"Fuel is the biggest four-letter word in the industry," Billy Glover, director of environmental performance strategy at Boeing Commercial Airplanes, said in an interview with Reuters.

"Fuel efficiency is an economic issue, but it's also an environmental one," said Glover, whose job involves looking at how Boeing can build planes that fly cleaner and quieter.

That means using less kerosene-based JP-8 fuel and looking at alternatives, he said.

"There are a number of feedstocks out there," Glover said, citing sugarcane, switchgrass, soybeans and algae. "Those are being looked at, and there appears to be some promise."

The US Air Force flew a B-52 bomber recently with two of its eight engines using a 50/50 blend of jet fuel and a synthetic alternative.

The test flight reflected growing calls for fuel alternatives for military use, a process likely to feed civil applications as well.

US CALLS FOR STUDIES

The US Department of Defense's technology development arm, the Defense Advanced Research Projects Agency (DARPA), in July asked for proposals on biofuel development.

It hopes to find a way to convert crop oil into a synthetic jet fuel that will achieve at least a 60 percent conversion efficiency by energy content and eventually a rate of 90 percent.

Glover said challenges posed by biofuels include the fact they are less stable when stored for long periods and freeze at a much higher temperature than jet fuel.

There's also infrastructure to be considered, with more than 10,000 airliners in operation all using engines designed for jet fuel.

"Jet fuel has sulphur in it, and this causes seals to swell, which is good. With biofuels without sulphur, you risk leaks."

South Africa already uses a 50/50 blend of jet fuel and coal-derived synthetic fuel, but Glover said this had its limits.

"It's not environmentally friendly at this stage," he said, citing the fuel's higher output of carbon dioxide emissions in production.

"What we're flushing out right now is 'What is viable?'," Glover said. "For now biofuels make more sense for land transport than air, but our job is to go find out."

Boeing's next airliner, the 787 due in 2008, is expected to use 20 percent less fuel and be 60 percent quieter than the 767 model it replaces.