When a transport aircraft lands, the wheel brakes do not simply receive a fixed hydraulic pressure and remain there until the aircraft stops. Modern braking systems combine pilot or autobrake commands with anti-skid control so that the commanded deceleration is achieved without allowing a wheel to remain locked on a surface that cannot support the requested braking force. FAA and EASA transport-aircraft certification material treats automatic braking and anti-skid as separate but interacting functions. [1] [2]
Autobrake is the deceleration-management layer: when armed and triggered under the required conditions, it applies wheel brakes automatically to pursue a selected deceleration target. Anti-skid is the tyre-runway protection layer: it monitors wheel behaviour and reduces brake pressure when a wheel approaches an unacceptable skid condition. EASA’s CS-25 guidance makes the relationship explicit by requiring anti-skid action to retain priority over automatic braking. [2]
Why an aircraft needs anti-skid
A rolling tyre can generate substantial braking force because the contact patch can transmit friction to the runway. If brake torque exceeds what the tyre-runway interface can support, wheel rotational speed begins to fall too quickly relative to aircraft ground speed and the tyre can skid. A locked or deeply skidding tyre can lose useful directional and braking capability and can suffer severe wear or damage. FAA braking-system guidance therefore includes anti-skid as a central part of transport-category brake-system certification. [1]
Anti-skid does not create friction. It attempts to use the friction that is available efficiently by modulating brake pressure. On a dry runway, the tyre may support high braking force before skidding; on a wet, snowy or icy runway, the available coefficient of friction can be far lower. EASA guidance requires anti-skid performance to be satisfactory over the range of runway friction and surface conditions for which the aircraft is certificated. [2]
Wheel-speed sensing
An anti-skid system needs information about how quickly each monitored wheel is rotating. Wheel-speed transducers provide this information to the control system. If a wheel decelerates in a way that indicates impending skid relative to the system’s reference logic, the anti-skid controller commands the associated brake-control valve to reduce pressure until the wheel recovers. FAA aircraft examples describe individual-wheel anti-skid control in which brake pressure is released on the affected wheel when a skid is detected. [3]
The exact reference algorithm varies between aircraft. A modern controller may use wheel-speed decay, comparison with other wheels, aircraft inertial information or other logic. It is therefore more accurate to describe anti-skid as detecting an impending or developing skid from wheel behaviour than to claim every airliner uses one fixed slip percentage or one identical calculation. [1]
Pressure is reduced, not increased, by anti-skid
Anti-skid sits between the brake demand and the physical brake. The pilot or autobrake system requests braking, but anti-skid can reduce the pressure delivered to a wheel when tyre adhesion is being exceeded. FAA rulemaking material explains the certification intent: the anti-skid function must be able to release a wheel that is going into a skid regardless of whether the brake demand came from the pilot or the automatic braking system. [4]
This means selecting a higher autobrake level cannot force the tyre to generate more friction than the runway provides. If the surface is sufficiently slippery, anti-skid will keep relieving brake pressure to prevent sustained wheel lock, and the achieved deceleration may remain below the selected target. [3]
What autobrake actually controls
An autobrake system is generally designed around aircraft deceleration rather than one fixed brake pressure. The selected setting corresponds to a target deceleration characteristic. After touchdown and the required activation conditions, the system increases or reduces brake pressure as needed to pursue that target. The FAA’s Boeing 737 braking-system explanation states that brake pressure is reduced when reverse thrust or spoilers provide additional decelerating force so that the selected overall deceleration is maintained. [3]
That is why reverse thrust can reduce brake use when autobrake is active. If the reversers and aerodynamic drag are already producing a large share of the target deceleration, the autobrake controller can demand less wheel-brake pressure. This is not the same as reverse thrust physically releasing the brakes; the deceleration-control system adjusts braking because the total aircraft deceleration has changed. [3]
Ground spoilers matter to braking
Wheel braking depends on normal force between the tyres and runway. After touchdown, ground spoilers or lift-dump devices reduce wing lift and transfer more of the aircraft’s weight onto the landing gear. That increases the normal load available at the tyre contact patches and allows the brakes and anti-skid system to make better use of runway friction. FAA braking discussions note that anti-skid performance benefits when aircraft weight is supported by the landing gear. [3]
