Q & A : Biodiesel Airplanes?

Treehugger Elizabeth Snow wrote us today, asking "Can airplanes run on biodiesel?" We thought that was a decent question, since a Boeing 747 run from Minneapolis to Amsterdam uses over 109,700 liters of fuel to make the trip. That's almost 8 times as much fuel per passenger mile as a bus, and over twice as much fuel/passenger mile as a car. With the global airline fleet sucking down this much fuel, it would be nice to know that it was renewable...Like so many other eco-questions, it appears that the answer to this one is "It Depends." Since biodiesel is so similar to regular diesel oil, an engine that runs on one will hardly notice the difference if you switch it. This is the reason that alternative fuel enthusiasts are so gung-ho about it; There are no heaters, tanks, tubes, or filters to install. Just fill up your tank.

But if your car craps out on biodiesel, all that happens is you stop on the road. The consequences get much more hairy when you're flying at 35,000 feet and the engine seizes up. To avoid this problem, a number of careful tests have been performed, including this one by Baylor University in 1998.

From what we've seen, biodiesel performs well in diesel aircraft engines, reducing emissions of NOx and unburned carbon. Diesel aircraft are propeller (piston driven) engines, because diesel fuel's stability(extremely high flash point and low volatility) and need for high pressure are prohibitive to use in a turbine, (like on a 747).

But in these aircraft applications, biodiesel is used as a blend, rather than 100%. This is necessary in order to reduce the effects of biodiesel's low temperature crystallization and thickening. If you were in an airplane at 20,000 feet, where the temperature is possibly well below freezing, and your fuel thickens to the point where it can't be pumped to the engine, that's a very bad situation.

So, Elizabeth, it doesn't look like you'll be riding in a biodiesel commercial jet flight any time soon. But, if you're a private pilot, or regularly fly in a diesel prop plane, it may be worth your while to look into biodiesel blends as a fuel.

If you've got additional information about biodiesel in planes, or other alternative airplane fuels, we'd love to have your comments.