Q & A from the Discord:
Q: So no plans a la: "Firepower type XY can't shoot at Aircraft at all!" Everyone will be able to shoot with a penalty unless unit has AA
A:The penalty means that it will be nearly impossible to hit a well-piloted aircraft. An aircraft with no other modifiers in place is only hit on a 6. If it moves but does not fire, you can't hit it at all.
So then you need to look to other means. Precision fire, guided missiles with a sensor lock, AA vehicles, etc.
The plan is that when aircraft are released, there will be AA ground vehicles in all flavours released along with them.
Plus, some aircraft will specialise in hunting other aircraft, while others will be ground attack.
And of course, aircraft are mighty fragile when you do hit them.