A soft hush fell over Maribor Airport as a sleek, four-seat aircraft, the HY4, lifted into the autumn sky. It was early September 2023, and though the air felt cool, the tension in the cockpit was warm: this flight could mark a turning point in aviation.
For over three hours, the world witnessed something new — a piloted electric aircraft flown entirely on liquid hydrogen. It was an hour of innovation, an hour of promise, and most importantly, an hour that carried hope.
A Flight That Changed Expectations
The HY4, developed by H2FLY (a company spun out of Germany’s DLR Institute of Engineering Thermodynamics), took off from Maribor, Slovenia, as part of a testing campaign under the HEAVEN consortium.
It wasn’t just one test flight: there were four flights in total powered by liquid hydrogen. One of these lasted over three hours — long enough to send a message: we may be closer than ever to emissions-free medium-range flight.
Earlier, HY4 had used gaseous hydrogen in its fuel cells. But gaseous hydrogen requires bulky, heavy tanks and infrastructure that limit range. With liquid hydrogen stored cryogenically, the plane’s range more than doubled — from about 750 km (with gaseous H₂) to 1,500 km.
What Made It Possible
Liquid hydrogen is hydrogen cooled to extremely low temperatures so it becomes a liquid — this means it’s much denser than its gaseous form, allowing more fuel to be carried with less weight and volume. For the HY4 flights, engineers had to adapt several cutting-edge technologies:
- A fuel cell propulsion system that converts hydrogen to electricity, which powers electric motors, producing only water vapor as a by-product.
- Cryogenic storage: keeping hydrogen very cold, insulating it, ensuring safety, avoiding boil-off.
- Flight-testing under real-world conditions — the HY4 demonstrator had to prove reliability, consistency, and safety over hours in air, not just minutes or ground tests.
Real People, Real Hope
Project coordinator for HEAVEN, Rau, reflected on the significance: “We reached over three hours of flight time in one test, with positive overall results bringing us closer to emissions-free, medium- and long-haul commercial flights.”
There was a mix of relief, pride, and cautious optimism among the engineers. For them, every minute in the air was data, every measurement a promise. The team transferred innovations from the automotive and space sectors into aviation; this cross-pollination meant that ideas long proven elsewhere were tested now under the open sky.
What This Means (And What It Still Doesn’t)
What It Signifies:
- This is not just a lab demo. It demonstrates that liquid hydrogen-electric propulsion is feasible for piloted aircraft in flights of real duration (3+ hours).
- Doubling the range (750 → 1,500 km) opens up possibilities for regional commercial service — flights that today use conventional fuel could someday use zero-emissions hydrogen fuel cells.
- It underlines that engineers are solving real hurdles: storage, safety of cryogenic liquid hydrogen, integration of fuel cells, flight stability over longer durations.
What Remains Challenging:
- Infrastructure: Liquid hydrogen fueling, storage, supply chains, and safety regulations all need global investment and standardization.
- Certification And Regulation: Aviation certification for new fuel types and propulsion systems takes time and rigorous testing.
- Costs: Green hydrogen production, cryogenic storage, and fuel cell systems are still expensive and need to reach commercial viability.
- Scale And Capacity: HY4 is still a demonstrator — commercial aviation will require scaling to larger aircraft and longer routes.
The Most Important Point
This flight proves that real, practical emissions-free flight with liquid hydrogen power is no longer a distant dream — it is now a living, breathing prototype that works. This is the breakthrough moment: from “could this work someday?” to “this works now, let’s scale it.”
It gives hope for cleaner skies, motivates policymakers to invest in infrastructure, and inspires engineers to keep innovating. For passengers, it offers the promise that one day, we may fly without leaving a carbon trail behind.
Looking Ahead With Optimism
In the months since this flight, aviation innovators around the world are building on the progress:
- The HEAVEN consortium continues test flights, gathering data and refining designs.
- Companies like ZeroAvia are working on retrofitting larger planes (up to ~19 seats) with hydrogen electric powertrains.
- Governments and industry leaders are supporting hydrogen in aviation policy, funding green hydrogen production, and exploring certification pathways.
If progress continues, commercial hydrogen-electric flights could be certified within the next decade for regional routes, with longer routes to follow.
Conclusion
Maribor’s clear air and calm horizon may have witnessed just another demonstration flight. But in the context of climate urgency and aviation’s emissions, this was a flight of significance. The HY4’s three-hour dance among the clouds whispered that clean aviation is not only possible — it is underway. And maybe, just maybe, the next time we fly, the plane under us will carry nothing but water and dreams aloft.
Sources:
Zeroavia
H2FLY
Good News Network
