Journey of the Dream to Fly

Brakes provide this critical stopping function during landings to enable airplanes to stop within the length of the runway. They also stop aircraft during rejected take-off (RTO) events, in which an attempted take-off is cancelled as the airplane is rolling down the runway before it lifts off from the ground due to engine failure, tire blowout, other system failure or direction from air traffic control.

Aircraft brakes also prevent motion of the aircraft when it is parked, limit its speed during taxiing and can even help steer the aircraft on the ground by applying different levels of braking force to the left- and right-side brakes.

Aircraft brakes work in conjunction with other brake mechanisms such as 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 opposes the motion of the aircraft. Air brakes and spoilers are flight control surfaces that create additional aerodynamic drag when deployed into the path of air flowing around the aircraft.

How aircraft brakes work

The most common type of brake used on aircraft is the disc brake. Disc brakes function by exploiting friction between rotating and stationary discs inside the brake.

Upon receiving a command signal to brake — from the pilot depressing a foot pedal or from the autobrake system — actuators in the brake move a piston to squeeze the discs together, generating a frictional force that slows the rotation of the wheel. The friction between the discs generates heat as the aircraft’s kinetic energy is converted to heat energy. In this function, the brakes act as a heat sink, absorbing tremendous amounts of heat as the aircraft sheds kinetic energy. During RTO stops, the temperature of carbon disc brakes can exceed 1,800° C.

Lets watch a small video to understand more. https://youtu.be/9qd5-2dKbD4?feature=shared

Brake materials

The most common aircraft brake disc material until around 1963 was steel. The introduction of beryllium as a disc material provided improved thermal properties at the cost of material handling difficulties due to the toxic nature of beryllium oxide.

Carbon brakes became widely available for commercial airplanes starting in the 1980s. Carbon brakes — made of, for example, carbon fibers in a graphite matrix — are lighter, more durable, less thermally sensitive and have higher energy absorption and faster cooling rates compared to steel brakes.

Compared to steel, carbon’s higher specific heat — the amount of heat that a unit mass of carbon absorbs to raise its temperature by one unit — enables reduced brake weight. Carbon’s higher thermal conductivity allows faster heat transfer and uniform heat distribution through the disc. Carbon also has lower thermal expansion, higher thermal shock resistance and a higher temperature limit than steel. The specific strength of carbon is relatively constant over a very wide range of temperatures, unlike steel and beryllium, which both exhibit steep declines in specific strength at high temperatures, dropping below carbon around 650° C (1,200° F).

Brake cooling

The GS100 aircraft brake cooler has a gas or diesel engine-powered fan that provides suction cooling for fast turnaround times. Source: Ground Support Specialists, LLC. (Click image to enlarge)The large amount of heat generated during aircraft braking is dissipated by means of passive or active cooling. Passive cooling occurs due to natural conduction of heat through the disc material and to surrounding components, radiation of heat off the brake and wheel assembly, and convection as air flows past and through the brake assembly. Active cooling is achieved by using fans that force air through the brakes. Fans are either built into the wheel or part of the external ground support equipment that is positioned against the wheel and removed once sufficient temperature drop has occurred.