The core challenge the researchers confronted is one of boundary characterization. Every battery thermal model depends on assumptions about how heat leaves the cell’s surface, and at sea level those assumptions are well rehearsed. Air density, dynamic viscosity, and thermal conductivity are treated as constants or simple functions of temperature, and convective heat-transfer coefficients follow familiar correlations. But when atmospheric pressure drops from 100 kilopascals to 50 kilopascals, the air density falls by roughly half, and the viscosity and thermal conductivity of the gas shift in ways described by Sutherland’s law and related temperature-correction formulations. A simulation that ignores these corrections will systematically misjudge how effectively the surrounding air can carry heat away from the cell, and the error compounds at higher discharge rates when more heat is being generated in the first place.

To build a model adequate to this environment, the team anchored their electrochemical description in the Newman–Tiedemann–Gu–Kim, or NTGK, framework, a semi-empirical approach that represents the battery through parameters related to the equilibrium potential and the equivalent conductance of the cell. The NTGK model is prized in battery engineering because it captures the essential electrochemistry without the computational burden of full porous-electrode theory, making it practical for the multi-scale multi-domain simulations needed to resolve temperature fields across an entire cell. The researchers embedded this electrochemical core within a coupled thermal-fluid simulation, and then made the crucial modification: at the convective heat-transfer boundary, they corrected the air density and viscosity for the actual ambient pressure and temperature, so that the fluid exchanging heat with the battery behaved like the thin air it truly was.

Validation came from experiment. The team compared their simulation predictions against measured battery surface temperatures and found an absolute error of less than 1.5 kelvin and a relative error of less than 5 percent in predicting the maximum surface temperature. The simulated temperature-rise trends tracked the experimental curves closely across the tested conditions. In a field where thermal models of batteries under nonstandard atmospheres often carry uncertainties several times larger, this level of agreement matters. It means engineers can now trust the model’s output when designing thermal management systems for aircraft battery packs or high-altitude installations, rather than treating the simulations as rough qualitative guides requiring extensive physical retesting.

The quantitative findings carry a message that is both reassuring and cautionary. Under identical atmospheric pressure, when the discharge rate increased from 0.5C to 2C, the battery’s temperature rise grew by 7.42 to 8.21 kelvin. That is a substantial thermal penalty for pushing a cell harder, and it confirms that discharge rate is the dominant lever on battery thermal response. By contrast, under the same discharge rate, when atmospheric pressure fell from 100 to 50 kilopascals, the overall battery temperature increased by only 0.49 to 1.37 kelvin. The effect of halving the ambient pressure is real but modest, roughly an order of magnitude smaller than the effect of quadrupling the discharge current.

Perhaps more intriguing is what the pressure change did not do. The temperature difference between the interior and exterior of the battery remained essentially constant at approximately 1.5 kelvin across the tested pressures, showing minimal impact from the reduced atmosphere. This is a technically significant observation. It suggests that low pressure does not dramatically alter the internal thermal gradients within the cell, which are governed mainly by the cell’s own thermal conductivity and the distribution of heat generation. Instead, the primary mechanism by which reduced pressure harms battery thermal performance is external: thinner air convects heat away less effectively, so heat accumulates at the surface and the whole cell runs slightly hotter, but the interior-to-surface temperature difference stays largely intact.

The physics behind this asymmetry is worth unpacking. Convective heat transfer at a battery surface scales with the properties of the boundary-layer air, and as density decreases, the mass flow of air available to absorb and transport heat diminishes even if the flow velocity is unchanged. The specific heat capacity per unit volume of the air drops in proportion to its density, so each cubic meter of thin air carries away less thermal energy than the same volume at sea level. Meanwhile, the heat generated inside the cell, which the NTGK model partitions into irreversible contributions from electrochemical resistance and reversible contributions from entropy change, depends on the discharge current and the cell’s state of charge, not on the ambient pressure. Discharge rate therefore controls the heat input directly, while pressure controls only the efficiency of heat rejection, which explains why the former dominates the thermal response.

For the emerging field of electric aviation, these results arrive at a critical moment. Battery thermal runaway hazards at cruise altitudes have become a focus of intense research, with prior studies documenting how cylindrical and pouch cells behave under the low pressures typical of civil aircraft flight, and how thermal runaway propagates differently in modules at reduced ambient pressure. The new work complements that hazard-focused literature by providing a validated predictive tool for normal operating conditions, the regime in which thermal management systems must function every flight. A designer of an air-cooled battery pack for an electric aircraft can now use the corrected convective boundary conditions to estimate how much extra temperature margin is needed at altitude, and the answer, according to this study, is on the order of one kelvin per halving of pressure at moderate discharge rates, a manageable but non-negligible correction.

