Retrofitted with a prototype hydrogen-electric powertrain, it performed as expected and marks a major milestone for the future of zero-emission aviation, according to the company.Hydrogen is considered among the most promising alternate fuel sources, and Thursday's demonstration marks a major step forward in ZeroAvia's goal of operating commercial routes with its technology by 2025.The engine prototypes have received experimental certificates from both the U.K.'s Civil Aviation Authority as well as the Federal Aviation Administration in the U.S. ZeroAvia is also working on a powertrain that can fly aircraft with up to 90 seats."This flight opened the gate and served as important proof-point that hydrogen-electric is viable, not just on the ground but also in the air, and it is now just a matter of getting certification and putting it in commercial operations," ZeroAvia Chief Commercial Officer Sergey Kiselev told CBS MoneyWatch."We have invested in hydrogen-electric aircrafts because it's going to take a portfolio of solutions to decarbonize air travel," Michael Leskinen, president of United Airlines Ventures, the airline's corporate venture fund that invests in emerging companies, told CBS MoneyWatch."