And I also have a second question: can I order (and if yes how?) an attack on an apparently empty hex ? The hex might in fact not be empty, just that I can't see inside because of FOW but I suspect ennemy presence. As I couldn't select an unit, i couldn't give the air attack order.
AFAIK and tested/seen myself, indirect fire works with artillery... As with ground units, you have to more or less reveal if something is there...
Artillery Dialog: This displays the artillery available in the current scenario. Note that Indirect Fire attacks are carried out at the beginning of the next turn.
Air Attack: This allows you to call in an Air Attack on the current Hot-Spotted Hex. A target marker is placed to denote the impending Air Attack, but the attack can be delayed for several turns. Once plotted, the Air Attack cannot be cancelled, and you are limited to one Air Attack per hex. If the Air Attack arrives when the targeted hex is out of Line of Sight to any friendly unit, the Air Attack will be recalled and be no longer available. There is also a small chance the Air Attack will strike a friendly unit as the Air Attack may be off target (randomly drifting one or two hexes).
So, you might be lucky not to be attacked by your own AF.