May be legal as long as all goes well. As soon as things go wrong, some insurer will remind you of the fine print insisting on good airmanship, taking all measures required for safety, acting in good faith &c &c and obviously letting 20 litres of gas loose in the cabin/cockpit doesn't match any of those.
But if you are searching for a law, forbidding verbatim "the carriage of a canister with a 20 litre capacity, holding combustibles of the petroleum type, on the back seats of any aeroplane" you might have to search for a long while.
Last edited by Jan Olieslagers; 23rd February 2011 at 16:43.