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 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.

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.