ATLANTA — The union that represents flight attendants who worked for Northwest Airlines before it was bought by Delta Air Lines is crying foul over Delta's failure to offer bigger sizes for its signature red dress uniform designed by Richard Tyler.

When he was hired to create his uniform collection, Tyler said he wanted them to "look sexy and great."

The Northwest chapter of the Association of Flight Attendants-CWA has filed a grievance with the world's largest airline operator, asking it to offer the red dress up to size 28. The union hopes the grievance will go to mediation in August.

The red dress currently is only offered up to size 18, though a Delta spokeswoman said the airline offers a range of outfits in other colors and styles up to size 28 that flight attendants can wear.

Patricia Reller, who handles grievances for the union's executive committee, said Friday that even if there was only one flight attendant who wore a size over 18, that person should be able to wear the stylish red dress.

"I think red is an eye-popping color and it's not subtle, and to me by not offering it in a size over 18, Delta is saying, 'We don't want you wearing that if you are over size 18,'" Reller said. "But the job isn't about being sexy. It's about safety."

Reller said she did not know how many pre-merger Northwest flight attendants want to wear the red dress but need a size bigger than 18.

"If the dress wasn't a tiny size 18, there would be less," Reller said. "It's a very small size 18, so that makes the numbers a lot larger."

Delta spokeswoman Gina Laughlin could not immediately say why the red dress is not available in bigger sizes while other uniform options are. But she said there have been few complaints.

"The majority of pre-merger Northwest flight attendants have given us very positive feedback about the Richard Tyler Collection," Laughlin said.

Delta hired Tyler in 2004 to do a makeover of its gray employee uniforms. Tyler, whose dresses have been worn by stars such as Julia Roberts, Heather Locklear and Jamie Lee Curtis, promised a classier — and in some cases, sexier — look for flight attendants, airport agents and other workers.

"I want them to look sexy and great, but you have to keep that classic look as well," Tyler told The Associated Press at the time.

The uniforms — including white blouses and navy dresses besides the red dress — debuted in 2006. Atlanta-based Delta acquired Northwest in October 2008. Pre-merger Northwest employees began wearing the Delta uniforms on March 30.