An airliner’s wheel brakes convert kinetic energy into heat. During landing, they work with aerodynamic drag, spoilers and reverse thrust. During a rejected take-off, they may be required to absorb far more energy in a shorter time. Two electronic systems help manage that task: anti-skid prevents individual wheels from locking, while autobrake commands a selected overall deceleration without the pilots having to hold a constant pedal position.
The systems are related but perform different functions. Autobrake decides how much braking the aircraft needs to achieve a target deceleration. Anti-skid decides how much pressure each wheel can accept without losing useful tyre-to-runway friction. If the surface is slippery, anti-skid can reduce pressure below the autobrake command. It always has the final protective role at the individual wheel.
Why a locked wheel is inefficient
A rolling tyre develops braking force through a small amount of slip between tyre rotation and aircraft groundspeed. Maximum friction normally occurs before the wheel becomes fully locked. Once locked, the tyre slides and can produce less controllable braking, reduced directional stability and rapid localised wear.
A locked wheel on a wet runway can ride on a layer of water. Dynamic hydroplaning occurs when water pressure lifts part of the tyre away from the pavement. Viscous hydroplaning can occur on a very smooth, contaminated surface at lower speeds. Reverted-rubber effects can arise when a locked tyre generates heat and steam under the contact patch.
Anti-skid attempts to keep each braked wheel near an efficient slip ratio. It does not create friction that the surface cannot provide. On ice or standing water, available braking can remain poor even when the system works correctly.
Wheel-speed sensing
Each controlled wheel has a speed transducer. The anti-skid computer compares wheel rotation with a reference representing aircraft speed. On many systems, the reference is built from the speeds of other wheels and filtered to account for normal differences.
A wheel that decelerates significantly faster than the aircraft is probably approaching a skid. The computer commands a valve to reduce hydraulic pressure at that brake. When rotation recovers, pressure is reapplied. Modern fully modulating systems adjust pressure continuously rather than simply switching it fully on and off.
The calculation must distinguish a genuine skid from wheel-speed changes caused by touchdown spin-up, runway roughness or sensor error. Software uses thresholds, rates and cross-comparison. Fault monitoring removes unreliable channels and alerts the crew.
Touchdown protection
Brakes should not be fully applied while the main wheels are airborne. Touchdown protection inhibits brake pressure until wheel spin-up, weight-on-wheels signals or a time delay confirms ground contact. This prevents a wheel from touching the runway already locked.
Spoiler deployment and landing-gear signals may also be part of the logic. The exact combination is aircraft-specific. A faulty wheel-speed sensor or ground signal can delay or remove normal braking, so alternate logic and crew procedures are provided.
Pilots still place their feet on the pedals during landing, but autobrake may control pressure until a pedal override threshold is reached.
What autobrake commands
A landing autobrake system usually offers several deceleration levels. The selected level is not simply a fixed brake pressure. The controller measures actual longitudinal deceleration and commands enough brake pressure to meet the target.
Immediately after touchdown, spoilers create drag and dump wing lift, increasing weight on the wheels. Reverse thrust adds deceleration. If those forces are strong, the autobrake needs less wheel-brake pressure. As reverse thrust is reduced and aerodynamic drag falls, the system may increase brake pressure to maintain the selected deceleration.
This closed-loop behaviour explains why brake temperature can differ between landings made with the same autobrake setting. Aircraft weight, touchdown speed, wind, runway slope, reverse thrust and exit point all change the required brake energy.
Rejected-take-off mode
Many transport aircraft have an RTO or MAX autobrake mode. The system arms during take-off and applies maximum available braking when defined conditions indicate a rejected take-off, such as thrust-lever retardation above a threshold speed.
The logic must avoid applying full brakes during a low-speed taxi correction. It therefore uses speed, thrust and ground signals. Once triggered, it commands high pressure while anti-skid protects individual wheels.
A high-speed rejected take-off can approach the brake system’s maximum certified kinetic-energy condition. Fuse plugs may release tyre pressure if wheel temperature becomes dangerously high. Fire crews may monitor the aircraft while the brakes cool.
Hydraulic pressure and metering valves
Pilot pedal movement is measured mechanically, hydraulically or electrically. A brake-control unit commands metering valves that regulate pressure to each brake. Normal braking may use one hydraulic system; alternate braking may use another or an accumulator.
