Lofting a 500,000-kg jumbo jet into the flight levels on pond scum? Then how about jet fuel refined from an inedible coconut grown in the Brazilian rainforest? Or a previously useless weed that grows in arid regions of the world?

As the need to address global warming becomes more widely accepted, all these bio-sources-as well as more conventional ones like maize and sugarcane-are being studied by the aviation and energy industries as alternative sources for liquid hydrocarbons necessary to fuel gas-turbine engines. Spurring this research, as well, are the current escalation of fuel prices, especially in North America, and the widespread conviction that fossil-based oil reserves are declining (i.e., that the world has reached so-called "peak oil").

In addition to reducing greenhouse gases (GHGs), such as carbon dioxide (CO2), carbon monoxide (CO) and nitrous oxide (NOx), which contribute to global warming, another incentive driving the search for alternative fuels is "oil security," or the necessity to ensure adequate supplies of jet fuel for both defense and commercial purposes as traditional sources of petroleum become scarcer. (Ongoing instability in the Middle East also plays into this scenario.) This has also reinvigorated interest in coal and natural gas liquefaction using the Fischer-Tropsch (or F-T) process developed at the Max Planck Institute in Germany in the 1920s and refined in South Africa during the apartheid era to produce synthetic fuels, or "synfuels."

The U.S. Air Force, for example, is particularly interested in Fischer-Tropsch-produced jet fuel for security purposes, given the huge coal reserves available in America, and has set a goal of powering its entire inventory of aircraft on a 50/50 mix of synfuel and petroleum-derived JP-8 by 2010. Accordingly, in 2006, the service conducted a demonstration of F-T synfuel in a B-52.

And as announced at last year's Paris Air Show, the Federal Aviation Administration has partnered with the USAF in a program to test the synfuel/JP-8 mix in the C-17 tactical transport so that data can be shared with U.S. airlines, already beleaguered by stratospheric prices of conventional Jet-A. (The C-17 is powered by a military derivative of the civil Pratt & Whitney PW2000 turbofan, making it an appropriate test bed for the program. P&W, which built the B-52's TF33 engines, also participated in the B-52 demo and is a partner in the C-17 tests.)

It should be noted that South African Airways has been powering its jetliners for decades with a blend of F-T synfuel and petroleum-based Jet-A, and last year Sasol Ltd. of South Africa received certification from the American Society of Testing and Materials (ASTM), a technical standards group, for a 100-percent synfuel approved for use in jet engines, the first F-T refiner to do so.

But the conundrum with synfuel is that it doesn't solve the emissions problem, not only producing only slightly less CO2 as petroleum-based fuel when burned, but also releasing copious amounts of the GHG when refined through the F-T process. While carbon sequestration may ultimately solve the processing drawback, it's clears the primary usefulness of synfuel in the short term will be energy security and that the future in terms of reduced GHGs lies with biofuels.

In the meantime, the parallel initiative for reducing aviation's GHG emissions lies with engine technology, given that the more fuel-efficient the powerplant, the less it will pollute into the atmosphere. Accordingly, the new generation of airline and business jet engines under development (and showcased here at Farnborough) promises claimed "step changes" in efficiency and lowered emissions over existing turbofans, as epitomized by Rolls-Royce's advanced Trent and BR725, Pratt & Whitney's Geared Turbofan (GTF) and its Canadian (PW&C) sibling's PW800, and General Electric's GEnx and so-far unnamed CF34 replacement, just to mention a few.

Worldwide, biofuel research embraces fatty acid methyl esters (or FAMEs) that can be refined into "biodiesel" fuel, hydrogenated vegetable oils and ethyl alcohols such as ethanol and butanol. Under intense study are feedstocks as diverse as corn, sugar cane and soy beans; switch grass and the weeds jatropha and camelina; oil from the Brazilian babassu palm; and algae-or the green "pond scum" cited at the beginning of this report.

The most common feedstocks for ethanol are corn (maize), sugar cane and soy. Both Brazil and the U.S. have developed thriving industries producing ethanol, the former from cane and the latter mostly from corn. In Brazil, in fact, some 40 percent of automobile fuel consumed is ethanol, and the government has mandated that any car sold in the country must be powered by an engine capable of multi-fuel operation. Meanwhile, Brazilian airframe OEM Embraer continues to produce a variant of its Ipaneme crop duster powered by an ethanol-fueled 320-hp Lycoming reciprocating engine.