While fires pose a dangerous threat in any given situation, they can be especially alarming aboard an aircraft. In fact, cabin and engine fires are some of the most dangerous situations a flight crew can face. Beyond an aircraft being equipped with smoke detection devices, fire blankets, portable fire extinguishers, an aircraft firewall to protect internal engine components, and other various fire prevention/mitigation instruments, one of the most important is an engine fire extinguishing system. As such, this blog will provide an overview of the engine fire extinguishing system, its distinguishing features, and importance within an aircraft.
Commuter aircraft certified under 14 CFR part 23 are required to have a one-shot fire extinguishing system while all transport category aircraft certified under 14 CFR part 25 are required to have two discharges, each of which produce a sufficient amount of agent concentration. The fixed fire extinguisher systems used in most engine fire protection systems are equipped to dilute the atmosphere with an agent that does not support combustion. Most systems contain perforated tubing or discharge nozzles to disperse the extinguishing agent. In the case of high rate of discharge (HRD) systems, they use open-end tubes to deliver a certain amount of the extinguishing agent in 1 to 2 seconds.
The most common extinguishing agent is Halon 1301, not only because of its enhanced firefighting capability, but for its relatively low toxicity. More than that, its noncorrosive nature makes it so it does not affect the material it contacts and requires no clean-up when dispersed. Some military aircraft use HCL-125, which the FAA is currently testing for use in commercial aircraft.
As a part of the engine fire extinguishing system, there are several fire extinguisher containers (HRD bottles) that store a liquid halogenated extinguishing agent and a pressurized gas, which is usually nitrogen. The containers can be made of stainless steel, titanium, and other various materials. Moreover, each container is equipped with a temperature/pressure sensitive safety relief diaphragm that prevents the pressure within the container from exceeding container test pressure in the case that the container has been exposed to varying temperature extremes.
The second kind of discharge indicator is differentiated by the yellow color of the disk. As the flight crew activates the fire extinguisher system, the yellow disk is released from the skin of the aircraft fuselage. Similarly, the maintenance crew takes this as visual evidence that the fire extinguishing container needs to be replaced.
A fire switch is also an important component that is generally installed on the center overhead panel or center console in the flight deck. When the fire switch is activated, there are a series of things that happen. Upon activation, the engine stops because the fuel control shuts off, the engine is isolated from the aircraft systems, and the extinguishing system is activated.
Lastly, the fire extinguishing system is designed with a visible and audible warning system that is installed in the cockpit to alert the flight crew. Beyond a loud horn, several warning lights are illuminated which let the flight crew know that an engine fire has been detected. Once the fire has been extinguished, the indications will cease.
While knowing the different components within the engine fire extinguishing system is important, it is paramount that the fire extinguisher containers, visual and audible warning systems, and fire switches are in optimal condition. With this in mind, routine checks and inspection of such components should not be neglected.
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