I worked for TSA and later for the county aviation dept. Needless to say, I spent thousands of hours at the airport and have seen many hysterical things happen to bags.
If you wear the helmet, it will be safe. As an added bonus, people will not attempt to talk to you. If the flight is not full you might even get an entire row to yourself!
If you carry it on, pack it in your bag. It will be fine. It does not take up too much room as you can stuff things where your head goes.
If you check it, do not strap it to the outside of the bag. If it gets hung up in machinery, it is gone.
At any rate, it is a helmet - not a Ming dynasty vase. Pack it inside a bag and it will be fine.