/air"pawrt', -pohrt'/, n.
a tract of land or water with facilities for the landing, takeoff, shelter, supply, and repair of aircraft, esp. one used for receiving or discharging passengers and cargo at regularly scheduled times.
[1915-20; AIR1 + PORT1, on the model of SEAPORT]
/air"pawrt', -pohrt'/, n. Naut.
a porthole designed to be opened to the outside air.
[1780-90; AIR1 + PORT4]

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Site and installations for the takeoff and landing of aircraft.

Early airports were open, grass-covered fields, called landing fields, that allowed a pilot to head directly into the wind to aid a plane's lift on takeoff and to decrease its speed on landing. In the 1930s heavier airplanes required paved runway surfaces. Larger planes needed longer runways, which today can reach 15,000 ft (4,500 m) to accommodate the largest jet aircraft. Air traffic is regulated from control towers and regional centres. Passenger and cargo terminals include baggage-movement and passenger-transit operations.

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also called  air terminal,  aerodrome , or  airfield 
 site and installation for the takeoff and landing of aircraft. An airport usually has paved runways and maintenance facilities and serves as a terminal for passengers and cargo.

Evolution of airports
      The requirements for airports have increased in complexity and scale since the earliest days of flying. Before World War II the landing and takeoff distance of most passenger-transport aircraft was at most 600 metres (2,000 feet). Additional clear areas were provided for blind landings or bad-weather runs, but the total area involved rarely exceeded 500 acres (200 hectares).

      It was not until the general introduction of heavy monoplanes for transport, such as the Douglas DC-3, during the late 1930s that extensive takeoff and landing distances were needed. Even then, the prewar airfields at New York City (La Guardia), London (Croydon), Paris (Le Bourget), and Berlin (Tempelhof) were laid out on sites close to the city centres. Because even transport aircraft of the period were relatively light, paved runways were a rarity. Croydon, Tempelhof, and Le Bourget, for example, all operated from grass strips only. Early airports were also major centres of leisure activity, often attracting more visitors than passengers. In 1939 La Guardia Airport attracted almost 250,000 visitors per month, reaching a peak of 7,000 in one day, compared with a maximum daily throughput of only 3,000 passengers. In 1929 Berlin's airport reported 750,000 visitors and boasted a restaurant that could seat 3,000 people on the roof of the passenger terminal. The status of prewar airports as major social centres was reflected in their design, especially where the requirements of catering, observation decks, and parking were paramount. Indeed, the requirements of aircraft and passengers were not at all dominant at early airfields.

      Much long-distance air transport was handled by the large seaplanes known as flying boats or clippers. These aircraft, though slow and of limited range, offered a level of comfort that was necessary for long-distance travel. Air terminal facilities were necessarily constructed close to large open stretches of water. La Guardia Airport and Santos Dumont Airport in Rio de Janeiro are examples of airports that still operate on sites originally chosen for their ability to handle large seaplanes. The large facilities at Southampton Water in the United Kingdom have now disappeared, but the artificial lake at Linate Airport near Milan, Italy, is still to be found close to the present administration facilities.

      The vast majority of airfields throughout the world are still relatively simple facilities. Even now, many have unpaved runways or at most lightly paved runways with tiny terminal or administration buildings, a rudimentary control tower, and crude landing aids. Such facilities can deal only with light aircraft and a negligible flow of passengers or freight. Heavy air traffic, on the other hand, is now almost entirely handled by sophisticated airport facilities that can accommodate the needs of crew, passengers, and freight and the great range of aircraft types that have evolved to meet the needs of modern air transport and general aviation.

      More than 100 airports around the world now handle at least 10 million passengers each per year; nearly half of these are in the United States. Dozens of airports regularly move more than 30 million passengers on a yearly basis—Chicago's O'Hare International Airport and the Hartsfield Atlanta International Airport in Georgia each handle more than 75 million. The Memphis (Tennessee) International Airport, the home airport of the FedEx Corporation's cargo service, and the Hong Kong International Airport are the world's largest cargo shippers, each of which handled nearly four million tons in 2007. In order to meet the increasing demand for air travel, large transport aircraft powered by multiple jet (jet engine) and turboprop engines have been built. Such aircraft require extensive ground facilities, runways, taxiways, fire-fighting and rescue services, passenger- and cargo-handling facilities, access to car parking and public transport, lighting, navigational and approach aids, and various support facilities such as catering, meteorology, and governmental inspection. In order to be attractively convenient, the complex of activities and facilities that make up a modern airport must be located sufficiently close to the main centres of world population. At the same time, they must be adequately distant, so that the environmental problems associated with the noise of large aircraft and the activities of large numbers of passengers, workers, and visitors do not become intolerable to the cities that are served.

Modern airports
 The largest airports in the world employ more than 100,000 workers each. They are immensely complex entities with regard to the physical facilities that they comprise, the organizations that are active within their boundaries, and the services that are provided in conjunction with their operation.

      Physical facilities include runways, taxiways, aprons, and strips, which are used for the landing and takeoff of aircraft, for the maneuvering and positioning of aircraft on the ground, and for the parking of aircraft in order to load and discharge passengers and cargo. For the safe landing and takeoff of aircraft, lighting and radio navigational aids are provided. These are supplemented by airfield markings, signs and signals, and air traffic control facilities. Support facilities on the airside of the field include meteorology, fire and rescue, power and other utilities, aircraft maintenance, and airport maintenance. Landside facilities are the passenger and cargo terminals and the access system, which includes parking, roads, public transport facilities, and loading and unloading areas.

