Safety Red Line: Thermal Runaway Risks and Aviation Safety Regulations for the 25kg Air Transport Limit on Lithium Batteries

In 2010, UPS Airlines Flight 6 crashed over Dubai due to a lithium battery fire in the cargo hold, killing all crew members; in 2022, a Qatar Airways cargo plane made an emergency landing after a lithium battery shipment in its hold suffered thermal runaway, resulting in complete destruction of the cargo hold. These tragic accidents have made the global aviation industry acutely aware that lithium batteries have become one of the most high-risk goods in air transport. To curb risks, the International Civil Aviation Organization (ICAO) introduced regulations as early as 2009, limiting the weight of lithium batteries in single or consolidated packages for air transport to 25 kilograms. This “safety red line” has since become a universal standard for global air logistics. However, with the rapid development of the new energy industry, demand for lithium battery transportation has surged. Some enterprises attempt to bypass the weight limit for efficiency, and new risks brought by battery technology iterations have compounded the challenges, leaving lithium battery air transport safety still under severe threat. Deeply understanding the thermal runaway risk logic behind the 25kg limit and strictly enforcing aviation safety regulations are crucial to ensuring air transport safety.

I. The 25kg Air Transport Limit: A “Safety Firewall” Born from Accidents

The weight limit for lithium battery air transport is not arbitrarily established but a scientific conclusion based on in-depth analysis of numerous accident data and risk assessments. Behind the 25kg standard lies the aviation industry’s adherence to the bottom line of “controllable risks,” with the core goal of minimizing hazards caused by lithium battery thermal runaway.

(1) Origin of the Weight Limit Policy: Regulatory Upgrades Driven by Accidents

Since the 1990s, with the popularization of electronic products such as laptops and mobile phones, the volume of lithium battery air transport has increased annually, accompanied by a surge in related safety accidents. In 1999, a United Airlines passenger plane made an emergency diversion due to a short circuit and fire of lithium batteries carried in the cargo hold; in 2006, multiple cargo flights were forced to return due to smoking and fire incidents involving lithium battery shipments. These accidents attracted high attention from ICAO. After three years of risk research, ICAO explicitly stated in the 2009 edition of the Technical Instructions for the Safe Transport of Dangerous Goods by Air (TI) that “the weight of lithium-ion batteries in single packages shall not exceed 35kg, and in consolidated packages shall not exceed 25kg.” In 2011, it further restricted the weight of single packages to 25kg and required all lithium battery shipments to be marked with transport classes of “Cargo Aircraft Only” or “Passenger and Cargo Aircraft Allowed.”

This policy was formulated based on a critical risk insight: when the weight of lithium batteries exceeds 25kg, the energy and heat released in case of thermal runaway will exceed the control capacity of the aircraft cargo hold fire suppression system. According to simulation experiments by the U.S. Federal Aviation Administration (FAA), when a 10kg lithium battery undergoes thermal runaway, the cargo hold fire suppression system can control the fire within 5 minutes; for a 25kg lithium battery, it takes 15 minutes to suppress the fire; while for a 50kg lithium battery, the fire will breach the cargo hold firewall within 8 minutes, triggering full aircraft combustion. Thus, 25kg becomes the critical point between “controllable” and “uncontrollable” by fire suppression systems.

(2) Global Coordination of Weight Limit Policies: From International Standards to National Implementation

ICAO’s 25kg weight limit standard has been transformed into mandatory national regulations worldwide through the Chicago Convention. In 2010, the Civil Aviation Administration of China (CAAC) issued the Regulations on the Air Transport of Lithium Batteries, fully adopting ICAO standards and refining operational requirements for lithium battery packaging, marking, and declaration; the European Union incorporated the 25kg weight limit into its unified regulatory framework through the European Regulation on the Transport of Dangerous Goods by Air; the U.S. FAA stipulated in Part 121 of the Federal Aviation Regulations that airlines violating lithium battery weight limit regulations will face fines of up to $250,000, and freight forwarders will have their operating qualifications revoked.

