What are engine fire detection and suppression systems and how do they work?

A fire of any kind is a great danger in flight because there is a risk that it will spread to the point of spiraling out of control. For this reason, aircraft have fire detection and extinguishing systems in areas of the aircraft where a fire is likely to occur.

Engines and the APU (Auxiliary Power Unit) are components that produce a lot of heat – some of the aircraft‘s most complex fire detection and suppression systems are placed near these components. Here we focus on engine fire detection and suppression systems. The APU system is exactly the same as the engines.


The engine fire detection system

In most large aircraft, the engines and APU are lined with firestop loops or cables where a fire is most likely to occur. Firewire is a tubular structure consisting of an electrode covered with an insulating material which is then covered with a steel tube. This firewire is connected to the fire detection computer or to the control unit.

Firewires are placed in areas of the engine where a fire is most likely. This is an example of firewire placement in the CFM engine. Photo: Airbus.

The control supplies a small voltage to the firewire, which remains constant with no change in temperature. However, if the temperature were to increase (due to a fire), the resistance of the insulating material would decrease, causing current to flow between the outer steel tube and the inner electrode. This increase in current is detected by the fire detection computer, which then gives an alert to the pilots in the cockpit.

FireWire cables are always connected to the detector on both sides. This way, even if the wire were to break at a certain point, the fire detection continues to work.

The firewire consists of an electrode buried inside an insulating material covered with a steel cover. Photo: aircraftsystemstech.com.

For redundancy and to prevent false fire alarms, two fire wires or loops are placed in the engine fire locations. These loops are kept close to each other. Depending on the nomenclature chosen by the aircraft manufacturer, they may be either called loop A and loop B, or loop 1 and loop 2. A fire alert is only issued when both loops detect a fire. A loop failure fault is transmitted to the driver if one loop detects a fire and the other does not. In this situation, the faulty loop is isolated from the system and the fire detection switches to single loop operation where fire detection by the remaining loop will be accepted by the system as a positive fire signal.

In some aircraft, such as the classic Dash 8s, the fire detection system is a gas type system. In this system, a tube is filled with helium gas under pressure and is sealed at one end. Inside this tube is a core material that gives off hydrogen gas when heated. The other end of the tube is connected to an intervention unit which is connected to the fire detection unit.

In the answering unit there are two switches. One of these switches, called the integrity switch, is closed with normal helium pressure in the tube. The next switch, called the alarm switch, remains open. When there is a fire, as in the firewire system, the tube heats up. This causes the nucleus to release hydrogen gas. As the hydrogen is released into the tube, which already contains the helium, there is an increase in gas pressure which activates the alarm switch, giving the fire warning.

How loops A and B are placed in the PW 123 motors of a classic Dash 8. Photo: Dash 8 ODM.

Similar to the Firewire system, the gas system also has dual loops for redundancy purposes and to prevent false fire alarms. If the tube should rupture and helium should come out of the tube, the integrity switch opens and a loop fault is reported to the pilot.

What does a gas type fire detector loop look like. Photo: Meggitt.com

The cockpit fire alarm system

The manner in which the fire alert is given in the cockpit varies from aircraft to aircraft. Typically, this is a visual and audible alert. Some older aircraft are equipped with an electric bell visible in the cockpit.

Pilots also have a way to test the fire detection system before flight. This is a mandatory test for the first flight of the day for a crew set. The test switch allows the system to simulate a fire by activating the wire or detection tube.

The fire alarm system of an Airbus A320. Photo: Airbus.

The fire extinguishing system

The fire extinguishing system on board the engine consists of fire bottles filled with pressurized extinguisher. The extinguisher of choice, for now, is Halon 1301 because it is non-corrosive and non-conductive. This prevents serious damage to engine components after it is extinguished. The only downside to halon is that it is an ozone depleting substance, and the industry is currently working on finding an alternative.

There are usually two cylinders per engine. In small planes, there may be two cylinders for two engines. However, in this case, the two cylinders must be able to empty their contents towards an engine in the event of fire. To release the extinguishing agent, the cylinders are equipped with electrically explosive cartridges. These cartridges are called squibs.

To give pilots and mechanics an indication of the integrity of the extinguisher bottles, relief discs are placed on the outside of the engines where they can be observed while lapping around the track. The discs are green when the tank pressure is within limits. In case the bottles are unloaded, the colors of the disc change. They are red in case of thermal discharge which occurs when the bottles are subjected to high pressure, possibly due to a rise in temperature. This causes the bottle to empty the extinguisher overboard. If the cylinders are unloaded by pilot actions, the disc will be yellow.

In some aircraft, if the fire extinguishers discharge, this is indicated in the cockpit. It is not necessary to check it from the outside.

Aircraft discharge indicator discs. Photo: ATPL of Oxford.

What if an engine fire were to occur in flight?

An engine fire is a serious event that requires immediate action by the pilot. If a fire is detected by the system, cockpit warnings are issued. This includes visual and audible alerts. The first action is to cancel the alarm, as this can be a major distraction.

The second step is to stop the motors. This forces pilots to bring the respective engine thrust lever to idle and then shut off the fuel supply to the engine. Once this is done, the next step is to spray the extinguishing agent. However, before that, it is very important to isolate the engine from all other aircraft systems. For this, the planes have a fire push button or a pull switch. When this switch is pressed, it performs the following actions:

  • Closes the low pressure (LP) fuel valve
  • Arm the firecrackers from the fire extinguisher
  • Closes hydraulic shut-off valves
  • Closes the engine air bleeder valve
  • Disables the electric generator driven by the engine

As soon as the motor is isolated and the squibs are armed, pilots can activate the squibs. When the primer control push button is actuated, the fire cylinder extinguisher is released to the engine fire zones. Depending on the severity of the fire, one bottle may not be enough to get rid of it. If it cannot be extinguished with one bottle, the second bottle can be unloaded.

There is no guarantee that firing all the bottles will extinguish the fire either. If not, pilots must immediately find a place to drop off the plane. An uncontrolled fire is the most dangerous thing a pilot could face in the air, as fires can be very unpredictable.

Delaying action to deal with engine fires has caused problems in the past. There are cases where delayed pilot actions have led to the burning of detection loops. When loops are completely burned, it can give pilots false hope that the fire is out, when in reality it is burning full force.

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