      Many organizations are involved in the operation of a modern airport. Overall management is usually in the control of an organization, authority, or company that holds a license to operate the facility. This license is granted subject to a judgment by the national civil aviation authorities that the managing body is fit and competent to run an airport within national and, if applicable, international laws governing safety and operations. While overall responsibility for efficient, safe, and legal operation lies with the airport management, many of the individual services at an airport are provided by other organizations. Such organizations include airlines; air traffic control authorities; ground handling companies; fixed-base operators; concessionaires; security organizations; governmental agencies responsible for customs, immigration, health control, and police; support companies providing flight catering, fueling, aircraft engineering, and maintenance; aero clubs; and flying schools. Since the early 1980s, when privatization began to sweep through civil aviation, terminal-operation companies have also become more frequent, such as those that own terminals in Birmingham, Eng.; Brussels; and Toronto.

      Airport services related to the aircraft are frequently referred to as airside. Many of these services are concentrated on the apron, or ramp, which is that part of the operational surface adjacent to the terminals where aircraft are maneuvered or parked. They include the apron handling of aircraft, airside passenger transfer to the aircraft, the handling of baggage and cargo, aircraft fueling, catering and cabin cleaning, engine starting, deicing, ground power and air-conditioning, and minor maintenance engineering. Other airside services are runway inspection, lighting and navigational aids, fire fighting and rescue, airside maintenance, and air traffic control (air-traffic control). Among the landside services are those related to ground passenger handling; these include check-in, security, customs and immigration, baggage delivery, information, catering, cleaning and maintenance, shops and concessionary facilities, automobile rental, ground transportation, porters, special help for the elderly and handicapped, automobile parking, and public transportation (including taxis). In addition, because airports employ such a large number of workers, extensive provision must be made for their daily requirements.

Site selection
Aeronautical and environmental factors
      Selecting a site for a new airport, or evaluating how well an existing site can be expanded to provide a new major airport, is a complex process. A balance must be achieved between aeronautical and air-transport requirements and the impact of the airport on its environment.

      From an aeronautical viewpoint, the basic requirement of an airport is that it have a relatively flat area of land sufficiently large to accommodate the runways and other facilities and that this area be in a locality free from such obstructions to air navigation as mountains and tall buildings.

      From the viewpoint of air-transport needs, airport sites must be sufficiently close to population centres that they are considered reasonably accessible to their users. Environmental (environment) considerations, on the other hand, dictate that the site should be far enough away from urban centres that noise and other deleterious effects on the population should be kept to acceptable levels. Furthermore, the airport should not destroy areas of natural beauty or other significance. These two sets of requirements, the aeronautical and the environmental, almost inevitably clash, with the conflict becoming more severe as the scale of the envisaged airport increases.

      The most modest airport facility—with a single runway, an apron, and a building that serves simultaneously as terminal, administration area, and control tower—can comfortably be built on a site as small as 75 acres, since it requires only a flat, well-drained area sufficient to accommodate a short runway and its surrounding safety strip. Larger and more modern airport facilities, on the other hand, require multiple runways of extended length, extensive terminal apron areas, and large expanses of land devoted to parking and landside access roads. For such an airport, a minimum area of 3,000 acres is likely to be required. Several major airports—such as Dallas–Fort Worth International Airport in Texas, King Abdul Aziz International Airport near Jiddah, Saudi Arabia, and Charles de Gaulle Airport near Paris—are built on sites well in excess of this figure.

The selection process
      The site-selection process for large airports can take many months; in some notable cases it has extended over many years. The procedure is complicated by the number of factors that must be taken into account. First, the operational capability of the site is assessed, particularly with respect to weather conditions such as wind, snow, ice, fog, and low visibility and also with respect to obstructions to air navigation around the airport, particularly on the approach and takeoff paths. The location of the facility in relation to air-traffic-controlled airspace is also operationally important. In addition, there must be an evaluation of the capacity of the available land to accommodate the expected configuration of runways and other facilities. Flat or very gently undulating land is necessary, because runways must be constructed according to restrictions on maximum allowable slopes—which in turn are governed by aircraft performance on landing and takeoff.

      Ground access to the airport is also considered. An evaluation is made of the distance from population centres, the regional highway infrastructure, public transport facilities (including railways), and the availability of land for parking. Development costs are also estimated, taking into account the nature of the terrain, soil and rock conditions, drainage requirements, and local land values.

      The environmental consequences of an airport development rank very high in any site-selection procedure. The impact of aircraft noise on the neighbouring population is often the most significant environmental factor, but, in many countries, account must also be taken of the impact on the flora and fauna of the area, pollution through chemical runoff into local groundwater, the presence of endangered species or significant cultural sites, and even undesirable changes in land use. Many governments now require that environmental analyses of airport development projects include evaluations of population relocation, changes in employment patterns, and distortion of existing regional land use and transportation planning.

Airfield layout and configuration
Operational requirements
 It is obvious even to the most casual observer that there is a large variation in the appearance and layout of airport facilities. Simple airports designed to accommodate light aircraft are essentially similar, but, as airports become larger and more complex, thus accommodating more passengers and cargo, their individual requirements affect their layouts and ensure that each becomes recognizably different.

