This seems to be a dead thread, but I'll post my thoughts anyway.
OK, pro for some type of aircraft: it would make the game more complex, maybe more interesting, add a new dimension and capabilities. It might proceed from the need for rapid transport (unless the colonies use a magnetically levitated rail-car system) among the spreading colonies.
OK, con: I agree that it might detract from the Outpost feel, but whether or not that happens needs to be tested in practice. Second, depending on the function of aircraft, there would need to be a whole new weapons balance, i.e. a tank simply cannot target a flying machine--it would be like trying to shoot a flying bird down with a rifle--cumbersome and ineffective at best.
Now, down to eath: if the planet in question has an atmosphere similar to New Terra's, then it would be like flying on Mars. It is possible, but not in the conventional sense. I read a book once (The Labyrinth of Night) that discussed future manned exploration of Mars in terms of the near future. The aircraft transported from Earth, if I remember correctly, used a modified gas turbine (I'm not sure) with internal fuel stores--a small, one-man affair. Because the atmosphere is thin, control surfaces (i.e. ailerons and elevons) are rendered useless. Aerodynamic control is supplied by the expulsion high-pressure gas through control nozzles--much like on the Space Shuttle.
Anyway, Knux, I hope you don't give up on a good idea simply because of popular opinon. I hope you do well on your game.
Best regards,
Axen