자동 브레이크 시스템

Overview

The autobrake system provides automatic, pre-selected braking upon landing or rejected takeoff, eliminating the response time delay associated with manual brake application and delivering consistent, controlled deceleration without requiring pilot pedal input. On landing, the system allows the flight crew to select a desired deceleration rate before touchdown — typically LOW, MED (medium), HIGH, or MAX on landing, and RTO (Rejected Takeoff) for abort scenarios — with the system automatically applying the brake pressure needed to achieve that rate as soon as it detects touchdown.

The autobrake system is particularly valuable on short runways, wet or contaminated surfaces, and during Cat III autoland approaches where the crew workload is high and consistent braking must be established immediately after touchdown without diverting attention to the brake pedals. RTO autobrake activation is especially critical: a rejected takeoff at high speed requires maximum braking force applied within fractions of a second after the abort decision, faster than manual application can reliably achieve.

How It Works

Prior to landing, the crew selects the desired autobrake mode using the autobrake selector switch on the centre console. When the aircraft touches down and the weight-on-wheels signal confirms ground contact, the autobrake system arms, and the anti-skid/autobrake control unit begins applying hydraulic brake pressure. The system uses a closed-loop deceleration control law: it measures actual aircraft deceleration from an inertial reference system or accelerometer, compares it to the target deceleration for the selected mode, and adjusts brake pressure upward or downward to maintain the commanded rate. The anti-skid system operates transparently within the autobrake loop, modulating pressure to individual wheels to prevent lockup while the autobrake maintains the overall deceleration command.

Autobrake disengagement occurs automatically if the pilot applies manual brake pedal force above a threshold (typically 200–400 N depending on aircraft), or manually via the autobrake selector switch. Once disengaged, braking reverts to full manual control. The system also disengages if any malfunction is detected in the sensors or control logic. On some aircraft, autobrake remains armed even if thrust reversers are deployed, with the system reducing brake pressure demand as reverse thrust contribution increases, providing optimised combined deceleration.

For RTO, the system is armed on the ground before takeoff roll. If the crew retards both thrust levers to idle simultaneously above a minimum speed (typically 85 knots), the system interprets this as a reject command and applies maximum brake pressure immediately. RTO autobrake does not require weight-on-wheels confirmation — the existing ground condition is inferred from takeoff-phase logic.

Key Components

• Autobrake Selector: Cockpit rotary selector providing OFF / RTO / LOW / MED / HIGH (and MAX on some types) positions. Selected before landing; armed state confirmed by ARMED annunciation.
• Brake and Steering Control Unit (BSCU) / Autobrake Computer: The digital controller executing deceleration closed-loop control. Receives deceleration feedback from the IRS, brake pressure commands from the selector, and weight-on-wheels from the PSEU.
• Deceleration Sensor: Longitudinal accelerometer or IRS-derived deceleration signal providing the actual deceleration feedback for the closed-loop control law.
• Brake Metering Valves: Electro-hydraulic servo valves modulating brake pressure in response to autobrake computer commands. Must respond rapidly enough to follow the control law without pressure oscillation.
• Disarm Sensors: Pedal force sensors (or travel switches) detecting manual brake application and triggering autobrake disengagement to prevent conflict between manual and automatic braking.

Aircraft Applications

• Boeing 737-800 — autobrake deceleration rates: 1 (LOW) = 1.0 m/s², 2 (MED) = 1.5 m/s², 3 (HIGH) = 3.0 m/s², MAX = maximum available; RTO at full pressure
• Airbus A320-200 — modes LO (1.7 m/s²) / MED (3.0 m/s²) / MAX; BSCU integration with anti-skid; autobrake arms automatically in some Cat III configurations
• Boeing 777-300ER — twelve braked wheels under autobrake control; deceleration rates similar to 737 but greater kinetic energy to absorb
• Boeing 787-9 — electric brake-by-wire enables finer pressure modulation in autobrake mode; software-configurable deceleration rates

Advantages and Limitations

The autobrake system eliminates the variability of manual braking and reduces pilot workload during the most demanding phase of flight. Consistent deceleration reduces passenger discomfort from uneven brake application and extends brake life by avoiding the peak thermal loads associated with late, hard manual braking. On contaminated runways, the autobrake system reacts faster than manual application and integrates seamlessly with anti-skid modulation to maintain optimum stopping force.

The primary limitation is that the system's deceleration target is fixed at the selected mode — it does not automatically adapt to runway length remaining. A crew selecting LOW autobrake on a short runway with a tailwind must monitor stopping performance and be prepared to manually apply additional braking if the aircraft will not stop in the available distance. Most operators publish stopping distance calculations for each autobrake mode, runway condition, and aircraft weight, and require crews to verify that the selected mode provides adequate stopping margin before committing to the approach. Inadvertent autobrake disengagement — by accidental pedal contact — can result in a sudden increase in stopping distance at a critical moment, a scenario that drives the relatively high force threshold required for disengagement.