How Things Work: Air Traffic Control

Handling over 80,000 flights each day, air traffic controllers in the United States guide aircraft from the ground to the sky and back to the ground.

Using radar and various types of computer software, traffic controllers are responsible for updating pilots on the weather, monitoring traffic flow, and ensuring that planes in the air and on the ground maintain enough distance between one another.

Between the time that an airplane takes off and lands, information concerning the flight of that airplane passes through many different controllers. In fact, the pilot submits a flight plan to the airport’s air traffic control tower at least 30 minutes before takeoff.

A flight data person, who is one of the controllers in the traffic control tower, reviews the weather and enters the flight data into a computer connected to the Federal Aviation Administration (FAA) network.

The flight data person also prints out a flight progress strip, a piece of paper containing the flight number, airline name, equipment and aircraft specifications, planned airspeed and cruising altitude, and intended route of flight.

Digitally or by hand, the flight progress strip travels to every air traffic controller involved in guiding the flight. Barring any conflicts, the flight data person alerts the pilot of the flight plan’s approval and transfers the flight progress strip to the tower’s ground controller.

For private pilots, on the other hand, the flight service station (FSS) handles information about the flight plan.

The ground controller, who also works in the traffic control tower, monitors all ground traffic, including aircraft on runways and others taxiing to and from runways. He radios the pilot to tell him when to leave the gate and which runway to use.

The local controller, on the other hand, directs planes so that they take off at safe distances from one another.
After giving the pilot clearance to take off, the local controller electronically transfers the flight progress strip to a terminal radar approach control (TRACON) station, and the flight is no longer in the airport’s control. TRACON stations, each covering an airspace about 50 miles in diameter, monitor planes flying to and from one or more nearby airports.

Following takeoff, the pilot turns on the plane’s transponder, a device that detects incoming radio signals and sends out its own radio signals in reply. The transponder finds a radio signal from a TRACON station on the ground, and the pilot sends a transmission to the station containing the flight number, speed, altitude, and destination. A TRACON departure controller receives the signal and gives the pilot information regarding weather and traffic.

Once the plane flies past the TRACON airspace, the departure controller transfers the information to controllers at an air route traffic control center (ARTCC). Airspace in the United States consists of 21 zones, each of which has its own ARTCC.

A team of ARTCC controllers keeps track of the plane as it flies through the zone. If the aircraft travels into another zone, the controllers pass the flight’s information to a different ARTCC.

At this point, once the plane is en route to its destination, changes to the flight plan might be made for several reasons, including bad weather and congestion. If a destination airport is especially crowded, an ARTCC controller instructs the aircraft to travel in a holding pattern, circling the airport until space opens up.

Aircraft approach their destination airport in single file, and each flight’s information moves to a TRACON approach controller as it crosses into TRACON airspace. The pilot maneuvers the plane in line with the runway, and the TRACON controller transfers the information to a local controller at the airport when the plane is within 10 miles of the runway.

The local controller on the ground clears the pilot for landing and assigns the pilot a taxiway to exit the runway. A ground controller then monitors the plane as it travels from the runway to the gate.

The air traffic control system relies heavily on radar, which allows control stations to view aircraft in 3-D. Radar relies on two mechanisms of detection, echo and Doppler shift, both of which are easy to understand in the context of sound.

In sound, an echo is a reflection of sound wave, such as when it bounces off a wall. The time it takes an echo to return to its source is the distance traveled divided by the speed of sound.

As for Doppler shift, one can consider a car horn as an example.

If you are standing 100 feet from a resting car that honks its horn for one minute, you will hear the honk of the horn for one minute from where you are standing.

However, if the car is driving towards you, the honk of the horn will sound higher pitched, and you will hear it for less than a full minute.

This effect, known as a Doppler shift, has to do with the fact that sound travels in waves. As the car approaches, the sound wave of its horn is compressed, meaning that it has a higher frequency, which results in a higher-pitched honk that lasts a shorter amount of time.Similarly, a car driving away produces a lower-sounding honk that lasts slightly over one minute.

Radar monitors radio waves using echoes and Doppler shifts. If a person were to direct a wave at a moving object, that person could record its echo to determine the object’s distance. In particular, the distance is determined by the time it takes for the echo to return to the person.

The speed of the moving object, on the other hand, can be calculated by measuring the Doppler shift (the faster the object is approaching, the higher the shift).