      The principal determinants of airport layout are the number of runways and their orientation, the shape of the available site, and constraints at the site both on the ground and in the air. The location and orientation of runways is governed in turn by the need to avoid obstacles, particularly during landing and takeoff procedures. For the largest airports, obstacles to air navigation must be considered up to about 15 km (10 miles) from the runways. Runway configurations must also ensure that, for 95 percent of the time, aircraft can approach and take off without either crosswinds or tailwinds that would inhibit operations. At the smallest airports, light aircraft are unable to operate in crosswinds greater than 10 knots; at all airports, operation in tailwinds in excess of 10 knots is not recommended by aircraft manufacturers (10 knots, or nautical miles per hour, is equal to about 12 statute miles per hour or 19 km per hour).

Runway configurations
 The operational capacity of an airport, which is usually defined as the maximum possible number of aircraft landings and takeoffs, is determined by the number of runways that are available for use at any one time. The vast majority of airports around the world have the simplest possible layout, a single runway. Where crosswinds would be high for an unacceptable proportion of operational time, a two-runway configuration is necessary, usually in the form of a main runway and an auxiliary crosswind runway. Depending on the shape of the site and the availability of land, the crosswind facility can take on a crossed configuration or an open or closed V layout. Where visibility is good and aircraft can operate under visual flight rules (VFR), operational capacity increases from the lowest level, crossed runways, through the closed V and open V configurations. However, in poor visibility or under certain conditions of very heavy air traffic, aircraft must operate under the strict instructions and rules of air traffic control and instrument operation; these are called instrument flight rules (IFR). Under such conditions, crosswind runways cannot be used simultaneously with main runways, so that the capacities of the crossed and V configurations are equivalent to that of a single runway.

 An increase in operational capacity under VFR is possible with the use of a close parallel runway configuration. Most very large airports must be assured of adequate capacity even under IFR conditions, and this can be achieved by separating the parallel runways by a minimum of 1,035 metres (3,400 feet), which was the distance approved by the International Civil Aviation Organization on Nov. 9, 1995. This independent parallel configuration permits simultaneous independent landings and takeoffs on both runways. Munich Airport exemplifies this type of configuration. Even greater capacity is possible using a four-runway configuration of independent close parallels, as is the case at Los Angeles International Airport. With such a configuration, even under IFR, it is possible for two aircraft to land simultaneously while two other aircraft take off. A number of the world's largest airports have master plans that feature eight runways in the form of independent close parallels supplemented by other close parallels that are capable of crosswind operation. However, with passenger aircraft increasing in size, most can now operate in crosswinds of 20 knots and above. This reduces the likelihood that configurations with four crosswind runways will ever be constructed.

Runway pavements (pavement)
      Until the introduction of heavy monoplane aircraft in the latter part of the 1930s, civil air-transport aircraft were able to operate from grass runways with takeoff distances of less than 600 metres (2,000 feet). The advent of heavy aircraft such as the DC-3 required the provision of paved runways; at the same time, takeoff distances increased to more than 900 metres (3,000 feet). The length requirements for runways continued to increase into the mid-1970s, when large civilian aircraft such as the Douglas DC-8 and some models of the Boeing 747 required almost 3,600 metres (12,000 feet) of runway at sea level. (Even longer runways were necessary at higher elevations or where high ambient air temperatures occurred during operations.) The trend toward increasing runway lengths caused many problems at existing civilian airports, where runways had to be extended in order to accommodate the new aircraft. Ultimately, pressure by airport operators and the development of turbofan jet engines arrested and finally reversed the trend. Since the 1970s, runway length requirements have actually decreased, and the takeoff and climb performance of civilian aircraft has improved substantially. This has brought a dual benefit in reducing the area of land required by an airport and also in reducing the area around the airport that is adversely affected by noise on takeoff.

      At all but the smallest airports, pavements are now provided for runways, taxiways, aprons, and any other areas where aircraft are maneuvered. Pavements must be designed in such a way that they can bear the loads imposed by aircraft without failure. A pavement must be smooth and stable under conditions of loading during its expected or economic life. It should be free from dust and other particles that could be blown up and ingested into engines, and it must be capable of spreading and transmitting an aircraft's weight to the existing subsoil (or subgrade) in a manner that precludes subsoil failure. Another function of the pavement is to prevent weakening of the subsoil by moisture intrusion, especially from rainfall and frost.

      Airfield pavements are of two types, rigid and flexible. Rigid pavements are constructed of portland cement concrete slabs resting on a prepared subbase of granular material or directly on a granular subgrade. Load is transmitted through the slabs to the underlying subgrade by flexure of the slabs. Flexible pavements are constructed of several thicknesses of asphalt or bituminous concrete layers overlying a base of granular material on a prepared subgrade. They spread the concentrated aircraft wheel loads throughout their depth until the load at the base of the pavement is less than the strength of the in situ soil. At all depths the strength of the pavement should be at least equal to the loads placed upon it by aircraft wheels. The choice of pavement type is often determined by economics. In some parts of the world, portland cement concrete is cheaper than asphalt; in other parts, the converse is true. For certain parts of the airfield, however, asphaltic concrete is an unsuitable material for pavement construction because of its vulnerability to damage by aviation fuel. Therefore, even at airports where flexible airfield pavements are generally in use, it is usual for concrete pavements to be used where aircraft stand on the aprons and at runway ends where fuel spillage is frequent.

