Is it possible to take a helium balloon on an aircraft, I am thinking of taking a balloon on for a birthday gift. I would be grateful if anyone knows, rather than ask an airline.
I would not recommend it, and would be surprised if an airline allows it. An aircraft cabin is kept at reduced pressure, and there is a danger of the balloon bursting.
While that might in itself not be very dangerous (and helium is an inert gas) the explosion might cause fear and alarm in the passengers.
Some of the helium balloons you can buy are not even made of rubber (which might expand a little without bursting) but of rigid thin printed plastic.
If the balloon is not inflated but is supplied with a separate helium canister, the same applies.
You really must ask the airline (or simply let it be confiscated).