Expert: Ray Wilkinson - 1/21/2009

Question
Can you please tell how does the braking system in an aircraft works?

Answer
Phew! Big question.
Aircraft brakes work in a similar way to car brakes, but there are a few major differences of course. With a car, the main thing is repeated braking, whereas with aircraft the brakes are applied once, followed by a long cooling period. However, the energy is much higher, as an airliner lands at about 150 knots and can weigh over 200 tonnes at landing.
The brake consists of a stack of discs within the wheel hub. Each alternate disc is attached to the wheel, and rotates with it. The interleaving discs are connected to the undercarriage leg and so don't rotate. When the brakes are applied, fluid pressure from the hydraulic system pushes pistons that squeeze the discs together, and friction pads on the faces turn the kinetic energy of the aircraft (the energy it has from its mass and speed) into heat, slowing the aircraft down. An anti-skid system detects if a wheel locks and momentarily releases the brake on that wheel to stop a skid.
On a large aircraft such as an airliner, there is a brake in each wheel of the main undercarriage gear (under the wings or fuselage) but not on the nose gear, which is dedicated to steering. Backup systems are provided in case of failure. A 'weight-on-wheels' switch holds the brakes off until the aircraft has landed, preventing skids. It's important that the weight comes onto the wheels as soon as possible after touchdown, so spoilers are often fitted to the wing to dump lift on touchdown, preventing bouncing and allowing the brakes to work quickly.
The aircraft also slows using aerodynamic brakes, such as deflecting the flaps to, say, 70 degrees to increase drag. Thrust reversers are also common - plates deflect the engine thrust forwards to help the aircraft slow down.
I hope this answers your question.