En Route and Descent
Safe Separation?
Safe vertical separation between aircraft is considered to be 1,000 ft (305 m) at altitudes below 29,000 ft (8845 m) and 2,000 ft (610 m) at altitudes above that. When aircraft are at the same altitude, safe horizontal separation is considered to be 5 miles (8 km). The Air Transport Association has recommended that these separations be reduced to use airspace more efficiently and reduce airport delays.
Once your plane has left TRACON airspace, it enters a sector of the ARTCC airspace, where it is monitored by at least two air traffic controllers. The radar associate controller receives the flight-plan information anywhere from five to 30 minutes prior to your plane entering that sector. The associate controller works with the radar controller in charge of that sector. The radar controller is in charge of all air-to-ground communication, maintains safe separation of aircraft within the sector and coordinates activities with other sectors and/or centers. The controllers must monitor the airspace at high altitude (above 24,000 ft/7320 m) and low altitude (below 24,000 ft). The center controllers provide your pilot with updated weather and air-traffic information. They also give directions to your pilot regarding such aspects as speed and altitude to maintain a safe separation between aircraft within their sector. They monitor your plane until it leaves their sector. Then they pass it off to another sector's controller.

The various air traffic control facilities encountered by a plane during its flight

Another controller, called the radar hand-off controller, assists the radar and associate radar controllers during times of heavy traffic, watching the radar screen and helping to maintain smooth air-traffic flow.

Photo courtesy NASA
ARTCC, showing various controllers

While you are enjoying your meal, snack, in-flight movie or the view outside the window, your plane gets passed from sector to sector and center to center. In each sector, center controllers radio instructions to the pilots. The path of your plane may have to be changed from the original flight plan to move around bad weather or avoid a congested sector. Your pilots may request a change in altitude to avoid or reduce turbulence. This back and forth between pilots and center controllers continues until you are about 150 miles (241 km) from San Francisco (your destination). At this point, the center controller directs all planes flying into San Francisco to move from high altitudes to low altitudes and merges the descending aircraft into a single file line toward the airport. The controller gives instructions to your pilot, such as changes in heading, speed and altitude, to place your plane in line with these other aircraft. Depending on traffic conditions, the controller may have to place your plane into a holding pattern, which is a standard route around each airport, where you wait until the airport can handle your arrival. The controller continues to give directions to your pilot until your plane is within TRACON airspace.

Traffic Management Advisor
To assist the center controllers in scheduling descents to airports within their airspace, the FAA uses Traffic Management Advisor (TMA) software developed by NASA and the Federal Aviation Administration (FAA). TMA assists the center coordinator in scheduling when each plane should arrive and what order it should be placed in for descent.

Photo courtesy NASA
A center coordinator consults the TMA displays.

TMA uses flight-plan information, aircraft performance data and wind predictions to compute, predict and schedule when a particular airplane should reach its destination. It also looks at the number of planes allowed to land within a given period of time at an airport (the airport's capacity) and compares it to the number of planes scheduled. If the scheduled number exceeds the capacity (rush alert), it calculates the number that can be landed safely and makes recommendations to the coordinator for adjustments in the air-traffic pattern. This information is passed on to the controllers, who then give appropriate directions to the pilots.