if they are set to "Flying" in their 'special' box, use
this setpos [(getpos this select 0),(getpos this select 1),heightoftheplanehere]
if u want it to change alititude in the course of the game, use
planename flyingheight 50.........or whatever altitude u want
hope this helps
EDIT: the 'this setpos' bit goes in the initialization box in case u didnt know.
The flyinheight bit goes either in the intit box or in a trigger or waypoint activation box.