The fine was imposed by the U.S. Department of Transportation over the storm-related delays of 16 Southwest flights on Jan. 1 and 2, 2014, at Chicago
A law that took effect in 2009 prohibits U.S. airlines from stranding passengers on a delayed flight for more than three hours without giving them the option of getting off.
During the heavy Chicago storm, Southwest had 16 flights that were delayed as long as four hours on the airport tarmac, according to federal records.
According to a consent agreement signed by the airline, delays caused by the storm were aggravated by the malfunction of a crew-scheduling system, leading to a shortage of staff.
Before the penalty against Southwest, the largest civil penalties assessed by the Department of Transportation for violating the tarmac delay rules were $1.1 million against United Airlines in 2012 and $900,000 against American Eagle in 2011.