Control tower

There is a trick but it's kinda pointless.

Connect to a multiplayer server.
Get into a tower.
Wait untill the server goes down or do something yourself to get disconnected like unplugging the network cable.
When the popup box appears tell flightsim you want to continue "flying".
You are now in the tower in single player mode.....

Sadly even though you can seen the computer generated trafic, you cannot communicate with it. So the whole operation is useless. Still I don't think it would be a lot of work to build something that would allow you to talk to the competer generated traffic.