Anti-Skid Braking System

Overview

The anti-skid braking system is the electronic control system that prevents individual main gear wheels from locking up during braking, maintaining each wheel near the peak of its friction curve to maximise stopping force on any runway surface condition. An aircraft wheel that locks and skids generates substantially less stopping force than a rolling wheel near the slip limit, and a skidding tire sustains flat spots or blowout within seconds. Anti-skid systems are standard equipment on all modern commercial transport aircraft and are credited with significantly reducing landing overrun accidents on contaminated runways since their widespread adoption in the 1960s.

The system is conceptually analogous to the ABS (Anti-lock Braking System) found in modern automobiles, but operates in a far more demanding environment: higher speeds, far greater energy levels, lower-friction surfaces (ice, standing water, rubber-contaminated runway), and the critical need to account for differences in wheel speed arising from crosswind, turning, or asymmetric runway contamination. Commercial aircraft anti-skid systems also incorporate touchdown protection (preventing brake application in the air) and locked-wheel protection (releasing brakes if a wheel stops entirely while the aircraft is still moving at speed).

How It Works

Each braked wheel is equipped with a speed transducer — a toothed rotor and inductive pickup generating an AC signal proportional to wheel rotational speed. The anti-skid control unit continuously monitors wheel speed from all braked wheels and computes a reference speed representing the expected rolling speed for the current aircraft ground speed. When a wheel decelerates faster than the control law threshold — indicating impending skid — the anti-skid modulation valve partially releases brake pressure to that wheel. As the wheel accelerates back toward reference speed, pressure is gradually reapplied. This pressure modulation cycle typically occurs ten to fifteen times per second per wheel, maintaining each wheel within its optimal slip ratio (typically 10–15 percent slip) where the tire-runway friction coefficient is maximum.

Locked-wheel protection activates when any wheel reaches near-zero speed while others are still rotating at ground speed, indicating a fully locked wheel. The system fully releases the affected wheel's brake pressure and maintains it released until wheel rotation resumes. Touchdown protection inhibits all braking until weight-on-wheels sensors confirm the aircraft is on the ground, preventing inadvertent brake application that could cause wheel damage if applied while the gear is still in the air.

Key Components

• Wheel Speed Transducers: One per braked wheel, generating sinusoidal AC voltage signals at a frequency proportional to rotational speed. Signal quality must be maintained even when wheels are partially hydroplaning.
• Anti-Skid Control Unit (ASCU): Central digital processor executing the anti-skid control laws for all wheels simultaneously. Typically dual-channel with cross-monitoring for fault detection.
• Anti-Skid Modulation Valves: One per wheel or wheel pair. Electro-hydraulic servo valves that reduce, hold, or increase brake pressure in response to ASCU commands within milliseconds.
• Reference Speed Calculator: Algorithm within the ASCU computing expected ground speed from wheel speed signals, inertial reference data, or a combination, to provide a stable reference against which individual wheel deceleration is compared.
• Selector/Shutoff Valve: Allows manual anti-skid system deactivation (e.g., for manual braking checks during maintenance), though normal operations always use anti-skid active.

Aircraft Applications

• Boeing 737-800 — four-wheel anti-skid with individual wheel modulation; integrated with autobrake; touchdown protection via squat switch
• Airbus A320-200 — BSCU (Brake and Steering Control Unit) integrates anti-skid, autobrake, and nose wheel steering; per-wheel modulation
• Boeing 777-300ER — twelve-wheel anti-skid; Brake System Control Unit (BSCU) manages all twelve channels independently
• Boeing 787-9 — electric brake-by-wire anti-skid; faster pressure modulation achievable with electric actuators than conventional hydraulic valves

Advantages and Limitations

Anti-skid systems can reduce landing distance by 20–30 percent compared to manual braking on contaminated surfaces, and they virtually eliminate tire flat-spotting from inadvertent lockup. They also extend brake and tire life by maintaining more consistent braking forces and preventing the extreme heat generation associated with skidding. The system operates transparently to the crew — the only indication is pedal pulsing feedback transmitted through the hydraulic system during active modulation.

A key limitation is hydroplaning: when a water film beneath the tire lifts the tire off the runway surface entirely, wheel speed drops to zero regardless of actual ground speed, and the anti-skid system incorrectly interprets this as a locked wheel and fully releases brake pressure. Some modern systems incorporate hydroplaning detection algorithms that compare wheel deceleration patterns with the expected characteristics of a hydroplaning event and modulate brake pressure accordingly, but hydroplaning remains a significant factor in wet-runway landing performance calculations. The system also cannot increase the available friction coefficient — on icy runways, even optimal anti-skid operation may yield very low deceleration rates, making runway length margins critical.