Two popular low-cost airlines are the latest to be held accountable by the U.S. government over chronically late flights.Southwest Airlines and Frontier Airlines are both facing legal actions by the Department of Transportation for operating flight schedules the government says were notoriously late. Frontier Airlines operated delayed flights between Atlanta and Phoenix, and Orlando and Houston over 63 times, and the airline will be responsible for a $650,000 civil penalty fine, with the opportunity for half of the fine to be waived if the airline doesn’t operate chronically delayed flights over the next three years, according to the consent order.