The Solar Impulse aircraft, a pioneering Swiss bid to fly around the world on solar energy, successfully completed its first test flight in western Switzerland on Wednesday.

"There has never been in the past an aeroplane of that kind to fly. It was a huge question mark for us and it's an extraordinary relief," said Bertrand Piccard, pioneering round-the-world balloonist who co-founded the project.

"Today for Solar Impulse it's an incredible milestone. It gives us confidence for the next flight and for the next missions," he added.

The high tech prototype had lifted off into blue skies at a speed of just 45 kilometres per hour (28 miles per hour) after running a few hundred metres down the runway at Payerne air base shortly before 10:30 am (0830GMT)

Propelled by four 10 horsepower electric motors, the gangling single-seater aircraft and test pilot Markus Scherdel slowly gained altitude until 1,200 metres (3,900).

After 87 minutes, the plane descended gracefully back to land.

"Everything worked as it should. The flight was very successful. We were able to fly the programme as planned and we are safe on the ground again," said Scherdel.

Following Wednesday's test, the 70-strong team which had worked seven years on the project is expecting to carry out other test flights to refine the prototype aircraft.

Organisers added that the team will also construct the actual plane that would undertake the world tour in five stages by 2013, and not 2012 as previously announced.

"We will continue test flights to improve the design of the second plane that would go around the world," said Andre Borschberg, a co-founder of the project, adding that construction on the aircraft would start next year.

"This summer, we want to show that we can fly night and day. This will happen in Payerne. Hopefully in May, June or July," he added.

The prototype, which is slightly smaller than the plane that will undertake the round-the-world flight, has a wingspan comparable to that of an Airbus A340 airliner but weighs as little as a family-sized car at only 1,600 kilogrammes (3,527 pounds).

Borschberg said the first test flight was primarily aimed at testing the complex aircraft's behaviour in the air.

"The success of this first flight allows us to envisage the further programme with greater serenity," he said.

The Solar Impulse prototype had briefly taken off for the first time in December for a controlled 400-metre hop about one metre above the runway, but a full flight had been delayed for weeks until weather conditions improved.

The aircraft's slender long wings are covered with about 12,000 solar cells that fuel its 400 kilogramme battery packs and the electric motors.

The tests are due to build up to a first non-stop 36-hour flight through darkness by the summer, followed by a five-stage flight around the world in 2013.

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