Solo pilot Andre Borschberg and the project team put their solar-powered plane back in its portable hanger at Nagoya on Wednesday, saying weather forecasted around the destination Hawaii was still too unstable.

Project spokeswoman Elke Neumann said a cold front would have endangered the fragile plane's safety.

The plane was diverted to Japan in early June while heading to its intended destination Hawaii from Nanjing in China.

Borschberg admitted "disappointment" on Wednesday but said the weather was still too unstable over the Pacific. For hours, he waited in the cockpit to take off.

'Not worth trying'

From Monaco, project co-founder Bertrand Piccard said he and Borschberg had discussed conditions be radio but decided in the end that a Wednesday departure from Nagoya was "not worth trying."

"He knows, like us, that it's better to get out of the plane on the tarmac in Nagoya than on the third day with a parachute overhead;" Piccard said.

He has been taking turns with Borschberg in flying the plane, which only seats one, solo.

Longest leg

The intended 8,175-kilometer (5,079 mile) flight across the western Pacific would be the longest leg of the team's planned round-the-world endeavor which began in Abu Dhabi on March 9.

The Solar Impulse is powered by more than 17,000 solar cells on its wings that recharge its batteries, enabling it to fly via propeller. The wide-span aircraft is not designed to withstand heavy winds, turbulence and rain.

The project is meant to demonstrate improved energy efficiency and clean power.

ipj/msh (AP, dpa, AFP)