American Airlines faced sharp questioning Monday from a federal appeals court over its effort to block the airline's passenger service agents from voting on whether to join a union.American argues that a union representation election among the workers can't move forward because the National Mediation Board used an outdated standard when it authorized a vote.But judges for the 5th U.S. Circuit Court of Appeals in New Orleans said the standard changed after the Communications Workers of America applied to represent the workers."You are basically moving the goal posts in the middle of the game," Judge Catharina Haynes told American's lawyer.The carrier sued the Mediation Board in May after it authorized the vote. The board "grossly exceeded" its authority by basing the decision on a "showing of interest" among 35 percent of the employees instead of the new 50 percent standard, American said."You already have ongoing proceedings," Judge Patrick Higginbotham told the airline's attorney. "It's a done deal. Now it's a question of who is going to vote for it."The agency argued that its decision to apply the lower standard was a "permissible interpretation" of the law. The district court "improperly intruded into the NMB's sphere," the mediation board said in papers filed with the appeals court.U.S. District Judge Terry Means in Fort Worth sided with American in June. Means ruled that the 50 percent standard applied because, while the application was submitted under the old law, the board approved the election after the new one went into effect. The board and the CWA appealed.The Fort Worth-based airline, which blamed its bankruptcy on high labor costs, has negotiated cost-cutting deals with unions representing flight attendants, mechanics and other workers. It won bankruptcy court approval to impose cuts on pilots, who rejected a contract offer.Marianne Auld, an attorney for American, told the three- judge panel that a ruling in favor of the board and union would hurt the airline.Holding an election after the district court ruled in American's favor "adds uncertainty" to the carrier's future, affects "public perception" and the airline's relationship with its employees, Auld said.The panel didn't indicate when it will rule.