You don't need to disassemble the Halfbike in order to use the bag. There's even a way to use the rear two tires as a wheeled luggage with the Halfbike bag.
It would be best if you check with your airline beforehand. You'll surely be able to place it in the cargo hold, just not sure about onboard luggage. The dimensions of the Halfbike folded are 100 x 29 x 46 cm / 40 x 12 x 18 inches. You need to add a few extra to account for the bag itself.