WASHINGTON (AP) -- United Airlines will pay more than $1 million in fines for stranding passengers on 13 planes for more than three hours on the tarmac last year in Chicago.

The Transportation Department says it's the biggest fine against an airline since 2010. That's when new rules barred airlines from stranding passengers on the tarmac for longer than three hours without giving them the opportunity to leave the plane.

The delays involve 13 United and United Express planes at Chicago's O'Hare International Airport. They happened on a day when severe thunderstorms and lightning caused several ramp closures and disrupted the movement of aircraft at O'Hare. Toilets weren't working on two planes.

United will pay the government only $475,000. The rest of the fine will go toward steps to prevent similar events.

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