ATLANTA — The Federal Aviation Administration has approved a transition plan involving the effort by Delta Air lines Inc. and Northwest Airlines Corp. to achieve a single operating certificate after they combine.

The plan outlines the methodology, processes, tools and timing to maintain the safety of the day-to-day operations and to achieve a single operating certificate over the next 15 to 18 months, the two airlines said in a statement Monday.

The carriers said the plan will help make the transition of operations smoother.

Shareholders at Atlanta-based Delta and Eagan, Minn.-based Northwest are scheduled to vote on the combination on Thursday. Northwest shareholders will be asked to approve Delta's acquisition of Northwest, while Delta shareholders will be asked to issue new shares as part of the combination.

Delta announced the stock-swap deal on April 14. If approved by shareholders and federal regulators, the combination will create the world's largest carrier.

Delta hopes to complete the acquisition of Northwest by the end of the year. A federal lawsuit seeking to block the combination is scheduled to go to trial on Nov. 5 in San Francisco.