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