game_ui
func_physbox
phys_thruster
Those are the three main items i think. Also, from reading your shout, I"m not sure if you can specifically read the "+" and "-" buttons... But it's been a while since I used these.
Linking them together can be a pain. Just a fair warning - the player doesn't like being inside a fast moving moving object. You will probably get thrown out of the plane to your death.
Might i recommend a "enter" and "exit" function:
Enter - "use" the plane, activate a point_viewcontrol so the player's view is locked to the plane (you can even read keyboard/mouse commands to cycle cameras if you wish)
Exit: By parenting a point_teleport to the plane on a certain side, whenever you "exit" the plane, you're just teleporting the player to this spot outside the plane.
So when you're riding the plane, the player is standing.. somewhere else in order to avoid gravitational catastrophe
BTW I've made a rudimentary player controlled flying plane before.
I can send you the map, but I'm at work for the next 8 hours so I'm of no help until after that.