It doesn't have to be. You need to be sure you have enough battery power available for at least six hours of flying time, just to be safe. If you use the 02 to get to the plane, you may want more. Carry your re-charging equipment with you so you can charge your batteries if you're delayed in take-off. You should have already called the health desk for your airline to tell them that you're flying with 02; otherwise it can hold you up at check-in and security. I haven't been asked for it in a long time, but it's a good thing to have your prescription for 02 with you. Most airlines would like you in the window seat. I find that most people are helpful and kind.