Delta and Northwest pilots have approved a joint labor contract that will take effect when the two airlines' proposed merger closes, clearing a major hurdle in the integration of the two airlines. But the pilots still need to resolve the complicated issue of merging seniority lists and are preparing for arbitration on the matter.

In voting that ended Monday, about 61.7 percent of Delta's pilots voted in favor of the agreement, according to the Delta pilots union. Among Northwest's pilots, about 86.8 percent voted in favor of it. Negotiators reached the deal in June, and voting on ratification began in July.

Delta chief executive Richard Anderson called the contract approval a "historic milestone," in a written statement Monday. The deal also gives the airline the ability to quickly gain benefits from combining flights through "code-sharing."

The approval "certainly removes one additional impediment" in the proposed merger, said Les Hough, an industrial relations consultant and former director of Georgia State University's Usery Center for the Workplace.

"It also probably means that the sort of merging-of-cultures issues that often present themselves in airline mergers may be less difficult."

The pilot groups at Atlanta-based Delta and Eagan, Minn.-based Northwest are represented by separate units of the Air Line Pilots Association.

The deal 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 agreement covers Delta's 7,000 pilots and Northwest's 5,100 pilots.

Hough, giving a possible reason for the lower approval rate by Delta pilots, said some Delta pilots may have felt less of a need for the joint agreement because they already had reached their own labor deal.

That agreement, ratified in May with a 78 percent approval rate among pilots that voted, did not cover Northwest pilots and is superseded by the new joint agreement.

The Northwest pilots, on the other hand, "felt a little bit left out in the cold without this," Hough said.

Delta's pilots union chairman Lee Moak wrote in a memo to members Monday: "It will be crucial that we put any differences behind us, and move forward with unity and resolve."

However, the labor agreement does not settle the issue of seniority integration, which governs how the two pilot groups' work would be merged for purposes of aircraft assignments and other work issues. It's one of the most complicated issues for pilots in an airline merger.

Both pilot groups have agreed to a three-member arbitration panel.

The Delta pilots union is preparing for the arbitration process, and Moak said in the memo to members that the union's merger committee "will aggressively and confidently present our case to the arbitration panel."

In the arbitration process the two pilot groups agreed to, the panel would issue a decision by Nov. 20.

Still, the Delta pilots union remains open to negotiations and believes the best outcome is "a fair and equitable negotiated solution," Moak said in the memo to pilots.

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.