The study also speaks to a broader methodological debate in battery modeling. Computational fluid dynamics approaches to battery thermal management have proliferated, covering pouch cells, prismatic cells, and full modules with liquid cooling and phase-change materials. But many of these studies implicitly assume sea-level atmospheres, and the authors’ demonstration that pressure-corrected air properties must enter the convective boundary to achieve sub-1.5-kelvin accuracy is a warning that extends well beyond low-pressure applications. Any simulation of an air-cooled battery system operating in a hot, dry climate, at elevation, or inside a pressurized cabin with altered air composition should apply the same rigor to its boundary conditions. The framework’s modular structure, combining the NTGK electrochemical core with corrected fluid properties, makes such extensions straightforward.

Funded by the National Natural Science Foundation of China and the Natural Science Foundation of Hunan Province, the work from the Hunan University of Technology and its collaborators provides a quantitative foundation for a question that has lingered as batteries have taken to the skies. The takeaway is crisp: if you want to keep a lithium-ion battery cool, the discharge rate you demand from it matters far more than the altitude at which you fly it, but thin air will quietly erode your cooling margin, and now you can calculate exactly by how much. As electric aircraft move from prototypes to passenger service, models like this one, validated against experiment and honest about their boundary conditions, will be part of the safety case that makes that transition possible.

Subject of Research: Thermal behavior and convective heat-transfer modeling of lithium-ion batteries under low-pressure conditions

Article Title: Convective heat-transfer boundary characterization and numerical simulation of lithium-ion batteries under low-pressure conditions

Article References: Zhang, Y., Huang, Z., Liu, S., Jiang, Y., & Shen, S. (2026). Convective heat-transfer boundary characterization and numerical simulation of lithium-ion batteries under low-pressure conditions. Ionics. https://doi.org/10.1007/s11581-026-07553-5

Image Credits: AI Generated

DOI: 10.1007/s11581-026-07553-5

Keywords: lithium-ion batteries, low pressure, convective heat transfer, NTGK model, multiphysics simulation, thermal management, electric aircraft, discharge rate, air density, battery temperature, numerical simulation, heat dissipation

The review examines electrified aircraft propulsion through four interconnected lenses: propulsion architecture, certification frameworks, system safety assessment methods, and the certification-critical technical challenges that emerge when the two former elements collide. This structure reflects a key insight of the analysis: the way a propulsion system is architected fundamentally determines how difficult it will be to certify. An all-electric design with a single high-capacity battery pack presents a fundamentally different regulatory problem than a hybrid-electric arrangement that couples a turbine engine with electric machines, power electronics, and a transmission network, or a turboelectric concept in which gas turbines generate electricity for distributed motors. Each configuration distributes failure modes, redundancy, and power flows differently, and regulators must evaluate each accordingly.

The authors analyze the major configurations in detail. All-electric propulsion, exemplified by NASA’s X-57 Maxwell program, replaces fuel with batteries and electric motors, promising zero in-flight emissions and radically simplified drivetrains. Hybrid-electric propulsion blends battery and turbine power, offering extended range while retaining some of the operational flexibility of conventional aircraft. Turboelectric systems use generators driven by gas turbines to supply electric motors without onboard energy storage, sidestepping some battery limitations at the cost of added conversion losses. Distributed electric propulsion, or DEP, spreads many smaller motors across the wing or airframe, enabling aerodynamic benefits such as higher lift coefficients and shorter takeoff distances, but introducing complex fault-tolerance questions when any single motor or its power electronics fails. The review shows that certification complexity grows in a strongly architecture-dependent way: DEP, for instance, requires demonstrating that the aircraft can safely tolerate motor failures and power distribution faults across an interconnected network, something conventional single-engine certification rules never anticipated.

On the regulatory side, the review surveys the approaches of the three major airworthiness authorities: the European Union Aviation Safety Agency (EASA), the Federal Aviation Administration (FAA), and the Civil Aviation Administration of China (CAAC). EASA has pioneered special conditions for electric and hybrid propulsion systems, including Special Condition SC E-19 for electric and hybrid propulsion systems and SC-VTOL-02 for small-category VTOL-capable aircraft. The FAA has issued advisory circulars for powered-lift type certification and maintains standards for rechargeable lithium cells and batteries under TSO-C179b, drawing on RTCA’s DO-311A minimum operational performance standards. CAAC has published special conditions for unmanned aircraft systems and, notably, has released airworthiness standards for powered-lift aircraft. These frameworks share a common philosophy: because existing regulations were written around combustion engines, novel propulsion requires applicants to demonstrate, through special conditions and means of compliance, that equivalent levels of safety are achieved even where prescriptive rules do not yet exist.

