Electric Aircraft: When the Flames Go Out, the Fire May Not Be Over

By Garth Calitz

The rise of electric aircraft is forcing the aviation industry to rethink one of its oldest assumptions: that extinguishing the visible fire means the emergency is under control.

The aviation industry is rapidly moving towards electric propulsion. From battery-powered trainers and light aircraft to hybrid-electric regional aircraft and eVTOLs, high-energy batteries are moving from the passenger cabin into the very heart of aircraft propulsion. That transition promises quieter, cleaner and potentially more efficient aviation. It also presents a new challenge for Aircraft Rescue and Fire Fighting (ARFF) crews.

A lithium-ion propulsion battery fire is not simply another aircraft fire. It can behave very differently from a conventional jet-fuel fire, and some of the techniques that have served aviation firefighting for decades may not be sufficient on their own when confronted with a large, damaged high-energy battery.

The issue is not whether firefighters can extinguish flames. They can. The more difficult question is whether they can stop the battery from continuing to generate heat after the flames have disappeared. That distinction could become one of the most important safety issues of the electric-aircraft era.

Lithium-ion batteries can hold significant energy in a compact form. Typically, advanced battery-management systems, cooling mechanisms, and protective electronics manage this energy effectively. However, if a cell becomes damaged, overheated, defective, or experiences an internal short circuit, it can enter a state called thermal runaway. The internet is full of videos showing Electric Vehicle fires, and a similar situation could occur with electric aircraft.

The cell begins generating heat faster than it can dissipate it. Its temperature rises rapidly, potentially causing it to vent gases, ignite or rupture. The heat can then trigger neighbouring cells. The result can be a chain reaction.

The Federal Aviation Administration (FAA) describes thermal runaway as capable of producing fires and, in some circumstances, explosions. Its research has demonstrated that heat generated by one cell can cause adjacent cells to enter thermal runaway. This is the fundamental difference between a conventional fuel fire and a battery event.

A firefighter dealing with a jet-fuel fire aims to eliminate the fire's fuel, oxygen, or heat. In the case of a battery thermal runaway, the fire might be generating its own heat internally. Thus, extinguishing the flames around the battery may not necessarily halt the event.

There is a common misconception that water and lithium-ion batteries should never be mixed. The reality is considerably more complicated. FAA fire-safety research identifies water as an effective means of cooling lithium batteries because water can transfer heat away from the cells rapidly. The objective is not simply to extinguish the flame; it is to prevent neighbouring cells from reaching the temperature at which they too enter thermal runaway.

That creates a major practical problem for aviation. A smartphone battery can potentially be flooded with water. A propulsion battery weighing hundreds of kilograms may be buried deep inside an aircraft, surrounded by structural protection, thermal barriers, electrical equipment and other systems.

Firefighters may therefore be pouring water onto a battery enclosure while the actual thermal event is taking place somewhere inside it. The challenge becomes one of heat penetration and sustained cooling, rather than simply extinguishing an external flame.

Perhaps the greatest danger is the battery that appears to have been extinguished. A thermal runaway event can proceed in stages; there may be smoke, then venting, then flames. The flames may disappear, and then another cell can ignite, repeating the entire process. FAA guidance for high-energy fires specifically describes the need to extinguish active flames and then cool the battery or device to prevent additional cells from entering thermal runaway.

This means that "fire out" cannot necessarily mean "incident over".

For an airport, that could require a completely different post-fire procedure. An electric aircraft involved in an accident might need to remain isolated and under observation for an extended period. Thermal imaging cameras and temperature monitoring could become just as important as conventional extinguishing equipment. A battery that has survived an accident may remain a source of stored energy long after the aircraft has stopped smoking.

FAA testing of lithium-ion batteries has demonstrated why conventional suppression methods cannot simply be assumed to solve the problem. In large-scale testing, suppression systems could knock down flames but did not necessarily stop thermal runaway or cell-to-cell propagation. The batteries continued producing flammable gases, creating the potential for fire escalation and explosion.

This is particularly significant because lithium-ion battery thermal runaway can produce gases including hydrogen and hydrocarbons. A damaged battery can therefore present two simultaneous hazards: Fire and gas.

A firefighter approaching an apparently contained battery incident could be dealing with an invisible accumulation of combustible gases. If those gases ignite, the resulting event could be considerably more violent than the original battery fire. That changes the way the accident scene needs to be assessed.

European regulators are already taking this issue seriously. EASA's certification guidance for eVTOL aircraft introduces the concept of an Explosive Fire Zone around propulsion batteries. The regulator is not simply treating the battery as another piece of electrical equipment. The certification framework recognises that thermal runaway can produce flame, heat, sparks, hot material, pressure and accumulated gases. The design response is therefore increasingly based on containment.

Rather than requiring firefighters to extinguish a lithium-ion propulsion battery fire directly, the aircraft should be capable of containing the event sufficiently to protect critical aircraft systems and occupants. EASA guidance states that, for lithium-ion propulsion batteries, the capability to extinguish the fire itself is not necessarily the certification objective because doing so may be impractical. Instead, the battery's explosive fire zone is designed to contain the consequences of thermal runaway, with controlled overboard venting permitted where appropriate.

That is a major philosophical shift. The aircraft is expected to protect itself first. The firefighter then manages the consequences. This means that electric aircraft safety increasingly begins long before the fire truck arrives.

Modern propulsion battery systems can incorporate:

• Cell-level monitoring;

• Battery-management systems;

• Temperature sensors;

• Pressure monitoring;

• Automatic electrical isolation;

• Thermal management;

• Fire and gas detection;

• Module-level containment;

• Fire-resistant barriers;

• Controlled venting;

• Thermal-runaway propagation protection.

