MMD and KTD are two airports very close to each others (12 km apart), but are not considered serving the same city both in AS and in real life, because the two airports are on two separated islands. And in real life there’re also flights that connect the two islands together.
However, in AS, if user try to schedule a flight between the two islands, it could result in error due to “aircraft performance”. After some tests, it is found out that, using Dash 8-400A as example, because of the way the game calculate speed of the aircraft,
if the flight is set to be 25 minutes long, the flight speed will be displayed as 0km/h,
if the flight is set to be 26 minutes long, then the flight speed will become 720km/h, which is too fast for the aircraft type
while if the flight is set to be 27 minutes long, the flight speed will become 360km/h, which is too slow for the aircraft type.
Therefore, it seems like it would require an aircraft which speed is either around 720km/h or 360km/h for such flight to be allowed to operate in AS, which seems quite unrealistic.