Helmet on board

[Grant Johnson]

Maybe not call them, you'll often find, as in many things, that "head office" has no idea what is actual practice on the ground.
We have carried our helmets on many flights all over the world, and never had an issue - but we do carry them in a helmet bag, not obvious, for protection and so as to not advertise them.
If they give you a hassle at the gate, escalate it to the top. A helmet is no more dangerous than a carry on bag, in fact much less so as it's a lot lighter than some of the monsters people carry on board!
Unless they can show you, in writing, that it's against airline regulations, you will win. And I'd be surprised if anyone took it that far as it's ridiculous.
Good luck and let us know what happens!