I noticed this started happening about 4-6 months ago. Before I never had a problem playing a downloaded movie on my IPad while on a plane without a WiFi connection.
Now 'every' movie I download onto the same iPad requires a WiFi connection to 'start' the movie. Without a WiFi connection you would hit the Play button and it would act like it was going to start playing for about half a second and then revert back to the original play screen again. It would just keeping repeating this every time I would hit Play.
As I was already on the plane when this happened, but before takeoff I was able to quickly turn on my Hot Spot on my iPhone and then turn on my WiFi on my iPad and quickly connect to my Hot Spot, then hit Play on the movie which would actually start the movie, Pause the movie, go back and turn off WiFi and set to Airplane Mode on iPad and then able to resume watching the movie that I had to 'jump start' using my hot spot withou t any further issue.
I have to do this every time now! Not sure why ITunes makes you 'authorize' your already downloaded movie 'Again' before you can watch it. Hope the above Hotspot trick works for others who have that as an option.