Navigational aids, lighting, and marking
      Only the simplest airfields are designed for operations conducted under visual meteorological conditions (VMC). These facilities operate only in daylight, and the only guidance they are required to offer is a painted runway centreline and large painted numbers indicating the magnetic bearing of the runway. Larger commercial airports, on the other hand, must also operate in the hours of darkness and under instrument meteorological conditions (IMC), when horizontal visibility is 600 metres (2,000 feet) or less and the cloud base (or “decision height”) is 60 metres (200 feet) or lower. In order to assist aircraft in approaches and takeoffs and in maneuvering on the ground, such airports are equipped with sophisticated radio navigational aids (navaids) and visual aids in the form of lighting and marking.

Navigational aids
      The most common form of navaid used for the approach phase of aircraft descent is the instrument landing system (ILS). This is a radio signal that is beamed along the centreline of the runway and at the correct angle of approach (usually 3° above the horizontal). The beam is intercepted by an approaching aircraft up to 24 km (15 miles) from the threshold of the runway. Information is given concerning position above and below the glide slope and deviation to the right or left of centreline; consequently, the pilot is able to determine from cockpit instruments a deviation of the aircraft from the proper approach.

      Additional approach information is given visually to the pilot in the form of lighting approach aids. Two systems of approach aids are in use: the visual approach slope indicator system (VASIS) and the more modern precision approach path indicator (PAPI). Both work on the principle of guiding lights that show white when the pilot is above the proper glide slope and red when below.

Airfield lighting
      Visual guidance to approaching aircraft is also provided by approach lighting systems, a configuration of high-intensity white lights running along the centreline of the runway and extending up to 600 metres (2,000 feet) beyond the threshold. At airfields where aircraft operate in very poor visibility, touchdown-zone lighting is provided over the first 900 metres (3,000 feet) from the runway threshold. These lights, set in patterns flush with the runway pavement, provide guidance up to the final moment of touchdown.

      The runway itself is strongly delineated by a variety of guidance light systems. The threshold is designated by a line of green lights, and the edges and centreline are delineated by white lights that shine toward the maneuvering aircraft at regular intervals. The pilot is warned of the approaching runway end by a line of red lights at the end of the usable pavement. Taxiways are delineated by blue edge lights and by green centreline lights that also appear at regular intervals.

Runway markings
      Considerable additional visual guidance is given to pilots by painted markings on the runway. The form of marking indicates at a glance whether radio instrument guidance is available at any particular airfield. On precision instrument runways, the runway edges are indicated by painted lines, and distances along the runway from the threshold are indicated by pavement markings. In addition, touchdown-zone markings are painted on the pavement immediately after the threshold, providing vital visual guidance during the moments immediately before touchdown when all lighting may be obscured by fog.

Air traffic control (air-traffic control)
 In the vicinity of airports—especially large airports, where in peak conditions as many as three landing or takeoff operations may occur every minute—the control of aircraft in the air is a difficult but extremely important operation. Aircraft require very large amounts of airspace, but at the same time the risk of collision must be set at very low, almost negligible, levels. Because aircraft are concentrated in the airspace around airports, acceptable levels of collision risk can be achieved only by strict adherence to procedures that are set out and monitored by air traffic control authorities.

      An aircraft in flight follows en route air traffic control instructions as it flies through successive flight information regions (FIRs). Upon approaching an airport at which a landing is to be made, the aircraft passes into the terminal control area (TCA). Within this area, there may be a greatly increased density of air traffic, and this is closely monitored on radar by TCA controllers, who continually instruct pilots on how to navigate within the area. The aircraft is then brought into the final approach pattern, at which point control passes to the approach controller, who monitors the aircraft to the runway itself. Once on the runway, the pilot is given instructions on ground maneuvers by the ground controller, whose responsibility is to avoid conflicting movements of aircraft in the operational area of the airfield. The ground controller gives the pilot instructions on reaching the apron stand position via the appropriate turnoffs and taxiways. Final positioning may be the responsibility of an apron controller. Departing aircraft go through a reverse procedure, whereby control is passed from ground control to departure control to terminal control area and, finally, to en route control.

Passenger terminal layout and design
Passenger requirements
 As passenger throughput at airports increases, the passenger terminal becomes a more important element of the airport, attaining a dominant status in the largest facilities. The passenger terminal may amount to less than 10 percent of the total investment in a small airport, but at large airports terminals often account for more than 70 percent of infrastructural investment. The design that is ultimately adopted depends principally on the passenger volumes to be served and the type of passenger involved.

 Passengers are frequently classified as business or leisure, scheduled or charter, originating or destined, and transfer or transit. Business travelers tend to pay significantly higher fares, and airlines usually wish to provide a high quality of service in order to attract such traffic. The passenger terminal at Heathrow Airport near London, for example, was designed to a very high standard of space and decor to attract just this type of passenger. Scheduled and charter passengers, meanwhile, tend to have very different needs in the terminal, especially at check-in and in the provision of ground transportation. Palma Airport, on the Spanish island of Majorca, has a landside that is designed to accommodate large numbers of charter tourists arriving and departing the airport by bus.

