Delta and Northwest pilots have approved a joint labor contract that will take effect when the two airlines' proposed merger closes.

In voting that ended Monday, about 61.7 percent of Delta pilots voted in favor of the agreement, according to the Delta pilots union. About 86.8 percent of Northwest pilots voted in favor of the agreement, the pilots union said.

Of 6,231 eligible Delta pilots, 5,120 voted, the union said. Of those that voted, 3,161 were in favor of the agreement.

Of 4,371 eligible Northwest pilots, 3,535 voted. Of those that voted, 3,067 voted in favor.

"This historic milestone marks the first time that a labor agreement has been reached in advance of the close of an airline merger," said Delta's pilots union chairman Lee Moak in a written statement.

The Delta and Northwest pilots groups are represented by separate units of the Air Line Pilots Association.

"This momentous vote broke the traditional merger paradigm," Northwest's pilots union chairman Dave Stevens said in a written statement.

The pilots agreement is a key element of the two carriers' merger plan. It brings Northwest pilots' pay up to the level of Delta pilots and includes a 5 percent pay raise in 2009 followed by 4 percent annual raises from 2010 through 2012. It also includes 3.5 percent in equity in the combined company for Delta pilots and 2.38 percent for Northwest pilots. The deal covers Delta's 7,000 pilots and Northwest's 4,500 pilots.

The labor agreement does not settle the issue of seniority integration, which governs how the two groups would be merged for purposes of aircraft assignments and other work issues.

The pilot groups have agreed to submit to binding arbitration if they can't come to terms on seniority integration through negotiations. A three-member arbitration panel has already been chosen.

Delta and Northwest's proposed merger is expected to close by the end of the year, pending regulatory and shareholder approval. The U.S. Department of Justice is reviewing the proposed deal. Delta and Northwest shareholders meetings are scheduled for Sept. 25.

In a memo to Delta pilots Friday, Moak said ratifying the contract would give pilots "the opportunity to secure our future in an increasingly difficult industrial and economic environment."