It sounds to me like the "business casual" dress code that the FAA has instituted is written such that your immediate manager has rather broad authority in interpreting and enforcing the code. You could attempt to educate your manager about the kilt being an acceptable article or clothing that meets the requirement for "business casual," You could point out other government agencies that do allow men to wear a kilt.

If this does not change your manager's opinion, then as another poster had mentioned, you might be able to get some help from your union. Another possibility might be to contact your congressman. But however you would elect to pursue this, in my opinion I would keep your manager informed. Your manager may not agree that the kilt is appropriate attire, but he/she should respect that fact that you are keeping them in the loop so that there are no unpleasant surprises later. Who knows, this alone might get you in the door with your manager saying, "ok, lets give it a try and see how it goes."

Darrell