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Treehugger discusses the possibility of bio-diesel planes. Apparently, you can just drop a little bio-diesel into low flying planes and travel for miles while wafting chip crease in your wake. However, for larger planes at higher altitudes they suggest a mixture. You don't want your biodiesel turning to Crisco at 30,000 feet.

Q & A : Biodiesel Airplanes? [TreeHugger]