A failed lithium-ion cell can heat neighbouring cells, vent flammable gases and threaten essential aircraft systems. This explainer examines propagation barriers, designed venting, DO-311A and EASA's propulsion-battery test approach, with the limits of early detection and certification evidence.
A thermal runaway event in one lithium-ion cell can expose its neighbours to hot gases, particles and enough heat to trigger further failures. In a propulsion battery, the engineering question is whether those effects can be isolated while the aircraft completes a safe landing. Preventing the initiating fault matters, but the safety case also has to address a cell that fails despite the protections.
The UK Civil Aviation Authority's CAP 3203 report, published in December 2025, reviews thermal runaway hazards and firefighting for eVTOL aircraft. RTCA DO-311A provides a battery-testing baseline. EASA's means of compliance for enhanced-category SC-VTOL propulsion batteries also allows an alternative test method based on protective layers and a justified, realistic worst-case event. The safety objectives include preventing hazardous propagation and protecting the aircraft, with controlled venting where the installation permits it.
A lithium-ion cell stores a lot of energy next to a flammable liquid electrolyte, separated by a thin polymer film. When the cell overheats, whether from an internal short, overcharge, external heating or mechanical damage, a chain of reactions follows. The protective layer on the anode breaks down, the separator softens and can melt, the electrodes short internally, and some cathode chemistries can release oxygen and intensify the reactions. The electrolyte vaporises and the cell vents a hot, flammable mix of gases.
Once the cell's heat generation outruns its cooling, the temperature climbs very quickly. The concern for a pack is what that heat and gas do to the cells around it.
A cell-level failure becomes an aircraft-level hazard when heat, flame or vented material compromises adjacent cells, power channels or essential structure. The risk depends on cell chemistry, module geometry, state of charge and the paths available for heat and gas to escape. The battery installation needs to limit those effects under the failure cases established in its safety assessment.
Thermal barriers, cell spacing and module partitions can slow heat transfer. Designed vent paths route hot gas and particles away from neighbouring cells and vulnerable aircraft systems. The enclosure and its installation must tolerate the resulting pressure and heat without hazardous rupture, fragment release or uncontrolled flame escape. A vent can discharge gases through an approved route; containment does not necessarily mean sealing everything inside the battery.
Barriers, venting hardware, containment structure and sensing add mass that does not store energy. Cell-level specific energy therefore cannot be used as a pack-level performance figure. The available margin depends on how the battery meets its thermal, electrical and structural requirements. PropulsionWatch's battery energy-density explainer covers the same distinction.
RTCA DO-311A is a useful baseline for rechargeable lithium batteries in aircraft, including a thermal-runaway containment test. EASA notes that this test was originally developed for batteries supplying other aircraft systems and may not represent the appropriate worst case for a large propulsion pack. Its MOC VTOL.2440 therefore sets out an alternative approach that tests a representative propulsion battery, its protective layers and its installation against defined failure cases. The site's standards tracker also covers ED-289, ED-312 on cell failure modes, and the FAA's AC 20-184.
Road-vehicle pack tests and aviation compliance tests do not establish the same safety case. An aircraft battery must be evaluated against its certification basis, flight-phase hazards and continued-safe-flight-and-landing requirements. Cell selection studies can inform the design, but they cannot substitute for installed, representative battery testing.
Voltage, temperature, gas and pressure sensing may reveal abnormal conditions before or during a runaway event. CAP 3203 notes that cell venting can precede thermal runaway by up to an hour in some circumstances. That is a possible warning interval, not an assured one: the fault mechanism, sensor location and response logic determine whether the crew or control system receives useful notice.
Battery management systems use voltage, temperature and current data to detect and limit conditions such as overcharge, excessive discharge and overheating. Fast turnaround charging can narrow thermal margins if the cells have not cooled sufficiently after flight. PropulsionWatch's piece on thermal management as a mission problem looks at that cycle.
No test campaign covers every scenario. The CAA report is explicit that extreme cases outside the testing and certification envelope, such as severe crash damage, cannot be ruled out. That shifts part of the safety case to what happens on the ground: rescue and firefighting teams at vertiports need procedures for lithium battery fires, which behave differently from fuel fires and can reignite after they appear to be out.
The evidence required is a representative installation test showing how a credible cell failure develops, where vented products travel and whether essential aircraft functions remain available for the required landing interval. The mass and volume needed to demonstrate that outcome are part of the usable pack-energy calculation.
Evidence note: CAP 3203 is CAA-commissioned research, not an aircraft approval. The testing approaches described here draw on EASA MOC VTOL.2440 and the DO-311A baseline referenced there. This article discusses general design and certification principles, not a verified battery installation from any manufacturer.
Sources: UK CAA, CAP 3203: Battery Thermal Runaway in eVTOL Aircraft, version dated 22 December 2025; EES Batteries (RSC), Key considerations for cell selection in electric vertical take off and landing vehicles: a perspective, 2025; EASA, MOC-3 SC-VTOL Issue 2, MOC VTOL.2440.
