A Department of Transportation rule implemented in April 2010 prohibits airlines from keeping passengers sitting on a tarmac for more than three hours — a rule it says an American Airlines affiliate has since violated so frequently that it’s now been slapped with a $900,000 fine, the first such fine handed down since the edict took effect.

In a statement, the department said American Eagle Airlines had tarmac delays exceeding the maximum wait time on 15 flights arriving at Chicago’s O’Hare International Airport on May 29, affecting 608 passengers.

American Eagle, which blamed the delays on airport congestion caused by a slow-moving weather system, must pay $650,000 of the $900,000 fine within 30 days, but up to $250,000 of the total amount can be used for passenger refunds, vouchers, and frequent flyer mile awards.

“We put the tarmac rule in place to protect passengers, and we take any violation very seriously,” Transportation Secretary Ray LaHood said. “We will work to ensure that airlines and airports coordinate their resources and plans to avoid keeping passengers delayed on the tarmac.”

[Associated Press]