Can't watch downloaded Apple TV rentals on a plane.

I have gotten conflicting answers from Apple support on this: Do I need an internet connection (wifi or cell) to I watch a movie I rented from Apple TV and downloaded to an iPad. Sometimes on an airplane I am unable to watch Apple TV movies I have downloaded -- I get a message that the movie is unavailable. Other times I am able to watch them. One Apple media services tech said I needed a connection to the server to start the 48-hour playback period, meaning I would have to start the movie before I boarded the plane. A separate tech support person said I could watch downloaded movies without a connection and that I simply had not finished downloading them, which was a real possibility. He was unsure how to confirm that the download had been completed.