To ensure policy implementation, countries have established a full-chain regulatory system covering “pre-approval, in-process inspection, and post-incident accountability.” For example, CAAC requires airlines to verify the “Lithium Battery Transport Appraisal Report” submitted by freight forwarders before accepting lithium battery shipments to confirm compliance with weight limits; airport security departments use a combination of X-ray machines and unpacking inspections to verify consistency between the actual weight of lithium battery shipments and declarations; any overweight shipments are immediately detained, and non-compliant enterprises are included in a credit blacklist. In 2023, CAAC investigated and handled 127 cases of overweight lithium battery transport, detaining a total of 3.2 tons of shipments, effectively curbing violations.

II. Thermal Runaway: The “Primary Culprit” of Lithium Battery Air Transport Risks

The core risk targeted by the 25kg weight limit is the unique “thermal runaway” risk of lithium batteries. This chain reaction triggered by uncontrolled internal chemical reactions in batteries is characterized by “sudden onset, rapid heating, and severe harm,” and its risks are further amplified in the special environment of air transport.

(1) Mechanism of Thermal Runaway: From Internal Failure to Chain Reaction

Thermal runaway in lithium batteries is essentially a process of “uncontrolled energy release,” usually triggered by three major factors: internal short circuits, overcharging, and mechanical damage. Taking lithium-ion batteries as an example, their internal structure consists of a positive electrode, negative electrode, electrolyte, and separator. During normal operation, lithium ions migrate orderly between the positive and negative electrodes; when the battery suffers mechanical damage such as extrusion or puncture, the separator ruptures, causing direct contact between the positive and negative electrodes and forming an internal short circuit. The Joule heat generated by the short circuit raises the battery temperature to over 150℃ within 1-2 seconds; high temperatures cause electrolyte decomposition, producing flammable gases such as hydrogen and carbon monoxide. When gas pressure exceeds the bearing limit of the battery casing, the battery swells and ruptures, and the leaked flammable gases mix with air to form a combustible mixture; meanwhile, the positive electrode material releases oxygen at high temperatures, further supporting combustion, eventually leading to ignition or even explosion.

More dangerously, thermal runaway exhibits a “domino effect.” When one battery cell undergoes thermal runaway, the released high temperature and flames heat adjacent cells, triggering more cells to fail and forming a “chain reaction.” Experimental data shows that when a 25kg lithium battery pack (composed of 10 2.5kg cells) undergoes thermal runaway, it takes only 3 minutes from the ignition of the first cell to full combustion of the entire pack. The released heat is equivalent to that generated by burning 20 liters of gasoline, sufficient to melt the aluminum alloy structure of the aircraft cargo hold.

(2) Risk Amplification in Air Transport Scenarios: Environmental Factors Exacerbate Hazards

The high-altitude, low-pressure, and vibrating environment of air transport further increases the probability and severity of lithium battery thermal runaway. During high-altitude flight, the air pressure in the aircraft cargo hold is only 60%-70% of that at ground level, making gases generated by thermal runaway inside lithium batteries more prone to expansion and rupture, accelerating gas leakage and combustion; meanwhile, although oxygen concentration decreases in low-pressure environments, oxygen released by the positive electrode during lithium battery thermal runaway can independently support combustion without extinguishing due to oxygen deficiency.

Vibration and impact during transportation are also important risk triggers. Severe vibrations during aircraft takeoff and landing may cause loosening of lithium battery packaging and poor electrode contact, leading to internal short circuits; if shipments are not securely fixed, lithium batteries may collide in the cargo hold, directly causing mechanical damage and triggering thermal runaway. In addition, temperature fluctuations in the cargo hold affect lithium battery stability—when aircraft pass through clouds, cargo hold temperature may drop from 20℃ to -10℃ in a short time. Sudden temperature changes cause electrolyte freezing and volume expansion inside batteries, damaging the separator structure and laying hidden dangers for thermal runaway.

(3) Risk Differences Among Lithium Battery Types: From Consumer-Grade to Industrial-Grade

Not all lithium batteries pose the same risks; thermal runaway risks vary significantly among different types and states of lithium batteries. In terms of battery type, lithium-ion batteries carry higher thermal runaway risks than lithium metal batteries because lithium-ion batteries use organic liquid electrolytes that are more flammable, while lithium metal batteries typically use solid or gel electrolytes; in terms of application scenarios, industrial-grade lithium batteries (such as electric vehicle batteries and energy storage batteries) are riskier than consumer-grade lithium batteries (such as mobile phone batteries and laptop batteries) due to their larger capacity and higher energy density, which release more energy when failing.

