Never had much use for these in my games. There are criminally few elementals to summon in 5th edition though. So I'd let "Summon Aerial Servant" be a new spell, scale down the stats of a air elemental, and say it can only really go on fetch missions. Give it a chance to succeed on a mission on a case by case basis. If it fails it comes back to try to kill the summoner as a full strength air elemental due to rage. Boom! Ahnkeg tommorow.