Well, I think with the dimensions it would be visually fine if you made a 5" or 6" circle of fondant or gumpaste and let it harden. Then place on top of the cake and then the airplane. You shouldn’t need anything more than that. To be honest with you thought, if it were me I’d wouldn’t worry about supporting it unless you made really small landing gear than could puncture the fondant. If your fondant has been on the cake for several hours and had a chance to harden some before you place the plane on, you should be good to go.
