Boeing 777 brake certification testing

Aircraft must land, stop in specified distance, and the brakes can catch fire, but the airframe must hold three minutes before fire-crew intervention without engine damage or hydraulic leak.

The brakes on big aircraft are actually carbon-carbon composite. Very expensive. It's possible they might use some magnesium in some of the calipers and the structure that holds the brake pads, but the brake pads themselves get so hot when you're trying to stop 450 tons rolling down the highway at a hundred miles an hour. You need to dissipate a lot of energy. I watched a program years ago on the certification process of the Boeing 777. They had to land the aircraft and stop it in such-and-such a distance, and then the brakes could catch fire — but they had to have at least three minutes before the fire crew could put it out, because in a real-world situation the fire department's not standing right there at the end of the runway. So you have to pass tests where you don't destroy the rest of the engines or have hydraulic leaks for three minutes. You can have a big flaming fire and your carbon-carbon composite brakes starting to burn. Carbon-carbon composite — that's what we use for cruise missile engines and things like that. Takes high temperatures, but they don't last very long.