I just recently took my daughter on a plane for the first time. She is also 6, and has ASD.
She is very visual, so I googled pictures of airplanes, and airports, to give her a general idea. We did a lot of talking and role-play before the trip. With her, I was more worried about how she would react going through security and that sort of thing than the actual plane trip, but it was a short flight (Dublin to Paris). She tends to be a highly anxious child, so I got a short rerm prescription for Diazepam (just in case) but I ended up not even needing it. I had her Nintendo DS and some coloring books and that sort of thing, but, to be honest, she was more fascinated that she could see the clouds out the window!
Obviously, you know your child best, but, generally speaking I think the key with a child with ASD is that they are prepared well in advance in whatever way suits them, whether it be a picture schedule, social story, etc.
If you would like, I could move this back to the special needs boards. I know there are a couple of other parents of children with ASD, but they might feel more comfortable replying in the SN board. Let me know