WASHINGTON: Pilot Andre Borschberg oversaw the construction of a solar plane that can fly through the night, but these days the entrepreneur is more concerned with the limits of man than of technology.

The Solar Impulse HB-SIA combines the wingspan of a passenger plane with the weight of a small car, but it can only hold one person, so the pilot must remain awake and alert for its record-setting day-long flights.

"Energy is not a question anymore," the 60-year-old told AFP while flying the experimental airplane he helped conceive, at a height of 5,000 feet (1,500 meters) over a wildlife preserve in Illinois as part of a US tour.

"The pilot is becoming the weak part of the chain."

Staying awake at the controls for 24 hours is a recurring test for Borschberg and his business partner, Solar Impulse chairman and fellow pilot Bertrand Piccard, who are taking turns flying solo on the multi-leg US journey.

"We are limited by our own personal endurance," said Borschberg, who uses meditation techniques to maintain his concentration and practices yoga breathing to sustain his awareness on long flights.

"We are so enthusiastic about what we are doing and so happy to be in the airplane that it carries us really, from the morning to the evening," he said.

"But of course we are all human. We have our moments when the energy is high and we have moments when it is more difficult."

The cockpit of the prototype HB-SIA is narrow, cramped and, by Borschberg's own admission, "not very comfortable, so that is something we are working on for the second airplane," the HB-SIB, due to start flight tests next year.

Another uneasy prospect has been how the pilot relieves himself during flight, since there is no bathroom and he cannot get up and walk around.

So many people posed this question that the team made a YouTube video called "Solar Impulse Behind The Scenes: Bathroom."

"Yeah, he's got a bottle," a spokesman confirmed to AFP.

Despite the physical challenges, Borschberg smashed a series of records aboard the plane three years ago, including the longest-ever flight in the history of solar aviation, which lasted 26 hours, 10 minutes and 19 seconds.

He proved the plane could fly in the dark on that same July 7, 2010 venture, which established the first ever nighttime flight and the highest altitude (9,235 meters) ever flown by a solar airplane.