Solar Impulse tests limits of endurance

André Borschberg pictured in the cockpit at the start of the 72-hour Solar Impulse flight simulation Keystone

The world’s first round-the-world flight by a solar-powered aircraft is a step closer to reality following the successful simulation of a 72-hour non-stop flight.

This content was published on February 24, 2012 - 16:28
Sophie Douez,

André Borschberg, who successfully piloted the Solar Impulse from Switzerland to Belgium and France last summer, emerged from the cockpit simulator on Friday morning rested and upbeat.

For the previous three days, Borschberg had lived – eating, sleeping and exercising - in the confined space of the cockpit, all while piloting the plane through a series of simulated landings and weather conditions.

“It was an extremely rich experience, everything worked well, the results were better than I had expected,” Borschberg told reporters gathered at Dübendorf airfield near Zurich.

The brainchild of Swiss adventurer Bertrand Piccard, the Solar Impulse project aims to complete a round-the-world journey beginning in April 2014 in an aircraft powered solely by energy from the sun.

The journey is expected to take about two months and while stops are planned on all continents, the longest flight leg – the crossing of the Pacific Ocean - could take up to five days and nights.

“It is important to know that we can fly many days, many nights with only one pilot,” Borschberg told “The plane is fully sustainable today with the energy it collects from the sun, we had to make sure that we can also make the pilot sustainable.”


In the 72 hours he was confined to the cockpit simulator, Borschberg slept 32 times for 20 minutes, for a total of around ten hours sleep.

“The question is to understand how much sleep is necessary,” Borschberg said. “We have to find a solution for the pilot that is a mix of rest techniques and micro-sleeps. We chose to do 20-minute micro-sleeps.” 

And while there was a palpable air of relief and celebration amongst the dozens of medical, nutritional, ergonomics and engineering experts who make up the Solar Impulse team, it seemed Borschberg was far from being the most tired of the group.

“I feel good, but certainly I also feel like I’ve got a good case of jet lag,” he said, adding that while he planned to take a short nap in the afternoon, he would wait the day out to resume normal sleeping patterns.

“In the cockpit, there is a clock, but I didn’t really know if it was morning, afternoon or night time and it was strange. I feel it now.”

The simulated flight was also important to test how the pilot’s body would react to the long hours seated in the cockpit with no possibility of standing upright or moving around freely.

Engineers have been working on the design of the cockpit for more than a year. The pilot’s seat is ergonomically designed and adjustable to allow for the pilot to be seated upright while flying, or in a more horizontal position during rest periods.

It is also designed in a way that gives the pilot enough room to undertake yoga and pilates exercises which Borschberg did every two to three hours.

“I came out without any muscle stiffness,” said Borschberg.  

“But the other thing is, when you are seated like that for a long period of time and you stand up again, you feel the force of gravity on the body. I didn’t have vertigo or anything like that, but I felt something.” 

As for those other essential bodily functions, for which there is a receptor under the seat: “Everything takes a bit of time, so you have to be patient!”

Onwards and upwards

With two years until the Solar Impulse sets off on its epic journey, there are still several issues to resolve.

Construction of a second plane which will actually undertake the round-the-world flight in 2014 is underway and expected to be completed early next year.

Before then however, Piccard will undertake a similar flight simulation over several days in the coming months and the team will embark on a series of test flights over the Mediterranean in the summer.

“It [Borschberg’s simulated flight] was really important because it was the first time that the team worked together, simultaneously, in the same place,” Piccard said.

“We did several different scenarios and I think we learnt a lot in terms of response and communication in cases of crisis. I think we are ready for this year’s flights.”

Solar Flight

While Piccard and Borschberg's flight would be the first to circumnavigate the earth, other groups have already attempted solar-powered flights.

Nasa achieved success in 1997 when its Pathfinder - a lightweight, unmanned, flying wing - built with the AeroVironment firm, climbed to 21,793 metres under its own power.

In 2001 the remotely piloted Helios aircraft reached an altitude of 29,524 metres - an unofficial world-record altitude flight for a solar plane. During a 2003 flight, Helios crashed near Hawaii.

The first really notable solar aircraft was AeroVironment's Solar Challenger, which crossed the English Channel in July 1981.

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Solar Impulse in figures

Seven years of work, calculations, simulations and tests were needed by the 70-person team to complete the aircraft.

The plane has the wingspan of a Boeing 747-400 and the weight of an average family car at 1,600kg.

It has 12,000 solar cells mounted onto the wing, which will supply energy to the four electric motors with a maximum power of 10 HP each.

During the day they will also charge the polymer lithium batteries (400kg), which will allow the aircraft to fly at night.  

A second plane – which will be undertake the round-the-world flight in 2014 – is currently under construction.

The ten-year project is expected to cost around SFr120 million ($133.6 million).

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