Rival unions are fighting over how to represent 24,000 flight attendants at the new American Airlines, which will become the world’s biggest carrier after merging with US Airways.

The dispute highlights the difficulty that goes along with combining unionized work forces in airline mergers. Pilots from US Airways and America West are still squabbling eight years after those airlines combined.

Cabin crews at American are represented by the Association of Professional Flight Attendants, or APFA, while their US Airways counterparts belong to the Association of Flight Attendants, or AFA. The two groups have been talking for months about how to work together.

But on Wednesday, APFA President Laura Glading said in a memo to her members that the US Airways group had given her an ultimatum to abandon those efforts and begin new talks on a proposal drafted by the US Airways side, which she refused to do.

The president of the US Airways group, Roger Holmin, fired back by saying the problem was a “concessionary” labor contract that Glading’s group negotiated last spring with US Airways management. The unusual deal was part of US Airways CEO Doug Parker’s strategy of winning support for a merger from American’s unions while he negotiated with reluctant American Airlines management.

Holmin said the APFA-US Airways agreement would give flight attendants terms that would be on par with United and Delta “and squander an opportunity for a better contract.”

American, owned by AMR Corp., and US Airways Group Inc. expect to close their merger by mid-December.

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