How to use the aviation in World War I
 

Hi,

I would like your help in World War I. I´ve completed the Entente Campaign, and I´m know in Level 3 of the German campaign. However, I could not use any aviation yet. When I click in the icon in the leftmost side of the screen, it does nothing, neither the “Caps Lock” key.

Does anyone knows how to use the aviation, or at what level it becomes available?

Best regards,