If you have about £15 to spare, buy Microsoft Flight Simulator X. It's quite good, and the missions are challenging, but not impossible. :D Basically, you don't get bored too quickly.
@TheWiseEyes Heh, it's a generic joystick version fault, as we have the same joysticks, if I remember correctly. My throttle won't stay at 0, it flickers, and the actual joystick control flickers too, for some reason. :I
Sandboxes I know of: Incredibots(The servers were shut down, the game is open source) http://incredibots.com Garry's Mod(only 8 € / 10-12 $)(Conversate with me if you wanna join in on Steam) Powder Toy From Dust Universe Sandbox