An airline's route map looks like a list of destinations, but it is really a set of aircraft rotations. Every route has to earn its place against the alternative use of the same plane.
The aircraft is the scarce asset
A jet costs almost the same whether it sits at a gate or flies. Financing, insurance and crew costs continue regardless, so idle time is the most expensive thing an airline can buy.
Route decisions therefore begin with utilisation. Planners ask how many hours a day a given aircraft can be kept moving, and which sequence of city pairs keeps it moving best.
A route that looks profitable in isolation can still lose, because flying it strands the aircraft overnight in a city where the next morning offers nothing worth carrying.
Demand has to be measured in both directions
Traffic between two cities is rarely balanced. Business demand may run heavily one way in the morning and back in the evening, while leisure demand concentrates into a single season.
Planners look at the whole year rather than the best week. A route that fills in summer and empties in autumn still has to cover its costs across both halves.
Connecting passengers change the arithmetic. A flight that would be thin on local traffic alone can work if it feeds a hub at the hour when onward flights depart.
Slots and ground facilities constrain the map
At congested airports the right to take off at a particular time is scarce and tightly controlled. An airline may want a route badly and simply be unable to obtain a usable time.
Ground handling matters as much. Gates, fuel supply, de-icing capacity and border facilities all limit how many aircraft an airport can turn around during a peak hour.
These constraints explain why some obvious city pairs go unserved for years while less intuitive ones appear. Availability, rather than desirability, frequently makes the decision.
Aircraft type narrows the options further
Each type has an economic sweet spot. A small narrowbody flying a long thin route pays a payload penalty, while a large widebody on a short hop wastes most of its capacity.
Range is not a fixed number either. Headwinds, runway length, altitude and air temperature all reduce what an aircraft can lift, which is why some routes operate only in cooler months.
The arrival of a new type often opens a batch of routes at once, because it makes a distance-and-demand combination viable that no aircraft in the existing fleet could serve.
Routes are tested and withdrawn quickly
Airlines treat a new route as an experiment with a defined observation period, commonly a full year, so that every season is seen at least once before judgement.
If bookings do not build in the expected pattern, the route is cut and the aircraft redeployed elsewhere. That is a normal commercial outcome rather than evidence of a mistake.
Withdrawal is far easier than entry, which is why capacity shuffles around the map so often while the underlying fleet stays roughly the same size from year to year.