Aircraft Braking Systems

Types of Aircraft Braking Systems

Light aircraft typically use single-disc, hydraulically actuated brakes controlled directly by pilot foot pedals. Large transport aircraft rely on multi-disc, multi-piston brakes powered by the main hydraulic system, capable of absorbing the high kinetic energy from heavy landings and rejected takeoffs. Increasingly, electrically actuated brakes are being introduced for their reliability and weight savings.

Brake Accumulator Function

A brake accumulator stores pressurised hydraulic fluid, providing emergency braking capability if the main hydraulic system fails. It allows for a limited number of brake applications—usually around six—enabling controlled deceleration even with total hydraulic loss.

Anti-Skid System Explained

The anti-skid system prevents wheel lock-up during braking by automatically reducing brake pressure if it detects excessive slip. This maintains optimal friction between the tyres and runway, maximising braking effectiveness and preventing skids, flat spots, or tyre failure. Anti-skid is essential for both manual and automatic braking modes, especially on slippery runways.

Autobrake System Operation

Autobrake systems allow pilots to pre-select a desired deceleration rate or maximum braking (such as RTO—Rejected Take-Off or MAX). The system activates when appropriate conditions are met (e.g., wheel spin-up and thrust levers at idle) and modulates brake pressure to achieve the selected deceleration, factoring in the effects of reverse thrust and spoilers. The three main autobrake modes are:

• OFF: System is inactive.
• ARMED: Ready to engage under set conditions.
• ACTIVATED: Brakes automatically apply pressure to achieve the selected deceleration rate.

Autobrake disengages if the pilot presses the brake pedals or manually de-selects the system.

Differential Braking and Other Features

Differential braking allows pilots to apply different braking forces to each main wheel, aiding in ground steering, especially during tight turns. Parking brakes use a hydraulic lock to hold pressure and keep the aircraft stationary, even when the main hydraulic system is off.

Carbon Brakes vs Steel Brakes

Carbon brakes are lighter, dissipate heat more efficiently, and last longer than traditional steel brakes, making them preferable for modern commercial aircraft. However, steel brakes are still found on older or smaller aircraft due to cost considerations.

Brake Failure and Emergency Procedures

In the event of a brake failure, the accumulator or emergency brake system provides backup. Anti-skid is usually inoperative in emergency mode, so full system pressure is applied, which can increase the risk of wheel lock-up.

Key Points for the ATPL Exam

• Autobrake and anti-skid systems are closely linked; anti-skid must be operational for autobrake to function correctly.
• Maximum braking (RTO/MAX) is used for rejected takeoff or shortest landing distance, with anti-skid protection engaged.
• The brake accumulator provides emergency braking if the main system fails.
• Pilot pedal input overrides and cancels autobrake.
• Carbon brakes offer superior performance over steel brakes in terms of weight and heat management.
• Differential braking aids in ground manoeuvring.
• The shortest stopping distance is achieved with maximum braking, anti-skid, reverse thrust, and spoilers deployed.