigments and appropriate solvents. Depending on their application, lacquers may contain resins, plasticizers, cellulose esters, zinc chromate, pigments, extenders and appropriate solvents. Rubber mixtures find common use in paints, fuel cell lining materials, lubricants and preservatives, engine mountings, protective clothing, hoses, gaskets and seals. Natural and synthetic oils are used to cool, lubricate and reduce friction in engines, hydraulic systems and machine tools. Aviation gasoline and jet fuel are derived from petroleum-based hydrocarbons. High-energy liquid and solid fuels have space flight applications and contain materials with inherently hazardous physical and chemical properties; such materials include liquid oxygen, hydrazine, peroxides and fluorine.
Many materials are used in the manufacturing process which do not become part of the final airframe. Manufacturers may have tens of thousands of individual products approved for use, although far fewer are in use at any time. A large quantity and variety of solvents are used, with environmentally damaging variants such as methyl ethyl ketone and freon being replaced with more environmentally friendly solvents. Chromium- and nickel-containing steel alloys are used in tooling, and cobalt- and tungsten carbide-containing hard-metal bits are used in cutting tools. Lead, formerly used in metal-forming processes, is now rarely used, having been replaced with kirksite.
In total, the aerospace industry uses more than 5,000 chemicals and mixtures of chemical compounds, most with multiple suppliers, and with many compounds containing between five and ten ingredients. The exact composition of some products is proprietary, or a trade secret, adding to the complexity of this heterogeneous group.
Facilities and manufacturing processes
Airframe manufacturing typically is done in large, integrated plants. Newer plants often have high-volume exhaust ventilation systems with controlled make-up air. Local exhaust systems may be added for specific functions. Chemical milling and large component painting are now routinely performed in closed, automated ranks or booths that contain fugitive vapour or mist. Older manufacturing facilities may provide much poorer control of environmental hazards.
A large cadre of highly trained engineers develop and refine the structural characteristics of the aircraft or space vehicle. Additional engineers characterize the strength and durability of component materials and develop effective manufacturing processes. Computers have taken on much of the calculating and drafting work that was previously performed by engineers, drafters and technicians. Integrated computer systems can now be used to design aircraft without the aid of paper drawings or structural mock-ups.
Manufacturing begins with fabrication: the making of parts from stock materials. Fabrication includes tool and jig making, sheet-metal working, machining, plastic and composite working and support activities. Tools are built as templates and work surfaces on which to construct metal or composite parts. Jigs guide cutting, drilling and assembly. Fuselage sub-sections, door panels and wing and tail skins (outer surfaces) are typically formed from aluminium sheets that are precisely shaped, cut and chemically treated. Machine operations are often computer controlled. Huge rail-mounted mills machine wing spars from single aluminium forgings. Smaller parts are precisely cut and shaped on mills, lathes and grinders. Ducting is formed from sheet metal or composites. Interior components, including flooring, are typically formed from composites or laminates of thin but rigid outer layers over a honeycomb interior. Composite materials are laid up (put into carefully arranged and shaped overlapping layers) by hand or machine and then cured in an oven or autoclave.
Assembly begins with the build-up of component parts into sub-assemblies. Major sub-assemblies include wings, stabilizers, fuselage sections, landing gear, doors and interior components. Wing assembly is particularly intensive, requiring a large number of holes to be precisely drilled and counter-sunk in the skins, through which rivets are later driven. The finished wing is cleaned and sealed from the inside to ensure a leak-proof fuel compartment. Final assembly takes place in huge assembly halls, some of which are among the world’s largest manufacturing buildings. The assembly line comprises several sequential positions where the airframe remains for several days to more than a week while predetermined functions are performed. Numerous assembly operations take place simultaneously at each position, creating the potential for cross exposures to chemicals. Parts and sub-assemblies are moved on dollies, custom-built carriers and by overhead crane to the appropriate position. The airframe is moved between positions by overhead crane until the landing and nose gear are installed. Subsequent movements are made by towing.
During final assembly, the fuselage sections are riveted together around a supporting structure. Floor beams and stringers are installed and the interior coated with a corrosion-inhibiting compound. Fore and aft fuselage sections are joined to the wings and wing stub (a box-like structure that serves as a main fuel tank and the structural center of the aircraft). The fuselage interior is covered with blankets of fibreglass insulation, electrical wiring and air ducts are installed and interior surfaces are covered with decorative panelling. Storage bins, typically with integrated passenger lights and emergency oxygen supplies, are then installed. Pre-assembled seating, galleys and lavatories are moved by hand and secured to floor tracks, permitting the rapid reconfiguration of the passenger cabin to conform to air carrier needs. Powerplants and landing and nose gear are mounted, and avionic components are installed. The functioning of all components is thoroughly tested prior to towing the completed aircraft to a separate, well-ventilated paint hanger, where a protective primer coat (normally zinc-chromate based) is applied, followed by a decorative top-coat of urethane or epoxy paint. Prior to delivery the aircraft is put through a rigorous series of ground and flight tests.
In addition to workers engaged in the actual engineering and manufacturing processes, many employees are engaged in planning, tracking and inspecting work and expediting the movement of parts and tools. Craftspeople maintain power tools and reface cutting bits. Large staffs are needed for building maintenance, janitorial services and ground vehicle operation.