In a high-stakes proceeding Thursday, the U.S. Court of Appeals for the D.C. Circuit appeared skeptical of the U.S. Department of Transportation’s (DOT) rationale for excluding Frontier Airlines from consideration for newly added flight slot exemptions at Ronald Reagan Washington National Airport (DCA).

At the heart of the DC Circuit Zero vs 40 DCA slot dispute is whether Frontier qualifies as a "limited incumbent carrier"—a designation reserved for airlines operating fewer than 40 regular runway slots at DCA, excluding temporary exemptions.

Frontier, which holds zero regular slots but operates at DCA exclusively via exemptions, argues it meets the criteria. However, DOT contends that an airline must hold at least one regular slot to fall under the “limited incumbent” definition—effectively disqualifying Frontier.

Chief Judge Sri Srinivasan challenged that logic, suggesting that zero is indeed less than 40. “The question is whether they’re incumbent,” he stated during oral arguments. “And the point is they are because they’re there.”

Judge Cornelia Pillard also raised concerns about excluding an active operator like Frontier on a technicality. She questioned whether Congress truly intended to treat airlines operating multiple flights through exemptions as “new entrants.”

The legal debate centers on the interpretation of a recent statute that authorized 10 new slot exemptions at DCA—five of which are designated for long-distance flights operated by limited incumbent carriers.

Counsel for Frontier, Aaron Schaer of Ballard Spahr LLP, argued that the natural reading of the law supports Frontier's inclusion, stating, “The much more natural reading would be to say, ‘Yes, I hold fewer than 40.’”

Representing the government, Simon C. Brewer of the U.S. Department of Justice maintained that equating slot exemptions with regular slots would undermine the statute’s integrity.

The case, Frontier Airlines Inc. v. DOT, case number 25-1002, could have significant implications for air traffic access and competition at one of the nation’s most regulated airports.