I usually turn mine off, not because the plane will crash, but because it'll waste it's battery looking for a signal from a tower that it can't find. That's actually part of the reason that there's so much variation in cell phone battery life is that if you're in an area of lower reception it'll take more power to transmit back to the tower. If there's no towers to respond a lot of phones (used to anyway, can't confirm 100% if they still do) would try to transmit at maximum power to get a ping back from the tower and start getting a connection to the network.