1. A flight is uniquely identified by an airline and flight number.
2. A flight is made up of one or more flight legs, each of which has a length.
3. For each flight leg, there is a departure airport and an arrival airport (required).
4. There is also a scheduled departure time and a scheduled arrival time.
5. A flight leg can have any number of associated instances.
6. A particular instance has actual departure and arrival information (time and airport; airport is required) on a particular day and has been assigned a particular airplane (required).
7. A flight leg instance is uniquely identified by the date, departure and arrival airport codes, airline, and flight number.
8. An airport is uniquely identified by a code, and also has a location consisting of the airport's name and the city and state where it is located.
9. A type of airplane is uniquely identified by its maker and model. It also has a range (how far it can fly).
10. Certain kinds of airplanes can only land at certain airports.
11. A particular airplane is of a particular type (required) and is uniquely identified by its ID.
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