Smoke and fire-gas production are determined by the chemical composition of the material, the fire environment, and in particular the available oxygen. Smoke is defined as the airborne solid and liquid particulates and gases evolved when a material undergoes pyrolysis or combustion (ASTM, 1994). Fire gases, according to ASTM (1994), are airborne products emitted by a material undergoing pyrolysis or combustion that exist in the gas phase at the relevant temperature. Smoke density is influenced by composition, the rate of burning or intensity of the fire and the degree of ventilation.

Fundamental flammability principles are as follows (NFPA, 1988):

  • An oxidizing agent, a combustible material, and an ignition source are essential for combustion.

  • The combustible material must be heated to its piloted ignition temperature before it will ignite or support flame spread.

  • Subsequent burning of a combustible material is governed by the heat feedback from the flames to the pyrolyzing or vaporizing combustible.

  • The burning will continue until the combustible material is consumed, or the oxidizing agent concentration is lowered to below the concentration necessary to support combustion, or sufficient heat is removed or prevented from reaching the combustible material to prevent further fuel pyrolysis, or the flames are chemically inhibited or sufficiently cooled to prevent further reaction.

Toxicity

The toxic gases and irritants that are present in all smoke should be considered potential dangers. Toxic products can cause both acute and delayed toxicological effects. It is the acute and extremely short-term effects that prevent escape from an aircraft fire by causing faulty judgment, incapacitation, and death. The irritants in the smoke can also interfere with the ability of passengers to escape by causing severe coughing and choking and by preventing them from keeping their eyes open long enough to find the exits. In addition, delayed effects, such as tissue or organ injury, mutagenicity, carcinogenicity, and teratogenicity, may ultimately lead to permanent disability and post-exposure deaths among accident survivors.

Toxic potency is "a quantitative expression relating concentration (of smoke or combustion gases) and exposure time to a particular degree of adverse physiological response (e.g., death on exposure of humans or animals).... The toxic potency of smoke from any material, product or assembly is related to the composition of that smoke which in turn is dependent upon the conditions under which the smoke is generated" (ASTM, 1994). The LC50 (the concentration that causes death in 50 percent of the test organisms in a specified time) is a common endpoint used to assess toxic potency. In the comparison of the toxic potencies of different compounds or materials, the lower the LC50 (i.e., the smaller the amount of material necessary to reach this endpoint), the more toxic the material is.

FIRE SCENARIOS

For fire-resistant materials, self-sustaining interactions of individual materials are of minimal importance to the real fire scenario, even in terms of local ignition and sustained smoldering.1 Fire resistance is generally sufficiently effective to inhibit flammability under all but extreme heat loads resulting from either external source coupling or large-scale involvement. Thus, important issues are dominated by systems interactions to heat exposure from surroundings rather than the fire characteristics of individual materials themselves. However, the nonthermal hazards (visibility, production of irritant gases, and toxic product generation) are more strongly connected to the characteristics of individual materials in response to these surrounding interactions.

In general, the dominating initial heat source is that radiated or convected from surrounding fuel fires. In large-scale fires such as fuel pool fires, radiative heat transfer is expected to dominate the convective component (Hottel, 1959). In the case of aircraft interior fire characteristics, pool fire plume impingement on the inside of the cabin through open egresses or structural failures can also be important.

Once the fire spreads to the inside of the aircraft interior. the heat release from the burning of local materials can contribute to further evolution of the overall fire scenario. eventually leading to flashover. Understanding space flashover and predicting the likelihood of its occurrence is Critical in assessing materials response in aircraft fires. Flashover is the point at which most of the combustible

1  

Smoldering is the combustion of a solid without flame (ASTM, 1994) and is characterized by a glowing combustion supported by strong exothermic char oxidation reactions. Smoldering usually occurs in well-thermally-insulated, natural polymers such as cellulosic materials and wood products and rarely occurs in synthetic polymers. The cables used in transport aircraft are generally made of Kapton2® which generates a lot of char but it is not known to smolder. Phenolic composite and Tedlar (polyvinyl fluoride) constructions used commonly in interiors do not smolder nor do the fiberglass or polyimide foam materials used in cabin insulation. Smoldering is more often observed in buildings where cellulosic insulation or cotton fabrics are used in upholstered furniture.



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