Here is the full list of all 22 AX:EL - there are Air XenoDawn achievements.
Complete Mission 10 in a special way
Complete Mission 10
Complete Mission 8
Complete Mission 9
Complete Mission 7
Complete Mission 6
Complete Mission 5
Complete Mission 4
Complete Mission 3
Visit the hub 250 times
Complete Mission 2
Send 1000 enemies to the "otherside"
Visit the hub 100 times
Destroy 500 enemy vehicles
Visit the hub 50 times
Kill 250 enemies
Shoot down 100 enemies
Kill 50 opponents
Complete mission 1
Shoot down 10 enemies
Kill your first enemy
Visit the hub.