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.

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    For most crimes, in many jurisdictions, there's a required element of mens rea: if you didn't have any intent to commit a crime, nor negligently or recklessly allowed it to happen, you're not guilty. – Nate Eldredge Apr 20 '16 at 4:24
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    I don't think anyone would bring dry ice or liquid nitrogen onto a plane, so that leaves us with rocky road, which is basically ice cream, right? So if you have the sword mold on your lap and someone is eating ice cream drops it on your sword, is that going to make it become sword? I'm trying to think of a scenario in which the mold could become a sword. – Revetahw Apr 20 '16 at 16:02
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    You mean the cargo area where all the suitcases are? The one that's completely sealed off from the passengers? If so, there's no problem. It's legal to check in a real sword there. – Revetahw Apr 20 '16 at 16:07
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    Ok... Well ice cream doesn't really vary a lot in temperature. So it sounds like accidentally freezing this sword on the plane is implausible in the first place. It would have had to be done on purpose, like if he also brought something to cool it with. If normal ice cream can actually freeze the sword. Then it would be careless to keep the sword mold up when somebody is eating ice cream. But taking the sword mold onto the plane wouldn't really be careless, as it's extremely unlikely to actually freeze on the plane. – Revetahw Apr 20 '16 at 19:35
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    I would suggest a less implausible example, such as a piece of glass (bottle, framed picture) breaking. – phoog Jun 21 '16 at 23:22

In the UK possessing it would not be unlawful as you would have "reasonable excuse" to do so.

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