American Airlines, Philadelphia's largest airline, has been fined $1.6 million for violating a U.S. Transportation Department's rule of keeping passengers on planes for more than three hours without letting them get off and return to the terminal.

The Transportation Department cited American for 20 flights at Charlotte International Airport in Feburary 2013, six flights at Dallas Fort Worth International Airport in February 2015, and one flight at Shreveport Regional Airport in Louisiana in October 2015. Southwest Airlines received a similar $1.6 million fine for keeping passengers on planes in 2015.

Authorities said the tarmac delays in Charlotte occured during a snowstorm on flights operated by US Airways, which merged with American in December 2013. Heavy snow and rain also contributed to the delays in Dallas Fort Worth, the agency said. The delay in Shreveport was "partially due to the carrier's mismanagement of personnel and resources."

American will be credited $905,000 of the $1.6 million fine for compensating passengers on the affected flights and for costs of acquiring and operating a surveillance system at Charlotte and Dallas-Fort Worth airports "to monitor the location of each aircraft on the airfield," the Transportion Department said.