So airlines and plane manufacturers are working to develop planes that can run on cleaner fuels like hydrogen, which produces water vapor instead of carbon dioxide when burned.Aviation is considered one of the most difficult industries to clean upThe two companies used a converted Rolls-Royce AE 2100-A regional aircraft engine for the ground test conducted in the UK.Another major hurdle for hydrogen-fueled flight, according to the International Air Transport Association, will be to introduce and certify new aircraft designs.For instance, a Boeing 747 jumbo jet would need more than 1 million liters of hydrogen to get roughly the same range as 250,000 liters of jet fuel, The Guardian reports.A 2020 European Union report estimated that hydrogen-powered passenger planes for routes up to 3,000 kilometers (1,864 miles) could hit the market by 2035."