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Once you have 5 gates rented at a chosen airport, you may open a hub there. In AE, hubs do increase maintenance costs but produce connecting PAX. Connecting PAX can increase the amount of total revenue of the hub or hubs the aircraft is flying from/to. Hubs also let you have more than 5 gates at the chosen airport. To create a new hub, go to the manage hubs tab. Select the airport you want to open a hub at (remember you must have 5 gates). Then click open hub. You will need to pay a starter fee varying on which airport your opening the hub at.

If you want to close a hub, all you need to do is go to the manage hubs tab, select the hub you want to close, and click close hub. After you pay the fee to shut the hub down, you have no hub at the airport anymore.