At a large connecting airport, flights do not arrive evenly through the day. They come in waves, and the spacing of those waves is the single most important design decision in the timetable.
A bank is a deliberate cluster
Airlines group arrivals into a tight window, hold them briefly, then push a matching group of departures out together. That cluster is called a bank, and a hub may run several each day.
The logic is combinatorial. If thirty aircraft are on the ground at once, passengers from any one of them can reach any of the other twenty-nine destinations with a single stop.
Spreading the same thirty flights evenly across the day would carry identical passenger numbers but sell far fewer connections. The wave exists to multiply the routes an airline can offer.
Minimum connection time sets the gap
Between the arrival wave and the departure wave sits a gap sized by how long a passenger and a bag need to cross the terminal. That figure is published and is airport-specific.
It varies with geometry more than with distance. A connection within one terminal may need well under an hour, while a transfer between terminals with a security re-check needs considerably longer.
International connections usually need more because of immigration and customs. At some airports arriving passengers must reclaim bags and check them again, which adds a step no walking speed can shorten.
Bags follow a separate and slower path
A transfer bag does not travel with its owner. It is pulled from the hold, sorted, and driven to another aircraft, often across a considerable distance of apron.
That path has less slack than the passenger's, which is why a tight connection can be made by a traveller whose suitcase misses it. The bag was always the slower half.
Airlines size the connection gap around the bag, not the walk. When a bank runs late, baggage handling is usually the constraint that decides which flights can still be held.
Waves explain why terminals swing between extremes
A hub terminal alternates between crowded and near-empty in a way a point-to-point airport never does. Retail, security lanes and immigration desks all have to be staffed for the peak.
This is why queues at a hub can look unmanageable at one hour and disappear at the next. The staffing follows the bank structure rather than a smooth daily curve.
It also shapes what is open. Food outlets in a wave-driven terminal cluster their hours around arrival banks, and a traveller arriving between waves finds a strangely quiet building.
The structure decides which delays matter
An aircraft arriving late into a bank threatens dozens of onward connections at once, not just its own passengers. The cost of waiting is weighed against the cost of misconnecting everybody aboard.
Airlines therefore make hold-or-go decisions centrally, using the number of passengers at risk on each side. A flight with many connecting travellers is more likely to be held than a full one.
The same structure makes early-morning reliability disproportionately valuable. A hub that starts its first bank on time usually recovers through the day, and one that does not rarely does.