The typical image people have of air traffic control (ATC) is that of a group of people in an airport tower who coordinate aircraft activity by staring at radar screens that use points of light to represent aircraft. While not fundamentally incorrect, this isn't a fair representation of the extent of ATC operations. This article will flesh out that simplistic image and introduce you to the equipment, technologies, and procedures that go into keeping aircraft and air travelers safe in the air and on the ground. We'll look at the way air traffic control is organized, and explore the communication technologies that air traffic controllers use to keep in touch with air crew and ground personnel. We'll also look at the radar technologies used to keep track of aircraft, and we'll end with a brief look at some next-generation technologies.

Safety and wake turbulence

The primary mandate of air traffic control is to ensure the safe transport of people and cargo by keeping aircraft at a safe distance from each other and expediting the flow of traffic. Air traffic controllers have access to sophisticated radar systems that provide an overview of the airspace they control, and they have communication tools to coordinate flight paths with the air crew. Pilots lack the tools necessary to get an overview of the airspace, so they have to rely on air traffic controllers to guide the aircraft through congested airspace.

Since aircraft travel at significantly higher speeds compared to other common modes of transport, the time available for pilots to react to a dangerous situation can be quite short. Thus it's essential that flight paths are carefully planned and managed to minimize the risk of a collision. This is especially true around major airports where the density of aircraft in a given volume of airspace is higher than average.

Turbulence created by wingtip vortices and exhaust gases from jet engines can be significant when aircraft are in close proximity. This phenomenon, called "wake turbulence," can adversely affect trailing aircraft if the distance between them falls below a certain limit. This limit depends on the mass of the two aircraft. For instance, a light aircraft following a heavy aircraft is more susceptible to wake turbulence than a heavy aircraft in the same situation. Therefore, aircraft approaching an airfield have to be carefully sequenced in a manner that takes such factors into consideration. Additionally, weather conditions such as low cloud, heavy rain, or snow blizzards can mean that pilots can't see other aircraft in the vicinity and have to use their instruments and instructions from air traffic control to navigate.

Types of ATC

Air traffic controllers are organized into various groups, each of which is in charge of handling a distinct portion of the aircraft's flight. Each group has a designated airspace that it controls, and aircraft are handed off to the next group of controllers as it approaches the limits of the prior group's airspace. The airspace controlled by each group is further divided into sectors that are themselves handled by individual controllers. The way these groups are organized varies from country to country and depends on the extent of controlled airspace and number of aircraft handled.

The Potomac TRACON facility controls approaches and departures in the airspace surrounding Baltimore Washington International, Washington Dulles, and Washington National airports (Credit: FAA

The tower controllers are the most visible group. From their vantage point on the airport tower, they have a visual overview of all the important parts of the airport tarmac, such as runways and taxiways. Tower controllers monitor the airspace surrounding the airports and keep track of approaching and departing aircraft. At well-equipped airports, they may even have access to surface movement radar systems to monitor aircraft and support vehicles as they move on the ground.

Once the aircraft is in the air and clear of the airfield, tower control hands the aircraft off to a departure controller. These controllers are typically based at facilities a good distance from the airport. With the use of surveillance radars, they are able to monitor air traffic around the airport. These facilities are called Terminal Radar Approach Control (TRACON) facilities in the US. In an area like New York, where airports are close to one another, a TRACON facility can service multiple airports. The controllers here ensure that the planes approaching and departing the airspace they control are following designated flight paths and speeds. TRACON facilities also sequence the aircraft that are coming in to land, in order to ensure that they are adequately separated to minimize any wake turbulence effects. Departure controllers also need to take into account aircraft that may be flying through their airspace, and keep them separated from aircraft landing or taking-off.

As the aircraft exits the TRACON airspace, a facility known as an Area Control Center (ACC) takes over. These facilities monitor the aircraft's flight while in controlled airspace through remote radar stations. Each ACC will have a designated airspace that it supervises. An aircraft may fly through multiple ACC sectors as it flies to its destination, with each ACC handing off control of the aircraft to the next ACC as it exits the former's airspace. Once an aircraft gets closer to its destination airport, the ACC controllers hand off responsibility to the approach controllers at the local TRACON, who guide and sequence the aircraft to the active runway, and finally to the tower controllers.

Listing image by Andy Potter