Employees at Southwest Airlines and AirTran Airways hit two milestones as work forces are combined following the merger of the two carriers.

Dallas-based Southwest announced Friday that flight instructors from Southwest and AirTran have reached a tentative agreement to combine employee seniority lists. Southwest's flight instructors are represented by the Transport Workers Union, while AirTran's flight instructors are non-union.

Reaching the negotiated deal for the 70 flight instructors at Southwest and the 18 flight instructors at AirTran avoids arbitration, Southwest said.

Separately, the executive boards of the unions that represent mechanics at Southwest and AirTran agreed to send onto members their own deal to combine seniority lists and create a single labor contract, Southwest announced Thursday. The Aircraft Mechanics Fraternal Association represents about 1,800 employees at Southwest, while the International Brotherhood of Teamsters represents about 400 aircraft mechanics at AirTran.

Both the mechanics and the flight instructors will review the deals over the next several weeks before putting them up for a ratification vote. In general, seniority can be particularly important in the cyclical airline industry.

The two moves are additional steps forward in Southwest's acquisition of AirTran, a deal the carriers closed in May.