At the heart of the safety substantiation process lies system safety assessment, formalized in industry standards such as SAE ARP4761A for safety assessment and ARP4754A for civil aircraft and systems development. These methodologies require identifying failure conditions, classifying their severity from catastrophic to minor, and demonstrating that the probability of catastrophic events falls below stringent numerical thresholds, typically on the order of one in a billion flight hours. The review highlights how model-based safety analysis and newer techniques such as System-Theoretic Process Analysis (STPA) are being integrated into this process to handle the systemic and software-driven failure modes that electrified architectures introduce. Digital twin technology, which creates high-fidelity virtual replicas of physical systems, is emerging as a complementary tool, supporting condition monitoring, prognostics and health management, and fleet-level surveillance of battery and electric drivetrain health throughout an aircraft’s service life.

Battery safety emerges from the review as perhaps the single most consequential certification-critical challenge. Lithium-ion cells can undergo thermal runaway, a self-accelerating exothermic failure in which a overheating cell ignites neighboring cells and propagates through the pack. In an aircraft, where batteries constitute a large fraction of the vehicle’s mass and sit close to passengers and critical structure, a thermal runaway event is treated as a design condition that must be contained. EASA has published dedicated guidance on thermal runaway for propulsion batteries, and the review traces the mitigation hierarchy: preventing cell-level failure through cell chemistry and quality control, arresting propagation through physical barriers and thermal management, and ensuring the aircraft can tolerate a complete loss of one battery segment. Recent research on battery thermal runaway prevention and on safety assessment for propagation mitigation, cited in the review, shows that containment design must be substantiated analytically and experimentally, a demanding requirement for packs large enough to propel transport-category aircraft.

High-voltage integration poses its own distinct set of hazards. Electrified aircraft will operate power networks at voltage levels far above those in conventional avionics, in an environment subject to altitude-induced low pressure, temperature extremes, vibration, and electromagnetic interference. Insulation degradation, partial discharge, arcing, and electromagnetic compatibility failures all threaten both the propulsion system and neighboring aircraft systems. The review points to environmental qualification standards such as RTCA DO-160G and software and hardware assurance frameworks including DO-178C and DO-254 as the established means of demonstrating that power electronics, controllers, and cabling can survive the airborne environment and that their software performs to the assurance levels that catastrophic failure conditions demand. Fault tolerance adds another layer: propulsion systems must detect failures rapidly and reconfigure, and in distributed architectures the loss of a motor changes not only thrust but flight dynamics, requiring coordinated control allocation strategies that themselves must be certified.

A recurring theme in the review is that architecture decisions made at the conceptual design stage ripple forward into every certification document an applicant will ever file. Redundancy provisions, segregation requirements, failure detection latency, and the sheer number of system interfaces all scale with architectural complexity. The authors therefore advocate architecture-aware certification strategies, in which certification implications are evaluated early and systematically as part of the design process rather than retrofitted after an aircraft configuration is frozen. They argue that this integration of certification thinking into architecture selection is now the primary challenge facing the sector, more so than improving any individual component, because today’s batteries, motors, and power electronics are increasingly technically viable while the pathway to proving whole-system safety remains immature and expensive.

The stakes of solving this challenge are considerable. International commitments, including the International Civil Aviation Organization’s long-term aspirational goal for international aviation, and the rapid growth of the advanced air mobility sector depend on certifiable electric propulsion. Programs such as NASA’s X-57, which documented its electric aircraft certification and airworthiness approach in detail, offer early lessons on how novel propulsion can be substantiated under existing frameworks, and scaled flight testing of distributed propulsion concepts continues to generate evidence on failure detection and control reconfiguration. But the review’s message is sobering and, at the same time, constructive: electrified flight will reach the market not simply when the technology works, but when the industry and regulators together can demonstrate, architecture by architecture, that it is safe. Bridging that gap, the authors conclude, will require certification strategies designed hand in hand with the propulsion architectures of the future.

Subject of Research: Airworthiness certification and safety assessment of electric and hybrid-electric aircraft propulsion systems

Article Title: Airworthiness certification and safety assessment of electric and hybrid-electric aircraft propulsion systems: architectures, frameworks, and critical challenges

Article References: Wang, X., Cao, A., Liu, Y., Hong, L., Zhou, B., Yuan, X., Liu, B., Bao, W., & Guo, J. (2026). Airworthiness certification and safety assessment of electric and hybrid-electric aircraft propulsion systems: architectures, frameworks, and critical challenges. Aerospace Systems. https://doi.org/10.1007/s42401-026-00526-6

Image Credits: AI Generated

DOI: 10.1007/s42401-026-00526-6

Keywords: electric aircraft, hybrid-electric propulsion, airworthiness certification, system safety assessment, distributed electric propulsion, battery thermal runaway, high-voltage integration, fault tolerance, EASA, FAA, CAAC, digital twin