How Do Aircraft Brakes Work?
Aircraft brakes provide a critical stopping function during landings, enabling planes to stop within the length of the runway. They also stop aircraft during rejected takeoff events, which are attempted takeoffs that are canceled before aircraft lift off from the ground due to engine failure, tire blowout, or another type of performance malfunction. When aircraft are parked, brakes prevent any type of movement. They also limit aircraft speed during taxiing and can even aid in steering the aircraft on the ground by applying different levels of braking force on the left and right side brakes.
Brakes in aircraft, however, do not work alone. In fact, they work in conjunction with other brake mechanisms, including thrust reversers, air brakes, and spoilers. Thrust reversers are surfaces that are deployed into the path of the jet blast from the engines to redirect propulsive thrust in a direction that goes against the motion of the aircraft. Meanwhile, air brakes and spoilers are flight control surfaces that produce additional aerodynamic drag when deployed into the path of air flowing around the aircraft.
The most common type of brake utilized on aircraft is the disc brake. Disc brakes work by taking advantage of the friction between rotating and stationary discs inside the brake. Upon receiving a command signal to brake, usually from the pilot depressing a foot pedal or from the autobrake system, actuators in the brake move a piston to compress the discs together, generating a frictional force that slows the wheel’s rotation. The friction between the discs creates heat as the aircraft’s kinetic energy is converted into heat energy. During this function, the brake serves as a heat sink, which absorbs a majority of heat as the aircraft releases kinetic energy.
The discs are mounted on a carrier assembly consisting of a torque tube, which resists and transmits brake torque to the landing gear structure. Sandwiched between the carrier assembly housing and a backing plate, the discs are arranged in an alternating pattern of pixels, disc rotors, and stators. The notches and grooves around the perimeter of each rotor correspond with the geometry of the wheel so that they rotate simultaneously. The stators, on the other hand, are attached to the torque tube which keeps them stationary. Cylindrical clearances within the carrier assembly hold actuator pistons.
Whenever the brakes are applied, the disc material experiences wear due to the frictional forces. A wear indicator in the form of a pin protrudes out of the carrier assembly and indicates the thickness of the stack of discs. After many brake applications, materials are worn away and the discs become thinner, necessitating replacement over time. Carbon disc brakes are quite common and they are generally composed of carbon fibers in a graphite matrix. They are generally lighter, more durable, less thermally sensitive, and have higher energy absorption and faster cooling rates. Compared to steel, carbon’s high specific heat provides reduced brake weight.
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