Gloucestershire-based eco startup ZeroAvia is celebrating its fifth successive hydrogen-electric test flight.

The pioneering firm, based at Kemble Airfield, says the news is another step in its vision to deliver on-airfield hydrogen refuelling and regular domestic passenger flights of zero-emission aircraft in the coming years.

ZeroAvia aims to bring to market a zero-emission system capable of flying 20-seat aircraft 300 nautical miles by 2025.

As an update on progress, the company says it is now approaching the midway point in the first phase of a flight testing programme for the retrofitted Dornier 228.

A spokesman said: "We are happy to have achieved the maximum speed allowed under the permit to fly. During three loops of Cotswold Airport, test pilot Jon Killerby and flight test engineer James Yapp reached a top speed of 150 knots."

"In all five flights so far, the hydrogen-electric engine has performed as the team have expected and hoped. Over the next few weeks, we will begin to test at progressively higher altitudes. This testing all helps in our progression towards a certifiable design and the targeted entry-in-service date of 2025.

"Back in January, following ground-based high speed testing in September 2022, aviation history was made at Keble with the projects's maiden flight.

"The 19-seat Dornier 228 testbed aircraft took to the skies above England's Cotswolds with the leftside propeller powered by a hydrogen-electric powertrain, making this the largest aircraft flown with hydrogen-electric propulsion at the time," a spokesman said, adding it was "a huge step towards a truly clean future for aviation The future of flight is renewable hydrogen, and we're so proud to be leading the way!"

On Monday, ZeroAvia signed a deal with Alaska Airlines to develop a zero-emissions service.

Visiting the company in Alaska, ZeroAvia CEO Valery Miftakhov answered questions from reporters following a demonstration of his company's 1.8 megawatt Megacore electric motor.

ZeroAvia plans to retrofit a retired 76-seat Q400 with electric motors driven by hydrogen fuel cells.

And in March, the firm signed a deal to develop commercial routes using ZeroAvia's hydrogen-electric powertrain systems on routes from Skellefteå Airport in Northeast Sweden.

James Peck, chief customer officer at ZeroAvia, said: "Aviation will become a larger proportion of emissions as other sectors abate, so (Sweden) will need to see true zero-emission air travel that goes beyond combustion fuels. Bold projects such as the one planned for Skellefteå are crucial in this endeavour."

As the Keble tests proceed, Birmingham Airport watches the results in anticipation.

Simon Richards, chief finance & sustainability officer, Birmingham Airport, said: "We are thrilled to partner with ZeroAvia on creating solutions to the main challenge of our generation – protecting the future of our planet."

The airport could conceivably see the first hydrogen-powered domestic passenger flight taking off in a few years, he added, describing the progress as "mind-blowing".