The sun had yet to rise over Abu Dhabi when a giant aircraft, silent as a moth, settled gently on the runway at Al Bateen Executive Airport and into the history books. Monday marks five years since <i>Solar Impulse 2</i>, powered by sunlight and with wings longer than a Boeing 747, completed its 42,000-kilometre voyage around the world. Piloted by Bertrand Piccard, 63, and entrepreneur Andre Borschberg, 68, the flight was completed without any fuel beyond the sun’s rays, using four electric motors powered by more than 17,000 solar cells. Despite its 72-metre wingspan, the aircraft weighed no more than a Toyota Land Cruiser. With a pressurised cabin, it could reach an altitude of 8,500 metres but generally cruised at 1,800 metres. On a full charge, it could fly throughout the night. <i>Solar Impulse 2</i> left Abu Dhabi on March 9, 2015, on its way around the world. It spent about 23 days in the air, travelling at an average speed of about 70 kilometres an hour. When Mr Borschberg landed the aircraft in Kalaeloa, Hawaii, on July 3, 2015, it was discovered that the batteries had overheated during the flight from Japan. The rest of the journey was postponed until the following year. The circumnavigation was not without challenges, including turbulence caused by the hot desert air as the plane made the final approach to the UAE capital on July 26. It was several months behind schedule, delayed by bad weather in China and Japan and battery repairs in Hawaii. By then it had already broken several records, including the longest solar-powered flight by time and distance at 117 hours 52 minutes and 7,212 kilometres. After landing at Abu Dhabi, Mr Piccard predicted electric passenger aircraft would fly within 10 years. What then seemed an impossible dream is now edging towards reality. Several short-haul commuter aircraft are being developed, including projects by Boeing and Airbus, the world’s largest aircraft manufacturers. Mr Piccard and Mr Borschberg founded the <a href="http://foundation.it/">Solar Impulse Foundation</a> in 2003. It aims to find 1,000 solutions to environmental problems using non-polluting energy. Its aviation company, H55, was launched in 2017. This month, Mr Piccard and Mr Borschberg marked the anniversary by taking to the air in a very different aircraft, but one also powered by electricity. The Bristell <i>Energic</i> is a single-engine battery-powered flight trainer. Built by H55, it could become one of the first commercial electric planes. It was the first time the men had flown an electric aircraft together. Their combined weight would have been too much for <i>Solar Impulse 2,</i> which they piloted in turn. “It was so good to be back in electric flight mode with my old partner,” Mr Piccard said on Twitter. That alone illustrates the technological leap since their global flight five years ago. Other alternatives to fossil fuels include hydrogen fuel, which emits only water vapour. Mr Piccard said his latest aircraft was “the perfect proof that clean and efficient technologies allow us to do so much better for the environment and all fields of the industry”. “This low-carbon flight reminded me how great it is to fly without noise," he said.
