Carbon Brakes in Aviation

How They Work

Aircraft brakes function as friction energy converters: they transform the kinetic energy of a moving aircraft into heat through friction between a stack of rotating and stationary discs. The brake assembly on each main gear wheel contains alternating rotor discs (rotating with the wheel axle) and stator discs (stationary, held by the torque tube). When the brakes are applied, hydraulic pressure forces the disc stack together, creating friction that decelerates the wheel and thus the aircraft.

The total kinetic energy that must be absorbed during a rejected takeoff (RTO) is enormous. A Boeing 777 at maximum takeoff weight (352 tonnes) accelerating to V1 (approximately 160 knots, 296 km/h) has a kinetic energy of about 1.7 gigajoules — equivalent to the energy released by over 400 kg of TNT, all of which must be absorbed by the brakes in approximately 30 seconds. Even a normal landing from approach speed involves absorbing several hundred megajoules.

Carbon vs. Steel Brakes

Until the 1980s, all aircraft used steel brakes: steel rotor and stator discs with steel or sintered metallic friction linings. Steel is strong and relatively inexpensive to manufacture, but its heat capacity and operating temperature range are limited for the most demanding applications. Steel brakes must be replaced frequently — their wear rate at high temperatures is significant — and their weight is considerable (steel brake assemblies for a 747 can weigh several hundred kilograms per wheel).

Carbon-carbon (C/C) composite brakes replace all metallic components with a carbon fiber matrix in a carbon resin (hence "carbon-carbon"). The material properties are extraordinary:

• Specific heat capacity: Nearly double that of steel — carbon can absorb twice as much energy per kilogram before reaching dangerous temperatures.
• Operating temperature: Carbon discs can operate continuously at 1,000°C and survive brief excursions above 2,000°C. Steel brakes are limited to approximately 600–700°C before their strength degrades rapidly.
• Weight: Carbon brakes weigh 30–40 percent less than equivalent steel brakes. On a 747 with 16 main wheel brakes, the weight saving exceeds 400 kg per aircraft.
• Wear rate: In service, carbon brakes typically last 1,500–2,500 landings versus 400–800 for steel. Despite higher initial cost, carbon brakes can be more economical overall.

Heat Dissipation

After a heavy brake application — especially an RTO — carbon brakes can reach temperatures above 1,500°C. Thermal sensors monitor brake temperature, and aircraft may be subject to brake cooling requirements before takeoff can be attempted again (typically 30–40 minutes for a full-energy stop). Wheel well fire suppression systems provide protection if a wheel bearing or hydraulic line catches fire from brake heat.

Carbon brakes dissipate heat primarily through radiation and convection. After an RTO or maximum-energy stop, the brakes glow orange-red and cool slowly over 20–30 minutes. Unlike steel brakes, which fuse and may catch fire if brake system failure leaves them dragging during taxi, carbon discs in fire conditions tend to oxidize rather than support combustion — an important safety characteristic.

Brake temperature indicators in the cockpit (or visible brake temperature monitors on the ground) allow crews to monitor cooling progress and calculate when the aircraft has met the brake energy certification requirements for the next takeoff. Many aircraft use brake-mounted temperature sensors connected to the ECAM (Airbus) or EICAS (Boeing) systems.

The Autobrake System

Autobrake is an electronic system that automatically applies the brakes on landing or during an RTO, providing controlled deceleration without pilot manual input. Pilots select an autobrake setting before approach (typically LOW, MED, HIGH, or MAX on Airbus; RTO, 1, 2, 3, 4, MAX on Boeing systems). After main gear touchdown and confirmation of ground spoiler deployment, the autobrake applies a calculated hydraulic pressure to achieve the selected deceleration rate.

Autobrake serves several purposes: consistent deceleration reduces passenger discomfort compared to manual braking; it frees pilots to attend to thrust reversers, communications, and rollout control; and in the RTO case, MAX autobrake applies maximum braking automatically the instant the crew rejects the takeoff from above approximately 80–90 knots, providing the fastest possible response without human reaction time delay.

Brake-by-Wire

The Boeing 787 takes braking a step further with electric brake-by-wire (electromechanical actuators, or EMAs). Instead of hydraulic pressure moving pistons to compress the brake disc stack, the 787 uses electric motors driving ball-screw actuators in each brake assembly. There is no brake hydraulic system on the 787 — braking is entirely electrical, controlled by the electronic brake and steering control unit (BSCU).

Brake-by-wire offers advantages: the 787 eliminates hundreds of meters of hydraulic brake lines and their associated fluid, weight, and maintenance burden. The system provides more precise, faster control of braking force, improving anti-skid effectiveness. Each wheel has independent, rapid modulation capability similar to an automotive ABS but optimized for aircraft dynamics.

Tire Interaction

Brake performance depends critically on the tire-runway friction coefficient. Optimal braking — maximum retardation without skidding — requires maintaining the wheel in a partially locked condition with just enough slip between tire and pavement to maximize the friction force. The anti-skid system monitors wheel deceleration rates and modulates hydraulic (or electric) pressure to prevent lockup, controlling skid within a narrow optimal slip ratio window of approximately 10–15 percent.

On contaminated runways (wet, slush, ice), the achievable friction coefficient drops dramatically — a wet runway may provide only 50–60 percent of the friction available on a dry surface. Anti-skid effectiveness on contaminated surfaces is limited by the physics of the tire-runway interface, not the brake system itself. This is why wet runway accelerate-stop distances are substantially longer than dry, and why runway condition assessments are communicated to crews before every landing.