Dallas, US - BBC - A solar-powered plane aiming to cross the US from the West Coast to the East Coast has begun its third leg.

This will take the Solar Impulse vehicle from Dallas to St Louis in a flight expected to last 21 hours. The HB-SIA craft, which has the same wingspan as an Airbus A340 but weighs just 1.6t, took off from Dallas Fort Worth at 04:06 local time (09:06 GMT). It is due to land at Lambert-St Louis International Airport at about 01:00 local time on Tuesday (06:00 GMT).

The pilot duties on the “Across America” bid are being shared by the two leaders of the project - Swiss nationals Bertrand Piccard and Andre Borschberg. This third leg is being undertaken by Mr Piccard. The Solar Impulse team is having to erect a temporary hangar at St Louis to house the plane when it arrives. Recent storms damaged the building that was originally assigned the task. An inflatable hangar will now be used to park the aircraft.

The Trans-US journey is billed as the first cross-continental, solar-powered flight. It is the last showpiece with the prototype aircraft before Mr Piccard and Mr Borschberg attempt a trans-oceanic flight and an eventual around-the-world flight in 2015.

The aeroplane already holds records for the first international flight of a manned solar-powered plane in 2011, and first inter-continental flight in 2012.

The aircraft completed the first leg of its trans-American bid - between San Francisco and Phoenix - in early May, in a flight lasting 18 hours. The second leg - from Phoenix to Dallas - was completed in late May. This trip covered a distance of 1,541km (958mi) - a record for a manned solar-powered plane.

The Across America project coincides with the Piccard’s and Borschberg’s Clean Generation Initiative, an effort to encourage policy-makers and businesses to develop and adopt sustainable energy technologies.

About solar plane

* Wingspan - 63m (208ft)

* Weight - 1,600kg (3,500lb)

* Covered with 11,628 solar cells

* Carries 400kg (900lb) of lithium-ion batteries

* Maximum cruising altitude of 8,500m (28,000ft)