Unions representing aircraft mechanics at Southwest Airlines and AirTran Airways reached a tentative seniority integration agreement. The voting process will start on May 21 and Southwest hopes to announce the results on June 21, according to an airline industry source.

This is big progress for Southwest because in late February, the two unions representing the mechanics had rejected the airline’s seniority integration proposal. The next step would have been for the Aircraft Mechanics Fraternal Association (AMFA) and the International Brotherhood of Teamsters (IBT) to file jointly for arbitration, but Southwest had hoped for a resolution before arbitration began.

AMFA represents 1,600 mechanics at Southwest and the IBT represents about 400 at AirTran.

The airlines’ pilots, flight instructors and flight attendants already have completed their seniority integration negotiations—with the pilots finishing the process in December, followed by the other two groups in the first quarter.

AirTran became a wholly owned subsidiary of Southwest on May 2, 2011.

The FAA granted Southwest its single operating certificate on March 1, which cleared regulatory hurdles and enabled the two airlines to conduct operations under a single set of FAA specifications.

Southwest recently started converting and painting AirTran’s Boeing 737 aircraft to Southwest’s configuration and color scheme

Southwest hopes to finish integrating the two airlines in 2014.