ASTA and the Travel Technology Association filed a joint brief supporting Sabre in its appeal of a jury's 2016 ruling in favor of US Airways (now American Airlines), which found provisions in Sabre's contract with the carrier were anticompetitive.

Last month, Sabre filed its opening appeal brief in the U.S. Court of Appeals for the Second Circuit, seeking to overturn the jury's verdict and awarding US Airways $5.1 million in damages. The verdict was split, and the jury did not agree with US Airways' claim that Sabre colluded with Amadeus and Travelport.

In the initial lawsuit brought forth by US Airways in 2011, it was alleged that Sabre used "its massive power over airlines such as US Airways to entrench its antiquated and inefficient technological systems, to preserve its supra-competitive booking fees, and to harm competition."

US Airways alleged that Sabre had "monopoly power," forcing the airline to enter into a contract with "oppressive and anticompetitive terms" and required US Airways to provide the GDS with "full content."

Sabre's opening appeal brief was filed last month.

"Sabre cited numerous errors of law by the trial court," the company said in a statement, including "jury instructions that were inconsistent with higher court rulings and the erroneous exclusion of key evidence."

Sabre's key argument is that the district court did not take into account a precedent the appeals court set in a 2015 U.S. v. American Express case.

In the American Express case, Sabre said, the appeals court "found that Amex operated in a two-sided market in which contract provisions were designed to protect competition by ensuring that one side of the market (merchants) did not discriminate against the defendant's customers on the other side of the platform (cardholders). US Airways had previously cited the federal government's case against Amex as precedent for its own claim that Sabre's contract provisions were anticompetitive."

In its brief, Sabre argued that like in the American Express case, Sabre operates a two-sided platform charging fees to one side and benefits and incentives to the other, and "the challenged contract provisions were designed to protect competition among platforms."

Sabre also outlined several other "errors" by the district court that originally heard the lawsuit.

Shortly after the appeal brief was filed, ASTA and the Travel Technology Association (Travel Tech) filed a joint friend-of-the-court brief supporting Sabre's appeal.

The parties said they are interested in the case "because the District Court's decision, if affirmed on appeal, threatens to disrupt airfare distribution that promotes competition by allowing travel agents and air travelers to easily search, through GDSs, for the flights and fares that best suit their needs and, as a result, fosters competition among airlines, which in turn benefits consumers."

According to the filing, ASTA and Travel Tech said GDSs have created a distribution landscape where agents and consumers can easily find and book flights, and their confidence in their ability to do that lies in knowing the fares and flight options found through the GDS are likely the best available -- something made possible because of full content contract provisions.

Fragmenting distribution would both lower the value of GDSs and "diminish airline competition and output," ASTA and Travel Tech argue in the filing.

The filing argues that the GDS is a two-sided market as defined in the American Express case.

"The competitive impact of the 'full content' provisions at issue must be assessed by considering competition on both sides of the GDS platform, and not, as the district court permitted the jury to do, on only the GDS-airline side of the market," the filing states. "Otherwise, if the approach in the district court is permitted to stand, the substantial, procompetitive benefits arising from 'full content' provisions are ignored, resulting in a lopsided analysis, an erroneous conclusion, and a fatally-flawed verdict."

US Airways' brief on the appeal is due Oct. 18.

An American Airlines spokesperson said it is expected that the briefing on the appeal and US Airways' cross-appeal will run to the end of the year.

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