A Brief Introduction to the APU Fire Detection and Extinguishing System

With the aircraft engine operating at extremely high temperatures, there is always a risk of fire in the engine bay. As such, aircraft employ a fire detection and extinguishing system for both the main engine and Auxiliary Power Unit (APU). As a small engine that operates separately from the main engine, the APU is used to provide constant power to less demanding items, such as heating and air conditioning units, and as a source of electrical power for starting the engine. Nevertheless, similar to the main engine, the APU must have a fire protection system. This system is similar to the engine fire protection systems, but with several key differences. Here, we will be discussing these differences, alongside other key details about the APU fire detection and extinguishing system.

How Does It Work?

The fire protection systems on aircraft work by using one or more of the following: overheat detectors, rate-of-temperature-rise detectors, and flame detectors. These devices will be placed in potential fire zones on the plane, with the APU compartment counting as a fire zone in itself. As such, the APU compartment is separated from the rest of the airplane by a firewall. The APU system uses the same fire mitigation principles as the main engine, including minimizing the potential for ignition, utilizing firewalls, providing a means to extinguish the fire, and more. When you separate the main function of the APU fire protection system, it can be split into two central parts: the fire warning and fire bottle discharge.

APU Fire Warning

In the event of an APU fire, the APU fire detection system gives fire warnings and automatically stops the APU so that the fire can be taken care of without further damaging the system. When this happens, an APU fire warning light activates to identify the correct fire switch to use to extinguish present flames. Once initiated, the fire switch solenoid releases a mechanism so that it can be pulled up by an operator. If the APU is still running at that point, it will automatically stop when the fire switch is pulled, isolating the APU from other airplane systems to avoid causing further damage.

Fire Bottle Discharge

If shutting off the APU does not put out the fire, the next step is to discharge a fire extinguishing agent. This means that, if the fire warning does not go away with the switch out, the switch should be re-inserted and put to the left or right DISCH position. One should hold the switch against the discharge stop for one second to ensure that it fully registers in the device. This action fires the bottle squib and releases the fire extinguishing agent into the APU compartment. Finally, one should verify that the APU bottle discharge light comes on to make sure the extinguishing agent has been discharged.


The APU fire detection and extinguishing system combines a number of different elements which can work automatically in response to signs of a fire. They may also be triggered manually by the crew, and there are certain processes to be aware of when initiating a fire bottle discharge. Aside from knowing these practices, it is also key to use high-quality components which can function well in demanding environments aircraft constantly encounter.

Here, on ASAP Aerospace, we offer an array of aircraft parts at the top of their class. Moreover, we are proudly an AS9120B, ISO 9001:2015, and FAA AC 00-56B accredited enterprise, meaning that we go to great lengths to uphold a high standard of service and quality for our customers. If you are currently on the search for reliable aircraft parts, we invite you to browse our online catalog of available products and make use of our Instant RFQ service to receive competitive quotes for your comparisons. Our team of experts are available 24/7x365 to answer customer inquiries and will reply to submitted RFQ forms in just 15 minutes or less!


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