Biofuels were a major theme at last week's Farnborough International Airshow, with manufacturers, energy companies and trade associations teaming up to highlight some of the most promising technologies for greener travel. "Five years ago, drop-in biofuel for aviation was impossible," says Billy Glover, managing director of environmental strategy at Boeing Commercial Airplanes. "Four years ago, it was unlikely. Three years ago, we thought 'maybe there's something here,' but now we've got through most of the technical barriers. We are really ahead of where anybody could have predicted."