Remember that your database must know who the pilot was, and who the co-pilot was, for each individual flight. It is not sufficient for it to know that “these two people were the crew members, but I don’t know which one was the pilot and which one was the co-pilot that day.”

(Hint: instead of a many-to-many association, where “many” is “exactly 2,” use two different many-to-one associations, one for the pilot, and one for the co-pilot.)

The same thing is true for departure and arrival airports. It’s not enough for your database to know “Frontier Airlines 77 connects Boston and Detroit, but I’m not sure whether it goes from Boston to Detroit or from Detroit to Boston.” It must know that the origin airport is Detroit, and the destination airport is Boston.