ZeroAvia, the Hydrogen Fuel Cell in a Dornier Two Twenty-Eight, and Why the Cleanest Battery on Earth Is a Tank Full of the Universe's Simplest Gas

ZeroAvia has flown a 19-seat Dornier 228 commuter aircraft with its left engine replaced by a hydrogen fuel cell and electric motor, and the only thing coming out the back is water vapor. The first flight took place in January 2023 at Cotswold Airport in the west of England, and the aircraft has flown repeatedly since. The bet is straightforward: fly airplanes on the lightest element in the universe, and produce zero carbon emissions at the tailpipe.

Why bother with hydrogen at all?

The problem with electric aviation has always been the battery. Lithium batteries are heavy, and - crucially - they don’t get lighter as you use them. When you burn 100 pounds of avgas, your airplane arrives 100 pounds lighter. When you drain a battery, it weighs exactly what it did at takeoff. You haul dead weight the entire flight.

Engineers measure this as energy density: usable energy per kilogram. A good lithium battery pack today delivers around 250 watt-hours per kilogram. Jet fuel delivers roughly 12,000 watt-hours per kilogram - a gap of about 48 to 1 in favor of fossil fuel.

Hydrogen occupies a fascinating middle ground. By weight, it is the best chemical fuel there is - nearly 40,000 watt-hours per kilogram, almost three times the energy of jet fuel pound for pound. Nothing else comes close.

If hydrogen is that good, why aren’t we flying it already?

Because weight is only half the story. The other half is volume, and by volume hydrogen is terrible.

Hydrogen is the smallest, lightest gas in existence. To carry enough of it to matter, you have two options. You can compress it to enormous pressure - 5,000 to 10,000 pounds per square inch in thick, heavy tanks. Or you can chill it to a liquid, which lives at minus 423 degrees Fahrenheit, roughly 20 degrees above absolute zero - the coldest thing you will ever strap to an airframe.

So the fuel is a featherweight, but the tank to hold it is a monster. That is the central engineering tension in the entire hydrogen story, and every serious company in the field is fighting the same battle.

How does a hydrogen fuel cell actually work?

There are two ways to use hydrogen on an aircraft. You can burn it in a modified turbine, roughly the way you’d burn kerosene - the path Airbus has studied closely. Or you can run it through a fuel cell. ZeroAvia chose the fuel cell.

A fuel cell is not combustion - nothing is burning. It is an electrochemical device. Hydrogen feeds in one side, ordinary air the other. Inside, a membrane strips the electrons off the hydrogen atoms and forces them to travel the long way around, through a wire, to reach the oxygen on the far side. That flow of electrons through the wire is your electricity. When the hydrogen and oxygen recombine, they produce one thing: water.

The full chain looks like this:

• Hydrogen plus air enters the fuel cell.
• Electricity comes out and spins an electric motor.
• The motor turns the propeller.
• Warm, clean water vapor exits the tailpipe - no carbon dioxide, no unburned lead, no soot.

You get a clean electric drivetrain and a quiet motor, but you refuel in minutes instead of recharging for hours, and you carry far more range for the weight.

What has ZeroAvia actually flown?

In January 2023, ZeroAvia flew the Dornier 228 - a 19-seat, twin-engine commuter - with its left powerplant replaced by a full hydrogen-electric system: fuel cells, batteries for peak power, and an electric motor driving the propeller. The right engine stayed conventional as a safety backup, because you do not flight-test a brand-new propulsion system without one. The company has continued flying it, accumulating the hours regulators will eventually require.

This was not a stunt. Both the Federal Aviation Administration (FAA) and the UK’s Civil Aviation Authority (CAA) have been working with these programs. The endgame is a certified powertrain that can be bolted onto a real regional airplane carrying paying passengers.

What is ZeroAvia’s business strategy?

The smartest part of the plan is what ZeroAvia is not doing: building a clean-sheet airplane. That path leads to bankruptcy, as the industry has watched happen repeatedly. Instead, the company builds a powertrain - an engine - that retrofits into existing airframes that already hold certification.

Its first product is the ZA600: 600 kilowatts, roughly 800 horsepower, aimed at 9-to-19-seat aircraft like the Cessna Caravan, the Twin Otter, and the Dornier - the workhorses of regional and island flying. A larger system in the 2-to-5-megawatt class targets the 40-to-80-seat regional turboprops, the ATRs and Dash 8s of the world.

