Flammable liquids
Whenever people complain about banning liquids on airplanes they always seem to talk about using the liquids to create an explosive, which just seems way too complex.
My fear has always been a flammable liquid.
My only involvement with aircraft is as a passenger, but it seems to me that nothing is quite as dangerous as a serious onboard fire.
What would happen if someone poured a liter of something like ether around and ignited it inside an airliner at high altitude? I assume the oxygen would be consumed rapidly which would make fighting the fire more difficult.
Perhaps modern aircraft are more resistant to fire than I think.