Within this chapter you have two main parts. One is the fire detection, the second part is the fire extinguishing. Fire detection is done by means of fire loops, smoke is detected true smoke detectors. It goes without saying that fire is the biggest threat of all for an airplane of any shape or form. But for an airliner it’s even a bigger threat because of his volume, fuel quantity on board, and last but not least, the amount of passengers! There for it is always been a big concern of the world’s main airplane manufacturers that fire onboard is quickly detected and extinguished. To protect the passengers, crew, and plane. How they do this critical job you can find out in this chapter.
Fire loops more exactly, are the component that’s responsible for detecting a fire. Within the loops there are a few different kinds, the working principal is somewhat different.
The main thing to know is that loops work with the principal of electric resistance.
Low temperature gives high resistance so no output out of the loop, but when ever the temperature rises because of a fire or an overheat of some sort the resistance drops down permitting electricity to run trough the loop giving an output to the warning system at the flight deck.
These warnings are both aural and visual by means of a fire alarm and a bright red light illuminates all at the flight deck. Then the flight crew can take corrective action to extinguish the fire.
The Engine Fire handles of a Boeing 727, notice the wheel well light left to handle Nr 1. In case of a fire the light goes on an will stay on as long the fire isn't out. During this time the fire bell keeps on going. The flight crew will pull the specific handle an close off fuel, hydraulics, and air between the engine that's on fire and the rest of the airplane. If the fire is still present they push the push button underneath the fire handle to use the extinguishing bottle. This they can repeat.
Off course these loops are only used in specific area’s. Those area’s only where fire or an overheat can occur.
For example: the main engines or the APU (Auxiliary Power Unit) are potential fire hazards, because fuel and electricity come together on the same component. Infact, on an engine or an APU you have fuel, oxygen, ignition systems, electricity, heat, all thing you only may bring together in a specific way and sequence. So it is mandatory that these components are monitor in respect to fire protection.
Click image to zoom in or out.
NOTE: static electricity is also a possible ignition source. Because of this, all components all over the airplane have bonding wire’s attached, so the static electricity caused by friction between two materials, like fuel through a fuel line or air gliding over the outside surface of the airplane, gets rooted back to ground.
Both main and nose gear wheel wells are also equipped with fire loops. This because wheels and brakes accumulate a lot of heat on their take-off run. Because of the fact that lots of airliner models have a fuel tank above the main wheel well, also called the “centre fuel tank” it’s most important that these loops detect an overheat as soon as possible. So the crew can cycle the gear down in the airstream to cool off. And if necessary turn back to the airport. Tires are, like everybody knows, made of rubber, and that compound is especially hard to extinguish. Therefore the flight crew will always treat a wheel or brake fire with the same safety precautions as an engine fire.
The cargo compartments are also a possibility for an overheat or a fire. Maybe you already experienced it at the check-in desk, the pretty lady informs you that it’s not permitted to bring certain thing onboard. Also known as “dangerous goods”, battery’s were the most common thing in the past (before 9/11) that was revoked from entering the plane, but of the constant terror threat now also liquids like drinking water, tooth paste or baby food, hair gel, etc. all these items are now also on the already long list of things not permitted on board an airplane.
Dangerous Goods List
The extinguishing of any fire can be done by two theories:
trying to kill the fire by use of water, foam, sand, etc., or
by excluding oxygen true use of incombustible gasses sprayed in a closed area.
The last mentioned is the most practical for aviation use. The gasses are stored inside a bottle mounted inside the structure of the airplane. This bottle, called a “fire bottle”, contains a gas under high pressure which is sealed by a diaphragm, above this diaphragm is a cartridge mounted, which will explode when the flight crew operates the switch connected to the specific cartridge. This explosion will cause the diaphragm to shatter and as a result of this the gas exits via this way driven by his high pressure true the lines rooted to e.g., the engine that’s on fire. The engine is covered by cowlings which are especially seal for this purpose, keeping the gas inside the engine section causing the flames to die. Because these gasses replace the oxygen the flames and the fire can not keep on going. In case one bottle discharge is not enough, on most airliners, a second bottle can be used to do the trick.
NOTE: normally there’s a minimum of one bottle per engine installed on the specific airliner. Depending upon the size of the engine used and the manufacturer.
The APU and the cargo compartments have there own designated bottle(s). Nice to know: the bottles for the cargo compartments are much larger diameter, and contain off course more gas.
Fire is the big threat, but not the only one which is dangerous. Smoke has proven to be also a big hazard inside an airplane, especially the flight deck. Because, when the flight crew lose there consciousness, and lose control over the plane all together. This has sadly happened several time already in the past. Because of this oxygen masks are installed in such a way the crew can put them on within a few seconds. Because of that fact smoke should always be detected. This is done by means of smoke detectors installed in different areas and ventilation ducting.
These detectors have two basic working principals, they work as follows,
1. The ionization system; Ionization is the physical process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. This process takes place inside the detector. Inside there are two compartments, of which one is used as a reference.
Okay, let’s explain this an easier way. Just imagine this, in one hand you have a closed bottle with clean clear air inside that’s ionized, in your other hand you have an open bottle in which the surrounding air can go in and out and that’s also ionized inside the bottle. The difference in-between both is measured and when-ever the difference get’s above a curtain level,…bingo, smoke alarm!
2. Photoelectric system; Smoke produced by a fire affects the intensity of a light beam passing through air. The smoke can block or obscure the beam. It can also cause the light to scatter due to reflection off the smoke particles. Photoelectric smoke detectors are designed to sense smoke by utilizing these effects of smoke on light.
In plain English, you have a flash light shining at person “A” with your eyes shut, that person “A” can see the light as long as nobody is blocking it. As soon as that happens, person “A” will shout at you he can’t see the
light no longer, at that exact moment you know something is blocking you're ray of light. This is exactly what happens inside this type of detector, causing the smoke alarm to be triggered of the specific area where the detector is installed. And like the saying goes, no smoke without fire!
Article last modified on 10/06/2015