When you allow aircraft to reach the edge of the screen they will turn back. Use this to your advantage but be careful as it can be dangerous if they wander too far off course.
Fast Forward Button
When you have several planes in the air at once it is best to not use the fast forward button as it will likely cause calamitous results.
Routes
It is crucial that you keep checking the routes planes are taking and if necessary redraw them so multiple aircraft don't all try and land at once.
