M.C.
I think it's perfectly normal for her to be nervous, and nothing that you and she can't handle. I started flying alone when I was 10, and I remember it as being great fun--I felt very important and grown-up, and LOVED having the trip alone to see my grandma or cousins. It made me feel very special. All that being said, it is also a big deal for a young girl. I would talk her through EVERYTHING that is going to happen, and relate it to things that are familiar to her--tell her you'll go to the airport just like when you fly together, you'll go through security with her, etc.... Talk her through every step, making clear that the only time she WON'T be with someone she knows is when she's actually sitting on the plane. The airlines usually sit unaccompanied kids near the flight attendants, so she'll almost always have someone nearby to ask for help if she needs it. I was recently on a flight with my DD as a lap child--who also usually get seated near flight attendants, at least on American--and we were next to a young boy (I think 8) who was flying from Dallas to Boston to see his dad (his parents were divorced). He and my DD played together, and the flight attendants gave us lots of attention, and it went great!
You might also want to have your DD help you pack a special carry-on bag for the trip. Take her out to the dollar store, or big lots, or wherever, and pick out special snacks, new coloring books, or whatever will be fun for her on the trip. When you get home from the store, have her help you pack them in a backpack or other small bag she can carry, then put the bag aside for the trip. She'll look forward to the chance to play with her new stuff, which she'll know will only happen on the plane! If you're willing and able to spend a little more money, you could also take her out to buy a new outfit for her plane trip--again, it will give her something to look forward to, and help her feel special!
Good luck! I know it's probably as nerve wracking for you as for her, but she'll be fine, and it's a great experience for both of you to have her stretch her independence!