A U.S. Court of Appeals for the First Circuit panel seemed skeptical Monday of overturning an order that undid a partnership between American Airlines and JetBlue, which the lower court deemed anti-competitive.

During arguments in the airlines’ appeal, Chief Judge David Barron questioned if their abandonment of certain routes under the deal led to so many fewer flights as to harm consumers in violation of federal antitrust law. Under the court’s analysis, the U.S. Department of Justice must first show that the airlines’ so-called Northeast Alliance would have “substantial anti-competitive effect.”