Introduction
Flying at 35,000 feet is not just a quick way to get from point A to point B, it is also the safest [1]. Statistically, you are far more likely to encounter danger in any other form of transportation than you are while flying. Aircrafts are engineered to manage extreme conditions, and every flight is operated by a highly trained crew that is using state-of-the-art technology. Safety checks are rigorous, and pilots are prepared to handle challenging and unexpected situations.
However, with modern technologies perpetually transforming our everyday lives, new risks that need careful attention and management are emerging. One such rising concern is the fire risks associated with lithium-ion batteries. These powerful, compact energy sources have become essential in everything from smartphones to electric cars, and are not even exempt from the ritual of relaxing above the clouds.
While there has been plenty of coverage on battery fires in electric vehicles and other devices, the aviation industry also presents unique challenges with respect to lithium-ion battery safety. Therefore, let’s dive into the layers of complexity surrounding lithium-ion batteries and explore how we can keep our skies even safer in the face of this growing risk.
The Rise of Lithium-Ion Batteries in Aviation
The global demand for lithium-ion batteries (LIB) has surged (see Figure 1), with an impressive annual growth rate of 27%, spanning sectors from consumer electronics to electric vehicles, and aviation is no exception [2]. As industries increasingly shift towards more efficient energy solutions, the value and necessity of LIBs are soaring. This widespread adoption stresses the critical role these batteries play in powering the future. However, it also brings new challenges, particularly in aviation, where safety is paramount.
Figure 1: Global Li-ion battery cell demand, in GWh [2].
Furthermore, with the significant effort and investments in advancements of this technology, the cost of manufacturing Li-ion batteries have dropped significantly, all while the energy density has increased (see Figure 2). This only makes the current situation more challenging.
Figure 2: Battery cost and energy density since 1990 [3].
With passengers carrying multiple lithium-ion-powered devices like smartphones, tablets, laptops, and power banks, the number of batteries per flight has skyrocketed (pun intended). Each extra device introduces a potential point of failure, and it only takes one damaged or malfunctioning battery to trigger a challenging incident mid-flight.
Figure 3: LIB fire in the overhead compartment of a China Southern Airlines plane [4].
As the market grows, so does the risks associated with manufacturing defects, improper battery handling, and unintentional passenger misuse, all of which can increase the likelihood of battery fires. Additionally, the ongoing push for electric and hybrid-electric aircrafts is set to introduce even more LIBs into the skies. While this represents an exciting leap toward sustainable aviation, it also amplifies the risk, thus making it crucial for airlines to stay vigilant about safety protocols and battery management.
Fireworks in the sky – A different meaning all together
In the past decade, the Federal Aviation Administration (FAA) reported a total of 488 battery fires in the aviation industry involving American aviation companies [5]. This statistic also include a rising frequency of incidents related to lithium-ion batteries (see Figure 4), which is not surprising as such devices are becoming increasingly common in both modern aircrafts and in the passengers' personal belongings. This trend is of relevance for the growing safety challenges associated with the widespread use of LIB devices in aviation [6].
Figure 4: FAA reported these numbers of battery fires per year (created with numbers from [5]).
Furthermore, the extensive use of LIB devices extends far beyond passengers carrying personal electronics. Today, crew members and pilots rely heavily on such batteries for essential equipment, including communication devices, navigation tablets, and emergency tools. While these devices are vital for modern aviation operations, they also introduce multiple points of potential risk on board.
Beyond personal devices, lithium-ion batteries are now also intricately integrated into the design and operation of aircrafts themselves (see Figure 5), as they are playing crucial roles in powering various systems and technologies, thus making the situation more complex due to their presence in multiple locations throughout the aircraft [7] (see Figure 6).
Figure 5: LIB fires in an aircraft's seat control system [7].
About 53% of American flyers stored their LIB-powered devices in overhead compartments or checked luggage away from sight [8]. This diverse usage and placement of lithium-ion batteries onboard make it crucial for the aviation industry to implement comprehensive monitoring, maintenance, and fire prevention strategies tailored to every part of the aircraft.
Figure 6: Different locations a LIB-powered device or equipment can be found in an aircraft [7].
