Runway

This article is about airport runways. For information on fashion model runways, see catwalk. For the ITV quiz programme "Runway", see Runway (game show).

File:Bogrunway.JPG

Runway 13R/31L of El Dorado International Airport, Bogotá, D.C., Colombia.

Aerial picture of a runway of Chennai International Airport, Tamil Nadu, India.

A runway is a strip of land on an airport, on which aircraft can take off and land. Runways may be a prepared surface (often asphalt, concrete, or a mixture of both) or an unprepared surface (grass, dirt, or gravel).

Orientation and dimensions

Runways are generally numbered according to the magnetic direction in which they point (referred to as the "runway heading"), rounded to the nearest ten degrees and then divided by ten. Each digit is pronounced separately for clarity in radio communications. For example, Runway Three Six would be aligned in roughly a 360 degrees direction (i.e. magnetic north), Runway Nine would be used for a runway with a 94 degree-alignment (i.e. close to magnetic east), and Runway One Seven for 168 degrees. Each runway can be used in either direction, and hence has two numbers, each 180° apart. Thus, Runway One Zero (100°) becomes Runway Two Eight (280°) when used in the opposite direction and Runway One Eight (180°) becomes Runway Three Six (360°). Runways in North America that lie within the Northern Domestic Airspace are, because of the magnetic north pole, usually numbered according to true north.

In United States civil aviation, numbers for runways less than 100° are often given as single digits; e.g. Runway Nine or Runway Four Right. In United States military and ICAO operations, numbers for runways less than 100° include the leading "zero", e.g. Runway Zero Two or Runway Zero One Left.

If there is more than one runway pointing in the same direction (parallel runways), each runway is identified by appending Left, Center and Right to the number — for example, Runways One Five Left (15L), One Five Center (15C), and One Five Right (15R). Runway Zero Two Left (02L) becomes Runway Two Zero Right (20R) when used in the opposite direction (derived from adding 18 to the original number for the 180 degrees when approaching from the opposite direction).

At large airports with more than three parallel runways (for example, at Los Angeles International Airport in Los Angeles, California or Hartsfield-Jackson International Airport in Atlanta, Georgia), some runway identifiers are shifted by 10 degrees to avoid the ambiguity that would result with more than three parallel runways. In Los Angeles, this system results in Runways Six Left, Six Right, Seven Left, and Seven Right, even though all four runways are exactly parallel (approximately 69 degrees).

For fixed wing aircraft it is advantageous to perform take-offs and landings into the wind to reduce takeoff roll and reduce the ground speed needed to attain flying speed. Larger airports usually have several runways in different directions, so that one can be selected that is most nearly aligned with the wind. Airports with one runway are often constructed to be aligned with the prevailing wind.

Runway dimensions vary from as small as 800 feet (240 m) long and 25 feet (8 m) wide in smaller general aviation airports, to 16,000 feet (4,800 m) long and 250 feet (80 m) wide at large international airports built to accommodate large passenger jets. Runway dimensions are measured in feet in the United States and Canada, and meters are used elsewhere in the world.

"Sections" of a runway

• The Runway Strip is the cleared, grassy area around the paved runway. It is kept free from any obstacles that might impede flight or ground roll of aircraft, although the grass is not always necessarily in good condition. The grass is often marked with white cones or gables.

• The Runway is the entire paved surface, which typically features threshold markings, numbers, centerlines, and overrun areas at both ends.

• Blast pads, also known as overrun areas or stopways, are often constructed just before the start of a runway where jet blast produced by large planes during the takeoff roll could otherwise erode the ground and eventually damage the runway. Overrun areas are also constructed at the end of runways as emergency space to slowly stop planes that overrun the runway on a landing gone wrong, or to slowly stop a plane on an aborted take-off or a take-off gone wrong. Blast pads are often not as strong as the main paved surface of the runway and are marked with yellow chevrons. Planes are not allowed to taxi, take-off or land on blast pads, except in an emergency. Although blast pads are supposed to be a minimum 1000 feet long, most airports in the US do not meet this requirement.

• The Displaced threshold is the point at the end of the runway. In major airports, it is usually marked with white paint arrows that lead up to the displaced threshold (see diagram). Smaller runways may not have markings to indicate the displaced threshold. A displaced threshold may be used for taxiing and takeoff but not for landing, because obstacles just before the runway, runway strength, or noise restrictions may make the area unsuitable for landings.

Runway lighting

Runway lighting is used at airports which allow night landings. Seen from the air, runway lights form an outline of the runway.

