The U.S. Transportation Department is suing Southwest Airlines and has fined Frontier Airlines over "chronically" delayed flights, disrupting passengers' travel and plans.

The calculation includes cancellations and diversions. "When an airline knows that a particular flight is consistently late, it is essential that the airline adjusts its schedule," the DOT said in its lawsuit,

Transportation Secretary Pete Buttigieg said airlines have a legal obligation to ensure that their schedules list realistic departure and arrival times.

The Department of Transportation on Wednesday sued Southwest Airlines, alleging the carrier operated chronically delayed flights, and fined Frontier Airlines for late-arriving flights.