Honeywell hopes that by year-end subsystem tests will be ongoing at multiple locations as an EU-funded project to develop a 1MW-class hydrogen fuel cell powertrain moves towards its final stage in 2026.

But despite progress, Honeywell executives are uncertain of future investment from the EU’s Clean Aviation body to fund flight tests of the system under a second phase.

Project NEWBORN – next-generation high-power fuel cells for aviation applications – led by Honeywell from its European research and technology facility in Brno, Czech Republic, has been running since 2023. Clean Aviation has contributed €33.3 million to its total €43.9 million cost.

NEWBORN’s timeline calls for delivery of a full-scale liquid-hydrogen powertrain – including electric motors and propeller – for ground testing by end-2026, raising the integrated system to technology readiness level (TRL) 4.

As that deadline approaches, Honeywell and its 13 consortium partners have begun a series of critical subsystem performance-validation tests.

“We are actually at a very interesting phase of the project,” Ondrej Kotaba, senior technology fellow at Honeywell, who is overseeing the effort, told reporters at a media event in Brno.

“We are head over heels – like working 12 hours a day right now – testing at three different sites and by the end of the year we will be testing, in parallel, different systems at four different sites.”

Currently being put through their paces are the anode recirculation loop which feeds hydrogen through part of the fuel cell, the air supply working at simulated altitude, and the thermal management system. These are to be brought together towards the end of the year and then tested as an integrated power-generation system.

Also later this year, testing of the electric propulsion system is to begin. This consists of a University of Nottingham-developed megawatt-class electric motor and inverters integrated with power conversion and distribution components from the Fraunhofer Institute, and batteries supplied by Slovenia’s Pipistrel Aircraft.

But production of the motor has been slightly delayed as the first iteration developed an oil leak requiring it to be remanufactured, says Kotaba. “That component is now back on the test bench and currently undergoing testing of the machine itself.”

Testing of the cryogenic hydrogen fuel tank and distribution system is to be underway in early 2026.

Additionally, consortium member Powercell has already validated the performance of its 300kW fuel cell stack. “It fully matched the predictions from the design phase,” Kotaba says.

The stack is one of three that will be used in the complete system to deliver a nominal output of 720-750kW of usable power.

“We are quite certain that 720 is something we will be able to achieve at altitude [and] 750 on the ground is easily achievable,” he says. The altitude targeted is 25,000ft, although the optimal altitude for “many of the platforms” may be slightly lower.

However, with maximum power output needed set at 1MW, NEWBORN has opted to use batteries to make up the 250kW difference, for flight phases with high power demands such as take-off. During cruise, the power-generation system recharges those cells.

The decision to incorporate batteries also allows the fuel cell stack to be designed so it ”can run without a power margin”, Kotaba says, optimising it instead for the majority of flight phases.

While fuel cell stacks themselves are a relatively mature technology – albeit these are significantly more powerful – one of NEWBORN’s goals is to ensure they are suitable for aerospace applications. A key design consideration has been to enable the stacks to run at a higher temperature than normal to facilitate rejection of waste heat.

“This actually works with 100° Celsius coolant to enable as high [as possible] temperature difference between the coolant and the environment,” he says.

As a result, radiators and air intakes can be smaller, cutting weight and drag from the air-cooled system.

Honeywell is using an ethylene glycol-water mix as the coolant but Kotaba says the long-term plan is to switch to an alternative medium.

“We know that we need to do a side project on that to improve the weight of the thermal management system – the system and its coolant are the heaviest parts of the whole [powertrain], not the stack,” he says.

NEWBORN has also opted for a variable-pressure system to increase the durability of the fuel-stack to achieve a 20,000h operating target.

But development of the thermal management system has been complicated by the insolvency last year of original project partner Reaction Engines.

Kotaba declines to speculate on its plans, but in the interim, Honeywell – which is responsible for the overall system integration, control laws, health monitoring and the control hardware – has used commercial off-the-shelf parts.

Reaction Engines’ administrators have been seeking a buyer for its assets, including the intellectual property, and in May disclosed a deal was near to completion.

Although NEWBORN is due to culminate next year with ground tests to take place at Pipistrel’s facility in Gorizia, Italy, project partners are already contemplating how to take the system to flight test in the 2028 timeframe.

To achieve that, further development of components will be required, alongside greater levels of integration, raising the whole system to at least TRL6. In addition, work will be needed to integrate the powertrain into the as-yet undisclosed test aircraft.

Kotaba declines to say whether Pipistrel will also be providing that test aircraft, but images revealed by Honeywell show what he describes as the “targeted first commercial application” – a twin propeller-powered platform with the power-generation system located in the rear of the fuselage.

Additionally, NEWBORN is working with the European Union Aviation Safety Agency to establish a preliminary means of compliance for the system under the regulator’s CS-23 framework for commuter-category aircraft with up to 19 seats. That process should be concluded this year, he says.

A similar agreement for CS-25 transport-category aircraft should follow in 2026, but Kotaba believes a “special conditions” certification framework is someway off, particularly for larger aircraft, due to continued gaps in the data, such as the reliability of fuel cells over the longer term, or material compatibility to extended exposure to hydrogen.

How any test flight is funded will depend on Clean Aviation. The body included no hydrogen-focused topics within its third call for proposals released earlier this year and while the fourth call is being prepared, there is so far no clarity on the direction it will adopt.

Kotaba is confident there “will be a fuel cell continuation” in Clean Aviation’s second phase but says “the question is how exactly the topic will look… And that we don’t know.”

On top of which, Clean Aviation’s focus is on larger regional-class aircraft rather than those with 19 seats or fewer.

“Clean Aviation is clear about supporting only technologies that are directly supporting regional aircraft or larger,” Kotaba says. “Our opinion is that [hydrogen] will not get to CS-25 unless it is first proven in the CS-23 world.”

Although Kotaba sees customer demand, citing “expressions of interest from various airlines” for hydrogen-powered 19-seaters, that does not necessarily translate into public funding.

“Clean Aviation doesn’t fund CS-23, so depending on how the goal is written, we’ll see,” he says.

Nonetheless, he says the powertrain being developed through NEWBORN can be upscaled to supply the roughly 10MW total output required for a 100-seat, 1,000nm (1,850km)-range airliner. Airbus is targeting those range and payload performance parameters from the initial aircraft it has outlined under its ZEROe initiative.