This is one reason the landing-deceleration sequence is a coordinated aircraft system rather than a wheel-brake problem in isolation. Spoilers, wheel brakes, reverse thrust, aerodynamic drag and runway friction all contribute to the rate at which kinetic energy is removed. [1]
Why autobrake does not simply command maximum braking
Most landings do not require maximum wheel-brake effort. A moderate, predictable deceleration can provide a comfortable and repeatable rollout while reducing brake energy and wear. Aircraft therefore offer selectable autobrake levels on installations designed for that purpose. Higher settings generally request greater deceleration, while maximum or rejected-take-off modes are reserved for more demanding circumstances according to aircraft procedures. [3]
On a slippery runway, however, the difference between settings can become less meaningful because available friction becomes the limiting factor. The FAA notes that maximum achievable deceleration on a wet or slippery surface can be reached even at a setting below the nominal highest landing level, because anti-skid limits the effective brake pressure to what the tyres can transmit. [3]
Touchdown protection
Applying high brake pressure to a wheel before it has spun up on the runway could create an immediate skid or tyre damage. Many transport brake systems therefore include touchdown protection or logic that inhibits braking until the system has evidence that the aircraft is on the ground and the relevant wheels are rotating appropriately. FAA examples describe touchdown protection as part of anti-skid/brake-control architecture on transport aircraft. [5]
The exact triggering inputs vary by aircraft. Wheel spin-up, landing-gear load signals, air/ground logic and throttle or spoiler conditions may contribute. A general explanation should not claim one universal trigger sequence for every airliner. [1]
Rejected-take-off braking
Many autobrake systems include a rejected-take-off mode designed to command very high braking automatically when the aircraft is above specified conditions and the take-off is rejected. The operational objective is different from landing autobrake: instead of delivering a comfortable selected rollout deceleration, the system is intended to help stop the aircraft within the certified accelerate-stop performance assumptions. [1]
The exact arming criteria, trigger logic and disarm conditions are aircraft-specific and belong in the flight-crew operating manual. The generic principle is that autobrake can provide automatic brake application for both landing and rejected-take-off cases when the installed system is designed to do so. [5]
Pilot braking has priority
Automatic braking is not intended to prevent the pilot from taking manual control. Certification and aircraft design provide a means for pilot brake input to override or disconnect autobrake according to the system logic. EASA guidance separately requires that anti-skid continue to protect the wheels whether the braking demand originates automatically or manually. [2]
This creates a clear hierarchy: the crew can replace automatic deceleration demand with manual braking, while the anti-skid layer continues to prevent the commanded pressure from producing unacceptable wheel skid. [4]
Brake energy is the hidden limitation
Brakes stop an aircraft by converting kinetic energy principally into heat in the brake heat sinks. A heavy aircraft at high speed contains an enormous amount of kinetic energy, and brake assemblies have certified energy limits. FAA AC 25.735-1 contains test and analysis guidance for brake energy, rejected-take-off conditions and other transport-aircraft braking requirements. [1]
Autobrake can help distribute braking in a predictable way, but it cannot increase the physical energy capacity of the brake. Aircraft performance calculations, brake-temperature monitoring and cooling procedures address the thermal consequences of the stopping event. This is separate from the brake-fan topic: fans cool brakes after or between operations; autobrake and anti-skid determine how braking is commanded during the stop. [1]
Carbon brakes and control systems
Many modern transport aircraft use carbon heat-sink brakes because carbon systems can absorb large amounts of energy with favourable weight characteristics. The braking-control principles remain the same: the brake actuator generates clamping force, autobrake commands deceleration and anti-skid modulates pressure to protect tyre adhesion. FAA AC 25.735-1 references specific design and testing standards for carbon heat-sink brakes as part of transport braking certification. [1]
Hydraulic versus electric brake actuation
Most traditional transport aircraft use hydraulic pressure to actuate their wheel brakes, while the Boeing 787 is a notable example of electric brake actuation. The control concept should not be confused with the energy source. Anti-skid and autobrake logic can exist whether the final actuator is hydraulic or electric; what changes is how the commanded braking force reaches the brake. EASA CS-25 describes braking requirements in a way that covers electrical, pneumatic, hydraulic and mechanical transmitting elements. [2]
This article therefore focuses on control logic rather than a particular actuator architecture. A 787 electric-brake system and a hydraulically braked airliner can both use automatic deceleration and anti-skid principles while implementing them through different hardware. [2]
