Flight Attendants have asked American to block porn via internet access offered on board the aircraft. Of course this is pretty laughable if you really think about it. Anyone who is willing to view porn on a full airplane likely has enough on their hard drive, and several DVD’s that will last the entire flight. A big advantage to the local material is no waiting for it to download, and more viewing time. This isn’t anything new.
Of course you can also try visiting a list of plane crash videos, sure to freak out someone on the plane, especially if you turn the volume up.
You can also settle for movies like Alive (1993), Air Force One (1997), Snakes On A Plane (2006), Executive Decision (1995), or if you really want to be daring, United 93 (2006). Airplane! (1980) is a favorite, though likely not very controversial.
Or you could use a VPN, easy enough to pipe data through your own home, and around their likely-DNS based filtering.
Airlines rules are intentionally written very vague so that they can make decisions at whim and remove or prosecute passengers. Even a political message on a shirt is enough. I recall a story with visible merchandise from a competing airline being enough as well.