The pilot of a solar-powered aircraft attempting a round-the-world voyage has said the longer the plane is grounded in Japan the closer the project will come to failing this year.

Swiss adventurer Andre Borschberg said the Solar Impulse 2 will continue its journey from Nagoya Airport as soon as the weather improves, but he said it now needs a clear run if it is to complete the journey as planned.

"We have question marks," Borschberg told The Japan Times on Wednesday. "We certainly do not have a lot of time in reserve. We ate our margin, to be totally honest.