Last week, the United States Transportation Department issued American Airlines a $1.6 million fine for violating a rule regarding long tarmac delays.

The fine was a result of a Transportation Department investigation which found the airline allowed several domestic flights to remain on the tarmac for more than three hours without allowing passengers an opportunity to get off the plane.

“Our tarmac rule is meant to prevent passengers from being trapped in aircraft on the ground for hours on end,” said Anthony Foxx, secretary, Transportation Department. “We will continue to take enforcement action as necessary to ensure passengers are not kept delayed on the tarmac for length periods of time.”

The fine is matched by the penalty against Southwest Airlines in 2015 for violating the same rule. The department will deduct $602,000 for compensation American Airlines issued to passengers and another $303,000 for what the airline spent on surface management and surveillance to monitor aircraft location.

The rule states all airlines in the United States operating aircraft with more than 30 passenger seats are not allowed to let a plane remain on the tarmac for more than three hours without giving passengers the opportunity to deplane.

The airline must also provide food, water, working lavatories and medical attention when needed. The fine includes flights operated by US Airways and its regional partners.