Interesting question for our modern times.
When I flew with my parents growing up, my mother always made sure I was in suit and tie as a child. Of course, those were the Pan Am days.
As an adult, when I travel for business, I'll just wear my business suit. Unlike most business travelers, I always keep my jacket on and never hang it up. I have things in my jacket pockets that I'd prefer to keep on my persons during the flight.
For non-business travel, I just wear a sports coat and slacks, but without the tie.