Fire Extinguishing Agents and Hand Extinguishers

No single extinguishing agent suits every fire. Water is excellent on burning paper and dangerous on burning fuel; powder smothers a brake fire but would blind a pilot on the flight deck; halon stops almost any flame but does not cool what is left behind. Aircraft therefore carry several agents, each where the likely fire calls for it, and crews are trained to match the extinguisher to the fire.

This article covers the agents themselves and the hand fire extinguishers carried in the cabin and on the flight deck. The fixed systems that use them are described in engine fire protection and cargo and lavatory fire protection, and the crew's handling of a fire on board in in-flight fire, smoke and fumes.

The fire triangle

Combustion needs three things at once, known as the fire triangle:

• fuel, anything that burns: kerosene, oil, hydraulic fluid, paper, fabric, plastics;
• heat, enough to reach the ignition temperature, from a spark, a hot surface or the fire itself;
• oxygen, normally from the air.

Remove any one side and the fire goes out. Each agent attacks a different side. Water cools, removing heat. Carbon dioxide and foam smother, removing oxygen. Closing the fuel valves, as in an engine fire drill, removes the fuel. Halons do something more: they interrupt the chemical chain reaction in the flame, which is why they act so fast.

Extinguishers are rated for classes of fire. The US classes used in FAA guidance are:

The European standard EN 2 letters the classes differently, with Class C for flammable gases and no class for electrical fires, so the pictograms on an extinguisher's label are the safer guide.

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Halon extinguishing agents

Halon is the name for a family of halogenated hydrocarbons containing bromine. The number gives the atoms in each molecule, in the order carbon, fluorine, chlorine, bromine:

• Halon 1211, bromochlorodifluoromethane or BCF (CF2ClBr), is the agent of the classic cabin and flight deck extinguisher. It discharges as a heavy blanketing mist that can be aimed at the base of a fire.
• Halon 1301 (bromotrifluoromethane), or BTM (CF3Br), is used in fixed systems for engines, APUs and cargo holds. It discharges as a vapour that spreads through an enclosed space but is harder to direct, and it has very low toxicity.

Both work mainly by chemistry. In the heat of the flame the halon releases bromine atoms, which capture the free radicals that carry the combustion chain reaction; the bromine is released again and repeats the process, so a small amount of agent stops a large flame. The dense vapour also blankets the fire. A halon fire extinguishing agent is non-conducting, non-corrosive and leaves no residue, so it can be used on electrical fires without damaging equipment, and Halon 1211 neither freezes nor causes cold burns. It is also good at preventing re-flash while it remains concentrated.

Its limitations follow from how it works:

• It does not cool. A deep-seated fire in seat foam or fabric can flare up again once the halon has dispersed, so it is followed up with water and watched for the rest of the flight. A lithium battery fire likewise needs water or another non-alcoholic liquid after the halon, to cool the cells.
• Its fumes are harmful in a confined space. Halon and the products it forms in a fire must be cleared by ventilation. Before a halon extinguisher is discharged on the 737's flight deck, all crew members there put on oxygen masks with 100 % oxygen and EMERGENCY selected, and after a full extinguisher has been emptied the agent may take up to 7 minutes to dissipate.
• It depletes the ozone layer. Under the 1987 Montreal Protocol, production of halons has ended. Aviation continues to use recycled stock while replacement agents are introduced, and halon-free hand extinguishers are already approved on types such as the A320 family.

Carbon dioxide extinguishers

A carbon dioxide (CO2) fire extinguisher holds liquefied CO2 under pressure. It discharges as a cold gas and snow at about −78°C and smothers the fire by displacing the oxygen around it. CO2 does not conduct electricity and leaves no residue, so it suits electrical and flammable liquid fires.

Its drawbacks limit it mainly to ground use. In a confined space the gas that displaces oxygen from the fire also displaces it from the people fighting it, so it can asphyxiate, and the cold discharge can cause cold burns to the skin. On a running engine the gas is quickly blown away. ATPL texts identify CO2 extinguishers by a black label on a red or black body.

Dry powder extinguishers

A dry powder fire extinguisher discharges a fine powder, the dry powder extinguishant, typically potassium bicarbonate, that smothers the fire. It is very effective on flammable liquid fires, and special powders are the agent for burning metals.

On aircraft its classic use is on wheel and brake fires. Powder has no cooling effect, and that is its advantage there: water or foam on red-hot brake parts can cool them so suddenly that they crack or burst, and a hot wheel may explode. The standard ATPL answer is that dry powder is the only suitable extinguishant for a wheel or brake fire.

Powder is not used inside the aircraft. Its cloud blinds anyone nearby, and its residue can put serviceable electrical and avionic equipment out of action. The operational rules therefore forbid dry chemical extinguishers on the flight deck and in any compartment not separated from it, and they are not used in the cabins of pressurised aircraft.

Water and water-glycol extinguishers

Water puts a fire out mainly by cooling it, which makes it the right choice for Class A fires in paper, fabric, wood and seat cushions, the materials of a cabin. A water-glycol fire extinguisher contains water mixed with glycol as an antifreeze, so that it still works at temperatures down to about −20°C, for example in a cabin cold-soaked overnight. On the 737 the cabin water extinguisher is pressurised by a CO2 cartridge when its handle is turned fully clockwise, and it is used on fabric, paper and wood fires only.

Water must never be used on electrical fires, where it conducts and endangers the user, or on burning liquids and grease, which it spreads. In the cabin it complements halon: halon knocks the flames down, and water then cools the burnt material so that it does not reignite.

Foam extinguishers

A foam fire extinguisher discharges a foam that floats on a burning liquid. The blanket shuts out oxygen, cools the surface and stops the vapour from reigniting, which makes foam the agent for fuel spills and fuel fires.

Foam is a ground-use agent. It is messy, conducts electricity and would do more harm than good in a cabin. Its home is the airport rescue and fire-fighting service, whose vehicles use aqueous film-forming foam (AFFF), and the fuel spill procedures of refuelling (see refuelling safety and fuel emergencies). ATPL texts give foam extinguishers a cream label.

Hand extinguishers in the cabin

Hand fire extinguishers are the crew's first line of defence against a fire in the cabin or on the flight deck. The EASA rule, CAT.IDE.A.250, sets their number by passenger seating:

In addition, at least one extinguisher containing Halon 1211 or an equivalent agent is kept on the flight deck, suitable for fluid and electrical fires, and one is placed in each galley that is not on the main passenger deck. The cabin extinguishers are distributed through the cabin. The US rule, 14 CFR 121.309, uses the same seating scale and also requires an extinguisher in each Class E cargo compartment. Light aeroplanes that carry an extinguisher usually have a halon or halon-replacement type, because it works on the likely fuel and electrical fires without blinding the pilot or damaging the avionics.

Using one well matters as much as carrying it:

1. Before flight, check that it is in place, secured and charged: the pressure gauge of a halon extinguisher shows an acceptable range, a recharge range and an overcharged range.
2. Choose the agent: halon or equivalent for electrical and liquid fires and most cabin fires; water for burning paper, fabric and seats, never for electrical or liquid fires.
3. Aim at the base of the fire, not the flames, in short bursts.
4. Follow up: cool deep-seated fires and lithium batteries with water, watch the area for the rest of the flight, and ventilate to clear halon fumes.

Crew members fighting a fire wear protective breathing equipment, which supplies oxygen for at least 15 minutes, and the flight crew keep their oxygen masks on; both are described in in-flight fire, smoke and fumes.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.