Solar-powered plane leaves Egypt for final leg of world-tour
Piloted by the project's co-founder Bertrand Piccard, Solar Impulse 2 is expected to arrive in al-Bateen Airport in Abu Dhabi on Tuesday.
"It's a project for energy, for a better world," Piccard told reporters before taking off.
The 58-year-old Swiss pilot, who initiated the project 12 years ago, said that Egypt was where he first began thinking of making a circumnavigation using only solar power.
The plane, which runs only on solar power, began its round-the world journey in Abu Dhabi in March 2015, with stops in 16 cities to raise awareness on the viability of renewable energy.
The single-seat aircraft, no heavier than a car but with the wingspan of a Boeing 747, is fitted with 17,000 solar cells on its wings, running on battery-stored power during nighttime flights.
It typically travels at a mere 48 kilometres (30 miles) per hour, although its flight speed can double when exposed to full sunlight.
Piccard and Swiss entrepreneur and pilot Andre Borschberg have taken turns flying the plane on its 35,000-kilometre (22,000-mile) trip around the world.
Borschberg piloted the flight's 8,924 kilometre Pacific stage between Nagoya, in Japan, and Hawaii.
The flights across the Pacific and Atlantic each took five days to complete each, pushing the pilots to their physical limit as they flew the Solar Impulse 2 solo.
"The question isn't the plane," Borschberg said in April, "It's the question of the pilots being in the right physical and mental state to complete the flight."
Solar Impulse 2 had been scheduled to leave Egypt last week, but the flight was delayed because of winds and Picard falling ill.
Hot temperatures in the Middle East will test the limits of the plane and can cause thermals and turbulence, forcing Piccard to wear an oxygen mask for extended periods of time.
"It's a very, very hot region... it's going to be an exhausting flight," Piccard said.
Borschberg told journalists that the heat would be a new challenge for the plane.
"Technically it's close to the limits that we have set in terms of temperature, so that's something which we did not experience before," he said via Skype from mission control in Monaco.
"But with the temperature profile that we see over the coming days, we should be all fine."
Borschberg and Piccard have said they want to raise awareness of renewable energy sources and technologies with their project, although they do not expect solar-powered commercial planes any time soon.
"There will be passengers very soon in electric airplanes that we will charge on the ground," Piccard had said when the plane arrived in Cairo.
"On the ground you can charge batteries and you can have short haul flights, maybe 500 kilometres with 50 people flying in these planes" in a decade, he predicted.
Agencies contributed to this report