Landing is one of the most critical phases of flight. During touchdown, aircraft are subjected to immense kinetic energy that must be safely dissipated through braking. To ensure effective braking while preventing wheel lockup and skidding, modern aircraft are equipped with anti-skid systems. These systems are comparable to the anti-lock braking systems (ABS) used in automobiles, but are designed to handle the unique demands of aviation such as high landing speeds, heavy weight, and variable runway conditions.
Purpose of the Anti-Skid System
The primary objectives of an aircraft anti-skid system are:
To prevent wheel skidding during landing and ground roll.
To maintain maximum possible braking efficiency by ensuring optimal friction between tires and runway.
To minimize tire wear and damage.
To maintain directional control of the aircraft on the runway.
To reduce stopping distance, especially on wet or slippery runways.
Principle of Operation
The anti-skid system works by continuously monitoring the rotational speed of each wheel during braking. If the system detects that a wheel is about to lock up (rapid deceleration or no rotation), it automatically reduces brake pressure momentarily. Once the wheel regains speed and traction, braking pressure is reapplied. This cycle happens multiple times per second, ensuring the wheels are always rotating at a controlled rate relative to aircraft speed.
The system operates on the principle of maintaining the wheel slip ratio within an optimal range (typically around 10–15% slip), which provides maximum friction without skidding.
Components of an Aircraft Anti-Skid System
1. Wheel Speed Sensors
Measure the rotational speed of each wheel.
Provide input signals to the control unit.
2. Anti-Skid Control Unit (ASCU)
Processes signals from the wheel sensors.
Detects incipient skid conditions.
Commands brake pressure modulation accordingly.
3. Brake Control Valves / Servo Valves
Adjust hydraulic brake pressure based on control unit commands.
Increase, hold, or release pressure to prevent skidding.
4. Cockpit Controls and Indicators
Allow pilots to arm or disarm the anti-skid system.
Provide failure warnings if the system malfunctions.
Modes of Operation
Aircraft anti-skid systems typically operate in different modes depending on conditions:
Normal Mode: Provides full anti-skid protection during landing rollout.
Touchdown Protection: Prevents brakes from being applied before wheel spin-up to avoid tire blowout.
Hydroplane/Low-Speed Protection: Some systems reduce braking effectiveness at very low speeds to prevent wheel lock at taxi speed.
Fail-Safe Mode: If the system fails, braking reverts to manual control with no anti-skid assistance.
Advantages of Anti-Skid System
Increased safety during landing and rejected takeoff.
Reduced risk of accidents due to tire bursts or loss of directional control.
Lower maintenance costs by minimizing tire and brake wear.
Shorter and more predictable stopping distances.
Challenges and Limitations
Reduced effectiveness on icy runways where friction is minimal.
Dependence on accurate sensor signals and hydraulic system integrity.
Additional weight and complexity compared to conventional braking systems.
Conclusion
The anti-skid system in aircraft is a critical safety feature that ensures maximum braking efficiency while preventing wheel lockup during landing and ground operations. By continuously adjusting brake pressure based on wheel speed, it enhances safety, reduces stopping distance, and protects tires from excessive wear. Just like ABS in cars, but more sophisticated, aircraft anti-skid systems are vital for safe flight operations, especially in challenging runway conditions.