The United states immigration laws are quite specific with regards to foreign born permanent residents who wish to become US citizens....and, have been for quite a long time.
Simply put...you have to reside continuously in the United States for a specified period, and NOT travel overseas, except very occasionally.
Now, a foreign born permanent resident who also happens to work for a US airline therefore may not qualify, IF that individual flies, on a regular basis, overseas.
Yep, thems the rules, like it or not.