Brake Systems in Aircraft

Brake System Actuation: Hydraulic and Electric

Most aircraft brakes are actuated hydraulically, especially in large transport aircraft. Hydraulic pressure from the main hydraulic system is directed to the brakes via pilot-operated toe pedals. Some modern aircraft are shifting towards electrically actuated brakes, which offer advantages such as reduced weight, easier installation, and elimination of hydraulic leaks.

Types of Aircraft Brakes

• Single-Disc Brakes: Common on light aircraft, these use a rotating disc and stationary caliper with friction pads. Pedal input generates hydraulic pressure to clamp the pads onto the disc.
• Multi-Disc Brakes: Standard on large aircraft, these feature alternating rotors (attached to the wheel) and stators (attached to the landing gear). Multi-piston hydraulic actuators apply pressure, allowing the system to handle much greater braking energy.

Basic Operating Principle

When the pilot presses the brake pedals, hydraulic (or electrical) pressure forces pistons to press friction material against the rotating discs. This converts kinetic energy into heat, slowing the aircraft.

Anti-Skid Systems

Anti-skid systems prevent wheel lock-up and skidding by modulating brake pressure automatically. This maintains maximum braking effectiveness and directional control, especially on wet or slippery runways.

Brake Accumulator Function

A brake accumulator stores hydraulic pressure, providing emergency braking capability if the main system fails. It also helps smooth out pressure fluctuations during normal operation.

Brake Energy Limitations

Brakes are limited by the amount of kinetic energy they can absorb and dissipate as heat. Excessive use (e.g., during heavy landings or rejected takeoff) can overheat brakes, reducing effectiveness (brake fade) and potentially causing fire. Pilots must monitor brake temperatures and respect brake energy limits to avoid damage or loss of braking.

Parking Brake

The parking brake locks the brakes in place when the aircraft is stationary, typically by trapping hydraulic pressure in the brake lines. It is not designed for stopping a moving aircraft.

In-Flight Brake Use

Wheel brakes are not used in flight, but some aircraft have in-flight braking systems for special purposes (e.g., rotor brakes on helicopters). The main purpose of wheel brakes is for deceleration and stopping after landing or during rejected takeoff.