There are different classifications of fires differentiated by the type of material that is burning. In general terms, there are four primary fire types:
- Class A - Ordinary combustibles (solid material fires) - wood, paper, plastic etc,
- Class B - Flammable liquids or gases - fuels, alcohol, aerosols,
- Class C - Electric fires,
- Class D - Combustible metal fires - magnesium, potassium etc.
Fires of all classifications cannot be dealt with in the same way. As a consequence, the airport rescue and fire fighting services (RFFS) must have a variety of fire suppression tools at their disposal. This article identifies the three primary categories of fire extinguishing agents currently used by the RFFS in dealing with aircraft fires. These categories are:
- Primary Agents
- Supplementary Agents
- Other Agents
For information about the portable fire extinguishers carried on board of the aircraft together with the automated aircraft fire extinguishing systems see the dedicated article on SKYbrary: Aircraft Fire Extinguishing Systems
Foam is the primary agent used for extinguishing aircraft fires. Foam fire suppressant consist of a combination of bubbles of a lower specific gravity than that of hydrocarbon fuels or water. The foam has strong cohesive qualities and is capable of covering and clinging to vertical and horizontal surfaces. Aqueous foam cools hot surfaces by its high water retention ability and can flow freely over a burning liquid surface to form a tough, air-excluding blanket that seals off explosive and flammable vapours from access to air or oxygen. Good-quality foam should be dense and long lasting, capable of resisting disruption by wind or draft, stable to intense thermal radiation, and capable of re-sealing in event of mechanical rupture of an established blanket.
Supplementary agents are also referred to as secondary agents. Agents that fit into this category are carried on rescue vehicles to handle unique fire fighting requirements most common to airport fire fighting use. Supplementary agents are employed either singly or in combination with foam to accomplish particular aircraft fire fighting operations such as a three dimensional running fuel fires.
This class of agents include:
- Dry Chemical
- Halotron® I
- Carbon Dioxide
In general there are other special-use fire extinguishing agents available to airport fire fighting services. In particular, those agents used to combat Class D fires (Combustible metals), such as magnesium fires, are referred to as combustible metal agents. Under certain fire situations “wetting agents” also may be appropriate. These agents can be either in the form of liquid or powder. A wetting agent is defined as a chemical compound that, when added to water in proper quantities, materially reduces its surface tension, increases its penetrating and spreading abilities, and might also provide emulsification and foaming characteristics. These agents should not be mixed in any primary agent tanks.