Anti-skid valves sit in the pressure path and can reduce commanded pressure. They cannot increase pressure above the source or pilot/autobrake command. If hydraulic supply is lost, remaining braking depends on alternate design.
Carbon brakes are common on modern airliners because they tolerate high temperature and offer weight advantages. Steel brakes remain on some types. Both require correct wear measurement and cooling management.
Spoilers and weight on wheels
Braking force depends on normal load. A wing still producing lift reduces the weight carried by the tyres. Ground spoilers deploy after touchdown to reduce lift and transfer load to the landing gear.
If spoilers fail to deploy, stopping distance increases even though the brakes and anti-skid work normally. Autobrake may command more pressure, creating more heat, but it cannot fully replace missing wheel load and aerodynamic drag.
Spoiler logic usually considers wheel spin-up, strut compression and thrust-lever position. Manual speedbrake deployment may be required after an automatic failure.
Reverse thrust interaction
Reverse thrust helps deceleration but is not part of the wheel anti-skid loop. Autobrake senses the combined result and adjusts brake demand. Strong reverse thrust can reduce brake energy and temperature.
Certification rules do not always credit reverse thrust in dispatch landing-distance calculations. Operational calculations may treat it according to aircraft approval, runway condition and operator procedure. Crews must not assume reverse thrust will compensate for poor braking action.
On contaminated runways, directional effects and engine-ingestion risk influence use. Reverse thrust becomes less effective at low speed.
Runway friction and anti-skid efficiency
Regulatory performance models distinguish fully modulating, quasi-modulating and on-off anti-skid systems. Fully modulating systems can use available friction more efficiently because they adjust pressure closely around the skid threshold.
Even the best system depends on tyre condition, inflation pressure and pavement texture. Grooves and macrotexture help water escape. Rubber deposits can polish the touchdown zone and reduce wet friction.
Runway condition reports give crews a standardised assessment, but actual friction can vary along the surface. A local patch of slush or water can cause one wheel group to release while others continue braking.
Crosswind and asymmetric friction
In a crosswind, pilots use rudder, nose-wheel steering and differential braking as speed decreases. Anti-skid operates independently at each wheel, which helps preserve directional control when one side encounters poorer friction.
A split-friction runway, with ice on one side and better pavement on the other, is particularly demanding. The higher-friction side can produce a yawing moment. Anti-skid optimises each wheel but does not automatically guarantee zero yaw.
Autobrake commands overall deceleration. The flight crew remains responsible for directional control and may disconnect autobrake with pedal input.
Brake temperature
Braking energy becomes heat in discs, wheels and tyres. Temperature sensors provide indications or estimates. After a heavy landing or rejected take-off, temperatures can continue rising for several minutes because heat moves from the discs into surrounding components.
Brake fans may accelerate cooling on the ground. Operators use temperature limits before take-off because overheated brakes can affect retraction safety and rejected-take-off capability. A brake hot enough to trigger a caution may still provide stopping force, but the next high-energy event could exceed limits.
Cooling time depends on ambient temperature, wind, fan use, brake wear and energy absorbed. Dispatch software or charts calculate minimum turnaround time after high-energy braking.
Fuse plugs and tyre safety
Aircraft wheels may contain fusible plugs designed to melt at a defined temperature. They release tyre pressure before heat causes a more violent wheel or tyre failure.
A deflated tyre after a hot stop can therefore be evidence of a protective function. Personnel avoid approaching overheated wheels from the side because of potential failure direction. Airport rescue services use thermal monitoring and established cooling procedures.
Nitrogen inflation reduces oxygen content and limits pressure changes and fire risk compared with ordinary air. Maintenance controls inflation and checks for heat damage.
Brake wear and serviceability
Carbon brake wear is not always linear with energy. Some carbon materials wear differently at low and high temperatures. Airlines may prefer operational techniques that bring brakes into an efficient temperature range without exceeding limits.
Wear pins or electronic sensors indicate remaining material. A brake near its wear limit may still be airworthy within limits but has less life for future cycles. Maintenance planning balances removal opportunity against risk of an unscheduled limit.
A dragging brake, leaking actuator or damaged anti-skid valve can cause abnormal heat. Comparing temperatures across wheels helps identify a fault.
