Fire Extinguishing Systems
Onboard systems designed to extinguish fires which occur either in the air or on the ground.
For information on detecting and fighting fires in the cabin, see also the separate article: "Passenger Cabin Fire"
Four types of fire extinguishing installations are found on commercial transport aircraft.
· Portable Extinguishers installed at specified locations in both the main cabin and the flight deck
· Hold fire extinguishing systems (with automatic detection)
· Engine fire bottle extinguishing systems (with automatic detection)
· Toilet waste bin bottle extinguishing systems
Fires on board aircraft which occur within the aircraft cabin or flight deck - or are potentially directly accessible from them - arise in one of three ways:
· Fires that involve energized electrical equipment - in aircraft cabins typically IFE (In Flight Entertainment) systems in the passenger cabin, electrical equipment in the galley or avionics equipment in the flight deck or under floor avionics bay, or Personal Electronic Devices (PEDs) carried by passengers.
· Fires in ordinary combustibles such as cloth, paper, rubber, and many plastics - in aircraft cabins typically in furnishings
· Fires in flammable liquids, oils, greases, tars, oil-base paints, lacquers, and flammable gases - in aircraft cabins typically galley oven fires
Portable extinguishers present a special challenge since they must be capable of extinguishing a range of fire types - solid materials such as cabin fixtures and furnishings, flammable liquids and electrical fires.
Halon 1211 extinguishers have entirely replaced the previous combination of two different types of portable extinguisher - Carbon Dioxide and Water Glycol - on new-build aircraft and no other single extinguisher-type has yet been identified as a satisfactory alternative to it.
NOTE: Crews must be aware that the toxicity of the Halon gases, especially the combination which makes up Halon 1211, is such that use in confined spaces requires care to minimize 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 discharge to eliminate this risk, but for flight crew use on the flight deck, this will not be an option and risk awareness is the only defence.
Where the dual fit of extinguisher is encountered on older aircraft, it is essential that Water Glycol extinguishers are used on solid material fires and Carbon Dioxide extinguishers on liquid or electrical equipment fires.
The minimum dispatch requirement for aircraft portable fire extinguishers is determined by the capacity of the aircraft cabin and is specified in the Aircraft MEL.
Relevant authorities (e.g. EASA in the EU, FAA in the US) specify the requirements for hand held fire extinguishers in terms of:
· Minimum number of hand held fire extinguishers to be carried on board. This depends on the size of the aircraft, the number of passenger seats, the number and type of cargo compartments, etc.
· Hand held fire extinguisher distribution within the aircraft (e.g. number of extinguishers in the cockpit, cabin, cargo compartments, etc.).
· Mounting and marking (e.g. there should be a special sign in case the fire extinguisher is not clearly visible).
· The need for regulatory approval and acceptable means of compliance.
· Restrictions regarding the extinguishing agents used (e.g. cut-off and end dates for Halon-based extinguishers).
· Extinguishing agent quantity requirements (e.g. minimum amount of agent per extinguisher).
· Health considerations (e.g. toxic gas hazard minimization).
Hold fire extinguishing systems are usually activated as a flight crew response to abnormal heat detection in an aircraft hold, and usually operate in a dual function. Part of the available fire suppression capability is deployed in an ‘instant’, or ‘knock-down’, discharge of extinguishing agent and the remainder is deployed more gradually over a longer period of up to an hour, to assist in preventing re-ignition or at least providing partial fire suppression, to provide more time to get an aircraft with a continuing hold fire warning back on the ground. Various alternatives to Halon 1301 have been examined including water misting, inert gas and dry powder, either alone or in combination. The FAA has developed minimum performance standards for these systems and it has been demonstrated that although water misting alone is unable to pass the exploding aerosol can fire test, a combination of water misting and inert gas (nitrogen) discharge may be more effective. However, for such a solution to be viable, a means of on-board nitrogen generation will be needed.
Fire Bottles in engine compartments are usually electrically operated after manual selection by the flight crew based upon automatic fire detection. In the airborne case, APU fire bottles are similarly activated but it is usual for automatic APU fire detection during ground operation to trigger automatic shutdown and fire extinguisher activation. Until recently, the most common extinguishing agent was Halon 1301 for all Engines/APUs fitted to civil transport aircraft. However, Halon 1301 is no longer manufactured and has been banned (for new systems) since 1994; often they are now replaced by HFCs (Hydrofluorocompounds).
Toilet waste bin fire extinguishers are activated automatically if heat detectors in the vicinity are activated. Toilet Smoke detector activation does not trigger waste bin fire extinguishers. Alternative extinguishing agents to Halon 1301 have been approved for use in fixed toilet waste bin systems and have also been, uniquely in terms of the search for Halon alternatives, shown to be more effective than Halon 1301 units whilst being the same size. Since only a documentation change is required to fit these alternative extinguishers, they have been used for retrofit as well as in new-build aircraft.
Equipment has been introduced designed to deal specifically with lithium battery fires in PEDs; Lithium ion batteries (Li-ion) are used to power PEDs such as cellular phones, portable tablets, EFBs and digital cameras; Li-ion batteries are rechargeable. Non-rechargeable lithium batteries (Li-metal) are similar to Li-ion, but use a different electrode material – metallic lithium.
All lithium batteries present a potential fire hazard. These batteries are carried on aeroplanes as cargo, within passenger baggage, and by passengers directly. Like some other batteries lithium batteries are capable of delivering sufficient energy to start an in-flight fire. Lithium batteries present a greater risk of an in-flight fire than some other battery types because they are also unable to contain their own energy in the event of a catastrophic failure.
Once extinguished, a lithium battery fire – or a fire in a PED powered by lithium batteries – requires containment and continued cooling. Halon 1211 or water fire extinguishers are effective at extinguishing the fire and preventing its spread to additional flammable materials. After extinguishing the fire, dousing the electronic device with water or other non-alcoholic liquids cools the device and prevents additional battery cells from reaching thermal runaway. Containment devises are now available and where these are equipped, crews should receive specific training in how to use them to greatest effect.
These issues are discussed in some detail in the RAeS document "Smoke, fire and fumes in transport aircraft, past history, current risks and recommended mitigations - Part 1:References". See also the separate articles: "Aircraft Fire Risk from Battery-powered Items Carried on Aircraft" and 'Personal Electronic Device Fire - Cabin Crew Checklist".