      Some airports have a very high percentage of passengers who are either transiting the airport (i.e., continuing on the same flight) or transferring to another flight. At Hartsfield Atlanta International Airport in Georgia and at Chicago's O'Hare International Airport, for example, two-thirds of all passengers transfer to other flights and do not visit the cities where the airports are sited. These passengers have special needs but usually only on the airside of the terminal. There is no need to provide parking or ground transportation to the city for such passengers; they will, however, need transit lounges and other areas such as transit check-in desks.

      Airports that receive a large number of transferring and transiting passengers are referred to as hubbing airports. At a hub, aircraft arrive in waves, and passengers transfer between aircraft during the periods when these waves are on the ground. By using a “hub-and-spoke” network, airlines are able to increase the load factors on aircraft and to provide more frequent departures for passengers—at the cost, however, of inconvenient interchange at the hub.

Terminal designs

Open apron and linear designs
 The oldest and simplest layout for passenger terminals is the open apron design, in which aircraft park on the apron immediately adjacent to the terminal and passengers walk across the apron to board the aircraft by mobile steps. Frequently, the aircraft maneuver in and out of the parking positions under their own power. As airports grow, however, it is impossible to have large numbers of passengers walking across the apron. In this case, it is common to have terminals designed to the linear concept, with aircraft parked at gates immediately adjacent to the terminal itself. Usually, air bridges are employed for transferring passengers directly between the terminal building and the aircraft. The limitation of the linear concept is usually the long building dimensions required; these can mean long walking distances for transferring passengers and other complications related to building operation. In practice, building lengths tend to be limited to approximately 800 metres (2,650 feet). Examples of the linear design occur at Kansas City International Airport in Missouri, U.S., Munich Airport in Germany, and Charles de Gaulle Airport near Paris.

Pier and satellite designs
 Where one building must serve a larger number of aircraft gates, the pier concept, originally developed in the 1950s, has been found very useful. Frankfurt International Airport in Germany and Schiphol Airport near Amsterdam still use such terminals. In the late 1970s, pier designs at Chicago's O'Hare and Atlanta's Hartsfield successfully handled in excess of 45 million mainly domestic passengers per year. However, as the number of aircraft gates grows, the distances that a passenger may have to travel within a pier-type terminal become exceedingly long, passenger circulation volumes become very large, and the terminal itself can become uncomfortable and unattractive to use. In order to cut down walking distances, some terminals, beginning in the 1960s, were designed on the satellite concept. Frequently, passengers are carried out to the satellites by some form of automated people mover or automatic train. Some satellite designs were very successful—for example, at Orlando and Tampa in Florida, U.S.—but to some degree the concept has fallen out of favour, having been found difficult to adapt to the changing size of aircraft and wasteful of apron space. Los Angeles International Airport originally had all its aircraft served at satellite buildings, but during the 1980s all satellites were converted to pier structures.

Transporter designs
 In the early 1960s the transporter concept originated as a method of reducing aircraft maneuvering on the apron and of eliminating the need for passengers to climb up and down stairways in order to enter or exit the aircraft. In a concept derived from much older designs (such as that at Linate in Milan, where ordinary apron buses are used), passengers are brought directly to the aircraft by a specialized transporter vehicle. Mobile lounges used at Dulles International Airport near Washington, D.C., and at Jiddah's King Abdul Aziz International Airport have bodies that can be raised and lowered to suit the exact height of the terminal floor and the aircraft sill. However, passenger loading and unloading times are lengthened, causing turnaround delays, and aircraft are more likely to be damaged by the heavy lounges. For such reasons, this type of design has not proved popular with either passengers or airlines.

Remote pier designs
 The remote pier was introduced at Atlanta's Hartsfield in the early 1980s. In this concept, passengers are brought out to a remote pier by an automatic people mover and there embark or disembark in the conventional manner. The system has proved very efficient for handling transfer passengers, but the long distances involved in the terminal layout necessitate the use of a sophisticated people-mover system. The design of the terminal at Stansted Airport near London incorporates this concept.

Unit terminals
      The term unit terminal is used wherever an airport passenger terminal system comprises more than one terminal. Unit terminals may be made up of a number of terminals of similar design (e.g., Dallas–Fort Worth and Kansas City in the United States), terminals of different design (e.g., London's Heathrow, Pearson International Airport near Toronto, John F. Kennedy International Airport in New York City), terminals fulfilling different functions (e.g., Heathrow, Arlanda Airport near Stockholm, Barajas Airport near Madrid), or terminals serving different airlines (e.g., Paris's Charles de Gaulle, John F. Kennedy, Dallas–Fort Worth). The successful operation of unit terminal airports has often required the design of rapid and efficient automatic people movers such as those at Changi Airport in Singapore, at Dallas–Fort Worth, and at Houston Intercontinental Airport in Texas.

The apron area
      One of the important requirements in the design of a terminal complex is minimizing the time needed to service an aircraft when it is transiting an airport. This is especially important in the handling of short-haul aircraft, where unproductive ground time can consume an unacceptably large percentage of flight time. The turnaround time for a large passenger transport between short-haul flights can be as little as 25 minutes. During this period, a large number of service vehicles circulate on the apron. Therefore, an important aspect in the efficient operation of an airport facility is the marshaling of ground service vehicles and aircraft in the terminal apron area. Such an operation can become extremely complex at some of the world's busiest airports, where an aircraft enters or leaves the terminal apron approximately every 20 seconds.

