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I know there are airports with more than one ATC Tower, though I only know it from my own country (SCHIPHOL - EHAM). This airport has two towers called Tower-Center and Tower-West. Tower-West is built because of a sixth runway (18R - 36L), that wasn't clearly visible from Tower-Center. (They found this out after completion of the sixth runway.)

How common is it an airport decides to built a second tower, and are there a certain rules or limitations before the decision can be made?

Next to that; what are the practical consequences for pilots and ATC's? Do they not only switch between startup- and ground-controllers but also between several ground controllers?

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At larger airports it is fairly common to have more than one tower, but it all comes down to how well the tower controllers can see airplanes. Sometimes they have different towers for ground control frequencies and tower control frequencies, and the position of the tower is optimized for the appropriate task. For very large airports, they may need towers in different areas of the airport to properly see airplanes at each runway.

As far as practical consequences it doesn't matter much whether they have one or more towers. At very busy airports they split up the tower/ground controllers/frequencies because one person can only control so many airplanes at a time. For instance, there may be one controller for the North runway and one for the South runway. In this case, there could be only one ground controller, or there may be two or even more, depending on the complexity of the ground operations. With multiple ground frequencies, if you need to taxi from one controlled area to another, the controller that you are talking to will tell you to hold short of a particular point and contact ground on the other frequency. Once you contact the second controller, they will clear you to continue taxiing.

In short, it is very specific to the local conditions, and they do what they have to in order to properly control the traffic.

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How common is it an airport decides to built a second tower, and are there a certain rules or limitations before the decision can be made?

It's fairly common where there's more than one runway, and it has a lot to do with visibility from the ATC Tower cab. Under optimal conditions, ATC would need to be able to see all airfield paved areas with a minimum Line of Sight angle which varies from regulator to regulator.

This can sometimes prove difficult for runway thresholds if their elevation and distance to the tower location make this angle too low. Big terminal buildings (or any other building for that matter) can also be an obstacle to visibility, especially on apron areas.

The decision to go for more than one tower, from an operational point of view, comes down basically to the need to have control of all airfield areas. Building an additional control tower is not only very costly, but also adds complexity to ground operations which is always an undesirable (albeit inevitable) effect. In some airports apron control is transferred not to another tower, but to follow me personnel under the supervision of ATC, for example.

As for the practical consequences for pilots and ATC, Lnafziger has done a better job answering your question than I could have hoped to achieve.

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These days building more than one ATC tower in an airport is not advisable because of availability of many technological tools that help you to extend your surveillances over the far corner of the airport and prevent runway incursion. This is called A-SMGCS (Advanced-Surface Movement Guidance and Control System) depending on the need of airports there are 4 levels using radar and modern surveillance plus data links techniques for making visible aircraft and vehicles, I believe that is cheaper and less complex than making a new tower which in its turn would be an obstacle for flight operation.

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    $\begingroup$ You cannot rely on A-SMGCS for all tower tasks. Obstacles on runways, such as wildlife, birds, vehicles [without transponder], or even persons will not show up. You also cannot provide visual separation if you cannot see the runway, meaning that capacity would potentially be reduced. In short, a tower controller needs to be able to [visually] see the runway they is working with. $\endgroup$
    – 60levelchange
    Aug 20, 2016 at 9:28
  • $\begingroup$ While I like your comment; but in rare occasions and because of lack in airport layout or miss calculation of tower location or poor visibility with long duration, and…. the situation may suggest additional ATC towers. As I mentioned there are 4 levels of A-SMGCS with different capability and sensors can give you required possibility. Take into account remote tower technology which meet many ATC tower requirements and prevent runway incursions, it may suggest a business case and cost benefit study for the optimum salutations. $\endgroup$
    – M.Sadegh Dayjoori
    Aug 21, 2016 at 12:00

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