Aircraft Anti-skid Systems

Operating Principle

Anti-skid systems monitor the rotational speed of each main wheel and compare it to the aircraft's ground speed. If a wheel decelerates too quickly—indicating an impending skid—the system reduces hydraulic brake pressure just enough to allow the wheel to regain speed, then reapplies pressure to maintain optimal braking. This process keeps each wheel operating at the threshold between maximum friction and the onset of a skid.

Key Functions

• Slip Ratio Regulation: The system maintains a specific slip ratio (difference between aircraft speed and wheel speed) that delivers the highest possible braking force for the surface conditions.
• Locked-Wheel Protection: Prevents individual wheels from locking up, reducing the risk of tyre flat spots and blowouts.
• Touchdown and Bounce Protection: Inhibits brake pressure application during initial touchdown and after a bounce until wheels are spinning at the correct speed, avoiding premature wheel lock.
• Hydroplane Protection: Detects excessive slip caused by water and reduces brake pressure to restore tyre-to-surface contact.

Impact on Performance and Failures

With anti-skid, braking distances are minimised and tyre wear is reduced. If the system fails, landing distance can increase by up to 50%, and the risk of skidding, tyre damage, and loss of control rises. Pilots cannot match the precision of anti-skid modulation, so careful manual braking will never achieve the same performance.

Integration with Autobrakes

Autobrake systems rely on a functioning anti-skid system to regulate brake pressure and maintain the desired deceleration rate. If anti-skid is unserviceable, autobrake is also unavailable, and manual braking must be used.

Operational Considerations

Anti-skid systems are active during takeoff and landing but typically deactivate below taxi speeds (around 20 knots) and when the parking brake is set.