Notably, “used lithium batteries” and “damaged lithium batteries” carry the highest risks. Used lithium batteries have aged electrode materials and degraded separators after long-term use, with an internal short circuit probability over 10 times higher than new batteries; damaged lithium batteries may have hidden internal damage and are highly prone to thermal runaway during transportation. ICAO therefore specifically stipulates that the air transport weight limit for used and damaged lithium batteries should be further tightened to 10kg, restricted to cargo aircraft only and prohibited on passenger aircraft.

III. Global Aviation Safety Regulations: Building a “Protection Network” for Lithium Battery Air Transport

The 25kg weight limit is just the “tip of the iceberg” of lithium battery air transport safety regulations. To comprehensively prevent risks, ICAO and national regulatory authorities have established a strict safety regulatory system covering the entire process of “packaging, marking, declaration, and transportation,” forming a multi-layered risk protection network.

(1) Packaging Regulations: The “First Line of Defense” for Physical Isolation

Packaging is critical to preventing the spread of lithium battery thermal runaway. Regulations require lithium batteries to use specialized packaging that is “leak-proof, puncture-resistant, and spark-proof.” Specifically, individual lithium batteries must first be placed in sealed plastic or metal containers filled with cushioning materials (such as foam or bubble wrap) to prevent collisions during transportation; for consolidated packages of multiple lithium batteries, fire-resistant partitions must be installed between each cell to prevent one failing cell from igniting others; the entire package must be waterproof to prevent electrolyte leakage from corroding aircraft structures.

Packaging requirements vary for different lithium battery types. For example, lithium-ion battery packaging must have “ventilation holes” to release gases generated during thermal runaway and prevent package explosion; lithium metal battery packaging requires additional “short-circuit prevention devices” to avoid electrode contact with external conductors. After packaging, a series of tests including “drop tests, vibration tests, and thermal tests” must be conducted to ensure integrity under extreme transport conditions.

(2) Marking and Labeling: “Information Windows” for Risk Warning

To facilitate identification and emergency response, lithium battery shipments must bear standardized markings and labels. According to ICAO requirements, each lithium battery package must be marked with “Lithium Battery” or equivalent, indicating battery type (e.g., “Lithium-Ion Battery,” “Lithium Metal Battery”), rated capacity, and rated voltage; consolidated packages must display an orange diamond-shaped “Dangerous Goods” mark with black borders and symbols, no smaller than 100mm×100mm; additionally, “Cargo Aircraft Only” labels (if prohibited on passenger aircraft) or “Passenger and Cargo Aircraft Allowed” labels, as well as “directional arrows” indicating correct package orientation, must be affixed.

Stricter marking requirements apply to used and damaged lithium batteries, which must additionally display “USED” or “DAMAGED” and “Heat Sensitive” labels to remind handlers to handle with care. These markings and labels not only serve as “information windows” for risk warning but also provide critical basis for risk identification by airport security, freight forwarders, and airlines.

(3) Declaration and Approval: “Access Checkpoints” for Source Control

Lithium battery air transport implements a “pre-transport declaration” system. Shippers or freight forwarders must submit a Dangerous Goods Transport Declaration Form and Lithium Battery Transport Appraisal Report to airlines before shipment. The declaration form must detail shipment name, weight, quantity, packaging method, and transport route, signed by the declarant; the appraisal report must be issued by qualified third-party testing institutions, certifying that lithium batteries meet air transport safety requirements, comply with the 25kg weight limit, and are free from damage or leakage risks.

Airlines must complete review within 24 hours of receiving declaration materials, focusing on verifying weight compliance, packaging conformity, and appraisal report validity. Airlines have the right to refuse non-compliant shipments; high-risk lithium battery shipments (such as used batteries or large-capacity industrial batteries) require additional approval from civil aviation authorities before transport. In 2023, global airlines rejected 56,000 lithium battery shipments due to non-compliant declaration materials, effectively blocking risks at the source.

