What Are Engine Fire Detection & Extinguishing Systems And How Do They Work?

A fire of any sort is a big hazard when flying because there is a chance of it spreading to the point where it becomes uncontrollable.

A fire of any sort is a big hazard when flying because there is a chance of it spreading to the point where it becomes uncontrollable. Because of this, airplanes have fire detection and extinguishing systems in the areas of the aircraft where a fire is likely to occur.

The engines and the APU (Auxiliary Power Unit) are components where a lot of heat is produced - some of the most complicated fire detection and extinguishing systems in the aircraft are placed near these components. Here, we focus on the engine fire detection and extinguishing systems. The APU system is exactly the same as the engines.

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

The control supplies a small voltage to the firewire, which remains constant with no changes 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 a warning to the pilots in the cockpit.

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

For redundancy purposes and to prevent false fire alarms, two firewires or loops are placed in the engine fire locations. These loops are kept close to one another. Depending on the nomenclature chosen by the aircraft manufacturer, they can be either referred to as loop A and loop B or loop 1 and loop 2. A fire warning is only given out when both loops detect a fire. A loop failure fault is given to the pilot 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 goes into a single-loop operation where the detection of fire by the remaining loop will be accepted by the system as a positive fire signal.

In some aircraft, such as the Dash 8 classics, the fire detection system is a gas-type system. In this system, a tube is filled with Helium gas under pressure and is sealed on 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 a responder unit which is connected to the fire detection unit.

In the responder 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, like in the firewire system, the tube heats up. This causes the core to release hydrogen gas. As hydrogen is released to the tube, which already contains the helium, there is an increase in gas pressure which activates the alarm switch, giving out the fire warning.

Similar to the firewire system, the gas system also has dual loops for redundancy purposes and to prevent spurious fire alerts. If the tube were to break and helium was to release out of the tube, the integrity switch opens, and a loop fault is given to the pilot.

How the fire alert is given in the cockpit varies from aircraft to aircraft. Typically, it consists of a visual and an audible alert. Some older aircraft are fitted with an electrical bell which can be seen in the cockpit.

The pilots are also provided with a means to test the fire detection system before the flight. This is a mandatory test for the first flight of the day for a crew set. The test switch enables the system to simulate a fire by activating the sensing wire or tube.

The onboard engine fire extinguishing system consists of fire bottles that are filled with fire extinguishant under pressure. The extinguishant of choice, for now, is Halon 1301 because it's non-corrosive and non-conductive. This prevents severe damage to the engine components after it is extinguished. The only drawback of Halon is that it is an ozone-depleting substance, and the industry is currently working to find an alternative.

There are usually two bottles per engine. In smaller aircraft, there may be two bottles for two engines. However, in this case, both bottles must be able to empty their contents to one engine in a fire event. To release the extinguishant, the bottles are fitted with cartridges that can be exploded electrically. These cartridges are known as squibs.

To give the pilots and the engineers an indication of the integrity of the fire bottles, discharge disks are placed outside the engines where they can be observed during the walkaround. The disks are green when the bottle pressure is within the limits. In case the bottles are discharged, the disk colors change. They are red in the event of a thermal discharge which occurs when the bottles are subject to high pressure, possibly due to increased temperature. ​​​This causes the bottle to empty the extinguishant overboard. ​​​If the bottles are discharged by the pilot's actions, the disk will be yellow.

In some aircraft, if the extinguishers discharge, it is indicated in the cockpit. There is no need to check it from the outside.

An engine fire is a serious event that calls for immediate action from the pilot. If a fire is detected by the system, cockpit warnings are given out. This includes visual and aural alerts. The first action is to cancel the alarm, as it can be a major distraction.

The second step is to shut down the engines. This requires the pilots to bring the respective engine thrust lever to idle and then shut off the fuel to the engine. Once this is done, the next step is to spray the extinguishant. However, before that, it is very important to isolate the engine from all the other systems of the aircraft. For this, aircraft have a fire push button or a pull switch. When this switch is operated, it does the following:

As soon as the engine is isolated and the squibs are armed, the pilots can activate the squibs. When the squib control push button is operated, extinguishant from the fire bottles is released to the engine fire zones. Depending on the severity of the fire, one bottle might not be enough to get rid of it. If it cannot be extinguished with one bottle, the second bottle can be discharged.

There is no guarantee that firing all bottles will extinguish the fire either. If it does not, the pilots must immediately find a place to set down the aircraft. An uncontrolled fire is the most dangerous thing a pilot could face in the air, as fires can be highly unpredictable.

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

Journalist - An Airbus A320 pilot, Anas has over 4,000 hours of flying experience. He is excited to bring his operational and safety experience to Simple Flying as a member of the writing team. Based in The Maldives.