because it tends to settle and blanket the fire. CO₂  is a dry, noncorrosive gas, which is inert when in contact with most substances and will not leave a residue and damage machinery or electrical equipment. CO₂  is a nonconductor of electricity regardless of voltage, and can be safely used in fighting fires that would present the hazard of electric shock. CO₂ extinguishes the fire by diluting and displacing its oxygen supply. If gaseous CO₂    is directed into a fire so that sufficient oxygen to support combustion is no longer available, the flames will die out. CO₂  has limited cooling capabilities, and may not cool the fuel below its ignition temperature. It is more likely than other extinguishing agents to allow reflash. Therefore, the fire fighter must remember to stand by with additional backup extinguishers. NOTE: CO₂ is not an effective extinguishing agent for fires in materials that produce their own oxygen supply, such as aircraft parachute flares or fires involving reactive metals, such as magnesium and titanium. Halon 1211 Halon is a halogenated hydrocarbon. Halon 1211, known chemically as  bromochlorodifluoromethane,  is colorless and has a sweet smell. Halon attacks the fire by inhibiting the chemical chain reaction. Halon decomposes upon contact with flames or hot surfaces above 900°F (482°C). Halon 1211 is used for twin agent (AFFF/Halon 1211) applications on board flight and hangar deck mobile fire-fighting equipment. For flight and hangar deck fire-fighting procedures, you should refer to NAVAIR 00-80R-14, NATOPS U.S. Navy Aircraft Fire-Fighting and Rescue Manual. Potassium Bicarbonate (Purple-K-Powder or PKP) Potassium bicarbonate (PKP) is a dry chemical principally used as a fire-fighting agent for flammable liquid fires. When PKP is applied to fire, the dry chemical extinguishes the flame by breaking the combustion chain. PKP does not have cooling capabilities on fire. PKP is highly effective in extinguishing flammable liquid (class B) fires. Although PKP can be used on electrical (class C) fires, it will leave a residue that may be hard to clean. Also, when combined with moisture, it may corrode or stain the surfaces it settles on. PKP does not produce a lasting inert atmosphere above the surface of a flammable liquid. Therefore, its use will not result in permanent extinguishing if ignition sources, such as hot metal surfaces or persistent electrical arcing, are present. Reflash of the fire will most likely occur. The ingredients used in PKP are nontoxic. However, the discharge of large quantities may cause temporary breathing difficulty and, immediately after the discharge, it may seriously interfere with visibility. Q12-1. What are the four elements necessary to produce fire? Q12-2. What is the "fire point" of a substance? Q12-3. What is the "flash point" of a substance? Q12-4. What are the four classes of fire? Q12-5. What  are  the  primary  fire-extinguishing agents used aboard naval ships? FIRE-FIGHTING EQUIPMENT LEARNING OBJECTIVE: Recognize the various systems and equipment used for aircraft fire-fighting on board ships and shore activities. In assisting the crash fire fighters, you will use very specialized equipment.  A crash crew must bring its equipment  into  action  with  every  pump  nozzle delivering at its maximum capacity. Fire-fighting equipment is discussed in the following text. FIREMAIN SYSTEM You must get acquainted with the firemain system throughout your ship. You should know the location of the firemain and the riser piping that carries water to the upper decks. You must be able to identify the plugs where hoses can be attached to the mains. You must know the location of all pumps, valves, and controls in the vicinity of your duty and berthing stations. Fireplugs have outlets either 1 1/2 or 2 1/2 inches in diameter. Some plugs are equipped with wye gates that provide two outlets, each are 1 1/2 inches in size. In some cases, a reducing connection is used so that a 1 1/2-inch hose can be attached to a 2 1/2-inch outlet. Connected to the fireplugs and stored in adjacent racks are two lengths of either 1 1/2- or 2 1/2-inch diameter hose. The 1 1/2-inch hose is used on smaller ships and below decks on larger ships. This hose is made up in 50-foot lengths, with the necessary end 12-4