How air traffic control works

Air traffic control is the service that keeps aircraft apart, moves them in an orderly sequence and gives them safe routes through the air and on the ground. It is not one person in one tower. A single flight can pass through five or six different control positions, and each one hands the aircraft to the next.

This guide follows a flight from the gate to the runway and back to the gate, and explains the jobs along the way. The sequence is broadly the same across the world, although the names of the units and the details of the rules change from country to country.

Who does what

Control is split by place and by job. An airport and the airspace around it are usually run by several units, each with its own radio frequency.

• Clearance delivery (at busy airports) gives the departing crew the first part of the route and the departure procedure before the aircraft moves.
• Ground control guides aircraft and vehicles on the taxiways and apron, and tells them where to stop and which way to go.
• Tower controls the runways. It gives takeoff and landing clearances and watches the runway and the approach.
• Approach and departure control handles aircraft in the terminal area, usually within a few dozen miles of the airport. It sequences arrivals, vectors them onto the final approach and sends departures up to their cruising route. In the United States this unit is called a TRACON.
• En route control looks after aircraft in the cruise, between the terminal areas. In the United States these are the Air Route Traffic Control Centers, known as "centers". Their airspace is large and they are often far from the airports they serve.

At each boundary, the controller tells the crew which frequency to switch to. The phrase "contact departure" or "contact approach" is that handover, and it is one of the most common calls on the radio.

Before the aircraft moves

Long before a departure, the airline files a flight plan. The plan lists the route, the planned altitude, the aircraft's type and its equipment. Air traffic control uses the plan to plan the sequence of departures and arrivals, and to tell neighbouring centres what to expect.

At busy airports, flights are also managed by time. Many airports have a limited number of arrival and departure slots each hour, and a flight that is late may have to wait for a slot to open. Flow management units watch the whole network for this. In the United States, the FAA's Air Traffic Control System Command Center does this job. In Europe, EUROCONTROL's Network Manager does it for the whole network. EUROCONTROL

From gate to runway

When the crew is ready, they ask for pushback. The aircraft is pushed back from the gate by a tug, and the crew starts the engines. The ground controller then gives taxi instructions, which name the taxiways to use, the places to stop and the runway to go to.

Taxiing is slow and detailed work. Taxiing aircraft are told to "hold short" of a runway until it is clear, and each crossing needs a clearance from the tower. Runway incursions, where an aircraft or vehicle enters a runway without clearance, are among the most serious ground risks, so the ground controller's job is largely about keeping the paths separate.

At the runway, the tower gives the takeoff clearance. The aircraft lines up, waits if another aircraft is ahead, and then takes off. Before the clearance, the tower has checked that the runway is clear, that the wind allows the chosen runway, and that the departure path is free of other traffic.

Climbing away

After takeoff, the aircraft flies the departure procedure, which takes it away from the airport on a published route and to a safe height. Once it is clear of the immediate airport area, the tower hands it to departure control. The departure controller may give radar headings and altitudes to keep the aircraft apart from others that are climbing or descending nearby.

Climbing traffic is kept apart vertically and horizontally. In much of the world the usual vertical rule is at least 1,000 feet between aircraft. Horizontal minima depend on the radar in use, and near airports they are commonly three to five nautical miles. SKYbrary

Cruise and descent

The aircraft is then handed to en route control. It flies at its cruising altitude along airways, or on more direct routes where those are allowed, with its track set by the flight plan and small changes from controllers when the traffic demands it.

As the aircraft approaches its destination, the en route controller hands it to the arrival controller at the terminal area. The arrival controller sequences it into the stream of other arrivals. This is where most of the holding and speed changes happen, because the runway can take only so many landings each hour.

The controller may use radar headings, called vectors, to bring the aircraft onto the final approach course in the right order. Speed instructions help set the gaps between aircraft. Where traffic is heavy, the controller may send an aircraft to a holding pattern until it can be fitted into the sequence. Holding patterns explained covers how that works.

Landing and back to the gate

The approach controller hands the aircraft to the tower. The tower gives the landing clearance, usually with the wind and the runway in use, and it watches the approach until the aircraft has landed and cleared the runway.

After landing, the aircraft taxies off the runway. The tower tells the crew to contact ground control, and the ground controller guides it to the gate or to a parking stand. The flight is over once the engines are shut down, though the next departure may already be waiting.

Who is responsible for safety

Controllers are responsible for separating aircraft, giving them clear instructions and watching for conflicts. The crew flies the aircraft. The pilot in command can refuse an instruction that they think is unsafe, and the crew can ask for a different clearance at any point. Both sides check each other. Controllers repeat back what the crew has said, and the crew reads back the instructions they have been given.

The rules behind all this come from national authorities and from ICAO, the International Civil Aviation Organization, whose Annex 11 sets out the standards for air traffic services. The details are set by each country and each airspace, so the same route can have different procedures in different places.

The same ideas in Short Final

Short Final puts you in the controller's seat at one airport, such as Atlanta. Each plane comes in from an edge of the screen, and you draw a path from it to a runway. You keep the planes apart, which is the core job of every controller in this guide.

The game also shows the parts that real control needs. Departures wait at gates, push back, taxi and take off. Arrivals taxi to a gate after landing. The radio strip at the bottom of the screen gives clearances and traffic calls in the words controllers use, which is covered in the phraseology guide.

The game is simplified. There are no slots, no flow management and no centres, and the planes do not follow published procedures. What it does keep is the central idea: many aircraft, one runway, and a controller who keeps each one clear of the others.

What to remember

• Control passes from one unit to the next at every boundary, and the handover is one of the most common radio calls.
• Ground, tower, approach and en route each have a different job, with their own frequencies.
• Flow management spreads traffic over time, so arrivals may be held or given a slot.
• Controllers separate aircraft; the crew flies the aircraft and can refuse an unsafe instruction.

Sources

• EUROCONTROL: the European Organisation for the Safety of Air Navigation
• SKYbrary: the aviation safety knowledge base
• ICAO: the International Civil Aviation Organization
• FAA Air Traffic Control handbook, JO 7110.65