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,

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

The lawsuit alleges Southwest illegally operated chronically delayed flights and disrupted passengers' travel plans.