The fire triangle and the fire tetrahedron
[National Fire Protection Association]
- The fire triangle identifies the three needed components of fire:
- fuel (something that will burn)
- heat (enough to make the fuel burn)
- and air (oxygen)
- All three components must be present to have a fire. Fire will burn until one or more of the components are removed. Traditional fire extinguishing methods involve removing the fuel, heat, or oxygen.
- In more recent years, a fourth component – the uninhibited chain reaction – has been added to explain fire. This chain reaction is the feedback of heat to the fuel to produce the gaseous fuel used in the flame. In other words, the chain reaction provides the heat necessary to maintain the fire. The addition of this fourth component (which forms what is called the "fire tetrahedron ") more accurately describes the mechanism for fire suppression by clean agent halon replacements which break up the uninhibited chain reaction of combustion.
You need all four components for a fire, if you get rid of one the fire goes out.
Classes of Fire
[National Fire Protection Association] Fire classifications based on fuel type:
- Class A: Ordinary combustible materials, such as wood, cloth, paper, rubber and many plastics. They burn with an ember and leave an ash. Extinguish by cooling the fuel to a temperature that is below the ignition temp. Water and other extinguishing agents are effective.
- Class B: Flammable liquids (burn at room temperature) and combustible liquids (require heat to ignite). Petroleum greases, tars, oils, oil-based paints, solvents, lacquers, alcohols, and flammable gases. High fire hazard; water may not extinguish. Extinguish by creating a barrier between the fuel and the oxygen, such as layer of foam.
- Class C: Fuels that would be A or B except that they involve energized electrical equipment. Special techniques and agents required to extinguish, most commonly carbon dioxide or dry chemical agents. Use of water is very dangerous because water conducts electricity.
- Class D: Combustible metals, such as magnesium, titanium, zirconium, sodium, lithium and potassium. Most cars contain numerous such metals. Because of extremely high flame temperatures, water can break down into hydrogen and oxygen, enhancing burning or exploding. Extinguish with special powders based on sodium chloride or other salts; also clean dry sand.
- Class K: Fires in cooking appliances that involve combustible cooking media (vegetable or animal oils and fats).
Causes of Aircraft Fires
[AC 120-80, ¶3.]
- Wiring Failures. A majority of hidden in-flight fires are the result of electrical arcs along wire bundles. In most cases, the electrical arc acts as the initiating event, igniting other surrounding materials. The surface of insulation materials is often a conveyer of these initiating events, as contamination from spillage, accumulated dirt/dust, lubrication or corrosion inhibitors on these surfaces can promote flame spread (uncontaminated insulation materials are generally very fire resistant). In other instances, the re-setting of a tripped circuit breaker can overheat wiring, ultimately leading to failure and arcing, causing the same chain of events.
- Electrical Component Failures. Electrical motors can overheat, bind, and fail, and possibly ignite surrounding materials. The spread of fire in these instances is exacerbated by an accumulation of contaminants in the immediate area.
- Lightning Strikes. Although very infrequent, there have been instances in which a lightning strike has initiated a fire. In these instances faulty or contaminated insulation material contributed to the fire.
- Bleed Air Leaks. Aircraft with systems that use air from the engine (bleed air) depend on a series of pneumatic lines to deliver the air supply. A failure of any of these supply lines, if left unchecked, can cause high temperatures in the surrounding area and damage to the aircraft's equipment, wiring, and associated components. High temperature bleed air leaks have been known to cause in-flight fires and structural damage.
- Faulty Circuit Protection. A malfunctioning CB that does not open (trip) when an abnormally high current draw is detected may cause the affected unit or associated wiring to overheat and ignite.
To this Advisory Circular list we can add:
- Electronic entertainment systems and galley equipment can smoke internally and ignite, or cause an overload to a power supply system which then smokes and possibly ignites. In either case, the remedy is to remove the electrical power.
- Galley fires resulting from food preparation can ignite in ovens, food holding areas, even trash bins.
- If aircraft potable water systems are operated at reduced quantities and allowed to run dry, water heaters could overheat.
- Smoking materials.
- Lithium batteries
- Aircraft air conditioning system can be a source itself or can telegraph fumes from the engines.
- Lavatories can be a source of fire either from toilet motors or something left in trash containers.
- Undeclared HAZMAT can ignite spontaneously or as a result of reduced cabin pressure.
Indications of Hidden Fires
[AC 120-80, ¶4.]
- Abnormal Operation or Disassociated Component Failures. Failure or uncommanded operation of an aircraft component may indicate a developing fire. Electrical connections and the components themselves may have been damaged by a fire in the area of the component or at any point along its power supply line. For this reason cabin crewmembers should report all failures of electrical items to the flight crew in accordance with company policy.
- Circuit Breakers. Circuit Breaker(s) tripping, especially multiple breakers such as entertainment systems, coffee makers, etc., may be an indication of damage occurring in a hidden area common to the affected components.
- Hot Spots. Hot spots on the floor, sidewall, ceiling or other panels should be immediately investigated.
- Odor. This may be one of your first indications of an impending fire. Never ignore a strange odor; you need to identify its source as soon as possible.
- Visual Sighting – Smoke. Smoke coming from vents or seams between interior panels, especially from the ceiling area, is a sure sign of a problem, and you should take immediate action to determine the source.
[AC 120-80, ¶6.]
- Which type of fire extinguisher should I use? Immediate and aggressive action when confronted with a potential fire is much more important than delaying while you attempt to classify a particular fire. As a general rule Halon fire extinguishers are your best choice since Halon is classified as a multipurpose (Class A, B, & C fires) agent. Halon extinguishes fires by chemically interrupting a fire's combustion chain reaction, rather than by physically smothering the fire. Approved Halon-type extinguishers are three times as effective as Carbon Dioxide (CO2) extinguishers with the same weight of extinguishing agent.
- When a fire is discovered, the initial focus should be on extinguishing the fire and then following-up with the appropriate class of fire extinguisher. Generally, you should consider using the first available extinguisher rather than delaying your fire fighting efforts while you locate the most appropriate extinguisher. After you have initially suppressed the fire or exhausted the first fire extinguisher, you should use the preferred extinguishing agent for the class of fire to maintain control or extinguish the fire.
- There are a few exceptions to this general guidance. A water fire extinguisher (H2O) should not be discharged directly into a CB panel or an electrical outlet. Nor should water be used to combat a liquid fire (e.g., grease or fuel) that is pooled or has collected on a non-porous surface. The use of an H2O extinguisher on a fire fueled by flammable liquids is acceptable if the surface has absorbed the liquid, such as gasoline poured on a seat or other absorbent material.
Fire extinguisher Preflight
"TSS - There, Secure, Sealed"
- Halon — TSS, security pin in place, gauge reads in the green zone.
- Water — TSS, breakaway wire intact.
Fighting fires with handheld fire extinquishers
[AC 120-80, Appendix 4, ¶d.]
- Best results in fire fighting are generally obtained by attacking the base of the fire at the near edge of the fire and progressing toward the back of the fire by moving the fire extinguisher nozzle rapidly with a side-to-side sweeping motion.
- The effective discharge time of most hand-held fire extinguishers ranges from 8-to- 25 seconds depending on the capacity and type of the extinguisher. Because of this relatively short effective time span, the proper selection and use of the fire extinguisher must be made without delay.
- Care must be taken not to direct the initial discharge at the burning surface at close range (less than 5-to-8 feet) because the high velocity stream may cause splashing and/or scattering of the burning material.
- Ventilate the compartment promptly after successfully extinguishing the fire to reduce the gaseous combustion and gases produced by thermal decomposition.