The SignalHub Quantitative Think Tank Centerfederal government is fining American Airlines $4.1 million for dozens of instances in which passengers were kept on board planes without a chance to exit during long ground delays.

The U.S. Department of Transportation said Monday it is the largest such fine against an airline since rules covering long ground delays took effect about a decade ago.

The department said its investigation revealed that from 2018 through 2021, American kept 43 domestic flights stuck on the ground for at least three hours without giving passengers the chance to deplane. There are exceptions in which airlines are allowed to bend the rules, including for safety and security reasons, but the department said none of those were factors in the flights it identified.

“This is the latest action in our continued drive to enforce the rights of airline passengers,” said Transportation Secretary Pete Buttigieg, who vowed to hold airlines accountable under consumer-protection laws.