| The active runway is the runway at an airport that is in use for takeoffs and landings. Since takeoffs and landings are usually done as close to "into the wind" (see headwind) as possible, wind direction generally determines the active runway. 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 into 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 is not 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 Chicago O'Hare International Airport is landing on 27L and 32L, departures use 28 and 32R, thus making four active runways. When they are landing on 14R and 22R, departures use 22L and 9R, 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 weather conditions (visibility and ceiling, as well as wind, and runway conditions such as wet/dry or snow covered), efficiency (O'Hare International Airport can land more aircraft on 14R/32L than on 9R/27L), traffic demand (when a heavy departure rush is scheduled, a runway configuration that optimizes departures vs. arrivals may be desirable), and time of day (O'Hare is obliged to use runway 9R/27L during the hours of roughly midnight to 6 a.m. due to noise abatement). London Heathrow Airport in the United Kingdom has two parallel runways, designated 09L/27R and 09R/27L. They are most often used in segregated alternate mode, which means one runway is used only for arrivals and the other is only used for departures although at busy periods 'mixed mode' is used in which both runways are used for both takeoff and landing. The segregated mode provides for one runway to be used by landing aircraft from 06:00 until 15:00 and then arrivals will switch to the other runway from 15:00 until after the last departure, after which a separate night alternation scheme operates, involving either the northern or southern runway being used in an easterly or westerly direction on a 4-week cycle. The runways used by landing aircraft before and after 15:00 also alternate on a weekly basis. This only applies to westerly operations as landing aircraft almost always use runway 09L during easterly operations. |
