Why do airlines cancel flights?

Weather is the leading cause. Storms, snow, ice, dense fog or crosswinds beyond limits reduce an airport's capacity, and when demand exceeds what the runway can handle, air traffic control imposes restrictions: ground delays and, if the situation persists, cancellations. In Europe, Eurocontrol manages these restrictions across the whole network; in the United States, the FAA's command center does. Strikes or controller shortages have the same effect as a storm: fewer flights per hour than planned. Airlines prefer to cancel the least full flights in advance so the rest can depart on time.

A technical fault discovered before departure is not always fixed in time, and without a spare aircraft the flight is scrapped. Large airlines keep backup aircraft at their bases, but at secondary airports there is no alternative. The crew is the other bottleneck: pilots and cabin crew have legal limits on duty hours, and an earlier delay can leave them without enough time to complete the flight. Illness, strikes or simply a winter with a lot of flu deplete staff reserves, especially during peak season.

A flight with very few passengers can be canceled and its travelers rebooked onto another one, although European passenger rights rules penalize this with compensation. More common is the knock-on effect: an aircraft that operates five legs in a day drags down the following ones if the first is canceled. That's why cancellations cluster in the afternoon and at connecting airports. In Europe, regulation 261/2004 requires the airline to offer alternative transport and, if the cause is not extraordinary, to pay compensation of €250 to €600 depending on the distance.