Brake-system redundancy
Transport-category certification requires continued braking capability after specified failures. EASA CS 25.735 requires the brake system and associated components to be designed so that, after certain single failures or loss of an operating-energy source, the aircraft can still be brought to rest within defined limits. [2]
Aircraft therefore may have normal and alternate brake power sources, accumulators or other backup arrangements depending on design. Anti-skid itself must also fail in a controlled manner: EASA guidance states that failures making the anti-skid ineffective should not prevent manual braking and should normally be indicated to the crew. [2]
Why a wheel can momentarily speed up under anti-skid
When anti-skid releases brake pressure on a decelerating wheel, tyre-road friction can accelerate that wheel back toward the rotational speed corresponding to aircraft ground speed. The controller can then reapply brake pressure. This modulation can repeat rapidly as runway friction changes. The objective is not perfectly constant wheel speed; it is to remain near the maximum usable braking region without sustained lockup. [1]
From the cockpit, the crew normally experiences the result as aircraft deceleration rather than manually feeling each individual pressure cycle. The brake-control electronics are operating much faster than a pilot could individually manage multiple wheel brakes. [3]
Contaminated runways change the whole system
Water, slush, snow and ice can reduce tyre-runway friction, which reduces the braking force available before anti-skid must intervene. Anti-skid can optimise the available friction but cannot restore dry-runway grip. This is why runway-condition reporting and landing-performance assessment remain essential even on aircraft with advanced brake control. [2]
The FAA notes that as runway conditions become more slippery, wheel braking becomes less effective and the aircraft may depend more heavily on reverse thrust and spoilers for total deceleration. In an autobrake landing, the controller can demand braking, but anti-skid still limits the wheel-brake contribution to the adhesion actually available. [3]
Directional control
Anti-skid is also important to directional control. If one wheel or one side of the aircraft locked while the other side continued to roll normally, asymmetric braking forces could create yaw. Certification therefore requires braking-system failures and anti-skid behaviour to be assessed for their effects on both stopping and directional control. [1]
Some brake systems control individual wheels while alternate modes may control wheel pairs. The FAA’s 737 example documents that distinction: normal anti-skid provides individual wheel protection, while the alternate system groups wheels differently. This is a type-specific example, not a universal architecture. [3]
Why autobrake can reduce brake wear
If a selected landing deceleration can be achieved partly through aerodynamic drag, ground spoilers and reverse thrust, the autobrake controller can reduce wheel-brake pressure rather than continuing to apply a fixed pressure. The FAA notes that this can reduce brake use while maintaining the selected deceleration. [3]
This benefit depends on the landing and selected setting. A demanding stop on a slippery runway may require the brake system to operate at the anti-skid-limited maximum for extended periods, leaving little opportunity to reduce brake energy. [3]
The pilot selects deceleration; the runway decides what is achievable
The most useful way to understand the system is as a chain of constraints. The pilot or autobrake system commands a desired level of deceleration. The brake-control system creates pressure or actuator force. Anti-skid checks whether each tyre can transmit that force without unacceptable skid. The runway surface determines the friction actually available. [1] [2]
Autobrake therefore does not guarantee a stopping distance by itself, and anti-skid does not magically create grip. Their engineering value comes from using the aircraft’s braking capability predictably and efficiently: autobrake manages the target aircraft deceleration, while anti-skid prevents that demand from becoming destructive wheel lock when tyre adhesion is the limiting factor. [3]
Verified Sources / References
- Federal Aviation Administration AC 25.735-1 — Brakes and Braking Systems Certification Tests and Analysis. Active FAA transport-category brake certification guidance.
- EASA Easy Access Rules for Large Aeroplanes — CS 25.735 Brakes and Braking Systems. European certification requirements and AMC for anti-skid and braking-system failures.
- FAA — Boeing 737-700 Braking System Technical Description. FAA explanation of autobrake target deceleration, anti-skid pressure reduction, spoilers and reverse-thrust interaction.
- FAA Braking Systems Harmonization Working Group — Transport Airplane Brake Requirements. Regulatory-development material explaining anti-skid priority over autobrake.
- FAA — McDonnell Douglas MD-82 Braking System Technical Description. FAA description of autobrake, anti-skid and touchdown protection concepts.
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