I led one of the UK/US vulnerability studies which tore apart the company manufactured theories. You can only imagine how keen they were to show how inert helium was whilst ignoring vulnerable systems and the pink fleshy things inside.
About the only positive I could state is that if an enemy was dumb enough to repeatedly target the big empty space then they may have a chance. Should the enemy choose to target the other things then it would get messy.
To tease the company we even showed how simple it would be to ruin their much vaunted helium envelope, just for fun. A helicopter laying an explosive rope along the top would do the trick. At one stage people were looking into how to defend against the threat of being boarded inflight. Anyway, a burst of fire or a missile or 2 into the flightdeck is much more traditional. If you target the engines and consult the metman you may even find a load of nice equipment downwind somewhere.
Still, these things are so slow that there is plenty time for the enemy to learn on the job.