(4) Transport and Emergency Response: “Safety Guarantees” for Full-Process Monitoring

During transportation, airlines must take a series of measures to ensure lithium battery shipment safety. First, lithium battery shipments must be loaded into “designated cargo holds” equipped with specialized fire suppression systems (such as Halon or water-based systems) and temperature monitoring devices that real-time track cargo hold temperature. If abnormal temperatures (exceeding 50℃) are detected, fire suppression procedures are activated immediately; second, lithium battery shipments must not be co-loaded with flammable, explosive, or corrosive goods, maintaining a minimum 1-meter safety distance to prevent cross-contamination and risk aggregation; finally, crew members must receive lithium battery emergency response training before flight, mastering initial fire suppression methods and evacuation routes.

In case of lithium battery thermal runaway accidents, emergency response must follow the principle of “control first, then extinguish.” Crew members first close the cargo hold ventilation system to prevent fire spread; activate the fire suppression system to suppress the fire; simultaneously report the accident to the ground command center via satellite phone and select the nearest airport for emergency landing. Ground rescue forces must prepare in advance to extinguish the fire with dry powder extinguishers and foam equipment immediately after aircraft landing, and cool residual lithium batteries to prevent re-ignition.

IV. Challenges and Countermeasures: Safety Upgrades in the New Energy Era

With the development of the new energy industry, demand for lithium battery transportation has exploded, posing new challenges to the traditional 25kg weight limit and safety regulations. Balancing safety with transportation demand has become an urgent issue for the global aviation industry.

(1) Emerging Challenges: Dual Pressures from Demand Growth and Technological Iteration

On one hand, the development of industries such as electric vehicles and energy storage stations has driven a surge in demand for large-capacity lithium battery transportation. For example, an electric vehicle power battery typically weighs 50-100kg, far exceeding the 25kg limit. Splitting such batteries for transport per current standards not only reduces efficiency but also increases packaging costs and risk points. To bypass the weight limit, some enterprises adopt illegal “split-and-conceal” methods, declaring a single battery as multiple small packages—an approach that could lead to catastrophic consequences if an accident occurs.

On the other hand, lithium battery technology iterations have introduced new risk points. For instance, although solid-state batteries are safer than traditional liquid lithium-ion batteries, they remain in the R&D stage, lacking unified transport safety standards; the risk characteristics of new batteries such as sodium-ion batteries are unclear, and existing testing methods and regulations cannot fully cover them. Additionally, the rise of cross-border e-commerce has increased personal shipments of lithium batteries. To save costs, some individuals fail to comply with packaging and declaration requirements, introducing hidden risks to air transport.

(2) Upgraded Risk Prevention Countermeasures: Integrating Technological Innovation and Management Optimization

Addressing new challenges requires upgrading risk prevention systems through both technological innovation and management optimization. Technologically, first, develop “smart packaging” technology by embedding temperature, humidity, vibration sensors, and GPS positioning modules in lithium battery packaging to real-time monitor battery status, issue immediate alerts for abnormalities, and transmit data to remote monitoring platforms; second, improve fire suppression systems by developing “high-efficiency fire-extinguishing agents” suitable for large-capacity lithium batteries to enhance control over large-scale thermal runaway accidents; third, establish a “lithium battery full-lifecycle traceability system” using blockchain technology to record production, transport, use, and recycling information, enabling risk traceability and responsibility identification.

In terms of management, first, implement “classified supervision” by formulating differentiated weight limits and transport standards based on lithium battery capacity, type, and risk level—for example, appropriately relaxing the weight limit for solid-state batteries to 35kg while tightening it to 5kg for used batteries; second, simplify approval procedures for “compliant enterprises” by implementing a “green channel” system for enterprises with a consistent record of safety compliance, shortening declaration review time and improving transport efficiency; third, strengthen “international coordination” to promote unified lithium battery testing standards, certification systems, and emergency response procedures globally, eliminating risk loopholes caused by regulatory differences.

Conclusion

The 25kg air transport weight limit—this “safety red line”—is a lesson learned by the aviation industry through bloodshed, embodying in-depth understanding of lithium battery thermal runaway risks and scientific risk prevention logic. In the new energy era, despite emerging challenges in lithium battery air transport, adhering to the principle of “safety first,” upgrading prevention capabilities through technological innovation, and consolidating safety barriers through strict regulation enforcement will enable us to meet industrial development needs while ensuring sustained air transport safety. After all, for the aviation industry, no efficiency improvement can justify sacrificing safety—this is the core essence of lithium battery air transport safety regulations and the bottom line that all practitioners must uphold.