[Sgt_Walrus]

You have no control over aircraft other than making the initial 'plot' to get them onto the map to do their sortie.
Certainly you cannot alter the height they attack from.

Helos on the other hand can be controlled by you like any other unit. Their is a tab in the unit screen where you can land, go low or high.

Edit: Oops, didn't see Cross' response before posting mine.