Aircraft Brakes — Page 129, Lesson 175

I want to walk you through the aircraft braking system now. Let's start with the fundamental idea, because everything else builds on it. In common with most braking systems, aircraft wheel brakes work by using friction between a fixed surface and a moving one to bring the aircraft to rest. That friction converts kinetic energy into heat energy. Kinetic energy is the energy of motion — the aircraft is moving, and we have to turn that motion into heat to stop it. And here's the scale of the problem: the amount of heat generated in stopping a large modern aircraft is enormous. The challenge of dissipating, or getting rid of, that heat has dogged aircraft designers and scientists for years. And as progress has been made in dissipating heat, aircraft have got faster and heavier — so the problem has actually gotten worse. You might think the ideal answer would be to build runways long enough that the aircraft never needs its brakes at all. But the prohibitive cost of building runways four or five miles long makes that a non-starter. Then there's reverse pitch on propeller-driven aircraft and reverse thrust on jet-engined aircraft — those provide a partial answer to the problem. But even with those aids, the need for normal braking still exists. So we can't escape the brakes. Now, all modern aircraft use what are called plate brakes, or disc brakes, operated by hydraulic systems as their means of slowing down or stopping. The system uses a series of fixed friction pads that bear on, or grip, one or more rotating plates — similar in principle to the disc brakes on a car. The number of friction pads and rotating plates is a matter of design and wheel size. A light aircraft could use a single-plate disc brake, whereas a typical arrangement on a large aircraft would be a multi-plate unit. In that multi-plate unit, the physical size of the braking area is increased by employing multiple brake plates sandwiched between layers of friction material. Now here's the key construction detail. The rotating plates are called rotors, and they are keyed to revolve with the outer rim of the wheel. The stationary plates, which carry the friction material, are called stators, and they are keyed to remain stationary with the hub of the wheel. So the rotors spin with the wheel, the stators stay still. When the brake is applied, hydraulic pressure pushes the actuating pistons, which are housed in the torque plate. Those pistons squeeze the rotors and stators together between the pressure plate and the thrust plate. The harder you press the brake pedal, the greater the braking force applied to the pressure plate by the pistons. And the torque generated by the brake unit — that twisting force — is transmitted to the main landing gear leg by a torque rod, also called a brake bar. Now, the materials matter a great deal. The friction pads are made of an inorganic friction material, and the plates are made of heavy steel with a specially case-hardened surface. And this is a critical safety point: it is that case-hardened surface which causes the plates to explode if they are covered with liquid fire extinguishant when they are red hot. So in the unfortunate event of a wheel or brake fire, the best extinguishant to use is dry powder — not liquid. That's a direct operational consequence of the materials we just talked about. Let me show you the wear markers on the friction material, because that's how you'll inspect these brakes in practice. So to tie it together: friction converts kinetic energy to heat, the rotors spin with the wheel while the stators stay fixed, hydraulic pistons squeeze them together, and the torque rod carries the braking force to the landing gear leg. And remember — if those steel plates are red hot, use dry powder, never liquid extinguishant.