Hybrid propulsion
Hybrid-electric propulsion at the regional aircraft scale introduces a multidimensional engineering challenge: integrating electrical and thermal power sources while satisfying stringent airworthiness requirements under CS-25/Part 25 regulations
In this context, the battery is a vital safety-critical, certifiable subsystem. Recent progress on the RTX Hybrid-Electric Flight Demonstrator underscores this, with H55 delivering a 200kWh aviation-grade Energy Storage System (ESS) that is structurally embedded into the certification strategy of the propulsion system.
The RTX demonstrator, led by Pratt & Whitney Canada and Collins Aerospace, targets up to 30% improvement in fuel efficiency through a parallel hybrid architecture combining a thermal engine and a 1MW-class electric motor. Achieving this level of efficiency gain depends not only on power density, but also on the ability to safely manage high-energy electrical systems under dynamic flight conditions.
H55’s ESS plays a central role by serving as a pre-validated certification baseline. Unlike many experimental battery systems, H55’s architecture has accumulated over 2,000 flight hours and undergone European Union Aviation Safety Agency (EASA) validation campaigns. This operational pedigree enables system integrators to leverage existing compliance evidence, effectively reducing certification risk and programme timelines.
ENGINEERING FOR CERTIFICATION
At the core of H55’s system is a modular, lightweight battery architecture designed explicitly for aviation constraints. The modularity allows distributed placement within the airframe, optimising centre-of-gravity management and structural integration, all of which are key considerations for retrofitting platforms such as the De Havilland Canada Dash 8-100 demonstrator aircraft. More critically, the system is engineered around a “certifiable by design” philosophy. This includes:
- Independent cell characterisation and screening to address variability in commercial lithium-ion cells
- Redundant safety layers, including thermal containment, fault isolation, and controlled failure modes
- Worst-case scenario testing aligned with regulatory expectations, rather than nominal operating conditions
This approach reflects a broader industry realisation: compliance cannot be retrofitted onto high-energy systems. Instead, certification requirements must inform architecture from the outset, particularly in areas such as thermal runaway propagation, electromagnetic compatibility, and system-level fault tolerance.
SCALING FROM CS-23 TO CS-25
A notable aspect of this programme is the scalability of H55’s technology. Originally developed for CS-23 (small aircraft) applications, the same underlying cell-level safety philosophy has been extended to a CS-25 hybrid-electric demonstrator. This transition is substantial, as scaling energy storage systems involves nonlinear challenges in thermal management, structural integration, and failure containment.
For example, increasing system capacity to 200kWh significantly amplifies heat generation and fault energy. Maintaining safety margins requires not just larger systems, but fundamentally robust design principles that remain valid at higher power levels. The RTX demonstrator thus serves as a proving ground for scaling certified architectures into the regional transport category.
CONTEXT WITHIN BROADER INDUSTRY ADVANCES
H55’s progress aligns with parallel developments across the hybrid-electric aviation ecosystem, particularly in high-power electric propulsion and energy systems.
At Fraunhofer IISB, engineers have recently developed a 750kW traction motor using hairpin winding technology, achieving a specific power density of approximately 8kW/kg. This represents a significant improvement over conventional winding techniques, enabling lighter and more compact propulsion units—an essential factor for hybrid-electric aircraft where weight penalties directly impact range and payload.
Similarly, National Research Council Canada is advancing integrated electric propulsion systems through coordinated research in power electronics, thermal management, and system integration. Their work emphasises end-to-end optimisation, including high-voltage distribution architectures and cryogenic or advanced cooling techniques to manage the thermal loads associated with megawatt-class systems.
On the energy storage front, initiatives such as the French-backed development of dual-use battery systems by Ascendance Flight Technologies highlight a growing emphasis on versatility and lifecycle integration. These systems aim to bridge aviation and ground-based applications, improving economic viability while addressing sustainability and supply chain considerations.
TOWARD FLIGHT TESTING
The RTX Hybrid-Electric Flight Demonstrator is now progressing toward full aircraft integration and flight testing. This phase will be critical in validating not just component performance, but also system-level interactions such as thermal coupling between propulsion elements, transient load management, and failure response dynamics.
Flight testing will also provide empirical data to refine certification frameworks, which are still evolving for hybrid-electric architectures. Regulators must address novel failure modes and system interactions that do not exist in conventional propulsion systems, making demonstrator programmes essential for informing future standards.
The RTX Hybrid-Electric Flight Demonstrator illustrates a key inflection point in electric aviation: the transition from experimental systems to certifiable, scalable technologies. By anchoring the propulsion architecture in a validated battery system, the programme reduces technical and regulatory uncertainty, accelerating progress toward commercial viability.
As parallel advances in motors, power electronics, and integrated systems continue, the convergence of these technologies will define the next generation of regional aircraft. In this landscape, energy storage systems will become central to both performance and certification, as well as the broader success of hybrid-electric flight.
