The first manned, hydrogen-powered plane has been successfully tested in the skies above Spain

The hydrogen-powered plane is capable of carrying two people

The first manned, hydrogen-powered plane has been successfully tested in the skies above Spain, its makers say.

The small, propeller-driven craft, developed by aviation giant Boeing, made three short flights at an airfield south of Madrid, the company said.

It was powered by hydrogen fuel cells, which produce only heat and water as exhaust products.

The tests could pave the way for a new generation of greener aircraft, the company said.

Boeing's chief technology officer John Tracy said the flights were "a historical technological success" and "full of promises for a greener future".

Small future

Three test flights of the two-seater aircraft took place in February and March at an airfield at Ocana, south of Madrid. The plane was modified to include a hybrid battery and fuel cell system developed by UK firm Intelligent Energy.

The fuel cells, which create electricity by combining oxygen and hydrogen, were used to power an electric motor coupled to a propeller.

During take-off the plane's batteries were used to provide an additional boost, but whilst in the air, the plane relied entirely on the cells.

Boeing said the plane has a flying time of 45 minutes but tests were limited to around half that time.

Although the test had been successful, the firm said it did not believe fuel cells could be the primary power source for large passenger aircraft.

However, it could be used as a secondary source of energy for large planes, according to Nieves Lapena, the engineer responsible for the test flights, but this may take some time to develop.

"In my opinion, we are talking about a delay of about twenty years," she said.

Green skies

Hydrogen-powered planes have been flown before, but never with a human pilot onboard.

In 2005, California-based AeroVironment successfully completed test flights of its Global Observer craft which was powered by liquid hydrogen.

Other companies are also seeking to develop more environmentally-friendly planes, amid concerns over their contribution to climate change.

Earlier this year, the airline Virgin Atlantic conducted the first commercial flight powered partly by biofuel.

And last year, defence firm Qinetiq flew a solar-powered plane for 54 hours, smashing the official world record for the longest-duration unmanned flight.

Zephyr, as the craft was known, could be used for military applications, as well as for Earth-observation and communications.

Other unmanned prototypes have been shown off by the American space agency Nasa.

However, in 2010, Swiss balloonist Bertrand Piccard plans to launch Solar Impulse, a manned plane in which he will attempt to circumnavigate the globe.

To carry the precious payload, the craft will have a huge wingspan of 80m (262ft), wider than the wings of the Airbus A380.

As the plane is piloted by only one person at a time, it will have to make frequent stopovers. The current plan is for the journey to be broken into five legs each lasting between four or five days.