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The aircraft brake system is the primary mechanism for decelerating the aircraft during the landing roll and rejected takeoff, and for controlling speed during taxi operations. Unlike automotive disc brakes, commercial aircraft brakes must absorb enormous kinetic energy in a very short time — a fully loaded wide-body aircraft at maximum landing weight carries kinetic energy comparable to several megajoules — while remaining within the structural limits of the wheel and tire assembly. This demanding requirement has driven the evolution from early steel brakes to the carbon-carbon (C/C) composite brakes now standard on most commercial jets.

Aircraft brakes are housed inside the main landing gear wheels, with no braking capability on the nose wheel. Each braked wheel typically carries a brake assembly consisting of a rotating stack of rotor and stator discs clamped together by hydraulic or electro-hydrostatic actuator pistons. Brake pressure is modulated by the anti-skid system and, optionally, by the autobrake system to maximise deceleration without wheel lockup.

Como Funciona

When the pilot depresses a brake pedal or the autobrake system activates, hydraulic pressure is applied to actuator pistons arranged around the circumference of the brake assembly. The pistons clamp the rotating disc stack — alternating rotors keyed to the wheel hub and stators keyed to the axle — together, generating friction that converts wheel rotational kinetic energy into heat. On carbon brakes, rotor and stator discs are manufactured from carbon-carbon composite, a material that maintains structural integrity and friction coefficient at temperatures exceeding 1,000 °C and has an energy absorption capacity several times greater than equivalent steel components at a fraction of the weight.

Heat management is the central design challenge. A maximum-energy rejected takeoff event can heat carbon brake discs to 1,400–1,800 °C, hot enough to ignite nearby materials if not contained by the brake heat shield assembly. Brake cooling fans are fitted on some aircraft to accelerate post-landing cooling. Fusible plugs in the wheel rim melt at a preset temperature to deflate tires before they burst from heat-induced pressure, preventing catastrophic wheel disintegration.

Componentes Principais

• Carbon-Carbon Rotor Discs: Rotating friction elements keyed to the wheel hub. Typically four to seven rotors per brake assembly, depending on aircraft size and energy requirement.
• Stator Discs: Stationary friction elements keyed to the axle torque tube. Alternate with rotors in the brake stack.
• Actuator Pistons: Hydraulically powered pistons, typically arranged in a ring of six to twelve, that apply axial clamping force to the disc stack. On electro-hydrostatic brake architectures, electric actuators replace conventional hydraulic pistons.
• Pressure Plate and Backing Plate: End structures that distribute clamping load evenly across the disc stack and react the reaction force against the axle.
• Brake Wear Indicators: Mechanical pins that protrude beyond the brake housing by a measurable distance proportional to remaining disc thickness, enabling visual inspection without disassembly.
• Thermal Fuse Plugs: Pressure-relief devices in wheel rims that melt at approximately 175–200 °C rim temperature to safely deflate tires before heat-induced failure.
• Brake Metering Valves: Proportional valves that translate pilot pedal input into calibrated brake pressure, feeding the anti-skid control valves downstream.

Aplicações em Aeronaves

• Boeing 737-800 — four-wheel main gear, carbon brakes standard; hydraulic System B primary, System A backup
• Airbus A320-200 — carbon brakes on all four main wheels; Green hydraulic system with Yellow backup; electric braking option on A320neo
• Boeing 777-300ER — six-wheel main bogies with twelve braked wheels; carbon brakes; largest kinetic energy absorption requirement in commercial service
• Boeing 787-9 — electric brake-by-wire (no conventional hydraulic brake lines to wheels); electro-hydrostatic actuators at each wheel

Vantagens e Limitações

Carbon brake assemblies offer a 40 percent weight saving over equivalent steel designs, improved friction stability at high temperatures, longer service life (typically 2,000–3,000 landing cycles versus 1,000–1,500 for steel), and superior energy absorption. Their primary limitation is sensitivity to cold temperatures: carbon brakes must reach a minimum operating temperature (typically 100–150 °C) before their friction coefficient stabilises, which means the first landing following a cold-soak at altitude may produce slightly reduced braking effectiveness until the brakes warm. This cold-brake characteristic drives operational procedures requiring judicious brake application on the first landing of the day in cold climates.

The Boeing 787's electric brake-by-wire architecture eliminates all hydraulic brake plumbing from the wheel area — a significant simplification that removes a potential source of brake fluid fire — and enables finer control granularity for anti-skid and autobrake functions. However, it introduces electrical actuator reliability as the critical failure mode and requires careful thermal management of the actuator electronics in proximity to the hot brake assembly.