There is a screenshot on the Facebook page where you can see the player is at level 11 and has a Tetra-M. Yet I am at level 12 and the biggest planes I have are Birchcraft. Or maybe is it just because they are the developers?
Every level yields 1 airport and 3 bux. Somewhere in the teen levels you start getting planes every other level.
After the Birchcraft is the Pearjet, then followed by the Aeroeagle. I don't know if the helicopters are a higher level unlock or unique to the "flight crew" challenges. (the casablanca challenge offered up Huey parts for example)
I'm level 18 atm, so much beyond that i'm unsure. Haven't found any parts i don't already have unlocked by buying the class 3 airports (red) or class 2 (blue) either, they just save you a few bux if your going to make a plane.