After failing to get enough support to file for union representation, the mechanics at Northwest Airlines now will tighten the bolts in a union-free division of the merged Delta Air Lines Inc.

The National Mediation Board on Thursday agreed to honor a request by the Aircraft Mechanics Fraternal Association (AMFA), which represents Northwest mechanics, to end its union designation.

In a letter to members dated Wednesday, the union said the move came after a recent authorization card campaign showed that "interest has come short of the required number needed" to file for a representation vote.

Northwest has fewer than 1,000 mechanics compared with Delta's 5,700, who are not unionized. The Northwest mechanic's ranks were crippled after an unsuccessful strike in 2005 led many workers to cross picket lines while the company hired hundreds of replacement workers.

As a result, only about 10 percent of Northwest mechanics were dues-paying members of AMFA, according to one member, making it a long shot for the remaining workers to garner the 35 percent support needed to call for a vote to unionize at Atlanta-based Delta.

The letter from AMFA National Secretary-Treasurer Steven Nowak, posted on the union's website, said freeing its members from representation "will clear the way for Delta to treat the combined work equally" based on previous agreements relating to seniority.

Delta said in a statement that it will immediately increase the pay rate for the Northwest mechanics to bring them in line with the rest of its workers.

In a memo Tony Charaf -- president of Delta's maintenance division, which calls itself TechOps -- called the switch "fantastic news for Delta and the new TechOps team. As I've said many times in the past, our flexible workforce has always been an advantage as we grow our global customer base."

Calls to AMFA Local 33's Dennis Sutton, who took part in negotiations, were not returned.

Although the issue with mechanics is now resolved, the 21,000 flight attendants and 22,000 airport ground workers have yet to be integrated. Both of those groups were in unions at Northwest, but are not at Delta.

The Association of Flight Attendants-CWA, which represents the former Northwest workers, hasn't yet scheduled a representation vote for the combined group, though elections are expected.

The ground workers also haven't agreed on how to combine. Those employees from Northwest belong to the International Association of Machinists and Aerospace Workers.

Delta shares dropped 2 cents Thursday to close at $5.69.

The Associated Press contributed to this story. Jackie Crosby • 612-673-7335