Lack of awareness among passengers – A major avenue of concern
A survey conducted in the United States in 2023 by UL Standards and Engagement [8] revealed significant knowledge gaps among consumers regarding the hazards and risks associated with their lithium-ion powered devices while traveling - see Figure 7. This lack of awareness can lead to counterproductive behaviours and ignorance of best practices for safety.
While all respondents reported owning at least one LIB powered device, more than 40% admitted to knowing little or nothing about the batteries in these devices. A similar trend emerged when users acknowledged that their choices in replacement chargers and batteries were primarily driven by affordability rather than certification or safety standards. This highlights a fundamental issue: a lack of awareness and understanding of the risks and mitigation measures related to lithium-ion batteries.
Figure 7: Key insights from the UL Standards and Engagement survey from 2023 [3].
The Science Behind Battery Fires
To truly grasp the magnitude of the fire risk associated with LIBs, we need to discuss the core problem, which is thermal runaway (or the lack of management thereof). This persistent challenge is frequently highlighted when discussing battery fires and refers to a chain reaction that can occur when a battery cell becomes overheated or damaged. In simple terms, thermal runaway is like a small spark setting off a series of fireworks (see Figure 9). Once one cell overheats, it can trigger neighbouring cells to follow suit, thereby leading to a cascade of heat, and, eventually, fire or even explosion [9].
Figure 8: Significant damage of an UPS plane in Philadelphia that was caused by ignition of Li-Ion batteries [10].
Moreover, LIBs are composed of multiple cells filled with a flammable electrolyte. These cells are tightly packed together to maximize the energy efficiency (through increased energy density), but this compact design might result in disastrous outcomes. A battery fire is rarely just a small flame; it releases harmful gases and can be extremely difficult to extinguish, particularly within the confined space of an aircraft (that also has limited egress options). The combination of fire, heat, and toxic emissions makes thermal runaway a significant risk in modern aviation safety.
Figure 9: Thermal runaway stages and associated problems [7].
Thermal runaway can become an even more pressing issue in LIBs due to inherent challenges such as flaws in design and subpar manufacturing quality. Inadequate insulation, poor assembly, or the use of low-grade materials can all compromise the quality of these batteries, which in turn increases the risk of overheating or damage.
Furthermore, the problem is often exacerbated by improper handling and usage by individuals. Mechanical abuse caused by dropping, crushing, or overcharging devices can weaken the structural integrity of the batteries, making them more vulnerable to thermal runaway and, ultimately, battery fires. These factors combine to increase the risk of battery malfunctions, particularly in the high-stakes environment of aviation, where safety is paramount.
Current Safety Measures and Their Limits
Given the risks associated with lithium-ion batteries, the aviation industry has implemented various safety measures designed to prevent and manage battery fires. One key protocol involves strict transport regulations for lithium-ion batteries with limitations on the number of batteries passengers can carry and the energy storage capacity capped at 100 Wh for rechargeable batteries [11]. In cargo planes, there are even stricter requirements, with many airlines refusing to transport certain high-capacity batteries altogether.
Additionally, aircrafts are now typically equipped with fire containment bags that can contain overheating batteries in the event of a fire. These are primarily used in passenger cabins, where devices like laptops or smartphones might overheat. Pilots and crew are trained to handle these situations quickly, placing the device into the fire containment bag to prevent further escalation. With the knowledge that the gases from a battery undergoing thermal runaway are very toxic, it is questionable whether one would want the pilots to be the ones engaging with the firefighting. Do we have a pilot on board?
Figure 10: A lithium battery fire mitigation bag [12].
A key area for improving fire safety in aviation is the early detection and suppression of fires caused by lithium-ion battery-powered devices. While fires in devices stored in the passenger cabin are easier to spot due to visible smoke and flames, those in the cargo present a more difficult challenge. Detecting and controlling these fires is a much more complex task.
In modern passenger and cargo planes, cargo areas are typically classified as Class C compartments, in line with the CS 25.857 regulation [13]. These compartments are equipped with smoke and fire detection systems, as well as built-in fire suppression mechanisms. Additionally, they feature fire-resistant linings to keep flames and smoke from spreading to other sections of the aircraft. However, many older cargo planes, which fall under the Class E category, lack these suppression systems, focusing instead on protecting critical systems from fire damage rather than suppressing or extinguishing the fire.
