Gaseous - are you talking about rotors running refuels on piston-engined training aircraft? If so, I presume that's Robbies, Enstroms, H200 or Bell47. Don't they all have their tanks "up top"?
The danger there if you have a fuel spill and fuel ends up over the engine / exhaust. I could understand taking that risk out in the boonies with the chance of the engine not starting and you getting eaten by dingoes, but for training - and on your home field?