Has the time come for the MRO market to back hydrogen-electric propulsion as the sustainable solution to powering green aviation, or should it keep its options open?

From the flygskam movement shaming people who travel too often, to concerns about celebrities flying their private jets off to promote environmental issues, the pressure is increasing on the aviation industry to do something about its carbon footprint.
2050 has been marked by many countries as a year to achieve net-zero emissions, but aviation has another benchmark in its sights. 
By the midway point of this century, an energy.transitions.org report estimates aviation could be responsible, if unmitigated, for as much as 22 percent of climate impact

Batteries' struggle with jet propulsion

It's clear aviation needs to use green fuel, or at least find an alternative way to power aircraft through the sky that does not emit CO2, NOx, contrails, and particulates.
In this day and age, nothing can be ruled out – the tech that enables us to fly cleanly in the future might not have been conceived yet – and as things stand, there are three strong candidates to replace the polluting fuels that currently power the aircraft:
  • Green hydrogen fuel derived from Power-to-X
  • Lithium-ion batteries
  • Hydrogen-electric engines
Green fuel is a nice idea, but the Power-to-X technology converting alternative energy into liquid fuel is still at a nascent stage. Denmark's target to produce enough green hydrogen to fuel all its domestic flights by 2030 illustrates the gigantic scale that would be needed to power all aircraft in the future.
Likewise, lithium-ion batteries have a long way to go. Over the last decade they have enabled us to increasingly drive electric cars, scooters and bicycles – to the extent they power 1 percent of all passenger cars in Europe – but in order to power aircraft they need to be incredibly heavy. Testing began in 2019, but no major strides have been made yet.
Considerable progress, however, has been made with electric engines. Last year, Canadian-based company Duxion Motors reported a successful test of its eJet Motor, the world’s first rim-driven jet propulsion motor​​.
Also last year, engineers at MIT created a 1MW motor of a weight and size comparable to current small aero-engines. The creation would be best suited to hybrid propulsion systems: electric and traditional, or electric and hydrogen.

Good vibrations on maiden flight

The potential of hydrogen-electric engines is so great that an ambitious timeline has already been drawn up.
'A hydrogen-electric engine can harness 60 times more energy than lithium-ion batteries – and for a significantly lower cost too. 
In September this year, a 40-seater aircraft powered by a hydrogen electric engine took to the skies in the US state of Washington – a successful 15-minute flight carried out by Universal Hydrogen that reached a height of 3,500 metres and performed with greatly reduced noise and vibrations. 
Universal Hydrogen, a Californian-based company founded in 2020, is one of several making serious headway in the sector – and they already have an order to convert 75 aircraft for US carrier Connect Airlines, which commented after the flight: 
“We are committed to being North America’s first zero-emission airline. This historic flight – taking hydrogen, which can be made with nothing but sunshine and emitting only water – is a key milestone on our journey.”

Big commitment from Germany to trials

Not to be outdone, Europe also staged hydrogen-electric trials this year – H2FLY, a German company based in Stuttgart, oversaw four flights taking off in Maribor, Slovenia.
The H2FLY aircraft uses liquid hydrogen to power a hydrogen-electric fuel cell system. Using hydrogen gas, the aircraft had a range of 750 km, but this doubled using cryogenically-stored liquid hydrogen (LH2), which enables lower tank weights and volume, thus increasing the range and useful payload.
A HY4 demonstrator aircraft was fitted with a hydrogen-electric fuel cell propulsion system powered by LH2. The longest of the four flights lasted three hours, prompting H2FLY co-founder Professor Josef Kallo to comment:
"This achievement marks a watershed moment in the use of hydrogen to power aircraft. Together with our partners, we have demonstrated the viability of liquid hydrogen to support medium and long-range emissions-free flight. We are now looking ahead to scaling up our technology for regional aircraft and other applications, beginning the critical mission of decarbonising commercial aviation.
Some pretty major German players are funding the development, including the federal ministries for economic, digital and transport affairs.  

