LPG fuel
 

Back in the day the Mi-8 helicopter had a LPG/LNG variant with 2 tanks mounted above the u/c outriggers*. these were covered with a layer of resin-impregnated fibre windings to contain fragments if an explosion occured. This made me think (shut up) given the inexorable rise of avgas prices, what about a system of LPG bottles at aerodromes which can be easily swapped out for refuelling instead of liquid fuel..... or a system such as the automotive one? Is the weight penalty too great (tank strength required as atmospheric pressure decreases with altitude) are there issues with icing? or over -complex and unreliable/too heavy/expensive fuel delivery systems? etc etc. who knows? I don't. *I saw a picture in one of Yefim Gordon's books. :p