Solar Impulse 2 postpones Pacific crossing due to bad weather

A solar-powered plane carrying no fuel has postponedits departure from central Japan for Hawaii.

Swiss pilot and project co-founder said the weather window for the flight early Wednesday appeared to have closed.

"We're stuck. We don't know what to do. The latest weather run shows the corridor we want to take has closed," saidBertrandPiccard, who is taking turns flying the plane with his co-pilot AndreBorschberg.

Borschberg, who is flying the Japan-Hawaii leg of the round-the-world journey was seen sitting in the plane's cockpit, looking patient but serious.

Piccard said the control center in Monaco was analyzing if the weather forecast was reliable. The plane must take off before the sun rises and it becomes too hot and windy to either leave or return to its mobile hangar at Nagoya's Komaki airport.

"We are really in the worst moment when we don't know what to say," Piccard said in an interview from Monaco.

Borschberg landed on June 1 while en route from the Chinese city of Nanjing to Honolulu. He was to fly the plane solo during the roughly five-day trip, taking short naps, doing yoga and meditating to endure the lack of sleep.

The airplane carries no fuel, so project engineers are using simulations to determine when it is safe to fly. The first Pacific leg is the riskiest because there is no place to land.

Borschberg has been waiting for a weather front stretching from Alaska to Taiwan to clear enough for him to resume the 8,175 kilometre(5,079 mile) journey across the western Pacific, the longest leg of the round-the-world journey which began in Abu Dhabi.

The length of the journey means that the weather forecasts the project's simulators use to decide if it is safe to embark are at their limits of reliability, he said.

"We don't want to take too much of a risk," he told reporters earlier in Tokyo. "Either the airplane is still flying and it gets to where it is going, or it's in the water and it's a failure. It's either 100 percent or 0 percent. There is no 50 percent. That is why we are so careful."

"I will really believe in this window to Hawaii when I will have crossed the point of no-return!" Borschberg tweeted on Tuesday.

Repaired and ready

On the ground in Nagoya, the plane was slightly damaged when a cover was tousled by the wind, but it has been repaired and ready to go for days.

The project's leader and co-founder, Bertrand Piccard, will be guiding Borschberg from a control center in Monaco. They are taking turns flying the plane solo.

The Solar Impulse 2 is powered by more than 17,000 solar cells on its wings that recharge its batteries, enabling it to fly. But bad weather and nights are challenges because diverting around clouds takes extra energy, and the aircraft is not designed to withstand rain, turbulence and heavy winds.

The aircraft travels at about the same speeds as a vehicle. At night, it descends from its maximum altitude of 8,500 meters (27,887 feet) to 3,000 meters (9,850 feet) to minimize power consumption as it draws from its batteries.

In the morning, the plane resumes producing power, but it needs to have enough left-over power to ascend to the daytime altitude.

The Solar Impulse project is meant to demonstrate the potential of improved energy efficiency and clean power, though solar-powered air travel is not yet commercially practical, given the slow travel time, weather and weight constraints of the aircraft.