The US Department of Transportation (USDOT) is suing Southwest Airlines for “illegally operating multiple chronically delayed flights and disrupting passengers’ travel,” according to a press release.

The USDOT’s investigation found that “Southwest operated two chronically delayed flights — one between Chicago Midway International Airport and Oakland, Calif, and another between Baltimore, Md. and Cleveland, Ohio — that resulted in 180 flight disruptions for passengers between April and August 2022,” per the release. “Each flight was chronically delayed for five straight months.”

A flight is considered chronically delayed if “it is flown at least 10 times a month and arrives more than 30 minutes late more than 50 percent of the time,” the USDOT says.

“Southwest is disappointed that DOT chose to file a lawsuit over two flights that occurred more than two years ago,” Southwest spokesperson Laura Swift says in a statement to The Verge. “Since DOT issued its Chronically Delayed Flight (CDF) policy in 2009, Southwest has operated more than 20 million flights with no other CDF violations. Any claim that these two flights represent an unrealistic schedule is simply not credible when compared with our performance over the past 15 years. In 2024, Southwest led the industry by completing more than 99% of its flights without cancellation.”

In addition, the USDOT is taking “enforcement action” against Frontier Airlines for operating chronically delayed flights. USDOT has fined Frontier $650,000 in civil penalties; the US Treasury will be paid $325,000, while the other $325,000 will be suspended if Frontier “does not operate any chronically delayed flights in the next three years,” the USDOT says.

Frontier Airlines spokesperson Jennifer F. de la Cruz declined to comment.

Earlier this month, USDOT announced a $2 million penalty against JetBlue for operating chronically delayed flights. The USDOT also fined Southwest Airlines in 2023 over a holiday meltdown that stranded millions in 2022.