The US Department of Transportation sued Southwest Airlines and fined Frontier Airlines for flight delays, seeking civil penalties against Southwest and imposing a $650,000 fine on Frontier, part of a broader crackdown on airline delay issues.

The Appeals Court ruled that while the DOT had the power to issue such rules, the rulemaking process had some procedural gaps.

Three U.S. senators said they want Frontier Airlines and Spirit Airlines to disclose whether they are manipulating seat fees by using customers' personal information to charge different fees to passengers on the same flight.

The DOT's moves come weeks after issuing a multi-million dollar fine against JetBlue.

Yesterday, the United States and U.S. Department of Transportation (DOT ... In addition, the DOT levied a fine against Frontier Airlines, which was also for chronically delayed flights.

Southwest is disappointed that DOT chose to file a lawsuit over two flights that occurred more than two years ago," a spokesperson for the airline said.

The U.S. Transportation Department is suing Southwest Airlines and has fined Frontier Airlines over "chronically" delayed flights, disrupting passengers' travel and plans.

The Fifth Circuit Court of Appeals struck down a junk fee disclosure rule, arguing that it violated an administrative law.