Step 1 group up all your fighter wings
Step 2 make sure they are all in 1 group
Step 3 Hold the D key on that group until it stops creating new groups
Step 4 you want to have all the air wings selected and Hold K now. ( this will train them but since you have air wings with no plains in it you would think it would give no xp but it actually gives double)
Step 5 hold K for around a month in game and by the end of the month you should have 500 xp and if not try again its a bit of a buggy system