How do you know if you have a fire on an airline jet? If you're flying a CRJ, or any other airliner for that matter, you most likely won't be able to see it. But you will know it's there. And, you'll know because of something called a fire loop.

What Is A Fire Loop?

A fire loop is a metal loop, just like what's pictured below:

It's called a loop, because it's shaped like one (good thinking, huh?). There's one loop in each main gear bay. They're located in several other parts of the aircraft as well, like the engines and APU, but we'll stick with the gear bay for this article.

The loop has an outer metal sheath, but what's happening on the inside is what really counts.

On the inside of the metal sheath, there are two wires that are separated by an insulating material. One of the wires is connected to a ground, and the other wire is connected to the aircraft's fire detection control unit.

So how do two wires that are separated with an insulator detect a fire? We'll get to that in a second, but first, why would you have a fire in your wheel well in the first place? Let's take a look.

Why Would You Have A Fire In Your Wheel Well?

Wheels and brakes can get really hot. In fact, during a normal 2 mile taxi and takeoff on a hot summer day, an airliner's wheels can get as hot as 300 degrees Fahrenheit. That's hot enough to barbeque.

In most cases, the heat isn't a problem. But, if there's any debris that gets caught in your wheels or brakes during takeoff, there's always the possibility (although rare) of a wheel fire as you retract the gear.

How Does The Fire Loop Detect A Fire?

So if you retract your gear and one of your wheels or brakes catches fire, how does the loop detect it?

The two wires inside of the loop are separated with an insulating material called a thermistor. The thermistor, which is a resistor, is temperature dependent. When it's cold, the thermistor resists electrical current, and the two wires on the inside are completely insulated from one another.

But if the material starts to heat up, like what would happen if there were a fire in the wheel well, the thermistor's resistance starts to break down. As it gets hotter and hotter, it no longer acts as an insulator between the two wires, and the ground wire and detection wire are no longer insulated from one another.

When that happens, the electrical flow through the wire that's connected to the fire detection system changes, and you get a warning message: "MLG BAY OVHT", as well as the aural alert "Gear bay overheat":

Putting It All Together

Fire loops are a rugged and reliable way to detect overheats and fires in places you'd never be able to see from the cabin. And with overheat and fire messages delivered directly to an EICAS screen, it's easy to determine what's happening to your plane, which lets you make decisions quickly and keep you, and your passengers, safe.