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Halon Fire Extinguishers
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|Category:||Fire Smoke and Fumes|
Halons are fire extinguishing agents which are gaseous when discharged in the aircraft environment. Halons have, until recently, been in almost universal use in aircraft fire extinguishers, both portable and fixed. They exist in two forms - as Halon 1211, BromoChlorodiFluoromethane (CBrClF2) also known as 'BCF', and as Halon 1301, Bromotrifluoromethane (CBrF3).
- Halon 1211 is used only in portable extinguishers and is a streaming agent
- Halon 1301 is used only in fixed extinguisher installations typically cargo holds or engines and is a total flooding agent.
Halons are electrically non-conducting and have been acknowledged as the most effective universal extinguishing agent for aircraft use. They work mainly by chemically interrupting the reaction described as the 'Fire Triangle' (Fuel-Oxygen-Heat) which must be sustained for a fire to continue. They do not produce residues and therefore do not cause secondary damage. However, the fumes are toxic if inhaled and all practicable precautions should be taken when they are used.
The chemical constituents in Halon gases, and the products of the reactions they induce when discharged on fires, have been identified as causing damage to the Ozone layer. As a result, their manufacture and use have been banned for many years in most countries and non-essential uses have been eliminated. However, the search for alternatives of comparable effectiveness has proved difficult and success limited, so they remain in wide use on board aircraft for most applications.
How Halon Extinguishers Work
Both Halon variants work by a combination of chemical and physical effects. The chemical effects, which are dominant in their overall effect, are achieved by the atoms in the gas directly inhibiting combustion in two different ways:
- Bromine, Iodine and Chlorine atoms act catalytically so that each atom participates repeatedly in the scavenging of important free radicals from the combustion gases.
- Fluorine atoms react with free radicals and form strong chemical bonds which neutralise combustion but can only do so once and are then “consumed.” The physical effects are both temperature reduction and dilution.
Temperature reduction occurs, whenever a non-reactive gas is added to a flammable gas, because the heat liberated by the reaction of oxygen molecules with a fuel source must be distributed into the overall environment. The rate of the combustive chemical reaction decreases rapidly with reductions in temperature and, if the concentration of added inert gas is high enough, the flame chemistry fails altogether.
Halon gas mixtures are not only inert but of low temperature when released from their pressurised state. Dilution is a simple matter of reducing the collision frequency of the oxygen and fuel source so that there is a reduction in chemical reaction rates. The magnitude of this effect, however, is relatively small compared to chemical inhibition and thermal effects, the former of these being the predominant one.
The toxicity of the Halon gases, especially the combination which makes up Halon 1211, is such that use in confined spaces requires care to minimise any inhalation of the discharged gases. Where a portable Halon extinguisher is used by cabin crew, it is usually recommended to consider donning a smoke hood before discharging it so as to eliminate this risk, but for pilot use on the flight deck, this will not be an option and risk awareness is the only defence.
Alternatives to Halon
The search for Halon alternatives for use on aircraft has been in progress for at least 20 years and has only recently led to potential success in the form of the product marketed as 'Halotron'. Extinguishing agents in common use generally are either formulations based on sodium bicarbonate - suitable only for fires involving flammable liquids and gases - or those known as ‘ABC’ which are usually based upon ammonium phosphate and are suitable for use on fires of all origins. Both types may be used safely on fires where electrical circuits are present. However, although these types are the most effective fire extinguishing agents in general use, they both leave a very fine, powdery residue after discharge which includes particles of sub-micron diameter which is difficult if not impossible to clean up. This makes them unsuitable for use on or in the vicinity of complex and often sensitive aircraft electrical systems. The ‘option’ of returning to the use of the far less effective dual extinguisher-type solution of Carbon Dioxide and Water Glycol has not found favour either. Although Carbon Dioxide, as an extinguishing agent, is both ‘clean’ and much cheaper than both Halon or potential Halon replacements, its overall effectiveness per unit of stored volume is significantly worse than that of Halon. In addition, it cannot be used on solid material fires and the method of achieving effective discharge on other fires is also more complicated, requiring a tube to horn delivery rather than nozzle discharge. Halon 1301, historically used in extinguishing systems on all Engines/APUs fitted to civil transport aircraft is no longer manufactured and has been banned (for new systems) since 1994; often they are now replaced by HFCs (Hydrofluorocompounds).
- Fire in the Air
- Passenger Cabin Fire
- In-Flight Fire: Guidance for Flight Crews
- Aircraft Fire Extinguishing Systems
- FAA Advisory Circular AC 120-80A In-Flight Fires;
- UK CAA: A Benefit Analysis for Enhanced Protection from Fires in Hidden Areas on Transport Aircraft;
- Lithium batteries: safe to fly?, C. Bezard et al., Airbus Safety First No. 21, pp. 22-41, January 2016;
FAA research reports