A runway (RWY) is a strip of land on an airport, on which aircraft can take off and land. Runways may be a man-made surface (often asphalt, concrete, or a mixture of both) or a natural surface (grass, dirt, or gravel).
In International Civil Aviation Organization (ICAO) and some United States military (such as Edwards Air Force Base) airports, numbers for runways less than 100° include the leading "zero", for example, Runway Zero Two or Runway Zero One Left. However in the United States at most civil aviation airports, numbers for runways less than 100° are often given as single digits; for instance, Runway Nine or Runway Four Right. This also includes some military airfields such as Cairns Army Airfield. This American anomaly may lead to inconsistencies in conversations between American pilots and controllers in other countries. It is very common in a country such as Canada for a controller to clear an incoming American aircraft to, for example, Runway 04, and the pilot read back the clearance as Runway 4. Users of flight simulation programs may note that those of American origin might apply U.S. usage to airports around the world. For example Runway 05 at Halifax (CYHZ) will appear on the FS program as the single digit 5 rather than 05.
Runway designations do change over time. Because the magnetic poles slowly drift on the Earth's surface, but the runways stay fixed, the magnetic bearing will change. Depending on the airport location and how much drift takes place, it may be necessary over time to change the runway designation. As runways are designated with headings rounded to the nearest 10 degrees, this will affect some runways more than others. For example,if the magnetic heading of a runway is 276 degrees , it would be designated Runway 28. If the magnetic heading changed upwards by 5 degrees, the Runway would still be Runway 28. If on the other hand the original magnetic heading was 284 (Runway 28), and the heading increased by only two degrees to 286, the runway should become Runway 29. Because the drift itself is quite slow, runway designation changes are relatively uncommon, and not welcomed, as they do require an accompanying change in a wide range of navigational and descriptive documents.
If there is more than one runway pointing in the same direction (parallel runways), each runway is identified by appending Left (L), Center (C) and Right (R) 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, Detroit Metropolitan Wayne County Airport in Romulus, Michigan,Hartsfield-Jackson International Airport in Atlanta, Georgia, and Denver International Airport), 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).
At Dallas-Fort Worth International Airport, there are five parallel runways, named 17L, 17C, 17R, 18L, and 18R.
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.
According to Transport Canada's regulations, the runway-edge lighting must be visible for at least . 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:
Control of Lighting System Typically the lights are controlled by a control tower, a Flight Service Station or another designated authority. Some airports/airfields (particularly uncontrolled ones) are equipped with Pilot Controlled Lighting, so that pilots can temporarily turn on the lights when the relevant authority is not available. 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 cost of having the lighting system 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 landing strip.
There are three types of runways:
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 that major settlements of the runway will occur over the years because of unstable ground conditions, 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 on 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 to as much as , including subgrade.
Historically, airport pavements have been designed by two methods. The first, Westergaard, is based on 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 on 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 on the 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.
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/32L than they can on 9L/27R), 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).
Although runway length may be of some academic interest, in terms of usability for air carrier operations, a runway of at least in length is usually adequate for aircraft weights below approximately . Larger aircraft including widebodies (Boeing 747, 767, 777, and 787; Airbus A310, A330, A340, A350, and A380; McDonnell-Douglas DC-10 or MD-11; and the Lockheed L1011) will usually require at least at sea level and somewhat more at higher altitude airports. International widebody flights may also have landing requirements of or more and takeoff requirements of +.
At sea level, 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 or 27R to request 27L (). It is always accommodated, although occasionally with a delay.
Any given aircraft will need a longer runway at a higher altitude due to decreased density of air at higher altitudes, which reduces lift and engine power. For example, New York JFK is at sea level, while Denver is over 5,000 feet, so the same aircraft with an identical load will require a longer runway at Denver. An aircraft will also require a longer runway in hotter or more humid conditions (see density altitude). Most commercial aircraft carry manufacturer's tables showing the adjustments required for a given temperature.