Airbus A320, Braking System
Airbus A320 Landing Gear & Braking System – BSCU Logic & Alternate Modes
Nov
The A320 braking system provides reliable braking performance, which is essential for any transport-category aircraft. Engineered to deliver predictable, safe stopping on every landing and rejected takeoff, the Airbus A320 braking system utilizes a multi-layered architecture built around carbon brakes, dual hydraulic sources, and an electronic control computer known as the Brake and Steering Control Unit (BSCU).
This system ensures consistent deceleration, allowing pilots to quickly and efficiently stop the aircraft even in demanding runway conditions, making safety features a key component of the A320’s design.
Main Components of the A320 Brake System
The A320’s main landing gear is fitted with carbon multi-disc brakes, chosen for their durability and stable performance under repeated high-energy stops.
Braking commands originate from the brake pedals or from the autobrake system, and the BSCU regulates normal braking and anti-skid control. The hydraulic system provides the muscle:
- Green system → Normal braking
- Yellow system → Alternate braking and the parking brake
- Yellow accumulator → Backup for emergency braking
The nose gear carries no brakes, so all stopping force is delivered through the mains.
Braking Modes and Hydraulic Systems
The FCOM identifies four operational braking modes:
Normal Braking
Powered by green hydraulic pressure, this is the default mode. The BSCU meters brake pressure electrically and provides full anti-skid and autobrake availability. Pilots do not see brake pressure in the cockpit during normal braking.
Alternate Braking
When green pressure is lost, braking automatically transitions to the yellow system.
- With anti-skid: BSCU still provides anti-skid control.
- Without anti-skid: Occurs after electrical or hydraulic failures. Pressure is then limited to 1,000 psi by the ABCU (Alternate Braking Control Unit), protecting the wheels.
Accumulator/Emergency Braking
If both hydraulic systems are unavailable, the yellow accumulator can provide at least seven full brake applications. Anti-skid is not available in this mode.
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Autobrake and Anti-Skid Protection
The A320 autobrake system offers LO, MED, and MAX modes, with MAX used primarily for RTO. Once armed, the system targets a selected deceleration rate, improving workload management and helping ensure a smooth stop.
The anti-skid system is continuously active above 20 kt. The BSCU compares individual wheel speeds to a reference value and commands brake release when slip approaches the optimum limit—maximizing aircraft brakes efficiency while preventing lock-up.
Brake Temperature Monitoring & Cooling
Brake temperatures appear on the WHEEL page. An amber indication occurs above 300°C, triggering ECAM alerts. Brake fans, when installed, provide rapid cooling, and takeoff must be delayed if temperatures exceed published limits.
Parking Brake Function
The parking brake, powered by yellow pressure or the accumulator, holds the aircraft during ground operations and maintains pressure for more than 12 hours under normal conditions.
Key Safety Features
A320 braking safety features include dual-channel BSCU redundancy, automatic changeover between green and yellow systems, anti-skid protection, accumulator backup, and fusible plugs that prevent tire burst in overheat scenarios.
A pilot who understands how normal, alternate, and emergency braking modes interact with the hydraulic and electronic systems gains a far clearer picture of how the A320 braking system consistently delivers safe stopping performance.
Airbus A320 Braking System – Frequently Asked Questions
1. How does the braking system of the Airbus A320 work to stop the aircraft safely?
The braking system of the Airbus A320 combines carbon multi-disc brakes on the main landing gear, dual hydraulic systems (green for normal brakes, yellow for alternate), and an electronic Brake and Steering Control Unit (BSCU). In normal mode, the green hydraulic system provides pressure with full anti-skid protection and autobrake capability, while the yellow system and accumulator provide alternate and emergency braking if green pressure is lost.
This layered design means the A320 is equipped with multiple independent ways to stop the aircraft quickly and safely, even after certain system or engine failures. In normal operation, brake pedal input is converted into electrical commands by the BSCU, which meters hydraulic pressure to each wheel, so the system ensures efficient and predictable deceleration on the runway.
2. How does the A320 anti-skid system prevent the wheels from locking up?
The A320 anti-skid system continuously compares each wheel’s speed to a reference speed calculated from inertial and air data. When a wheel decelerates too quickly, the BSCU commands a brake pressure release to avoid a full lock of that wheel, then reapplies pressure as grip returns.
In effect, it works like an advanced aircraft ABS: it modulates pressure to reduce skidding, protect the tires, and maintain lateral control so the plane tracks the runway instead of sliding sideways. This function is available in normal and in alternate braking with anti-skid, and it remains active down to about 20 kt groundspeed.
3. When does the autobrake system activate automatically, and which setting should pilots use?
On landing, the A320 autobrake system activates automatically when it detects ground spoiler extension, provided a LO, MED, or MAX autobrake setting is armed, green hydraulic pressure is available, anti-skid is powered, and at least one ADIRU is available. In a rejected takeoff, MAX will engage once spoilers deploy above a minimum wheel speed, delivering a high deceleration rate to stop the aircraft as quickly as runway conditions allow.
For routine landings, crews typically use LO or MED depending on runway length, contamination, and company SOPs. The system ensures a consistent, pre-selected deceleration so pilots can focus on directional control and thrust reverser management rather than constantly adjusting pedal pressure. On shorter or slippery runways, MED provides an efficient way to handle stopping performance with minimal workload.
4. What happens if normal braking is lost – can the crew still stop the plane efficiently?
Yes. If the green hydraulic system or BSCU channel for normal braking fails, the A320 automatically reverts to alternate braking using yellow hydraulic pressure, with anti-skid retained as long as electrical power and the BSCU are still available. If anti-skid is lost (for example, after turning the A/SKID & N/W STRG switch OFF), brake pressure is limited to about 1,000 psi to help avoid wheel lock and tire bursts while still allowing the crew to stop the aircraft.
In a complete hydraulic failure scenario, the yellow accumulator provides at least seven full brake applications, so the crew can still bring the plane to a halt, backed up by maximum reverse thrust from the engines. This multi-layer design—normal, alternate with anti-skid, alternate without anti-skid, and accumulator/parking brake— means the A320 braking system can handle multiple failure combinations and still deliver a controlled, efficient stop.
5. What type of brakes does the A320 use, and how do they compare to older steel brakes?
The A320 is equipped with carbon multi-disc brakes on each main landing gear wheel. Carbon provides a better heat-to-weight ratio than traditional steel brakes, which helps manage the very high energy levels seen in rejected takeoffs and short-field operations. High brake energy is one reason the manuals include brake-temperature limits before takeoff and recommend brake fans or cooling time after heavy braking.
Combined with autobrake, anti-skid, and thrust reversers on each wing engine, the carbon brake packs allow the aircraft to decelerate quickly yet smoothly, preserving tire life and passenger comfort. For pilots, this means the system ensures that even high-energy stops can be managed in a structured, predictable way rather than by “stomping on the pedals” at the last second.
A320 Basics, Limitations & SOPs
Complete A320 study guide by an Airbus pilot & instructor. Clear systems, logic & SOPs.
PDF • 365 pages • Lifetime updates