I hope you flush your lines and have another tap just before the filler attachment.
I suppose you could just open them outside for a min the right way up and the LPG vapourising would flush any light gas out.
But the whole thought of having Butane in an aircraft gives me the shivers. There have been enough boats gone up with leaks, then add in having O2 kicking around as well.
And I wouldn't like to place bets on what would happen if you played o2 over the fat on a bacon butty. Could make for an interesting mouth full.