• From a landing aircraft, the threshold is a pair of four green lights on each side of the runway (known as threshold lights). On precision runways, the threshold lights extend along the full width of the runway. The far end of the runway is a strip of red lights (arranged similar to threshold lights).
• White elevated edge lights run the length of the runway on either side. Taxiways are differentiated by being bordered by blue lights. On precision runways, the edge-lighting becomes yellow in the last 2000 feet of the runway.
• The centerline is indicated by white lights on some runways (notably precision runways), which may be coded alternately white and red at 3000 feet remaining and then purely red in the last 1000 feet nearing the far end of the runway.
• Furthermore, many runways equipped with instrument landing systems feature touchdown zone lighting. This consists of rows of white light bars (with three in each row) on either side of the centerline over the first 3000 feet (or to the midpoint, whichever is less) of the runway.

According to Transport Canada's regulations, the runway-edge lighting must be visible for at least 2 miles. Additionally, a new system of advisory lighting, Runway Status Lights, is currently being tested in the United States.

The edge lights must be arranged such that:

• the minimum distance between lines is 75 feet, and maximum is 200 feet;
• the maximum distance between lights within each line is 200 feet;
• the minimum length of parallel lines is 1400 feet;
• the minimum number of lights in the line is 8.

Runway markings

There are various runway markings and signs on any given runway. Larger runways have a distance remaining sign (black box with white numbers). This sign uses a single number to indicate the thousands of feet remaining, so 7 will indicate 7,000 feet remaining. The runway threshold is marked by a line of green lights.

Some airports/airfields (particularly uncontrolled ones) are equipped with Pilot Controlled Lighting, so that pilots can temporarily turn on the lights when they need them. This avoids the need for automatic systems or staff to turn the lights on at night or in other low visibility situations. This also avoids the costs of having hundreds of lights on for extended periods. Smaller airports may not have lighted runways or runway markings. Particularly at private airfields for light planes, there may be nothing more than a windsock beside a sod landing strip. At major airports, Runway Status Lights are being developed that give pilots more information on runway movements.

There are three types of runways:

• Visual Runways are used at small airstrips, visual runways are usually just a strip of grass, gravel, asphalt or concrete. Although there are usually no markings on a visual runway they may have threshold markings, designators, and centerlines. Additionally, they do not provide an instrument-based landing procedure; pilots must be able to see the runway to use it. Also, radio communication may not be available and pilots must be self-reliant.

• Non-precision runways are often used at small-medium size airports. These runways are always marked with threshold markings, designators, centerlines, and sometimes a 1000-foot mark (known as an aiming point, sometimes installed at 1500 feet). They provide horizontal position guidance to planes on instrument approach via radio beacons.

• Precision runways, which are found at medium and large size airports, consist of a blast pad/stopway (optional, for airports handling jets), threshold, designator, centerline, aiming point, and 500, 1000/1500, 2000, 2500, and 3000-foot touchdown zone marks. Precision runways provide both horizontal and vertical guidance for instrument approaches.

National variants

• In Canada, Australia, Japan, the United Kingdom, as well as some other countries all 3-stripe and 2-stripe touchdown zones for precision runways are replaced with one-stripe touchdown zones.
• In Australia, precision runways consist of only one 1-stripe touchdown zone, aiming point, and one 1-stripe touchdown zone. Furthermore, all non-precision and visual runways lack an aiming point.
• Some European countries replace the aiming point with a 3-stripe touchdown zone.
• Runways in Norway have yellow markings instead of the usual white ones. This also occurs on some airports in Japan.
• Runways may have different types on each end. To cut costs, many airports do not install precision guidance equipment on both ends. Runways with one Precision end and any other type of end can install the full set of touchdown zones, even if some are past the midpoint. If a runway has Precision markings on both ends, touchdown zones within 900 ft/270 m of the midpoint are omitted, to avoid pilot confusion over which end the marking belongs to.

Pavement

The choice of material used to construct the runway depends on the use and the local ground conditions. Generally speaking, for a major airport, where the ground conditions permit, the most satisfactory type of pavement for long-term minimum maintenance is concrete. Although certain airports have used reinforcement in concrete pavements, this is generally found to be unnecessary, with the exception of expansion joints across the runway where a dowel assembly, which permits relative movement of the concrete slabs, is placed in the concrete. Where it can be anticipated, because of unstable ground conditions, that major settlements of the runway will occur over the years, it is preferable to install asphaltic concrete surface, as it is easier to patch on a periodic basis. For fields with very low traffic of light planes, it is possible to use a sod surface.

The development of the pavement design proceeds along a number of paths. Exploratory borings are taken to determine the subgrade condition, and based upon relative bearing capacity of the subgrade, different pavement specifications are established. Typically, for heavy-duty commercial aircraft, the pavement thickness, no matter what the top surface, varies from as little as 10 in. (25 cm) to as much as 4 ft (1.2 m), including subgrade.

