The complementary extinguishing agent should be: a) dry chemical powders (classes B and C powders); or b) other extinguishing agents with at least the same firefighting capability.
Both principal and complementary agents should normally be provided at an airport. Principal agents produce a permanent control, i.e. for a period of several minutes or longer. Complementary agents have rapid fire suppression capability but offer a “transient” control which is usually only available during application.
Complementary agents do not generally have any substantial cooling effect on liquids or materials involved in fire. In a major fire situation extinguishment achieved by complementary agents may well only be transient and danger of “flashback” or re-ignition may occur when foam is not available to secure a fire. They are particularly effective on concealed fires (e.g. engine fires) in aircraft freight holds and beneath wings, where foams may not penetrate and on running fuel fire situations, on which foams are ineffective. They are known as complementary agents because while they may have the capability of rapid fire suppression (when applied at a sufficient rate), it is generally necessary to apply a principal agent simultaneously or at least before flashback can occur in order to achieve permanent control. Considerably improved complementary agents have become available in recent years, and constant studies are still being made in both the dry chemical and halocarbon fields.
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