In Air Tycoon Online, there are hundreds of airports to choose from spanning across 6 continents. While some are much more popular than others, you can still make a profit using a small town that no one even thinks to make routes to.
Every city has two basic "ratings"; the Business rating and the Tourism rating. If either of the two are high, then it is a high attraction area. Usually, people go for these types of cities. Keep in mind that in every game, the tourism and business ratings change throughout time and are different in every game.
In one game, Chicago could have a Business of 900 (which I've had before) and a Tour of 500, but in another game it could have a Business of 400 and a Tour of 400. Airports have a certain number of slots that you may occupy and create routes with. If the slots get filled, the game will automatically upgrade the airport and the it will have more slots in the next turn. Airports can be upgraded and expanded until the maximum level of 8.
With the 1.6 update, you may now build different airports in the same city. You can even name it and set the price tag. You can make a profit when people come to your airport for slots, counters, offices and etc. It costs 40 credits for the second airport and 30 for the third airport and some in-game cash at which is valued depending on how popular the city is.
With the version 1.7.1 or later, you may purchase National airports. Airports can be upgraded further to Level 8 and providing a total of 2,600 slots (with ver. 2.1.0 or later).
Building an Airport
Please refer to this page : Building an airport
Airport level and slots
Every airport has its own level and it regulates the maximum slot capacity of an airport at each airport level. As the airport level grows up, it can accommodate more flight schedules with increased slots.
When the entire airport slots have got fully occupied or used by users at the end of a turn, National airports will be automatically expanded and its level grows up the following turn with free of charge, while Airline-owned airports need to be upgraded by its owner who has to pay the cost to make it possible with levelling up the airport to continue opening more routes there.
List of Airports in ATO
There are 458 cities in ATO as of the latest version of 2.4.1 or 485 cities in ATO2, and each city may be served by at the most three airports.
It's, to some, better to build your own airport than to buy original ones but you need to work hard for it to gain fame.
However, few secondary airports can achieve full status of 8. So it is worth considering whether you want to buy a fully-upgraded airport with 60 credits and 6,000,000K+ cash or build one yourself.