EASA's VTOL guidance calls for detection systems that can include fire, gas, overtemperature, undervoltage and overpressure sensing. It also addresses containment and the protection of surrounding aircraft systems from heat, pressure, flames and hot material. This layered approach is critical.

The safest battery fire is the one that never reaches the stage where the airport's firefighters have to fight it. The emergence of electric vertical take-off and landing aircraft could make the problem even more complex. A conventional twin-engine aircraft has a relatively predictable architecture. An eVTOL may have numerous electric motors, distributed propulsion systems and multiple battery modules positioned throughout the airframe.

A crash could therefore create several separate high-energy fire zones. Instead of dealing with one engine fire or one fuel-fed area, emergency crews could potentially encounter several battery systems at different stages of failure.

EASA's certification framework specifically addresses thermal runaway at propulsion-battery and installation level, including scenarios involving multiple cells entering thermal runaway. This means ARFF crews will need to know far more than simply where an aircraft's engines are located.

They will need to know:

• Where are the batteries?

• Where are the high-voltage disconnects?

• Where are the vent outlets?

• Which areas are hazardous?

• Can water reach the battery?

• Which batteries remain energised?

• Where can firefighters safely approach the aircraft?

These answers will have to be aircraft-specific. Electric aircraft also introduce a fire scenario that traditional aviation has not had to manage on anything like the same scale: The aircraft is on fire while it is charging.

An electric aircraft sitting on the ramp can be connected to high-power electrical infrastructure while its propulsion batteries are being charged. An emergency could therefore involve the aircraft, battery system, charger, electrical supply infrastructure and surrounding aircraft simultaneously.

ICAO has already warned that aerodrome emergency plans and rescue-and-firefighting procedures need to be reviewed when electric aircraft are introduced. It also highlights the additional risks associated with battery storage, transportation and charging near aircraft operational areas. This is more than an aircraft issue; it becomes an airport infrastructure issue.

Charging areas may eventually require dedicated separation distances, emergency isolation points, battery-monitoring systems, specialist procedures and designated areas for damaged aircraft.

The transition to electric aviation will not happen overnight. For years, hybrid-electric aircraft could coexist with conventional aircraft. That could create an unusual firefighting challenge.

A hybrid aircraft could contain: jet fuel + batteries + high-voltage electrical systems + electric motors + conventional engines. A crash could therefore produce a conventional fuel-fed fire while a battery simultaneously experiences thermal runaway. Firefighters would have to manage both hazards.

The traditional ARFF response would remain essential for fuel, hydraulic fluid, composites and aircraft structure, while battery-specific procedures would be required for the electrical energy storage system. The result could be a much more complicated incident-command environment.

For South Africa, this is not a distant theoretical problem. Electric aircraft are likely to enter the market progressively through training, general aviation, specialised operations and eventually commercial air transport. The country therefore has an opportunity to prepare before electric aircraft become commonplace. The question is whether airport operators, ARFF organisations, municipal emergency services, aircraft operators and the South African Civil Aviation Authority are sufficiently prepared for the different characteristics of high-energy battery fires. 

A major international airport is one thing; a small general aviation airport or regional airfield is another. Not every airport has the same firefighting resources, water capacity, emergency equipment or specialist personnel. That raises several questions that deserve answers before the first serious accident:

• Do airports handling electric aircraft have battery-specific emergency procedures?

• Are firefighters trained to recognise thermal runaway?

• Do emergency crews know where the aircraft's batteries are located?

• Are high-voltage isolation procedures included in emergency response plans?

• Can the airport provide sustained battery cooling?

• Are thermal imaging systems available?

• Is there a designated isolation area for a damaged electric aircraft?

• How long should an aircraft involved in a battery incident remain under observation?

And perhaps most importantly:

Who is responsible for the aircraft after the visible fire has been extinguished?

These questions need to be answered in advance.

The most important lesson emerging from battery research is that firefighters should not be expected to solve a problem that the aircraft's design could have prevented or contained. The industry's approach is therefore increasingly based on layers of protection.

Detect * Isolate * Contain * Vent * Protect *Cool *Monitor

EASA's requirements reflect this philosophy, requiring propulsion-battery systems to demonstrate that thermal-runaway hazards can be managed and that surrounding aircraft systems remain protected.

This is good engineering. It also recognises an unavoidable reality:

A firefighter cannot physically reach every cell inside a damaged propulsion battery. Every aircraft can catch fire. The relevant question is whether the aircraft can control the consequences of the fire.

Electric aircraft manufacturers and regulators are already working on that problem through battery-management systems, containment, thermal-runaway testing, fire-resistant structures and controlled venting.

The next step is making sure the people waiting on the ground are equally prepared. ARFF crews have spent decades perfecting responses to fuel-fed aircraft fires. The electric era will require them to add another skill set: managing a high-energy chemical and electrical event that may continue long after the flames have disappeared. That means new training, new emergency procedures, new airport infrastructure and, potentially, new firefighting equipment.

The foam truck is not going away. Nor should it. But alongside it, the airport of the future may need thermal cameras, battery-specific rescue procedures, high-voltage isolation equipment, dedicated quarantine areas and a much better understanding of what is happening inside an aircraft that appears to be on fire.

The most dangerous electric-aircraft battery fire may not be the spectacular one. It may be the one that looks like it has been extinguished. Because with thermal runaway, the absence of flames does not necessarily mean the absence of fire.