How an airplane’s fire-fighting system works

The cabins of all passenger aircraft incorporate a fire-fighting system that serves to minimize the risk of flames and helps ensure passenger safety.

Two types of fires can affect an aircraft and its passengers: those in flight and those following an accident.

 A fire on board usually occurs in flight due to a failure of the system or of its components, or even for a maintenance problem. A post-accident fire, on the other hand, results from the ignition of the fuel that is released caused, for example, by an impact.

Aircraft are designed to prevent fires

The fire-fighting system is one of the most important aspects in the design, testing and certification of aircraft. In fact, aircraft design and certification regulations concern overall the materials, their chemical/physical specifications, the construction, detection and intervention systems and, finally, the operating procedures.

An aircraft is designed in such a way that the individual parts possess fire-fighting properties directly correlated to their integration in the aircraft architecture. This approach is based on certain principles: selection, separation, isolation, detection and control of the materials.

First, it is important to keep the three elements that contribute to a fire separate: fuel, ignition source and oxygen. Secondly, it is necessary to ensure the isolation  of potential fires from spreading to other parts of the aircraft and to keep the control of the situation.

An aircraft, therefore, usually uses both passive systems (such as the use of fireproof or self-extinguishing materials) and active systems (such as  fire-fighting systems).

Most of the materials used in the construction of the passenger compartment interior are self-extinguishing: for example, the material that insulates the electrical cables must be self-extinguishing. Even the thermal/acoustic insulation installed behind the internal cabin panels is designed with adequate fireproof properties to delay the onset of fires in the cabin in the event of an accident.

All fabric and plastic products on the plane are tested for flammability and toxicity. The seat fabrics and blankets that are usually distributed to passengers must be able to withstand a long time before igniting (about 4 minutes) to allow passengers to evacuate the plane in time.

The fire-fighting system of the aircraft

Despite all forms of prevention, both in the construction phase of the aircraft and in the operational one, it is not possible to totally exclude the occurrence of negative events. For this reason, aircraft are equipped with specific safety systems that have two purposes:

– detect the fire: the system promptly locates the flames, it is the prerequisite for implementing the safety procedures;
– intervene to control and extinguish it: it allows you to remove the fire, involving specific interventions.

The aircraft fire-fighting system has the task of protecting the aircraft from:

– fire;
– smoke, and
– overheating.

The areas protected by the fire-fighting system are:

– the powertrain (engines + drivetrain);
– the APU, i.e. the Auxiliary Power Unit, which is installed on board an aircraft;
– the cargo compartments;
– the landing gear compartments and the Leading Edge (LE)
– the toilet compartments, and
– the passenger cabin.

The fire detection system

There are different types of detection systems, which exploit different physical phenomena to recognize the risk of fire:

– temperature variations, which are detected with Thermo-Switch or thermocouple systems;
– the presence of smoke in the air that can be detected by photoelectric sensors, capable of highlighting changes in brightness or deviations of light beams.

Fire extinguishing devices

Fire extinguishing systems include a number of components:

– a series of cylinders containing an extinguishing fluid;
– a network of pipes distributed to the various areas of the aircraft, and
– selector valves to direct the fluid.

Each individual cylinder is connected to the pipeline in the instant the pilot activates an explosive cartridge, which causes the breakage of a diaphragm separating the cylinder from the piping network.

In the case of a fire warning to an engine, the pilot first checks for a false warning and then activates the fire handle.

In the passenger cabin of the plane there are also a series of portable fire extinguishers of various types: the cabin crew is trained to use them and to intervene in the event of a fire.

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