      Immediately after World War II, new airports were designed with their terminals entirely surrounded by runways, with access to the terminal areas provided by tunnels. Such layouts include John F. Kennedy at New York and London's Heathrow. Because many large airports now discourage general aviation use by small aircraft, making crosswind runways unnecessary, the newest designs (including airports at Dallas–Fort Worth, Munich, and Inch'ŏn, S.Kor.) have linear layouts of terminals set between parallel runways.

The design process
      The design of airport passenger terminals is a complex procedure normally undertaken only by experts in that area. Buildings are planned to accommodate comfortably the passenger flows that are forecast to occur at some future date. In addition, it is anticipated that terminals will have to operate at levels slightly over capacity for short periods. The measurements used are various, including such terms as standard busy rate and typical peak-hour passenger flow, but, by using these design procedures, it is possible to plan a facility that will have insufficient capacity for no more than 30 operating hours per year. Under such conditions, only about 5 percent of passengers would be subjected to inconveniences caused by lack of capacity.

Cargo facilities
      Less than 1 percent of all freight tonnage is carried by air. Nonetheless, this statistic significantly underestimates the importance of air freight because, in value of cargo moved, air transport dominates all other modes. For example, although Heathrow Airport handles only about a million tons of freight per year, in value of throughput it ranks as Britain's premier port.

      As is the case with passenger facilities, freight terminals vary greatly in the volumes of material handled. Consequently, the scale of the building facilities and the nature of the handling methods also vary. Because only 10 percent of air cargo is carried loose or in bulk, all modern air-cargo facilities are designed to handle containers. In countries where labour is cheap and where freight throughputs at the terminal are not high, freight-handling systems can still be economically designed around the manhandling concept. This is not feasible in developed countries, where labour costs are high. Even at facilities with small throughputs, freight is moved by mobile mechanical equipment such as stackers, tugs, and forklift trucks. At high-volume facilities, a mixture of mobile equipment and complex fixed stacking and movement systems must be used. The fixed systems, which require complex engineering design and maintenance, are known as transfer vehicles (TVs) and elevating transfer vehicles (ETVs).

      In the design of air-cargo facilities, special attention must be given to the handling of very heavy and oversized freight, perishables, urgent materials such as serums and human donor organs, high-value goods such as diamonds and gold, hazardous goods, and livestock.

      An area of very fast growth in the air-cargo business is specialized movement by integrated carriers such as the U.S.-based FedEx Corporation, which offer door-to-door delivery of small packages at premium rates. In its early years, this type of freight grew by more than 17 percent per annum. Cargo terminals for the small-package business are designed and constructed separately from conventional air-cargo terminals. They operate in a different manner, with all packages being cleared on an overnight basis.

Links to local ground transportation
      An airport should always be considered an interchange where different modes of transportation connect. Since the airport itself is not a primary destination, consideration must be given to access by surface vehicles. This is as critical a factor in airport layout and design as it is in the process of site selection. A large airport can quite easily generate in excess of 100,000 daily access trips by passengers and the same number of trips by workers, visitors, and suppliers. Such a scale of surface movement requires careful consideration of the design of internal circulation roads (roads and highways) and access highways to the city centre and to the economic hinterland served by the airport. Additionally, road-based access requires the careful design of drop-down and pick-up areas and of both long-term and short-term parking. Larger airports are able to sustain economically viable links to taxi, limousine, and bus services. In addition, many of the world's largest airports are linked to intercity, suburban, and metro-style rail systems.

      Peak traffic on airport approach roads tends to occur in the morning and evening, coinciding with other peaks of suburban traffic, so that it is difficult to forecast journey times between an airport and the downtown areas during rush periods. Even so, as major airports have gradually moved farther away from city centres, journey times to airports have increased. Road-traffic congestion has accentuated the problem, and the cost of delay has become critical in the economic comparison of air and surface transportation. In fact, large airports located on sites remote from the cities they serve have proved to be very unpopular with both passengers and airlines (e.g., Narita at Tokyo and, for many years, Mirabel at Montreal and Dulles at Washington, D.C.).

      A major argument for short takeoff and landing (STOL) aircraft (STOL airplane) lies in their lesser requirements for airport space, promising sites close to the central city. However, STOL airports have not proved economically viable, mainly because the aircraft used have been small, expensive to operate, and (at least in popular perception) uncomfortable. Furthermore, STOL aircraft have relatively low cruising speeds, which have limited their useful range. Helicopters, also known as vertical takeoff and landing (VTOL) aircraft, are in general too expensive for civil aviation transport, but the introduction of large tilt-rotor aircraft might revolutionize the use of centre-city airports.

Airport capacity
      The various facilities at an airport are designed to cope adequately with the anticipated flow of passengers and cargo. The flow that any particular facility can accommodate without serious inconvenience to the users is considered to be its capacity. Limits on the traffic that can reasonably be accommodated at an airport are reached in a number of ways. These include air traffic delays to landing and takeoff movements; congestion on runways, taxiways, and aprons; crowding and delays in terminal buildings; or severe congestion in such access facilities as parking areas, internal roads, and public transport.

