"In ALPA merger policy, seniority suffers from a negative legislative history, because if a seniority integration factor that was once recognized is later excised from the policy, that sends a loud message to the arbitrator," Seham says, adding that Northwest pilots " should not relegate their members to a crapshoot."

The arbitrator also took the position that US Airways, operating under bankruptcy protection, was in imminent danger of collapse. That wouldn't be a factor in Delta and Northwest arbitration.

The market has shown concern about Delta and Northwest since pilots failed to reach a deal over the weekend. On Monday, the Amex Airline Index rose slightly, but shares in Northwest lost 2.8%, while Delta fell 1.5%, partially reflecting investor doubts about the likelihood of a merger. All three declined on Tuesday.

"This is definitely working at its own pace," says William Swelbar, a researcher at MIT's International Center for Air Transportation. "But in the end, I am going to be surprised if Northwest pilots stand in the way of a deal."

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