How Does Air Traffic Control Keep Track Of Flights?

We often hear of the Air Traffic Control (ATC) when there are accidents involving aircraft. But what do the ATC do and how do they keep air traffic running smoothly? Let’s take a look. 

The Air Traffic Control, more popularly known as the ATC, handles aircraft movement right from giving it a route clearance for the flight to take off, which we’ll come to later,  and until the aircraft docks at the gate at the destination.

Aircraft fly in two broad types of airspaces — controlled airspace and uncontrolled airspace. All scheduled flights operate under the control of the ATC. There are a few smaller aircraft like training aircraft that may fly without the supervision of the ATC in uncontrolled airspaces, though the number is relatively low, especially in India.

Pilots and the ATC mostly communicate through voice on the radio on very high frequency (VHF). Some en route communication is also done through voice on High Frequency (HF) and now more modern methods such as a data link. 

Role Of ATC From Start To Finish

Let’s take the example of a flight from Mumbai to Delhi, one of the busiest routes in India. 

The aircraft first gets in touch with the ATC at the aircraft parking stand (e.g. the aerobridge) to get a route clearance. This is given to a flight to fly from its origin (in this case  Mumbai) to its destination (Delhi). It includes the final route/airway it is cleared on, departure instructions like the runway and heading (direction) to fly after takeoff, an initial altitude, any initial important frequencies that may be required, and a unique four-digit identification number called a Squawk Code. The Squawk Code is fed into a transponder on board the aircraft. The transponder is a piece of equipment that communicates with the radar equipment of the ATC so that the aircraft can be identified by the air traffic controller on their radar screen.

Once ready, the aircraft is then asked to contact the surface movement controller on a different frequency. This controller is responsible for the pushback of the aircraft from the gate/aerobridge and giving instructions to the aircraft to taxi from the gate up to the point before the aircraft enters the runway. Depending on the size of the airport, there could be multiple controllers managing different areas at the same airfield.

From here, the aircraft is asked to contact the tower where they are given clearance to enter the runway and subsequently a take-off clearance. There are many airports where the aircraft is only asked to keep a listening watch on the tower frequency and respond only when called. This is done to keep the amount of radio communication to the minimum as tower frequencies generally tend to be busy.

Once airborne, the aircraft contact area frequencies that are there to separate traffic laterally and vertically. They generally handle traffic that is outgoing as well as incoming. The number of controllers and frequencies largely depend on the size of the airspace being covered. Typically, each controller looks after either a block of altitude or a lateral zone. An example would be a controller handling all the air traffic from 1000 ft to 16000ft. The next controller would handle all aircraft from 16000ft to 28000ft and so on. Sometimes controllers manage geographical directions. For example, one controller handles all aircraft north of the airport and the other to the south of the airport. In a very high-density area, there could be a combination of both as well. The division is largely based on the amount of air traffic in the area.

At the end of one control zone, the aircraft is given the next frequency and asked to contact the next controller whose area they enter. So in our example, after the multiple controllers in the Mumbai airspace, they are asked to contact the Ahmedabad area controller as they traverse through that airspace. Then the aircraft is asked to contact the Delhi controllers till they are parked at the gate at the destination.

When flights are long and pass over water or land where there is no VHF radio coverage, the aircraft and controllers use HF radio, which has a longer range but is less reliable and clear than VHF. A more modern way to communicate in areas like these is through a data link, where the aircraft and controllers talk to each other through typed messages. These are mostly used in the cruise phase where the amount of communication is relatively much lower.

What Happens When A Flight Goes Off Radar? 

In the rare case of a total communication failure, there are rules that clearly define what the aircraft should do depending on the phase of flight they are in. An example would be the route and the flight altitude to be flown, approximately what time to reach close to the destination and what procedure to follow after reaching close to the destination. There are general rules that should be followed if an aircraft loses communication during the cruise and there may be a lost communication procedure for the airport the aircraft is departing or arriving at as well. All this is documented and available for the pilots to follow so that the pilot and the controller are in sync with each other. 

The Squawk Code on the transponder is also modified by the pilot to inform the controller that the aircraft has lost radio communication.

This is a brief overview of the way pilots and ATCs communicate. Anyone who operates on a radio frequency is trained and licensed to do so. All communication is supposed to be in a standard format and phraseology in English which is recognised and understood the world over. Although, there may be slight variations in the rules depending on the country and the region of the flight.

So until next time… CIAO or as pilots would say Charlie India Alpha Oscar!

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