Runway
October 20th, 2008
A runway is a strip of land on an airport, on which aircraft can take off and landing. 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 Two Left (2L) becomes Runway Two Zero Right (20R) when used in 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 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 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 is used at airports that wish to allow night landings. Seen from the air, runway lights form an outline of the runway. From a landing aircraft, the threshold is a strip of green lights (known as threshold lights) the far end is a strip of red lights. White elevated edge lights run the length of the runway on either side. Taxiways are differentiated by being bordered by blue lights. The centerline is often indicated by white lights, which may be coded alternately white and red and then purely red 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 on either side of the centerline over the first 30 feet of the runway. According to Transport Canada’s regulations, the runway-edge lights must be visible for at least 2 miles. Additionally, a new system of advisory lighting, Runway Status Lights, is being tested in 2006 in the United States.
The 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 there may have threshold markings, numbers, 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, numbers, centerlines, and sometimes an aiming point. 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, number, centerline, one 3-stripe touchdown zone (All countries) or two three stripe touchdown zone (All except US), aiming point, two 2-stripe touchdown zones (All countries) or one 2-stripe touchdown zone (All except US), and two 1 stripe touchdown zones. Precision runways provide both horizontal and vertical guidance for instrument approaches.
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