The primary fire suppression method for Class C cargo compartments uses Halon 1301, which is a powerful extinguishing agent. While the use of Halon is regulated by the Montreal Protocol, it is still allowed in aircrafts for fire extinguishing purposes. When a fire is detected, the system initially releases a 5% concentration of Halon to smother the flames quickly. Then, the system maintains a 3% concentration to keep the fire under control, with the agent flow adjusted to account for compartment leaks [14].
Figure 11: Typical metered fire extinguisher system with flow regulator [14].
However, a study by the FAA [15] has revealed a significant limitation related to the use of Halon 1301 for suppressing LIB fires. While the agent is effective at controlling flames, it lacks the cooling properties necessary to prevent battery cells from reaching thermal runaway and further propagation to other cells. Such a flameless development can lead to the release of large volumes of flammable gases, potentially triggering an explosion.
Additionally, the fire-resistant linings in cargo compartments were found to be vulnerable under these extreme conditions [16]. Fires fueled by LIBs can create an air pressure well above the 1-psi overpressure limit of these compartments (see Figure 11), leading to smoke leakage. In passenger planes, this poses a major hazard, causing visibility issues and the spread of toxic fumes. Worse still, the pressure can also lead to the leakage of Halon 1301, thus reducing its ability to control the fire.
Figure 12: Pressure rise due to four cells at 50% charge [15].
In summary, the current design of cargo compartments in modern aircraft is not adequately equipped to handle the intensity of lithium-ion battery fires, posing a serious risk of catastrophic failure.
Emerging Innovations: A Glimpse into the Future of Battery Fire Safety in Aviation
As the aviation industry becomes increasingly dependent on lithium-ion batteries, researchers and engineers are actively developing innovative solutions to tackle the associated safety challenges.
Key advancements include:
Solid-State Batteries: Unlike conventional lithium-ion batteries, solid-state batteries use a solid electrolyte rather than a flammable liquid, thus dramatically reducing the risk of a thermal runaway. While still in a developing stage, several large companies are investing heavily in this as they are seen as a safer, future alternative for aviation. Simultaneously, the advancements in sodium-ion batteries are also seen as as a viable option for further progress for battery technology in general.
Advanced Fire Detection: Advanced sensors that can detect early signs of overheating at the battery cell level, long before thermal runaway occurs are currently being tested. One notable advancement features an internal Resistance Temperature Detector (RTD) sensor, which has been shown to detect peak temperatures 7-10 times faster than traditional external RTD sensors, offering quick response times[17] .
Fire Protection Materials: Researchers are also exploring passive fire-resistant materials to enhance safety within aircrafts. When combined with automated suppression systems, these innovations can help airlines prevent and contain battery fires before they become critical.
Passenger Awareness and Regulations: Educating passengers on the risks associated with their personal devices and enforcing strict guidelines for carrying spare batteries or devices with damaged batteries can help reduce the chances of incidents caused by mishandling or improper storage.
Better Training and Awareness for Flight Crew Members: Novel procedures must be developed to educate crew members about the hazards and risks associated with the LIB-powered devices. Training should include clear guidance on effectively identifying and addressing fires in different locations within the aircraft, whether in the passenger cabin, cargo hold, or the cockpit.
Conclusion
In summary, while lithium-ion batteries power much of our modern world, their presence in aviation brings a unique set of challenges. The risk of thermal runaway and the associated hazards of fire, explosion, and toxic fumes pose a serious threat, especially in the confined spaces of aircraft cargo holds. Although fire suppression systems like Halon 1301 may help managing the flames, they lack the ability to prevent further damage from overheating, thereby emphasizing the need for more advanced safety measures.
Currently, thermal runaways in aircrafts is not a very frequent phenomenon, but as the aviation industry moves towards modern technologies and more battery-powered systems, continuous innovation in fire prevention, early detection, and suppression is needed. This article aims to make a contribution with respect to that by sparking a conversation about the need for critical advancements that are essential for maintaining safety at 35,000 feet. We are all better off with just a normal runway, rather than a thermal runaway.
Thanks for reading,
Sameed, Ulises and Grunde
References
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