Fly the Eco-Friendly Skies

It is possible to fly a jet on 100 percent biodiesel. Green Flight International and Biodiesel Solutions proved that much when they flew a 1968 Czechoslovakian Delfin L-29 jet on pure biodiesel made from french-fry grease last October. The jet flew to 17,000 feet, about half the cruise altitude of a modern airliner, over the Nevada high desert. Green Flight International intends to attempt a round-the-world flight later this year.

Yet not everyone was impressed with the flight. "I'd put that in the stunt arena," said Paul Adams, senior vice president of engineering at Pratt & Whitney. Adams noted that although jet engines will fly on almost any fuel for one flight, it doesn't mean that such fuels are viable in the long term. "In heavy rain, does the fuel stay lit?" Adams asks. "Does it affect the fuel pump, the O-rings? Does it work under all temperature conditions? Does it have the same reliability as jet fuel?"

And that is the primary hurdle for biofuels: the effectiveness of the entrenched competition, the fuel known as Jet A. "Any possible alternative fuel must be measured against Jet A," says James Hileman, a research engineer at the Massachusetts Institute of Technology and a member of the Partnership for Air Transportation Noise & Emissions Reduction group, or Partner, which is based at MIT. For half a century, this kerosene has been the fuel of choice in nearly every commercial airplane. Hileman cites some of its many virtues: "It's safe to handle. If you have a spill, it won't catch fire." It also has a very low freezing point—something you want when flying at altitudes where temperatures can plunge below –40ºF.

But perhaps most important, Jet A packs more energy into a gallon than other fuels. That's crucial because fuel makes up about a third of the weight of a commercial airplane. Energy-dense fuel means a lighter jet, which can fly farther on less fuel, which saves money. Jet fuel also doesn't take up much space, so it can be stored in the wings and not in the fuselage, leaving more room for customers and cargo.

Energy density isn't a problem with biodiesel, which holds almost as much energy per gallon as Jet A. (Ethanol, another widely touted petroleum replacement, fails this test: You need about 1.5 times the amount of ethanol to deliver the same amount of energy as Jet A.)

But biodiesel has other drawbacks. "It freezes at about the same temperature as water," Hileman notes, and when biodiesel freezes, "it's the consistency of the gravy the day after Thanksgiving." Boeing and Virgin hope to avoid this problem with biodiesel and Jet A blends. The fuel won't have the same carbon-fighting power as pure biodiesel, but at least the engines won't conk out at 40,000 feet.

Billy Glover, a managing director for environmental strategy at Boeing, explains that the aviation industry could gradually make the transition from Jet A to a biodiesel blend. "We foresee that within 10 years, there will be certain airports with fuel tanks where this [biofuel] blend will be available," he says. "When you fly to that airport, that's the fuel you'll get. When you fly to another airport, you might get a more conventional fuel."

Of course, the future of global aviation will not fly on french-fry oil alone. We still need to find a source for all this biofuel. Boeing has teamed with Air New Zealand and engine maker Rolls-Royce to test-fly biodiesel from an unconventional source in a jet engine later this year. One option is to create this biodiesel from algae. But only a few gallons of algae fuel have been produced to date, and Daggett expects that it will be a decade before it could receive industry approval. Says Hileman, "If you're a kid and you're thinking, 'OK, what can I do to save the world?' Well, the one way you can save the world is to find new, growable sources of fuel."