Fire Detection and Protection — Page 292, Lesson 371

Let’s pick up with the gas-filled detector, because this is the second major type of fire detection system you’ll see on turbine engines, and it works on a completely different principle than the fire wire we just covered. Here we have a continuous stainless steel tube. Inside that tube is a core material that absorbs gas. The tube is routed strategically around the engine, wherever a fire is likely to occur. Gas is forced into the tube under pressure, and it’s partially absorbed by that core material before the tube is sealed off. Now here’s the clever part. When the tube gets heated, the absorbed gas is released from the core material, and the pressure inside the tube builds up rapidly. That pressure rise is sensed by a pressure switch at the end of the tube. The switch sends a signal through a system control box, and that initiates a fire warning on the flight deck. But this system doesn’t just detect fire. It can also detect an overheat condition within the fire zones, which might be caused by a hot gas leak from a bleed air supply. So the same pressure rise that indicates fire can also indicate a dangerous overheat. Just like the fire wire, this gas-filled tube is positioned around the fire zones in a double loop. Both loops are required to detect a fire before a warning is given. That’s a redundancy feature — it reduces the chance of a false warning. Now, what happens if the tube gets breached? If the integrity of the tube is compromised and the gas escapes from the core, the same pressure switch that sensed the pressure rise from heat will now sense a drop in pressure. Instead of a fire warning, it signals a Loop Fault on the control panel or on the electronic systems display. And here’s a critical note you must remember as a professional: any fault within a fire detection system that could give rise to a false fire warning must be treated as a real fire. You never assume it’s a false alarm — you respond as if there’s an actual fire until you can prove otherwise. Now let’s talk about the fire test. Before flight, there must be a means to test the fire circuit. That’s the fire test selector on the flight deck. When you select it, indications identical to a real fire warning are displayed on all engines. This tests circuit continuity. Here’s the key point: if there’s a break in the fire warning system, no fire test will be given for that particular engine. Similarly, a leak in the gas-filled system will negate a warning. So the test itself verifies the integrity of the entire detection loop. On some aircraft, the system may be designed to give a warning if a single fire loop has failed, meaning the system is now operating on a single loop instead of the required double loop. Depending on the aircraft type, limited leg operations may be permitted in that single-loop mode — meaning you can fly a limited number of legs before the fault must be rectified. So to summarize what we have: the gas-filled detector uses a pressurized tube with an absorbent core. Heat releases gas, pressure rises, a switch senses it, and you get a fire or overheat warning. A breach causes a pressure drop and a loop fault. And the fire test verifies circuit continuity before every flight.