As far as I know, it is illegal for passengers to have weapons on a plane. Usually you need clearance to bring it as cargo as well.
What if you accidentally make a weapon while on a plane?
For example, let's say you have a sword mold with some pykrete fluid. By itself, this is not a weapon, just a sword mold with wet toilet paper in it. Now let's say that the airplane staff put that near something really cold of another passengers. This causes the pykrete fluid to freeze into pykrete. Now you have a fully functioning sword (see the link). Which of the following happen?
- You get in trouble since your cargo is now a weapon
- The airplane staff get in trouble since their actions created the weapon
- The passenger that brought the cold stuff is considered an accomplice to the above
- Ice Sword isn't allowed through security, and must stay on the plane until it melts
I'm looking at this in terms of regulator laws of the airplane industry. Taking into consideration specific rules for different airline companies is not necessary.