I think we should allow players to have an Premium Economy Class to make the game more realistic. In real life, there are airlines such as Virgin Atlantic, British Airways, Eva Air, Cathay Pacific and Air Transat all have Premium Economy Cabins. If you have Economy, Business and First Class cabins, why not Premium Economy?
If you search arround the forum, you will find the answer discussed a lot of times!
But to give a short answer:
It is a 5th demand calculation which means 25% more calculation time for each airport (at least)!
Additional in real life the three class system will be continued. Most airlines introducing a premium economy don’t have a first class. The business is upgraded to a first and the new premium eco is more or less the new business. Only a matter of naming.
Diamond Class
4,930 AS$/seat
Paradise on the wing; a bed and armchair in a private compartment with lockable door, minibar, and enough gadgets to fill an electronics store.