Fire Detection and Protection — Page 302, Lesson 373

We're starting a new topic now: fire detection and protection. Let's get into it. I want to walk you through what happens in the cockpit when a fire warning actually comes up, and then how the fire protection system physically puts that fire out. We're looking at the flight deck indications first, then the drill, then the hardware. So, the first principle is this: a fire warning in the flight deck must be attention-getting, but not startling. That's a deliberate design philosophy. You want the crew to notice it immediately, but you don't want to induce panic. So the format of the warning is built around that idea. There are three typical elements to the warning. First, an aural warning — that's a klaxon, or a bell, or a continuous repetitive chime sounding. Second, a master warning caption, which will literally say something like "No. 1 engine fire." And third, a steady red fire warning light in the appropriate engine display channel. So you get sound, a text caption, and a red light — all together, all pointing at the same engine. Now, on receipt of that fire warning, the drill must be carried out in strict order. The sequence matters. A representative drill looks like this: first, a means of cancelling the aural warning — you silence the bell or chime so you can think and communicate. Then, a sequence to shut off fuel, bleed air, electrics, and hydraulics to the engine. That's isolating the engine completely — cutting its energy and services. And finally, a means of discharging the fire bottles into the engine fire zones. Now, an important note here: that drill I just gave you is a generalization. The actual aircraft emergency checklist must be consulted for the specific aircraft. I'm giving you the representative shape of the drill, not the final word for any particular type. Let's look at the cockpit hardware. I want to show you the typical fire warning indicators and the panels where the crew actually operates this system. Now, once you've completed the shutdown drill and isolated all services to the engine, you move to fire protection itself. That's the extinguishing system. This system normally comprises fire bottles — usually two per engine — connected via piping to the fire zones. At the zones, the piping forms a spray ring, and from that spray ring the extinguishant is directed around the area. So the agent is sprayed in a ring pattern to cover the whole fire zone. There's a means of discharging the fire bottle on the flight deck, and its operation follows a sequence. Let me walk you through it step by step. First, the engine shutdown drill must be completed. That's the prerequisite. Then, an electrical cartridge, situated between the base of the fire bottle and the piping, is armed. When that cartridge is armed, the SQUIB light illuminates on the engine fire panel. So SQUIB is the indication that the cartridge is ready to fire. Next, pressing the AGENT selector fires the cartridge. That allows the fire extinguishant, under pressure, to enter the spray rings in the engine. So the agent flows from the bottle, through the piping, into the spray rings. Then, the pressurized extinguishant operates a low-pressure electrical switch, which illuminates the DISCH caption on the AGENT selector. So DISCH tells you the bottle has actually discharged — the agent has left the bottle and is being delivered. Now, in the event that a single fire bottle does not extinguish the fire, a second bottle is fitted. Its activation and indication are the same as I just described. And there's a regulatory requirement behind this: CS-25 1195(c) states that two discharges must be provided for each engine. That's a certification rule — you must have the capacity for two shots per engine. Now, older aircraft may have varying flight deck indications of a bottle having been fired. For example, an indicator fuse — that's a clear small bulb which turns red on bottle firing. So instead of a DISCH caption, you might have a little bulb that changes color. There are also physical indications that a bottle has been fired, though these may not be visible externally. They include an indicator pin on the bottle head, and a bottle pressure gauge. And note — panel access may be required to see these. They're not necessarily visible from outside. Finally, there's a safety consideration: in the event that a fire bottle has been subject to excess temperature or pressure, a thermal discharge may take place. That's the bottle venting on its own due to heat or overpressure. And the indications that the bottle has thermally discharged — well, that's where the excerpt cuts off, but that's the concept: the bottle can discharge itself if it gets too hot or overpressurized. So to tie it together: you have the warning — sound, caption, red light. You have the drill — cancel aural, isolate services, discharge bottles. And you have the hardware — bottles, cartridges, spray rings, and the indications that tell you what's happening. That's the complete picture of fire protection in the cockpit.