Fire Detection and Protection — Page 292, Lesson 367

Right, let's get into the Fire Detection and Protection chapter. We're starting fresh here, so I want to set the scene properly. First, the fundamental philosophy. By design, aircraft are intrinsically safe. That's the baseline—engineers build them to be safe. But the regulations demand that we cater for a "worse case scenario." We have to assume that something can go wrong, and we must be ready for it. So, to that end, a Fire Detection and Protection system must be fitted in three specific places: the engines, the APUs—that's the Auxiliary Power Unit, the small gas turbine in the tail—and the main wheel wells. These areas are formally defined as Designated Fire Zones. And I want you to remember the exact definition because it's a precise regulatory term. A Designated Fire Zone is: "Areas where a potential fire risk may exist following failure or leakage of any component or associated equipment." So it's not just where fire could start spontaneously; it's where a component failure or a fluid leak could create the risk. Now, to contain a fire once it starts, we establish fire zones in the engines and APUs. That means we build a series of fireproof bulkheads around them. These bulkheads physically isolate the fire so it can't spread to the rest of the aircraft structure. There's a critical design requirement here, and it's a subtle one. The fire detection system must be capable of providing rapid detection of a localized fire or an overheat condition. But—and this is the key point—it must not automatically operate the fire extinguishers. Detection and extinguishing are separate functions. The system warns the crew; the crew makes the decision to discharge the extinguisher. That's a deliberate design philosophy to prevent an automatic discharge that might not be appropriate. Let me also mention the Triangle of Fire, which is shown in Figure 15.1. It's the classic concept: fire needs three elements—fuel, oxygen, and heat. Remove any one of them and the fire goes out. We'll come back to that when we discuss extinguishing agents later. Now, let's move into the detection systems themselves. The detection method varies according to the position of the equipment. There are four methods, and we're covering the first two in detail right now. The first is the Melting Link Detector. This is found in older aircraft. Its construction is simple: a pair of contacts held apart by a fusible plug. A fusible plug is a plug made of a material with a low melting point. At a predetermined temperature, that plug melts. When it melts, the contacts are no longer held apart, so they close, and that completes the fire warning circuit—the warning lights up. But here's the major drawback, and it's a significant one. Once the fire has been extinguished and the temperature drops, the contacts will not open again. The plug has melted; it's gone. So the circuit stays closed, and you get a permanent fire warning even though there's no fire. That's why it's obsolete on modern aircraft, but you need to know it exists. The second method is the Differential Expansion Detector. This one is cleverer, and it operates on the principle of the differential rate of expansion of dissimilar materials. Let me explain that. Different materials expand at different rates when heated. This detector exploits that. Its construction: you have a pair of contacts mounted on a spring bow assembly. That whole assembly is fitted within an expansion tube, which is mounted on a base. So you have an outer tube and an inner bow. Here's the operating principle. When heat is applied, the expansion tube expands at a greater rate than the bow. Because the tube grows faster, it draws the contacts together. When the contacts touch, power is provided to the Fire Warning Circuit—you get your warning. Now, the beauty of this design is what happens when the temperature drops. The tube shortens again as it cools. That causes the contacts to open, and the warning is cancelled. So unlike the melting link, this detector is self-resetting. It gives a warning only while the overheat condition actually exists. This type of unit is often used as a monitor on Engine Cooling Air Outlets to provide what's called Internal Engine Overheat warning—abbreviated IEOH. So it's not just for detecting a fire; it's monitoring the engine's internal temperature through the cooling air outlets. One more important feature: this type of detector usually incorporates a short time delay before the warning is activated. Why? To prevent false warnings due to vibration. If the contacts momentarily touch because of vibration, the time delay ensures the warning doesn't trigger unless the condition persists. That's a practical engineering solution to a real-world problem. Let me show you the construction in Figure 15.2, which illustrates the differential expansion detectors. You can see the expansion tube, the spring bow, and the contacts. So to summarize what we've covered: we have the concept of Designated Fire Zones, the requirement for rapid detection without automatic extinguishing, and two detection methods—the melting link with its permanent-warning drawback, and the differential expansion detector with its self-resetting capability and time delay. Next, we'll look at the remaining two detection methods.