The deal will need antitrust approval, and integrating the work forces of fully unionized Northwest and Delta, where pilots are currently the only major unionized work group, will be tricky.

The joining of Atlanta-based Delta and Eagan, Minn.-based Northwest, if approved by regulators, will result in combined annual revenue of $31.7 billion, vaulting it ahead of Fort Worth, Texas-based AMR Corp.'s American Airlines for the top spot in the U.S.