Let's pick this up with the extinguishing agents — Page 302, Lesson 379

Let's pick this up with the extinguishing agents. We've already covered the dry powder type, so I want to carry that thought forward: dry powder is the agent that should not be used on electrical fires, and it's best known for its application against wheel and brake fires. As a powder, it has no cooling effect, and that's actually a deliberate advantage — because it doesn't cool, it reduces the danger of wheel explosions or the distortion of the brakes or wheels. Heat is what distorts metal, so not dumping cold on a hot brake assembly is what protects it. Now, beyond dry powder, there's a whole family of ground-use extinguishers. For ground use you have foam — that's the cream or red container with a cream label — water in a red container, carbon dioxide, BCF, and sand. Each one has a specific job and a specific limitation, and I want you to know them cold. Let's start with carbon dioxide, CO₂. It's stored in black or red containers with a black label. It is non-corrosive, and it extinguishes the flame by dissipating the oxygen in the immediate area — it smothers the fire by pushing the oxygen away. From the standpoint of toxicity and corrosion, it is the safest agent to use, and for many years it was the most widely used. But there's a catch: if handled improperly, it can cause mental confusion and suffocation. That's because it's displacing the oxygen you need to breathe. It also requires a stronger container than most other agents, and the reason is its variation in vapour pressure with changes of temperature — as temperature changes, the pressure inside the container swings, so the bottle has to be built to handle that. Now here's the critical limitation for you as a professional: the use of this agent in aircraft is not permitted. That's a hard rule. On the ground, though, carbon dioxide may be used against most fires, and it is particularly useful against engine fires because it will extinguish the fire without damaging the engine. It can also be used as a substitute for dry chemical against wheel and brake fires — but note the technique: it should not be sprayed directly onto the wheel, but alongside it, to blanket the wheel with a CO₂ cloud. That's the same logic as the dry powder — you're avoiding a thermal shock on the wheel assembly. Next, foam. Foam is the principal extinguishant for use on flammable liquid fires and propane. It blankets the flames by excluding oxygen — it lays a layer over the burning liquid and cuts off the air supply. Then water. Water is used on combustible material fires, and it extinguishes by cooling. But it must not be used on electrical, fuel, or brake fires. Electrical because water conducts, fuel because it spreads the burning liquid, and brake because of the thermal shock we just talked about. So the pattern to hold onto: each agent has a mechanism — CO₂ and foam smother by excluding oxygen, water cools, dry powder smothers without cooling — and each has a hard prohibition. CO₂ is banned in the aircraft entirely, water is banned on electrical, fuel, and brake fires, and dry powder is banned on electrical fires. That's the core of what you need to remember here.