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

Key Takeaways Southwest Airlines stock fell Thursday after the Department of Transportation sued the airline over a pair of "chronically delayed flights."The federal agency says Southwest operated two flights in 2022 that arrived later than scheduled more than half the time for five consecutive months.

Two popular low-cost airlines are the latest to be held accountable by the U.S. government over chronically late flights. Southwest Airlines and Frontier Airlines are both facing legal actions by the Department of Transportation for operating flight schedules the government says were notoriously late.