      At smaller one-runway airports, limits to capacity usually occur in the terminal areas, since the operational capacity of a single runway with adequate taxiways is quite large. When passenger volumes reach approximately 25 million per year, a single runway is unlikely to be adequate to handle the number of aircraft movements that take place during peak periods. At this point at least one additional runway, permitting simultaneous operation, is required. Airports with two simultaneous runways should be able to handle approximately 55 to 65 million passengers per year, and here, too, the main capacity problems are related to the provision of adequate terminal space. Layouts with four parallel runways are estimated to have operational capacities of well over one million aircraft movements per year and annual passenger movements in excess of 100 million. The main capacity constraints of such facilities are in the provision of sufficient airspace for controlled aircraft movements and in the provision of adequate access facilities. It is likely that many of the world's largest airports will face access problems before they reach the operational capacity of their runways.

      Large airports are actually urban complexes in which high-population activity centres are closely associated with very extensive paved areas. Typically a large airport can, on a daily basis, handle more than 100,000 passengers and support a working population of more than 50,000 employees. The sewage system of such an airport must cope with large daily flows of sanitary sewage effluent and, in addition, must accommodate runoff from rain and snow accumulating over several hundred acres of impervious pavement. The scale of the sewage problem at many large airports is such that some facilities have their own sewage treatment plants, especially for sanitary sewage. Because many airports are situated on low-lying ground, which is more likely to provide the flat land necessary for airstrips, the sewage system must often include extensive pumping facilities.

      Growing concern about the environment combined with the increasing scale of activity at many airports has meant that runoff water can no longer be drained directly into bodies of surface water such as rivers and lakes. In particular, deicing chemicals used on aircraft and airfield pavements and cleaning chemicals used in aircraft maintenance are serious contaminants of groundwater and surface water. Consequently, some airports are required to provide at least primary treatment of all runoff discharges, and there are legal restrictions on the nature of the chemicals that can be used. In order to prevent groundwater pollution, Munich Airport was designed to accommodate existing flows of surface water across the entire site and was also provided with extensive arrangements for the recycling of deicing chemicals.

      By the early 1960s, aircraft noise in the vicinity of urban airports had become a major problem. The cause of the problem was a rapidly increasing number of aircraft movements and the introduction of the first generation of turbojet aircraft with low climb performance, such as the early models of the Boeing 707 and the Douglas DC-8. Subsequently, public objections arose to the planned expansion of most urban airports. These objections often held up expansion for many years and, in cities such as London and Munich, ultimately severely modified the development of new airports. In addition, noise curfews were introduced at many existing airports, such as John F. Kennedy in New York, London's Heathrow, and Kingsford Smith Airport near Sydney.

      In response to national and international regulations aimed at certifying only quieter aircraft, major efforts have been made by aircraft manufacturers to reduce noise at the source. Successive generations of aircraft have been banned as they failed to meet increasingly severe requirements introduced by the International Civil Aviation Organization and the U.S. Federal Aviation Administration. The introduction of high-bypass turbofan engines and aircraft with high climb performance have helped considerably in reducing noise.

      Airports can diminish aircraft noise in a number of ways. Hours of operation can be limited through the use of night curfews, night noise having been found much more objectionable to the public than day noise. Noisy aircraft can be restricted or even banned. Runways can be selected to limit or spread noise more evenly over the community, and approach and departure routes can be designated over less-populated areas. Airlines can also be encouraged to modify approach and departure gradients and operating procedures in order to reduce engine thrust over highly populated areas. The performance of the operators is monitored, and offending operators have financial penalties imposed upon them. Using such methods, many major airports—even those that anticipate long-term growth in passenger numbers and aircraft movements—can significantly reduce the exposure of urban populations to aircraft noise.

Airport security
 Until the 1960s, airport security was relatively simple, requiring nothing more than civilian police to provide protection against conventional crimes such as theft, pickpocketing, vandalism, and breaking and entering. However, in the 1960s civil aviation became a recognized target for politically motivated crimes. These crimes came to include general acts of terrorism, such as mass shootings and bombings and, especially, aircraft hijacking.

      Although the first aircraft hijacking occurred in 1931 in Peru, such events were rare, with no more than a handful each year, and generally without any political motive. However, by the late 1960s, politically motivated hijackings to Cuba had become common. In 1969, for example, there were 87 hijackings worldwide, of which 71 were related to Cuba, which typically granted political asylum to the hijackers.

      The International Civil Aviation Organization (ICAO), which quickly recognized that passenger airliners had become political targets, responded in the decades of the 1960s and '70s with three major conventions covering “unlawful acts against civil aviation.” (See also air law: International regulation (air law).) To combat the crimes of hijacking and terrorism, the following international conventions established minimum conditions for appropriate security countermeasures to be adopted in an international context:
● Convention on Offences and Certain Other Acts Committed on Board Aircraft, commonly called the Tokyo Convention, was signed on Sept. 14, 1963, and went into force on Dec. 4, 1969—concerned with crimes on board aircraft, particularly any crime that jeopardizes the safety of the aircraft and its passengers;
● Convention for the Suppression of Unlawful Seizure of Aircraft, commonly called The Hague Convention, was signed on Dec. 16, 1970, and went into force on Oct. 14, 1971—concerned specifically with the offence of hijacking, with a recommendation that it should be made an extraditable offence for all member countries;
● Convention for the Suppression of Unlawful Acts Against the Safety of Civil Aviation, commonly called the Montreal Convention, was signed on Sept. 23, 1971, and went into force on Jan. 26, 1973—broadened the scope of The Hague Convention to include the crime of sabotage.

