Why Must Airplanes Wait on the Runway?
Many of you have probably experienced those few minutes of sitting in an airplane on the runway waiting for takeoff, annoyed and wondering what has taken them so long.
When an aircraft slices through the sky, its wings generate a disturbance called wake turbulence. This phenomenon consists of two key components: jet wash and wingtip vortices. Jet wash is the high-speed exhaust expelled by jet engines, powerful enough to flip cars, but fortunately, it dissipates relatively quickly. On the other hand, wingtip vortices, the swirling whirlwinds created by the wings as they generate lift, can linger in the air for up to three minutes.
If an aircraft enters the wake turbulence of a larger plane, it can lose control due to the rotating vortex, potentially leading to a catastrophic situation.
Now, picture the aircraft waiting patiently on the runway, positioned behind another. By giving a safe separation distance, we allow the wake turbulence from the leading plane to disperse harmlessly into the sky. This separation is vital for avoiding dangerous encounters with these atmospheric vortices.

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Note: "The appearance of U.S. Department of Defense (DoD) visual information does not imply or constitute DoD endorsement."


ADDITIONAL READINGS:
Air traffic control (ATC) is a service provided by ground-based air traffic controllers who direct aircraft on the ground and through a given section of controlled airspace, and can provide advisory services to aircraft in non-controlled airspace. The primary purpose of ATC worldwide is to prevent collisions, organize and expedite the flow of air traffic, and provide information and other support for pilots.

Air traffic controllers monitor the location of aircraft in their assigned airspace by radar and communicate with the pilots by radio. To prevent collisions, ATC enforces traffic separation rules, which ensure each aircraft maintains a minimum amount of empty space around it at all times. It is also common for ATC to provide services to all private, military, and commercial aircraft operating within its airspace, not just civilian aircraft. Depending on the type of flight and the class of airspace, ATC may issue instructions that pilots are required to obey, or advisories (known as flight information in some countries) that pilots may, at their discretion, disregard. The pilot in command is the final authority for the safe operation of the aircraft and may, in an emergency, deviate from ATC instructions to the extent required to maintain safe operation of their aircraft.

ATC is waiting for the safe separation between this plane and the plane that took off just ahead of it. The separation is to avoid wake turbulence problems. ATC does this to make sure the plane is ready to go and speed up departures during a “push”. That's a period when there's a lot of scheduled departures.

History
In 1920, Croydon Airport, London, was the first airport in the world to introduce air traffic control. The "aerodrome control tower" was a wooden hut 15 ft (4.6 m) high with windows on all four sides. It was commissioned on February 25, 1920 and provided basic traffic, weather and location information to pilots.

In the United States, air traffic control developed three divisions. The first of several air mail radio stations (AMRS) was created in 1922 after World War I when the U.S. Post Office began using techniques developed by the Army to direct and track the movements of reconnaissance aircraft. Over time, the AMRS morphed into flight service stations. Today's flight service stations do not issue control instructions, but provide pilots with many other flight related informational services. They do relay control instructions from ATC in areas where flight service is the only facility with radio or phone coverage. The first airport traffic control tower, regulating arrivals, departures and surface movement of aircraft at a specific airport, opened in Cleveland in 1930. Approach/departure control facilities were created after adoption of radar in the 1950s to monitor and control the busy airspace around larger airports. The first air route traffic control center (ARTCC), which directs the movement of aircraft between departure and destination, was opened in Newark in 1935, followed in 1936 by Chicago and Cleveland.[8] Currently in the U.S., the Federal Aviation Administration (FAA) operates 22 ARTCCs.

After the 1956 Grand Canyon mid-air collision, killing all 128 on board, the FAA was given the air-traffic responsibility over the United States in 1958, and this was followed by other countries. In 1960, Britain, France, Germany and the Benelux countries set up Eurocontrol, intending to merge their airspaces.

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