The Electrification of Aviation: Systems Engineering Challenges in Hybrid and Electric Propulsion

Commercial aviation has run on essentially the same propulsion physics for sixty years. Turbofan engines burn fuel, produce thrust, and bleed off pneumatic and hydraulic power to run the rest of the aircraft. The subsystem boundaries that aerospace systems engineers work within—propulsion, electrical, hydraulic, flight controls, environmental control—reflect that architecture. They were drawn deliberately. They stayed clean because the power sources stayed isolated.

Electric and hybrid-electric propulsion dissolve those boundaries. When the same energy source—a battery pack or a combined generator/battery bus—drives motors, powers avionics, provides thermal load to the cooling system, and feeds back energy during descent, the interfaces between subsystems multiply faster than most engineering teams have tooling to track. The challenge is not purely electrical. It is a systems integration problem of a scope that civilian aviation has not faced since fly-by-wire replaced mechanical control linkages.

This article examines where that complexity is concentrated, what new categories of failure it introduces, and how engineering teams are adapting—or struggling to adapt—their processes.

The Subsystem Boundary Problem

In a conventional turbofan aircraft, the propulsion system’s primary interface to the rest of the aircraft is mechanical thrust and a defined set of bleed air and shaft power offtakes. The electrical system is downstream. It consumes power; it does not generate propulsion. Flight controls interact with the engine through a relatively narrow channel: thrust commands, FADEC inputs, and a handful of protection functions.

A hybrid-electric architecture changes this in multiple directions at once.

The electric motor is simultaneously a propulsion device and a power sink. The battery pack is simultaneously an energy reservoir, a structural element (it has mass and location that affects CG), a thermal source, and—in failure cases—a fire hazard. The power electronics that connect them introduce switching dynamics that couple into the avionics power bus. The regenerative capability during descent creates energy flows that run backward through the system, which no legacy hydraulic or bleed-air architecture had to accommodate.

The result is that propulsion, electrical, thermal, structural, and flight control requirements are no longer separable at a meaningful architectural level. A requirement on battery discharge rate has thermal implications, which has cooling system implications, which has structural implications for the cooling loop routing, which has weight implications, which feeds back to the propulsion requirement. That chain of dependency exists whether or not your requirements management system can represent it. If it cannot, the dependency lives in someone’s head or in a comment in a spreadsheet, and it breaks when that person leaves the program.

The interface count reflects this directly. Programs working on hybrid-electric regional aircraft have reported interface requirement sets two to three times larger than comparable conventional aircraft programs at equivalent development stages. The interfaces are also qualitatively different—they carry bidirectional dependencies and time-varying behavior that a static interface control document was never designed to express.

Thermal Management as a First-Class Architecture Decision

In a turbofan aircraft, thermal management is real engineering work, but it operates in a relatively contained domain. Bleed air temperatures, nacelle cooling, avionics bay ventilation—these are hard problems, but they are largely local. The heat sources are concentrated and their locations are fixed by the propulsion architecture.

Electric propulsion distributes heat sources throughout the aircraft. Battery packs generate heat as a function of discharge rate and state of charge. Power electronics generate heat as a function of switching frequency and load. Electric motors generate heat as a function of torque and speed. All of these vary continuously during flight, and their thermal outputs interact with ambient conditions, airspeed, and altitude in ways that require coupled modeling to characterize.

The thermal management system must now satisfy requirements that come from at least four domains simultaneously:

Battery thermal limits. Lithium-based cells have both a minimum operating temperature (below which capacity drops and charge acceptance degrades) and a maximum safe temperature (above which runaway propagation becomes a design-basis event). The thermal system must hold the pack within this window across the entire operating envelope, including ground operations in hot climates, high-altitude cruise in cold ambient, and high-power climb profiles.

Power electronics reliability. Semiconductor junction temperatures directly determine device lifetime and failure probability. Thermal requirements on the inverter and motor controller are therefore reliability requirements and safety requirements simultaneously.

Cabin comfort and ECS interaction. The environmental control system is no longer working from essentially infinite bleed air enthalpy. It is competing for cooling capacity with the propulsion system. On a hot-day, max-power departure, these demands peak simultaneously.

Structural considerations. Cooling loop routing affects structural cutouts, weight distribution, and maintenance access. Thermal expansion of battery enclosures under cycling loads creates interface requirements with the airframe attachment structure that did not exist in any prior design.

Systems engineers who worked on conventional aircraft and moved to electric propulsion programs consistently describe thermal as the domain that broke their existing processes first. It is not that thermal analysis is new. It is that the coupling between thermal and every other domain means that a change anywhere propagates through thermal before it stops.

Battery Certification: Requirements Without Precedent

Certification of lithium-ion battery systems for primary propulsion in type-certified aircraft is, as of mid-2026, still being worked out between manufacturers and regulators in real time. The FAA’s Special Conditions process for large battery installations has produced useful structure, but it has not converged on a stable set of means of compliance that teams can build requirements against with confidence.

This creates a specific systems engineering dysfunction: requirements are being written against regulatory targets that may move before the aircraft reaches certification. Teams working on Part 23 and Part 25 electric aircraft programs have described building requirements hierarchies where the top-level certification requirements carry explicit uncertainty flags—a practice that has no clean analog in the structured requirements methods taught in most aerospace programs.

The substantive challenges cluster around several areas.

Thermal runaway propagation. A single cell entering thermal runaway is a manageable design-basis event. Runaway propagating through a module or pack is a different failure mode category entirely—one that involves chemical, thermal, and electrical coupling between cells that can defeat passive containment strategies within seconds. Demonstrating that propagation is contained requires test campaigns that are still being defined at the method level. The boundary between “analysis with test validation” and “test-demonstrated” is not yet clearly established for all configurations.