Historically, airport pavements have been designed by two methods. The first, Westergaard, is based upon the assumption that the pavement is an elastic plate supported on a heavy fluid base with a uniform reaction coefficient known as the K value. Experience has shown that the K values upon which the formula was developed are not applicable for newer aircraft with very large footprint pressures.

The second method is called the California bearing ratio and was developed in the late 1940s. It is an extrapolation of the original test results, which are not applicable to modern aircraft pavements or to modern aircraft landing gear. Some designs were predicated upon melding of these two design theories; they are empirical in nature and are not reliable. Another, more recent, method is an analytical system based on the introduction of vehicle response as an important design parameter. Essentially it takes into account all factors, including the traffic conditions, service life, materials used in the construction, and, especially important, the dynamic response of the vehicles using the landing area.

Because airport pavement construction is so expensive, every effort is made to minimize the stresses imparted to the pavement by aircraft. Manufacturers of the larger planes design landing gear so that the weight of the plane is supported on larger and more numerous tires. Attention is also paid to the characteristics of the landing gear itself, so that adverse effects on the pavement are minimized. However, in the final analysis, if plane weights continue to increase as they have in the past, it will be necessary to provide substantially stronger pavements than those that are generally in use in Europe and the United States. Sometimes it is possible to reinforce a pavement for higher loading by applying an overlay of asphaltic concrete or portland cement concrete that is suitably bonded to the original slab.

Posttensioning concrete has been developed for the runway surface. This permits the use of thinner pavements and should result in longer concrete pavement life. Because of the susceptibility of thinner pavements to frost heave, this process is generally applicable only where there is no appreciable frost action.

Active runway

FAA airport diagram at O'Hare International Airport. From left to right, runways 14/32 slant down, runways 4/22 slant up, and runways 9/27 are horizontal.

The active runway is the runway at an airport that is in current use for takeoffs and landings. Since takeoffs and landings are usually done as close to "into the wind" as possible, wind direction generally determines the active runway (or just the active in aviation slang).

Selection of the active runway, however, depends on a number of factors. At a non-towered airport, pilots usually select the runway most nearly aligned with the wind, but they are not obliged to use that particular runway. For example, a pilot arriving from the east may elect to land straight in to an east-west runway despite a minor tailwind or significant crosswind, in order to expedite his arrival, although it is recommended to always fly a regular traffic pattern to more safely merge with other aircraft.

At controlled airports, the active is usually determined by a tower supervisor. However, there may be constraints, such as policy from the airport manager (calm wind runway selection, for example, or noise abatement guidelines) that dictate an active runway selection that isn't the one most nearly aligned with the wind.

At major airports with multiple runways, the active could be any of a number of runways. For example, when O'Hare (ORD) is landing on 27R and 32L, departures use 27L and 32R, thus making four active runways. When they're landing on 14R and 22R, departures use 22L and 9L, and occasionally a third arrival runway, 14L, will be employed, bringing the active runway count to five.

At major airports, the active runway is based on existing weather conditions (visibility and ceiling, as well as wind, and runway conditions such as wet/dry or snow covered), efficiency (ORD can land more aircraft on 14R-22R than they can on 27R-32L), traffic demand (when a heavy departure rush is scheduled, a runway configuration that optimizes departures vs arrivals may be desirable), and time of day (ORD is obliged to use Runway 9L/27R during the hours of roughly midnight to 6 a.m. due to noise abatement).

An attempt to land at an active runway that fails and either leaves the runway path or overshoots is called a runway excursion.

Longest runways

Main article: Longest runways

Although runway length may be of some academic interest, in terms of usability for air carrier operations, a runway of at least 6,000 ft (1,820 m) in length is usually adequate for aircraft weights below approximately 200,000 lb (90,900 kg). Larger aircraft including widebodies (Boeing 747, 767, 777, and 787 (still in design)); Airbus A-340, A-330, A-350 (still in design), A-380 and A-310; McDonnell-Douglas DC-10 or MD-11; and the Lockheed L1011 will usually require at least 8,000 ft (2,430 m) at sea level and somewhat more at higher altitude airports. International widebody flights may also have landing requirements of 10,000 ft (3,048 m) or more and takeoff requirements of 13,000+ ft.

At sea level, 10,000 ft can be considered an adequate length to accommodate virtually any aircraft. For example, at O'Hare International Airport, when landing simultaneously on 22R and 27L or parallel 27R, it is routine for arrivals from the Far East which would normally be vectored for 22R (7,500 ft) or 27R (8,000 ft) to request 27L (10,000 ft). It is always accommodated, although occasionally with a delay.

See also

• Aviation