Timeline extends over next two decades

Nevertheless, the timelines outlined by hydrogen-electric aircraft developers do not envisage the aircraft replacing current fleets anytime soon.
For example, ZeroAvia, a British-American developer founded in 2017, has plans to start retrofitting existing aircraft with its hydrogen-electric powertrains in 2025 – starting with aircraft of 9-19 seats and a range of 300 nautical miles.
These will be followed by larger models in 2027 (40-80 seats; 1,000 NM), 2029 (100-200 seats; 2,000 NM) and 2032 (200 seats; 3,000 NM). But it is only by 2040 that it expects to retrofit an aircraft capable of intercontinental travel (200+ seats; 5,000 NM). 
Still, continental travel could be very different within a decade if development continues at the same rate as today. 
Other airlines taking major strides in the sector include: Ecojet, Alaska Airlines, Avolon, Delta Airlines, EasyJet, GOL, Icelandair, Japan Airlines, JetBlue, SAS, United Airlines, Vertical Aerospace, Virgin Atlantic, Viva Aerobus and Wideroe.

Future retrofitting scale is impressive

The retrofitting plans of ZeroAvia and others is of huge significance to the aviation supply chain industry. Already OEMs are showing a huge interest in getting involved.
After all, the retrofits are so much more than just replacing an engine and powertrain, as a power generation system that converts hydrogen into electricity needs to be installed along with hydrogen tanks. The nacelles and avionics both need modification too. 
The Aerospace Engineering Center department at the MHI RJ Aviation Group (MHIRJ) confirmed to aviationweek.com it will be providing MRO expertise to ZeroAvia – particularly in regard to engineering services, aircraft integration and certification support:
“We’re basically going to dovetail into ZeroAvia’s road map and strategic view to support them on the technical intricacies of being able to take a hydrogen concept and . . . industrialise and scale that so that, operationally, it becomes viable.”
The future scale of the retrofits has been compared by some in the industry to the conversions of passenger aircraft into freighter aircraft. 

Emerging tech presents safety concerns

Universal Hydrogen's modifications specialist is AeroTEC, which has set up a Hydrogen Aviation Test and Service Center at Grant County International Airport in Washington. It revealed to aviationweek.com:
“The area that would change the most is the powertrain, because you’re taking a turboprop engine, which has a clear set of inspections and maintenance activities, and you’re replacing it with a completely different type of powertrain, which is based on electric motor and fuel-cell technology.”
Safety has become a huge concern, as the liquid hydrogen cryo-tanks require cooling to -250C while the high-voltage electronics require 800-1,000 volts of electricity – more than enough to kill a worker who doesn’t follow procedure:
“We’ve learned a lot about high-voltage electronics, battery systems, battery management, battery cooling, propulsion, motor cooling and things that you don’t think about.”

Refuelling efficiency a big challenge

Another MRO making moves is Lufthansa Technik (LHT). In 2021, it launched a project to design and test hydrogen tech processes – and it is thinking big from the onset! 
It transformed a decommissioned A320 into what it describes as a “stationary laboratory”, where it is testing out internal cryo-tanks for liquid hydrogen, a fuel-cell system and a conditioning system. It is also considering the practicality of refuelling, repairs and potential incidents.
Additionally, it has created a digital twin to research digital health monitoring and predictive maintenance, and additional research is being carried out into ground infrastructure such as storage for liquid or gaseous hydrogen. 
However, progress brings more problems, LHT told aviationweek.com. For example, refuelling a hydrogen tank is a time-consuming process:
“How can we speed up refuelling in the future to the point where it enables practicable turnaround and handling times on the ground? And how do we avoid overfilling or wasting hydrogen during refuelling?”

Breakthrough needed for larger aircraft

Nevertheless, there are some MRO positives – experts believe the switch to hydrogen solutions could simplify engine maintenance, not complicate it. 
They predict reduced wear and tear – primarily due to a significant lowering of engine part temperatures. Reduced combustion and fewer rotating components play a part in this.
But the technology isn’t in place at the moment for larger plans, the experts concur – particularly in regards to cooling hydrogen for use in a combustion engine. A breakthrough is needed.
A Russian experiment in the 1980s, the Tupolev Tu-155, encountered the same problem. The result was an aircraft filled with tubes and refrigeration units with no room for seats, let alone passengers. 
This missing link in the future development of hydrogen-electric tech could leave the door open for green fuels that power conventional engines.
But for now, hydrogen-electric tech is still very much on its ascent, and aviation supply chain operators are best advised to keep it in the forefront of their future planning.


Like all sustainable means of powering aircraft, hydrogen-electric engines have a little turbulence to smooth out, but nothing that can't be fixed, concur most experts. Compared to batteries and green fuel, they present the most viable solution to making aviation green. It won't happen anytime soon, but if the industry gets onboard, it's a feasible target to work towards in 30 or 40 years' time.
Cover image credits: DLR/Fabian Walker/Creative Commons