Commuter aircraft certified under 14 CFR part 23 must be equipped with, at minimum, a one-shot fire extinguishing system, whereas all transport category aircraft are required to have a two-shot system. Regardless of the type of system in question, each produces ample agent concentration. An individual one-shot system can be utilized for auxiliary power units (APUs), fuel burning heaters, and other combustion equipment. Meanwhile, for “other” designation fire zones, two-shot systems are used, each of which generates sufficient agent concentration as well.
Fire Extinguishing Agents
Fixed fire extinguisher systems used in a majority of engine fire protection systems are generally designed to dilute the atmosphere with an inert agent that does not support combustion. To distribute the extinguishing agents, many systems opt for perforated tubing or discharge nozzles. In particular, high rate of discharge (HRD) systems utilize open-end tubes to deliver a specific amount of extinguishing agent in 1 to 2 seconds. When compared with other types, Halon 1301 serves as the most common extinguishing agent still used today because of its effective firefighting capability and relatively low toxicity.
Turbine Engine Ground Fire Protection
On a wide range of aircraft, there is a mechanism in place for rapid access to the compressor, tailpipe, or burner compartments. This usually consists of spring-loaded or pop-out access doors in the skin of various compartments. When facing internal engine tailpipe fires that occur during engine shutdown or false starts, the fire can be easily blown out by motoring the engine with the starter. To achieve the same ends, a running engine can be accelerated to a rated speed. If the fire persists, a fire extinguishing agent can be directed into the tailpipe. Keep in mind that excessive use of CO2 or other agents that have a cooling effect can shrink the housing on the turbine, causing the engine to disintegrate.
Fire extinguisher containers (HRD bottles) made of stainless steel store liquid halogenated extinguishing agents and pressurized gas, typically nitrogen. Based on design considerations, alternate materials like titanium are also common. In most cases, these containers have a spherical design that provides the lightest weight possible, but cylindrical shapes are also available where space limitations are a factor. Moreover, each container features a temperature/pressure sensitive safety relief diaphragm that prevents container pressure from exceeding container test pressure in the care of exposure to excessive temperatures.
Discharge valves are installed on the containers, and usually, a cartridge and a fragile disc-type valve are incorporated in the outlet of the discharge valve assembly. However, special assemblies that have solenoid-operated or manually-operated seat type valves are also available, and two types of cartridge disc-release techniques are utilized. The first are standard release types that take advantage of a slug driven by explosive energy to rupture a segmented closure disc. The second are used in high temperature or hermetically sealed units, those of which use a direct explosive impact type cartridge that applies fragmentation impact to rupture a prestressed corrosion-resistant steel diaphragm.
Typically, a vibration-resistant helical bourdon-type indicator is popularly used as a means to verify the fire extinguisher agent charge status, while a combination gauge switch provides a visual indication of the container pressure and emits an electrical signal if container pressure is lost.
Two-Way Check Valve
Two-way check valves are used in two-shot systems to prevent the agent in a reserve container from making its way into the previously emptied main container.
Discharge indicators provide an immediate indication of container discharge on fire extinguishing systems. There are two types of indicators, thermal and discharge, both of which are designed for aircraft and skin mounting.
Fire switches are located on the center overhead panel or center console in the flight deck, and when actuated, the engine stops and the fire extinguishing system is activated. Some switches are available as pull and turn types, while others are push-type switches with a guard.
Visible and audible warning systems are situated in the cockpit to alert the flight crew, many of which include horn sounds and one or several warning lights that illuminate to alert the flight crew that an engine fire has been detected.
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