Bitter sweet stop for sun-powered plane

Swiss pilot Andre Borschberg was in the air for more than 40 hours before he landed the Solar Impulse 2 at the Komaki airfield in Nagoya, Japan, a little after 11:30 p.m. (1430 UTC) Monday.

"This has never been done before. It's the longest flight ever in distance and duration for a solar-powered airplane," Bertrand Piccard, the co-founder of Solar Impulse 2, told DW.

The lightweight craft left Nanjing, China, early Sunday, embarking on what was to be a record-breaking journey of six days and five nights across the vast Pacific to Hawaii. But one day into the flight, the weather became a problem, and a decision was made to stop over in Nagoya, Japan.

"The plane would have gone through big clouds, heavy turbulence, rain, ice, and probably would have drowned," Piccard said.

"We are a little disappointed not to make it nonstop to Hawaii. But, on the other hand, we are elated to see that the plane works so well."

Borschberg and Piccard have been taking turns flying the plane since beginning their attempt to circumnavigate the globe using only solar energy in Abu Dhabi in March. They have so far stopped in Oman, India, Myanmar and China.

The 8,175-kilometer (5,079-mile) journey across the Pacific would have been the plane's first ocean crossing, and the trip's most challenging stint.

Predicting the weather

Suitable weather conditions could make all the difference to the success of the pioneering mission. While the aircraft is grounded in Japan, a team of meteorologists based at the Si2's mission control center in Monaco will try to identify a weather window that would allow the flight to continue to Hawaii in the next few days.

According to aviation safety consultant Keith Mackey, it's critical that the proper decisions are made when undertaking such an ambitious crossing.

"Once you get out of range of places to land, you better have the proper conditions, you better not find anything unexpected," he told DW.

The seventh leg across the Pacific of the round-the-world flight had already been delayed by more than a month due to the weather. Clear skies and tailwinds to push the plane make for perfect flying conditions. The craft is designed to be efficient - it weighs about the same as a car and has long wings that allow it to fly like a glider.

"Because of this, it's not as structurally strong as a commercial airliner would be, so it's not capable of flying in extremely turbulent weather," Mackey said.

The plane draws its energy from 17,000 solar panels that cover the wings and fuselage, charging four batteries, which in turn power the 17.5-horsepower electric motors. If the skies are too cloudy, the solar panels can't recharge the batteries.

Pacific first, then Atlantic

Piloting the plane over large stretches also requires constant attention. Both Borschberg and Piccard have trained in yoga and meditation to be able to cope with being cramped in the 3.8 square meter cockpit. They've also learned to sleep in 20 minute stretches in order to keep an eye on the flight controls.

Before the stop in Japan, the round-the-world journey had been scheduled for 12 legs, with a total flight time of around 25 days.

If the Si2 makes it to Hawaii, Borschberg and Piccard plan to fly on to Arizona in the United States, across the Atlantic Ocean, then back to Abu Dhabi before early August when the hurricane season sets in.

The two pilots spent 12 years developing the aircraft as part of a project to promote solar power and other forms of green energy.