An aviation rarity touched down in the Norway's second city of Bergen earlier this month. Alia had flown 100 miles (160km) in 55 minutes on battery power alone.
Built by US aerospace company Beta Technologies, the electric plane is designed for cargo operations - carrying up to 560kg (half a tonne) loads.
The flight had simulated a planned cargo route between the coastal cities of Stavanger and Bergen, and for the next few months test-flights will be carried out, as part of the country's move towards establishing low-emission aviation.
At the helm was pilot Jeremy Degagne, "If you do the drive, it's four and a half hours. And we did the flight in 52 minutes."
"It is an important milestone for Norway as an international test arena," says Karianne Helland Strand, a director at Norwegian airport operator, Avinor.
Alia can fly up to 400km (250 miles) on a single charge, and refuel in less than 40 minutes by plugging-in, just like an electric car.
The same fixed-wing model can be configured for medical transport or passenger travel with up to five seats, and this June it undertook the first electric demonstration flight carrying passengers into New York's JFK airport.
Beta, which counts Amazon as an investor and UPS as a customer, hopes to get US certification for its plane this year.
Alia is one of the most advanced projects, among dozens of firms exploring electric propulsion in aviation.
It would be one way of reducing the aviation industry's carbon footprint - which currently accounts for 3% of the world's greenhouse gas emissions.
However, the Pipistrel Velis Electro remains the only electric plane to receive full certification from European authorities, despite clearing that hurdle five years ago.
With a range of 185km and 50 minutes of flight time, the Slovenian-built Pipistrel is limited to training and not for shuttling passengers from A to B.
But successes like that have been overshadowed by a string of failures in electric aviation.
Even aviation giant Airbus has backed away from the market. In January it announced that development of its CityAirbus electric aircraft would be put on hold.
Range remains the major limitation to electric flight. Even the best lithium ion batteries are bulky and heavy, with much lower energy density than jet fuel.
They have "not improved significantly" over the past two decades, reckons Guy Gratton, an aviation expert and professor at Cranfield University.
For electric flight to take-off, a "revolution" in battery chemistry is needed, he says.
Given those limitations, some are looking at alternative technology.
Just as hybrid cars were a stepping stone towards electric vehicles; plane-makers are now also experimenting with hybrid technology.
Among the aviation start-ups trying to get electric passenger planes off the ground, is Heart Aerospace.
It recently shifted its entire operations from Sweden to the US, which its management said would help it focus "resources" and be closer to clients, including the airlines Mesa and United.
The firm has developed a 30-seater, prototype plane, the X1, which the BBC saw before it was shipped to the United States.
If all goes to plan during upcoming test-flights, it will become the largest battery-powered plane to fly. "It has about two tons of batteries in it," explained chief technology officer Benjamin Stabler.
For its real-world operations, though, Heart is adopting a fundamentally different design: a hybrid plane, powered by batteries, but carrying fuel as backup.
"You don't need as [many] batteries," argues Mr Stabler, which makes it lighter and cheaper, and also allows for more paying passengers.
"For a normal route, it would fly all-electric from takeoff to landing," he explained.
"If you want to go a longer distance, or if there's a diversion, you can switch over to the turbines."
The aircraft could travel 200km in electric-only flight. With the hybrid technology, which is scheduled for test-flights in 2026, it could fly 400km with 30 passengers, or up to 800km with 25, the firm claims.
"Public transport flying, quite rightly, requires a significant amount of energy reserve," says Prof Grattan.
"So hybridisation and the use of conventional fuels to carry safety reserves makes good sense," adds the professor, who has previously advocated this approach.
Heart isn't alone in this field.
US-based aerospace startup Electra expects its nine-seater hybrid plane to take flight by 2029, running on a combination of jet fuel and electric power.
Beta Technologies is also pursuing hybrid aircraft for defence and civilian purposes. Its first model was built in 2023, and later this year it plans to produce a plane that is not only hybrid but autonomous.
A fully electric foundation is necessary first argues Mr Hall, "you then layer on hybrid technology".
Hybrid systems have lower emissions than conventional aircraft and the electric motors would enable quieter take off and landing in urban areas.
It's still not clear what the future of aviation will look like.
Greener fuels such as sustainable aviation fuel (SAF) have attracted investment, along with hydrogen-based systems.
All will have to prove their commercial viability and safety, and much work needs to be done.
"This is a really challenging thing to do, electrifying aviation and removing the carbon," said Mr Stabler.
Courtesy: BBC
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