      These conventions resulted in many ICAO recommendations for the enforcement of greater security at airports. However, because the ICAO has no national jurisdiction, the organization's recommendations needed to be translated into individual national laws. Following the Tokyo Convention, the ICAO measures were widely adopted by most national civil aviation authorities, although the efficiency of the security procedures adopted varied greatly throughout the world. Countries that had no history of domestic civil terrorism became overconfident in their security measures, believing that only international flights were real targets for terrorist attacks. However, as terrorist acts continued to occur against passenger airliners, security measures gradually became less lax in most jurisdictions.

      Initially, the principal objective of security measures was to ensure that passengers could not board aircraft with weapons or explosives. Passengers were scanned with magnetometers (magnetometer) and suspicious individuals selected for body searches; carry-on baggage was routinely passed through X-ray machines. Public access to the aprons and operational areas was denied, except for authorized staff, as was unnecessary access to the nonpublic areas of the terminal. As control of passengers and carry-on baggage tightened, hijackings were increasingly replaced by acts of sabotage to aircraft, carried out by explosive devices secreted in baggage carried in the airplane's hold. By the late 1990s, the ICAO had produced recommendations that all hold baggage should be screened for explosive and dangerous devices. The operational areas of civil airfields were enclosed by security fences, with manned access gates and visual surveillance of much of the areas by closed-circuit television.

 In 2001, the September 11 attacks produced a sea change in much of the thinking surrounding airport security. In a period of two hours, a single terrorist organization wreaked an unprecedented level of destruction in the United States by using hijacked airliners as missiles. For the first time, civil transport aircraft, loaded with passengers and, most significantly, with a nearly full load of fuel, had been converted to destructive weapons.

      Authorities responded to these hijackings with an intensification of security procedures at airports around the world. Passenger and baggage search procedures were made significantly more thorough, involving more careful screening for known terrorists (including the creation of various no-fly and watch lists of risky individuals) and potentially problematic carry-on items. Passenger terminals increased the level and sophistication of security equipment, the number of staff employed in security procedures, and the space provided for security operations. As a result, recommended check-in times for departing international passengers at many airports became as much as three hours before scheduled departure.

      Hand baggage and checked baggage both became subject to strict scrutiny following Sept. 11, 2001. Many additional airports installed X-ray equipment, for spotting metal items in baggage or concealed in clothing, and massive electronic detection systems (EDS), which can detect trace molecules released by explosive materials. The massive weight of EDS equipment frequently requires structural modifications to existing buildings, and the size of the equipment often requires a reallocation of floor space. In many airports, installed security equipment now must be approved and certified by the national government.

      Another problem for security at airports is the possibility of a car or truck being loaded with explosives and detonated near people or facilities. In particular, the threat from such “car bombs” forced greater caution with the location and operation of passenger pick-up areas and airport parking facilities. Parking garages that were integrated into the design of the passenger terminal pose a special danger. At those airports where parking design results in a threat to the safety of the terminal building from potential car bombs, operational procedures have been reevaluated and changed. In many cases, parking facilities integrated into the terminal itself have been closed.

      The threat of terrorist attacks has meant that, for the foreseeable future and probably permanently, civil aviation cannot return to a situation of relaxed security. Eventually, access to airport terminals might require that all persons pass through some form of security check prior to check-in, that all passengers and baggage be thoroughly scrutinized for weapons and explosives, and that passengers even undergo profiling interviews to identify potential problem travelers.

Norman J. Ashford

Additional Reading
Norman Ashford and Paul H. Wright, Airport Engineering, 3rd ed. (1992), comprehensively sets forth the planning, layout, and design of passenger and freight airports, including heliports and short takeoff and landing (STOL) facilities. Robert Horonjeff and Francis X. McKelvey, Planning and Design of Airports, 4th ed. (1993), is a comprehensive civil engineering text on the planning, layout, and design of airports with strong emphasis on aspects such as aircraft pavements and drainage. International Civil Aviation Organization, Annex 14 Aerodromes: International Standards and Recommended Practices (1999), includes the internationally adopted design and operational standards for all airports engaged in international civil aviation.Christopher R. Blow, Airport Terminals, 2nd ed. (1995), provides an architectural view of the functioning of airport passenger terminals with extensive coverage of design case studies. Walter Hart, The Airport Passenger Terminal (1985, reprinted 1991), describes the functions of passenger terminals and their design requirements. International Air Transport Association, Airport Terminals Reference Manual, 7th ed. (1989), provides design and performance requirements of passenger and freight terminals as set out by the international airlines' trade association.Denis Phipps, The Management of Aviation Security (1991), describes the operational and design requirements of civil airports to conform to national and international regulations. Norman Ashford, H.P. Martin Stanton, and Clifton A. Moore, Airport Operations, 2nd ed. (1996), extensively discusses many aspects of airport operation and management, including administrative structure, security, safety, environmental impact, performance indices, and passenger and aircraft handling. Norman Ashford and Clifton A. Moore, Airport Finance, 2nd ed. (1999), discusses the revenue and expenditure patterns of airport authorities, methods of financing, business planning, and project appraisal. Rigas Doganis, The Airport Business (1992), examines the status of airport business in the early 1990s, performance indices, commercial opportunities, and privatization of airports.Norman J. Ashford

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Universalium. 2010.

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