Delta Air Lines' pilots began voting Thursday on whether to ratify an agreement that will ease the way toward the carrier's planned merger with Northwest Airlines in exchange for an equity stake and other perks.

The union's roughly 6,000 members will have until May 14 to vote on the proposed agreement, which would cover only Delta's pilots.

Pilots union Chairman Lee Moak said the proposal provides Delta pilots with a "far superior alternative" to the role unions usually play in a "traditional merger process."

During Delta and Northwest's months of on-again, off-again merger talks, the pilot unions at the two carriers tried but failed to reach an agreement on a joint labor contract and a plan for merging the seniority lists of the unions' 11,000 pilots. Seniority is a key issue for pilots, affecting their pay levels, work schedules and other lifestyle issues.

Northwest's pilot union, which is another unit of the same Air Line Pilots Association that represents Delta's pilots, has vowed to oppose the planned merger. The Northwest pilots are the only work group that hasn't been promised pay raises and equity in the new company, which would be the world's largest carrier. The two unions haven't resumed negotiations.

The proposed deal between Delta and the pilots grants a 3.5 percent equity stake and annual pay raises of 5 percent in 2009 and 4 percent in 2010-12 under an extended contract, in exchange for rule changes giving Delta more flexibility to integrate the two carriers' flight networks.

Merging airline unions have usually taken months or years to hammer out joint labor contracts and seniority pacts, resulting in stalled integration of the carriers and rancor among employees. Years after the 2005 merger of US Airways and America West, pilots at the two former carriers still work under different work rules and fight each other in court.

Thursday, Moak told union members that the agreement they are considering hasn't dealt with any of those contentious issues.

"You are not being asked to vote on a joint contract, a seniority list or a seniority list integration methodology," he said in the message posted on the union's Web site. "Those issues are still outstanding, and we look forward to working with the Northwest [pilots union leaders] to resolve them at the earliest opportunity."