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Bertrand Piccard, the athletic-looking chairman of the Solar Impulse initiative and one of two pilots of the Solar Impulse 2 propeller aircraft, appeared ready and eager to take off on the next leg of his trip during a stopover in San Jose Thursday, April 28.

Piccard flew Solar Impulse 2 by himself from Hawaii to San Francisco, crossing the Pacific Ocean in 62 hours and arriving April 23. He relied solely on energy drawn from the sun. All the upper surfaces of the experimental aircraft are covered with solar panels. That's a great deal of space, since the Solar Impulse 2's wings are as wide as a Boeing 747's.  

His next leg will run from Moffett Field in Mountain View to Phoenix.

"We will be flying very soon," Piccard promised in a blog post in the group's website early on April 28. Although he had been in the air for the better part of three days and two nights during the flight from Hawaii, Piccard said he "didn't wanted to stop" when he reached the Golden Gate Bridge.

In fact he descended to fly alongside the bridge and circled over several parts of San Francisco Bay before proceeding south to Moffett Field. Piccard appeared at the Silicon Valley campus of solar panel maker ABB in San Jose in an event for ABB employees that was also attended by a few members of the press.

Piccard landed at Moffett Field, instead of San Francisco or Oakland, because of its low traffic and the fact that the field is co-leased and operated by Google, one of the sponsors of his flights. That makes the air traffic control aspect a little simpler. Piccard approached the Golden Gate Bridge at what appears to be about 3,000 feet. It was dark by the time his experimental craft made it down the peninsula to Mountain View. The big slow-moving plane made an eerie sight as it touched down, as seen in this YouTube video.

Eight battery-powered lights on each wing, including the curving upward wing tips, marked its progress.

Plotting the Course

For the next leg of his trip, Piccard said in an interview with InformationWeek that he will fly from Phoenix to somewhere in the Midwest. His flight path is heavily dependent on the weather, since Solar Impulse 2 can't withstand too much turbulence from stormy weather.

From the Midwest, he will make his final hop across the US to an airport somewhere in New York, according to a trip projection posted on the Solar Impulse site. Piccard's next challenge is to cross the Atlantic.

Piccard plans each leg carefully ahead of time, in order to avoid other air traffic and turbulent weather, both of which could upend his delicate craft in flight. The Atlantic might not seem like too much of a challenge, given that he and fellow pilot Andre Borschberg have crossed the entire Pacific. But the sensitivities of the Solar Impulse as a flying machine would be enough to give many pause.

It's not supposed to bank off a level axis more than five degrees for safe flying. As a plane banks, some of the lift that used to hold it up in the air is applied to the turn. Solar Impulse can't afford to lose much of its lift. Asked by an ABB employee how steeply his aircraft can bank and remain airborne, Piccard answered, "Officially, its 8 degrees. Unofficially, I know it does 8.5 degrees," he answered.

Piccard was trying to take a 20-minute nap during his flight from Hawaii with the plan on autopilot when an alarm went off alerting him that the aircraft was approaching a flight limit. The operators in the Mission Control Center in Monaco had also noticed his flight going awry and were radioing him that he was in an unintended bank. He needed to reassert control through the foot rudder pedals. But his seat was all the way back in sleeping position. In a state of grogginess, he couldn't immediately recover and find the pedals.

"I have to get my seat up," he radioed back as concern deepened in mission control. He did so and righted the craft.

The slender parameters that Solar Impulse must stay within in order to keep flying were demonstrated on the leg of the trip that took it from Japan to Hawaii. That one was even longer, a five-day flight performed by Andre Borschberg, Piccard's fellow pilot.

Piccard is a psychiatrist by training. Borschberg is an engineer and former fighter pilot.

Borschberg had battery power to spare aboard Solar Impulse on the way to Hawaii, so he speeded up the revolutions of its four electrical engine propellers. That got him to his destination sooner, but drawing current out of the lithium ion batteries at a higher amperage heated the batteries up. The battery compartment is highly insulated, because part of a long flight's routine is to ascend to 28,000 feet during the day with surplus solar energy available, then descend through the night to 8,000 feet, using the descent to conserve power.

Battery Power

At 28,000 feet it is minus 30 degrees Celsius outside, said Piccard, so the batteries are insulated to protect them from diminished function at cold temperatures. "Because the battery insulation is so good, it trapped the heat, the batteries overheated. They were damaged... It was a human error," he added. Borschberg made it to Hawaii, but flight teams spent the next nine months replacing the batteries and retesting the aircraft.

Piccard said a cooling system had originally been rejected for the batteries, because it would have added more weight. As he took off from Hawaii, he had a lightweight cooling system for the batteries in place, which was operated manually by the pilot. It may be as big as a 747 in wingspan, but the Solar Impulse weighs only as much as a good-sized SUV, about 5,000 pounds.

Asked how much heavy radio equipment he needed to carry, Piccard said his team challenged a Swiss partner company to get the weight of its radio down from 50 kilograms, requiring a kilowatt of power, to 5 kilograms and 100 watts, or one-tenth of its previous specification. "They said they didn't know if they could do that, but accepted the challenge," he recalled. The company eventually delivered the needed equipment at the required weight and power.

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The Solar Impulse, however, isn't the sort of craft that many people would choose to fly in, particularly given some of the unknowns of its strange design, and even fewer would choose it for safe passage over something like the Pacific.

Piccard addressed that point in his talk to ABB employees. "When you are alone in a small cockpit in the middle of the Pacific at night, and the closest people who can help you are thousands of miles away, you might ask yourself, 'Why do I feel so good?'" he recounted, to chuckles in the audience.

Piccard said he feels good because he comes from a family of explorers, and he feels like he is carrying on the tradition. But he also said he realized in those long nights how much he and his engineering teams "have had to get out of our comfort zones and get away from the certitudes" that hold many other explorers back. His team had challenged the notion that a heavier-than-air craft couldn't fly continuously on solar power. His piloting the Solar Impulse on an around-the-world tour was proving them wrong.

He and Borschberg started their flights last March in Abu Dhabi.

"When you can't rely on external resources, that's when you realize you must depend on your inner resources," he said. By piloting the Solar Impulse 2, he wants to convince people they can invent a better future.

The Future of Flight

Within 10 years, he predicted, airplanes powered by batteries will be making short-flight commuter hops while carrying 50 passengers. The batteries would have to be charged with ground installations, not in flight, he conceded. But a cleaner, more climate friendly airline passenger system will one day be possible.

[Want to see another idea for use of solar power to expand the Internet? Read Facebook's Project ARIES Terragraph: Expanding Internet Through WiFi.]

Solar Impulse can't carry a passenger, and it can't reach its destination quickly, as, say, the British Concord once could. But Borschberg said on the group's website, "What's important is not to go fast, but to go forever. We can fly perpetually without refueling."

Piccard adds that politicians and the general public underestimate what can be accomplished with renewable energy. By ascending during the day and descending at night, he still has 10% of his battery power left when morning comes.

"We're trying to inspire people to do clean energy things. [...] Solar Impulse 2 can power its engines, charge its batteries, fly through the night and still reach sunrise," he said.