Southwest and American Airlines are in a regulatory feud about Cuba routes.

In filings with the US Department of Transportation obtained by the Charlotte Business Journal, American Airlines is requesting to shift its Charlotte (CLT) to Havana (HAV) route to depart out of Miami (MIA) because the CLT flight is only operating at 55% capacity, meaning each flight is only about half full. American already has routes out of Miami to Havana.

The problem with American’s plan is that once an airline route is approved by government officials, an airline can’t up and change the departure city because the flight isn’t performing well financially. The way the regulations are now, AA would have to first end its CLT-HAV route and then compete with other airlines to win another MIA-HAV slot, of which there are only 20 nationwide.

So, in its regulatory filing, American acknowledged this and asked for an exception to the rule. The paperwork asks the DOT to let airlines determine themselves from what cities to fly to Cuba. “This flexibility would allow American to bring additional benefits to the traveling public, while ensuring that all carriers can respond efficiently to further shifts in demand in the US-Cuba market,” AA told the Dallas Business Journal.

Southwest scoffed at this logic. “In essence, American’s Motion would substitute its own self-interest for the Department’s public interest decisions,” the airline wrote to the DOT in its own regulatory filing.

The two airlines — along with JetBlue — had also been competing for a Havana route from New York JFK that Delta announced it no longer wanted to fly in August. Southwest wanted to add a Tampa (TPA) to Havana route.

The DOT seemed to side with Southwest, saying that an additional Miami to Havana flight “would not promote competition or diversify our frequency allocations to the same degree as would a second Saturday flight at Tampa, a city that currently has just one daily flight.”