Autobrake disconnection
Autobrake disconnects when pilots apply sufficient pedal pressure, move a disarm switch or when a system fault occurs. Some aircraft also disarm if spoilers retract or thrust is advanced.
The disconnection should be clear through annunciation and pedal feel. Pilots then provide manual braking. Training emphasises that a selected autobrake level is not a guarantee; crew monitoring continues throughout the landing roll.
If autobrake fails before landing, manual braking and anti-skid may remain fully available. If anti-skid fails, autobrake may be restricted or unavailable because fixed automatic pressure could lock wheels.
Anti-skid failure
A complete anti-skid failure requires more cautious pedal application and can increase landing distance. The aircraft manual provides penalties and techniques. On a dry runway, braking may remain substantial, but wheel lock becomes possible.
A single-channel fault may affect one wheel while other channels remain active. The brake-control unit can isolate the failed channel. Crew messages identify the degraded condition as far as the system can determine.
Maintenance checks speed sensors, wiring, control valves and computers. A fault recorded only during high-speed operation may require targeted testing.
Brake-by-wire
Many modern aircraft use brake-by-wire. Pedal transducers send electrical demand to brake-control computers, which command hydraulic valves. This allows precise autobrake integration, anti-skid control and fault monitoring.
The system still needs a safe response to electrical failure. Alternate braking may use a simpler electrical channel, direct hydraulic path or accumulator. Certification requires that foreseeable failures do not produce uncommanded braking or loss of all stopping capability.
Electrical independence and software assurance are therefore as important as hydraulic hardware.
Landing-distance calculations
Performance calculations use aircraft weight, wind, runway slope, temperature, runway condition, flap setting, approach speed and system status. Anti-skid type and brake availability affect the result.
Dispatch landing distance and in-flight landing performance can use different regulatory factors. Operators add safety margins and account for reported runway condition. A wet or contaminated runway requires conservative assumptions.
Autobrake level mainly affects operational deceleration and runway-exit planning. Selecting a low level does not change the aircraft’s maximum available braking if the pilots take over.
Certification tests
Brake systems undergo energy, structural, stopping-performance and failure tests. The maximum kinetic-energy rejected-take-off test demonstrates the ability to stop at a demanding weight and speed with brakes at a specified wear condition.
Wet-runway testing or approved analysis establishes performance. Anti-skid tuning is evaluated across speeds and surfaces. Tests also examine brake vibration, landing-gear loads and tyre behaviour.
The aircraft must remain controllable after specified failures. Fire protection, wheel-well heating and tyre-pressure release are part of the safety case.
Maintenance testing
Technicians perform built-in tests, sensor-gap checks, valve tests and wiring inspections. Wheel-speed transducers can be damaged during wheel changes. Contamination in hydraulic fluid can affect fine-control valves.
After brake replacement, correct installation, bleeding and functional checks are essential. Cross-connected lines or incorrect components could reverse or degrade anti-skid action.
Reliability data tracks nuisance messages, hot brakes and rejected take-off events. Repeated faults may indicate a fleet-wide issue rather than isolated component failure.
Common misconceptions
Anti-skid is not the same as autobrake. Anti-skid can work during manual braking. Autobrake can command pressure but depends on anti-skid for wheel protection.
Autobrake does not necessarily use a fixed pressure. It targets deceleration and adjusts for spoilers, reverse thrust and drag.
A firm landing does not automatically mean maximum brake energy. Touchdown speed, rollout distance and braking command matter more. Conversely, a smooth high-speed landing followed by a short exit can create substantial heat.
Reverse thrust does not make brakes unnecessary. Wheel brakes are the principal controllable stopping system and are required for rejected take-off.
Conclusion
Anti-skid and autobrake form a coordinated control system. Autobrake asks for a selected aircraft deceleration. Anti-skid continuously limits each wheel to the friction available. Hydraulic valves translate those commands into pressure, while spoilers, reverse thrust and tyres determine the actual result.
The system protects tyres, improves directional control and makes deceleration more consistent, but it cannot defeat physics. Standing water, ice, poor pavement, excessive speed or missing spoilers still increase stopping distance. Safe operation depends on accurate runway information, correct performance calculation, maintained equipment and pilots ready to take manual control at any point in the landing roll.
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