Well, the first manned air balloons didn't exist until the late 18th century, but the whole heat can make light things float was known well before that. So I could see it being feasible that a group of people with a possibly dangerous route connecting them to the rest of civilization might try experimenting on the matter more than most, and, having magic might help speed the process along in the case of not needing to develop the same technology, though they would need to get the spells right. They'd probably be far more confident loading things into these airships than people. And would likely still be far more ready to transport expensive things the oldfashioned way, in case of a spell going wrong or the wind deciding to carry something away or it being too heavy. Likely they'd still need at least one witch on the ground following along as a sort of, magic tether to keep the ship going the way they want it to. Or to pull it quickly down to the ground should the weather act up and be a buffer against rough landings.

EDIT: Totally just wikiing this... But there's also an interesting theory that they were used to help make those ground figures in Peru. Not sure about that one, but it would put manned air balloons as possible 5th century creations. Though not in a European culture...