Fly short routes first, prove the technology on small aircraft, then scale up. On paper, the roadmap is close to perfect.

What are the real problems with hydrogen aviation?

Problem one: the tank. Compressed-gas tanks are heavy, bulky, and eat into the cabin. Liquid hydrogen offers far better range but demands managing a cryogenic fluid aboard an aircraft that vibrates, pitches, rolls, and sits out in sun and snow. Liquid hydrogen slowly boils off no matter how good the insulation - park the airplane over a long weekend and some fuel has literally evaporated. The industry calls it boil-off, and nobody has fully solved it. ZeroAvia is developing liquid hydrogen storage because compressed gas can’t give bigger airplanes the range they need. It’s necessary, and it’s genuinely hard.

Problem two: where the hydrogen comes from. Today the vast majority of the world’s hydrogen is made by cracking natural gas, a process that dumps carbon dioxide into the atmosphere - so-called gray hydrogen. Fly on gray hydrogen and your tailpipe is clean, but the pollution simply moved to a factory. The environmental case depends on green hydrogen, made by splitting water with renewable electricity. Green hydrogen is currently expensive and scarce, and building enough of it is an entire energy economy’s problem, not one ZeroAvia can solve alone.

Problem three: the airport. You cannot yet taxi up and request a top-off of liquid hydrogen. Every airport on a hydrogen route needs storage, handling equipment, trained crews, and safety procedures for a fuel that is invisible, burns with a nearly invisible flame, and leaks through gaps that would hold gasoline just fine. None of it is impossible - all of it is expensive and takes years.

Problem four: power delivery. Fuel cells excel at steady, cruise-level power but struggle with sudden bursts - and the most power-hungry moment of any flight is takeoff. These systems lean on a battery pack for the takeoff surge, then settle onto the fuel cell for cruise. It’s a sensible hybrid, but it means managing two energy systems, hydrogen and batteries, with the weight and complexity of both. There’s no free lunch - just a different lunch.

When will hydrogen airplanes actually carry passengers?

You will not board a hydrogen airliner for the holidays anytime soon. The near-term play is the small 9-to-19-seat systems. Certifying an entirely new class of propulsion is a long, grinding process - and it should be, because slow and skeptical is exactly what you want from a regulator when people’s lives are aboard.

The realistic window for the first small commercial hydrogen-electric operations is the back half of the 2020s into the early 2030s, and even that assumes certification, tanks, and fuel supply all line up. Large regional airliners running on liquid hydrogen are a 2030s story at the earliest, and likely later.

Why this matters for pilots

Regional aviation is the segment most exposed to pressure to clean up - and also the one that keeps small communities connected: the island route, the mountain town, the commuter hop no big jet will ever serve economically. If hydrogen works anywhere first, it works there.

Batteries and hydrogen aren’t enemies; they’re tools for different jobs. Batteries win for short hops, training, and anything that lands near a charger. Hydrogen wins when you need range, payload, and fast turnarounds - and you’re willing to pay for the tank and infrastructure. The future of clean flight is almost certainly not one chemistry but a mix, matched to the mission, exactly as we already choose between avgas, jet-A, and a glider’s total lack of an engine.

The hardest problem in hydrogen aviation was never the airplane. We can build the airplane - it’s flying right now. The hard part is everything around it: the tank, the fuel, the airport, the rulebook. In aerospace, the machine is often the easy part. The system it has to live inside is what takes the decade.

Key Takeaways

• ZeroAvia flew a 19-seat Dornier 228 on hydrogen fuel cells in January 2023 at Cotswold Airport, with one conventional engine retained as backup - a real, repeated flight, not a rendering.
• Hydrogen carries nearly 40,000 Wh/kg, about three times jet fuel by weight, but is terrible by volume, forcing either 5,000–10,000 psi tanks or cryogenic liquid at −423°F.
• ZeroAvia sells powertrains, not airplanes - the 600 kW ZA600 targets 9-to-19-seat aircraft, with a 2-to-5 MW system planned for 40-to-80-seat regional turboprops.
• The biggest obstacles aren’t the aircraft - they’re hydrogen tank boil-off, the scarcity of green hydrogen, airport infrastructure, and certification.
• First small commercial operations are expected in the late 2020s to early 2030s; large hydrogen regional airliners are a 2030s-or-later prospect.