How long would it take an airplane to fly around the world without using any fuel? About 22 days of actual air time, according to Fusion. Solar Impulse 2, an aircraft which is powered by solar energy, left Egypt on Sunday on the last leg of the first ever-fuel free flight around the world. The team behind it tweeted a few minutes ago that they have completed 91% of the final, last, conclusive flight. Reuters reports: Solar Impulse 2, a spindly single-seat plane, took off from Cairo in darkness en route to Abu Dhabi, its final destination, with a flight expected to take between 48 and 72 hours. The plane, which began its journey in Abu Dhabi in March 2015, has been piloted in turns by Swiss aviators Andre Borschberg and Bertrand Piccard in a campaign to build support for clean energy technologies. "The round the world flight ends in Abu Dhabi, but not the project," Piccard told Reuters a few days before takeoff. Solar Impulse flies without a drop of fuel, its four engines powered solely by energy collected from more than 17,000 solar cells in its wings. It relies on solar energy collected during the day and stored in batteries for electrical energy to fly at night. The carbon fiber plane, with a wingspan exceeding that of a Boeing 747 and the weight of a family car can climb to about 8,500 meters (28,000 feet) and cruise at 55-100 kph (34-62 mph).