How does a fire suppression system work?
The fire suppression system in an aircraft uses both passive and active systems to reduce and eliminate fires. Passive methods include the use of noncombustible materials, separation by routing, compartmentalization (use of firewalls), isolation, proper ventilation and drainage. The active methods include fire detection and extinguishing systems.

The fire extinguishing system in the aircraft consists of a number of components (excluding the fire detection system), which includes,


Portable (handheld) fire extinguishers in cockpit and cabin,
Fire extinguisher bottles for engines and APU
Cargo hold fire extinguisher systems and
Fire extinguishers for toilet waste bins.


All the transport aircraft are required to carry a minimum number of portable fire extinguishers depending on the number of passengers carried. The following table gives the minimum number of portable fire extinguishers to be carried in transport aircraft (the cockpit should have a minimum of one).



Table from FAA document Fire Protection Systems

In case of cabin (or cockpit) fire, the portable fire extinguishers (usually, $CO_{2}$ extinguishers are not used) are used by the crew to extinguish the fire.

The engines and APUs have their own fire extinguishing systems, which have a liquid halogenated extinguishing agent and pressurized gas stored in fixed (typically steel) fire extinguisher containers, which are discharged on pilot command.

The engines have their own fire extinguishing systems, which usually have a pair of fire extinguishing bottles (or a pair for each engine), which supply extinguishing agent (Halon 1301 or Hydrofluorocompounds) to the engine.



Image from Boeing Aero magazine- Fire Protection: Engines and Auxiliary Power Units

The fire detection system from each engine displays the status in the cockpit, which warns of any engine fire. In case the pilot pulls the fire handle, the fuel flow to the engine is shut off and an explosive cartridge is fired in the fire extinguishing bottle, releasing the agent into the engine.



Image from FAA document Fire Protection Systems

The APU fire suppression system is similar to the engine fire suppression system, though only a single fire extinguisher bottle is used. Again, the fire extinguisher operation is commanded by the cockpit crew and results in the shutting down of the APU and the source of air.



Image from Boeing Aero magazine- Fire Protection: Engines and Auxiliary Power Units

In ground, the APU fire extinguishing system can operate automatically.

The cargo hold fire extinguishing system is similar to the above, where the crew can activate the fire extinguishing system if the smoke detectors in the cargo hold activates the warning lights in cabin. The extinguishing system is of two types- a dump system that releases the extinguishing agent directly
when the cargo fire discharge switch is activated or a metered system, where metered bottles discharge slowly which keeps the correct concentration of extinguishing agent in the cargo compartment to keep the fire extinguished for 180 minutes.



Table from FAA document Fire Protection Systems

The toilet waste bin is the only place in the aircraft with an automatic fire extinguishing system (in addition to ashtrays), where the heat sensors (not smoke detectors) trigger fire extinguishers.



Some aircraft have On Board Inert Gas Generating System (OBIGGS), which is used to 'inert' the fuel tank using nitrogen (usually generated by membrane based gas separation).