Developers, usually, insert into their applications some hidden features. Often these features are useless, but sometimes they are complete applications.
In this case Google added in Google Earth a complete flight simulator.
To activate the simulator you have to type:
CTRL-ALT-A on windows
CMD-OPT-A on Mac OSX
Now you can choose between two aircrafts, and the Airport.
Now start the flight pushing the button "Start flight".