Solar Impulse 2 takes off for Abu Dhabi in final leg of round-the-world journey

Solar Impulse 2 has taken off for the last time on its historic round-the-world trip using nothing but the power of the sun. Piloted by the project’s co-founder, Bertrand Piccard, the zero-fuel aeroplane took off Cairo at 3 a.m. UAE time, and is expected to arrive at Al Bateen Airport in Abu Dhabi on Tuesday.

Mr. Piccard, who initiated the project 12 years ago, said that Egypt was where he first began thinking of making a circumnavigation using only solar power.

"I’m excited to come so close to the goal, but unfortunately there are still so many people we have to motivate before having a world running on the same clean technologies," Piccard said, according to a report in today's edition of the English language daily, The National.

The Masdar-sponsored plane first began its round-the-world journey in March 2015, when it took off from Abu Dhabi, and Piccard dedicated his first flight to His Highness Sheikh Mohamed bin Zayed Al Nahyan, Crown Prince of Abu Dhabi and Deputy Supreme Commander of the UAE Armed Forces, who he met 12 years ago. He said that the UAE has been a stalwart support for his adventure.

"Today, we are living the final moments of a once in a lifetime adventure contributing to setting a new milestone in aviation, one centred not on speed or height, but instead on exploring new clean and efficient technologies that can almost make it possible for the plane to fly with unlimited endurance, a week, a month, something that was never done," said Andre Borschberg, the project’s co-founder and co-pilot.

The final leg of the journey is expected to be as challenging as the previous 16, mostly due to the heat in the Middle East. The hot temperatures will test the limits of the plane and can cause thermals and turbulence, forcing the pilot to wear an oxygen mask for extended periods of time.

"We have never had to deal with temperatures so high before on our round-the-world tour, but a little challenge at the end of our mission is always good," Solar Impulse said on their blog.

The plane has been piloted by both Piccard and Borschberg around the world, stopping off at 16 cities to raise awareness on the viability of renewable energy.

The flights across the Pacific and Atlantic Oceans each took 5 days to complete, pushing each pilot to their physical limit as they flew the Solar Impulse 2 solo. "The question isn’t the plane," said Mr. Borschberg in April, "It’s the question of the pilots being in the right physical and mental state to complete the flight."

The warmer, thinner air above the Saudi desert also means Solar Impulse's motors will have to work harder to propel the vehicle forward. This will require careful management of the energy reserves in the plane's lithium polymer batteries, to be sure they can sustain the aircraft through the night hours.

"We thought it was going to be an easy flight because it's always good weather between Egypt and Abu Dhabi across Saudi. But actually, it's extremely difficult to find a good strategy," Piccard told reporters before the flight.