Why do planes have layovers?

Most layovers aren't because the aircraft can't make it there directly, but because there aren't enough passengers to fill a direct flight. An airline operating from a large connecting hub gathers travelers from dozens of origins onto a single aircraft bound for a distant destination, allowing it to offer daily flights to cities that, on their own demand, would only justify one flight a week. This model, known as hub and spoke, explains why flying from Bilbao to Bangkok means passing through Madrid, Frankfurt or Doha: the layover is what makes the route possible in the first place.

There are also layovers forced by physics. Every aircraft has a maximum range that depends on its load: the more passengers and baggage, the less fuel fits within the maximum takeoff weight. On very long routes or at hot, high-altitude airports, where engines perform less efficiently, a fuel stop may be the only way to operate with a full load. Until the 1980s, many flights between Europe and Asia stopped in Anchorage or the Gulf for this reason; today the Airbus A350-900ULR connects Singapore and New York, about 15,300 km, nonstop, but few aircraft and few routes allow it.

International flights are governed by bilateral agreements between countries that set which airlines may fly between them and how often. An airline may have the right to operate from A to B and from B to C, but not from A to C, making the layover a legal requirement. In addition, flight time duty regulations limit how many hours crews can work; ultra-long-haul flights require augmented crews and onboard rest areas, which raises operating costs and sometimes makes a layover with a crew change cheaper than a direct flight.