An airline is not a fleet of aircraft; it is a fleet tied to places. Every aircraft you own begins and ends its day at a base, every route has to touch one, and every base needs people on the ground to run the departures you schedule from it.
That is what stops an airline growing in a straight line. Aircraft can be bought in an afternoon. The ability to operate them has to be built.
A base is an airport where you have a presence: aircraft can be stationed there and routes can start there. Your first base is chosen when you found the airline, and it is also your headquarters.
Bases have levels. A higher level carries more of everything — more aircraft, more departures, more staff — and each level costs more than the last. Levelling a base is one of the few purchases in the game that never expires and never wears out.
One base is your HQ, and it carries more than the others do at the same level. That is not a bonus for its own sake: it means the shape of your network matters. An airline built around one strong home is a different company to one spread thinly across four cities, and the game should let you feel the difference rather than reading it in a table.
You can move the headquarters. It takes time — the move is not instant, and while it is in progress you are running an airline whose centre of gravity is somewhere between two airports. That delay is what makes moving a decision rather than a free optimisation. If your network has genuinely outgrown its origin, move; if you are chasing a small advantage, the transition will cost more than the advantage is worth.
Every departure needs people behind it: gate agents, dispatchers, planners, the ones who rebook a passenger when an aircraft goes technical. The game models this as a load on the base the departure leaves from.
The load is not per aircraft. It is per departure, plus an amount that grows with the length of the flight, plus a surcharge when the flight crosses a border. Two consequences follow, and both are intentional:
Each base has a capacity that grows with its level, and the HQ carries more than a regular base of the same level.
Going over capacity does not stop anything. Your aircraft keep flying and your passengers never find out. What happens instead is that you pay overtime, every week, for every point of load above what the base can carry.
This is on purpose. A hard ceiling would turn base levels into a gate you unlock; a running cost turns them into a judgement. Overtime for a few weeks while you save for the next level is a perfectly reasonable way to run an airline. Overtime forever is a slow leak that a growing airline stops noticing, which is exactly how real companies end up paying for it.
The upgrade pays for itself against the overtime it prevents, eventually. Whether "eventually" is soon enough depends on how hard you are running the base — which is the decision the mechanic exists to create.
See also Fleet, Facilities and Routes and operations.
Start your airline