Abbreviations
ULSE — UL Standards & Engagement
TRIP — Thermal Runaway Incident Program
ALPA — Air Line Pilots Association
FAA — Federal Aviation Administration
Key Terms
Thermal runaway — a dangerous chemical reaction inside a battery that causes a rapid rise in temperature and can produce smoke, fire, or an explosion.
Uncommanded evacuation — an evacuation started by passengers without an order from the crew.
Thermal containment product — special equipment used to isolate a hot or damaged device and help control heat and fire.
Crew resource management (CRM) — the effective use of communication, teamwork, information, and available resources by the crew.
Incidents involving lithium batteries on aircraft increased by 15% in the last year. Power banks have become the biggest source of these incidents, according to David Wroth, vice president of standards, technology, and operations at ULSE.
Wroth spoke about the scale of the problem and the steps pilots can take during the ALPA Air Safety Forum in Washington, D.C., on September 15.
Passengers now bring many rechargeable devices on board. ULSE research shows that the average passenger carries four rechargeable devices. A typical business passenger may have eight devices powered by about 10 lithium batteries.
As a result, a narrowbody aircraft may carry 700 to 800 devices, while a widebody aircraft may carry around 1,500.
Since 2019, ULSE has collected and analyzed thermal runaway incidents through TRIP together with airlines and other aviation safety organizations.
‘Dependency is Growing’
Power banks, or portable chargers, have become an important safety concern.
In 2025, a portable charger caused a fire that destroyed an Air Busan aircraft. Because the aircraft was still on the ground, everyone was evacuated and only minor injuries were reported.
Recent analysis showed that power banks were the leading cause of thermal runaway events in 2025. Many passengers carry them and use them during flights. Airlines are therefore giving passengers more information before departure and updating their rules on how power banks should be stored and used.
Lithium batteries are normally safe, but they can become dangerous if they are damaged, defective, counterfeit, crushed, overheated, or charged incorrectly.
In these conditions, a battery can enter thermal runaway.
Thermal runaway is not a normal fire. It is a self-sustaining chemical reaction in which heat causes more heat to be produced. The temperature can rise very quickly, and toxic and flammable gases can be released. This may lead to fire or even an explosion, and the process can develop within seconds.
Such incidents are still rare compared with the total number of flights. However, both the number and the rate of incidents are increasing as passengers bring more devices on board. In 2025, passenger aircraft experienced more than two thermal runaway events per week.
The problem is also linked to the growing dependence of passengers and crew on rechargeable devices during travel.
‘First Responders’
Many passengers have never seen a thermal runaway event and may not understand the risks of the batteries they carry. According to Wroth, fewer than 50% of passengers are aware of these risks.
During an incident, some passengers may panic or freeze. Others may stand up and try to move away from the fire. Some may try to help, while others may start recording the event on their phones.
These actions can make the situation more difficult and may interfere with the cabin crew’s response. If the aircraft is on the ground, passengers may also begin an uncommanded evacuation.
Cabin crew are the first responders in this type of event.
They identify the source of the smoke, inform the flight deck, and use a fire extinguisher to control visible flames. They then cool the device and continue to monitor it because the battery may catch fire again.
If necessary, the crew may place the device in a thermal containment product. Throughout the event, they continue to communicate with the flight deck.
Around 90% of these incidents occur in the cabin during flight. Most are brought under control before they develop into a serious fire.
Operational Disruptions
Lithium battery incidents can also cause significant operational problems.
ULSE estimates that almost one in five onboard thermal runaway incidents results in a return to the gate, a diversion, or an unplanned deplaning.
When an incident occurs, pilots may have to decide whether the flight can continue, return to the departure airport, or divert to another airport.
These decisions may have to be made at altitude, sometimes over the ocean, while the flight crew has incomplete information and the cabin crew is managing the situation behind them.
For this reason, Wroth said that pilots should treat cabin reports as important operational information. Reports of unusual odors, smoke, fire, or unusual passenger behaviour can help the flight deck understand how serious the situation is.
Crew resource management is one of the most important tools in this situation. Pilots should ask for specific information:
- What device is involved?
- Where is it located?
- Is there smoke?
- How thick is the smoke?
- Are there visible flames?
- What actions is the cabin crew taking?
Good information helps pilots make better decisions, and time is a critical factor.
FAA guidance also emphasizes that extinguishing visible flames is only one part of the response. Cooling and containment are also necessary because thermal runaway can continue even after the visible fire has been extinguished.