Just to let you know, AirportCityGame will be moving to superfast new servers on Friday afternoon. This will stop the slow running and downtime you may be experiencing when you visit.
For more information - Please see THIS thread.
How do I get skill stars and why do I need them?
- one star - Specialist;
- two stars - Master;
- three stars - Expert.
Each star also grants an additional reward to you, and their sum total opens new flights that become available to you.