Engine Fire Protection has two components, detection and extinguishing.
Engine Fire Detection
Detection is achieved by means of linear pneumatic sensing systems, often referred to as fire loops, which are gas filled pipes routed around potentially abnormal heat sources. If the temperature in the vicinity of the sensing element rises, the electrical resistance of the core material decreases and a warning indication can be triggered. They are duplicated to allow for continued detection if a single loop in the system becomes faulty. An open loop due to a short circuit fault will be detectable during daily pre flight testing or by annunciation of a fault in service. Physical damage to a loop such as a ‘pinching’ may lead to a false fire warning but would also produce an independent fault annunciation. The same principles apply to the protection of auxiliary power units (APUs).
Engine Fire Extinguishing
On modern commercial passenger jet aircraft, engine compartments are usually divided into two zones ‘1’ and ‘2’ for the purposes of fire protection. Two electrically operated extinguishers containing Halon 1301 or, in newer aircraft, HFCs (Hydrofluorocompounds), are available to each engine. These are sometimes installed in the nose cowling of each nacelle but may also be located within the fuselage and "shared" by the engines of a twin engine aircraft or be located in a dry bay in the wing and be "shared" by the engines on that wing of a multiengine aircraft. When activated, the contents of the extinguisher bottle are discharged into engine Zone 1, the engine fan assembly. Fires in Zone 2, the engine core, are extinguished by shutting down the engine. The flight crew engine fire drill includes shutting off the fuel and hydraulic fluid supplies to the engine concerned and the supply of oil, the other potentially readily-inflammable substance, is rapidly diminished since it is an isolated supply in each engine/APU.
Activation of engine fire bottles is normally also advised as a precautionary measure whenever an engine shutdown drill is carried out because of severe damage.
APU Fire extinguishers are activated by the flight crew in the same way as engine extinguishers - by manual selection upon receipt of a fire warning - when airborne, but automatically, and with accompanying automatic APU shutdown, in the case of fire detection during ground running.
If indications are available that an airborne engine or APU fire has been extinguished successfully, then the fire occurrence itself does not then influence the flight crew decision on how urgent it is to land the aircraft. However, in a twin engine aircraft, the shutdown of an engine will usually result in landing as soon as practical. Experience has shown that in certain circumstances, continued indications of engine fire may be presented after engine shutdown and the discharge of both bottles has resulted in the complete cessation of fire. If this occurs, then the flight crew cannot be absolutely sure that their action has fully extinguished the fire and are prudent to decide to ‘land as soon as possible’.