The U.S. Department of Transportation is stepping up enforcement of ongoing flight delays, filing a lawsuit against Southwest Airlines and fining Frontier Airlines.

The agency said it filed a lawsuit in U.S. District Court in California on Wednesday accusing Southwest of illegally operating long-delayed flights and disrupting passengers' travel plans. It said it was seeking "maximum civil penalties."

"Airlines have a legal obligation to ensure that their flight schedules provide realistic departure and arrival times for travelers," Transportation Secretary Pete Buttigieg said in a statement. "Today's action sends a message to all airlines , that is, the department is preparing to go to court to strengthen protection for passengers.”

The U.S. Department of Transportation said an investigation found that Dallas-based Southwest Airlines operated two long-delayed flights, including one between Chicago Midway International Airport and Oakland, California, and another between Baltimore, Maryland, and Ohio. between cleveland.

The agency said the two flights were delayed for five consecutive months, disrupting a total of 180 passenger flights between April and August 2022.

Southwest said in a statement it was disappointed the agency focused on flights from more than two years ago, pointing to its long record.

"Since the Department of Transportation issued its Chronically Delayed Flight (CDF) policy in 2009, Southwest has operated more than 20 million flights without any additional CDF violations. Any claim that these two The suggestion that flights represent unrealistic schedules over the past 15 years is not credible," the company said. "Southwest leads the industry by flying more than 99% of its flights with no cancellations through 2024."

The department imposed a $650,000 civil penalty on Frontier Airlines, of which $325,000 will be paid to the U.S. Treasury and the remaining $325,000 will be suspended if the airline does not operate any of the long-delayed flights for the next three years. Denver-based Frontier declined to comment.

Less than two weeks ago, the Transportation Department fined JetBlue $2 million for lengthy delays, the first time the department has fined JetBlue for lateness on a specific route. The agency attributed JetBlue's lengthy delays to "unrealistic scheduling." JetBlue later said the government that operates the air traffic control system was also responsible for the flight delays.

Aviation data provider Cirium said in a report released this month that Southwest ranked fifth among the 10 North American airlines it rated for on-time performance, with 77.8 percent of arrival flights on time last year and just under 77 percent of departures. Flight punctuality method. By comparison, No. 1 Delta Air Lines had an arrival score of 83.5% and a departure score of 83.7%.

Federal regulators increased scrutiny of Southwest last year after a series of incidents, including flying at extremely low altitudes while still miles from the airport. The Federal Aviation Administration said in July it wanted to ensure the airline complied with federal safety regulations. The FAA declined to provide details but noted that it continually adjusts its regulation of airlines based on risk.