I was on a plane yesterday (literally on airplane mode) and I finished a book I've been working on for a month, and prepped/wrote half of a presentation. Quite often I produce much of my writing on a plane.
I find myself very productive on a plane. Especially on cheap flights that don't have in-flight entertainment. Literally no distractions for a preset amount of time. You're not only offline, you're also physically stuck. Best way to make the time fly by is by being productive.