We don’t recommend flying with any bike in a soft bag, but frequent flyer, Aidan Prior, who regularly uses soft bags for transporting his bikes, has found an interesting way of flying with his Helios in a surfboard bag.
By removing the front wheel, seat posts and pedals, his Helios fits into the Komunity Project surfboard bag and weighs around 20kg all up (Aidan’s Helios is a hub gear geared heavy duty bike, so a bit less for some models). There is also the issue of maximum lengths, but BA, with whom Aidan is flying, and various others have a limit of 190cm which is almost exactly at the limit.