Catapult should be able to target air with artillery.

[Saiyuki]

DerSatan already mentioned the problems, resulting by both "air crash" and/or "ground splash". you would need an animated model, which would need to at least 10 times per second ( realistically: way often, because the shot is quite fast ) change its location and height ( for it to look good ) and checking each time if its new coordinates ( including height ) would match the air tank, starting at the catapult. as for the ground, you could easily check the height of the dummy model. moreover, one may need to detect if a friend or foe was hit, dealing the damage then or passing through.
in terms of triggering, it could surely be done in JASS, but could get kinda laggy ( for lower systems ). would you like to trade your current artillery shot with a custom artillery shot including laggs? is it really worth it?

i won't discuss this in here any further. keep in mind, it worked for all times, why changing a running system now? some things are intended.