State of charge and state of health prediction. An aircraft must know its available energy with enough accuracy to guarantee legal reserves. For a battery pack that has been through hundreds of cycles and has cell-to-cell variation from manufacturing and thermal history, characterizing available energy with aviation-grade confidence requires algorithms that are themselves certifiable software artifacts. This adds a software certification dimension to what looks like a hardware problem.

Fault detection and isolation. Battery management system (BMS) architecture must satisfy detection and isolation requirements derived from the aircraft-level safety assessment, but the failure modes of large-format lithium cells under aviation stress profiles are still being characterized empirically. Requirements written today may prove to be either under-conservative or unachievable as test data accumulates.

The systems engineering implication is that battery-related requirements need to be managed with explicit version control, dependency tracking, and change impact propagation. When a regulator updates a special condition or a test reveals a new failure mode, every downstream requirement that derived from the original assumption needs to be found and reassessed. In a document-based system, that sweep is done manually and incompletely. In a traceable, model-connected system, it is at least mechanically tractable.

New Failure Mode Categories

Traditional aviation safety analysis assumes that propulsion failures are discrete: an engine either produces commanded thrust or it does not, and the transition is rapid enough to be modeled as instantaneous. Partial thrust loss exists in the design space—one engine out of four—but within a given engine, the failure modes are largely binary at the function level.

Electric propulsion introduces failure modes that are neither binary nor instantaneous, and some of them have no direct precedent in aviation safety analysis.

Partial power degradation. A battery pack losing capacity due to low temperature or accelerated aging does not fail to zero thrust. It produces a continuously varying available power curve that depends on state of charge, current demand, and thermal state simultaneously. The flight management and flight control systems must respond to an available thrust envelope that changes shape during the maneuver, not just at its start.

Regenerative braking interactions. During descent, a regenerative motor configuration feeds energy back to the bus. If the battery is near full state of charge—as it would be after a descent from cruise on a short-haul route—this energy has nowhere to go. The system needs a controlled dissipation path, and the failure mode of that path interacts with both the flight control response and the thermal management system.

Power electronics faults with partial consequence. An inverter phase fault may degrade motor torque output without eliminating it. Modeling this for safety analysis requires extending FTA and FMEA methods to accommodate graded failure states, which the standard methods do not handle naturally.

Common-cause thermal events. In a conventional aircraft, an engine fire and an avionics bay overheat are unrelated events. In an electric aircraft where both are connected to the same battery pack and cooling loop, they may share causes or propagate through each other. The independence assumptions that underlie standard safety analysis methods need to be explicitly validated rather than inherited from prior architecture.

How Teams Are Adapting

The engineering teams making the most progress on these challenges share a set of practices that distinguish them from teams still struggling.

Model-based requirements from the start. Programs that began with a system model—even a simplified one—as the primary artifact for capturing requirements and their relationships have been better able to track the interface density that electric propulsion creates. The model forces interfaces to be declared explicitly. It does not allow an interface to live in a comment or a hallway conversation.

Deliberate thermal architecture reviews. Teams that treat thermal as a cross-cutting architectural concern, reviewed at every major system design review, have been more successful at catching thermal-driven requirement conflicts before they reach the hardware phase. Programs that left thermal to a subsystem team and reviewed it in isolation have consistently reported late-breaking conflicts that required architecture changes.

Explicit uncertainty management in requirements. For battery certification requirements specifically, leading programs are annotating requirements with their certification basis confidence level, tracking which requirements derive from stable regulatory text versus Special Conditions that may change. This is a process adaptation that most requirements management toolchains do not support natively—teams are building it on top of their existing tools or moving to tools that can represent requirements metadata more flexibly.

Interface requirements as first-class artifacts. The explosion in interface requirements between electrical, thermal, propulsion, and flight control domains has pushed several programs to treat interface requirements with the same rigor as system and subsystem requirements—full traceability, allocated owners, formal verification planning. This is not universally practiced even on conventional programs, but the interface density of electric propulsion makes the cost of not doing it visible quickly.

Tools built around graph-based requirement models rather than flat document hierarchies are showing practical advantages here. Platforms like Flow Engineering, which represent requirements and their relationships as a navigable graph rather than a document tree, allow engineers to ask which thermal requirements are affected by a change in the battery discharge profile—and get an answer that the tool derives from the model rather than from a manual search of hundreds of requirements documents. For programs where interface requirement sets run into the thousands and cross-domain dependencies are dense, that capability is not a convenience feature. It is the difference between a change impact sweep that takes two hours and one that takes two weeks.

Honest Assessment

Aviation electrification is real, the regulatory frameworks are maturing, and several programs are on credible paths to certification. The systems engineering challenges described here are not arguments against electrification. They are arguments for taking the systems integration complexity seriously from the start of a program, not as something to be resolved during detailed design.

The teams running into the hardest problems are those that imported their conventional aircraft development processes without modification and discovered mid-program that those processes were designed for a lower interface density than electric propulsion produces. The requirement traceability problem is not subtle—it becomes visible, loudly, when a late-phase change in battery thermal limits requires a manual search through thousands of requirements to find everything that has to change.

The teams making the most progress have invested in process adaptation before they needed it. That investment is not free. It requires tooling changes, training, and often a period where the new process feels slower than the old one. For programs developing novel propulsion architectures under certification pressure, that investment is justified by the cost of the alternative.

Electric aviation will happen. The question for systems engineers is whether their processes and toolchains will be ready when it does.