Anti-skid devices

[mitchnvic]

Hi: I've always been fascinated with the way modern airliners can be kept safely on the runway when landing in wet conditions - can anyone explain in layman's terms how an airliner's anti-skid device works?

[Bullethead]

G’day mitchnvic,

There is an optimum amount of ‘skid’ for maximum braking retardation on a dry surface, this of course differs for wet and contaminated surfaces. What the anti-skid system attempts to do is achieve and maintain this optimum skid under heavy braking conditions. The system does this by comparing actual wheel speed to the speed of the other wheels as well as to the inertial reference system speed and when it detects excessive skid the brake pressure is backed off a bit until the optimum is achieved. All this is re-calculated several times a second for each individual wheel, that’s right each wheel. You can have vastly different braking requirements over the width of the main undercarriage which on a B747 is over 30 feet wide so one side of the aeroplane can be braking harder than the other depending on the surface condition of the runway.

Incorporated into the ant-skid system on modern airliners is an autobrake system which, when selected, decelerates the aeroplane at a predetermined rate up to a maximum setting and the ant-skid function activates if required.

I hope this makes sense and explains it for you.

Regards,
BH.

[inbalance]

From the CJ3 Manual:


WHEEL BRAKES
The Model 525B uses hydraulically powered main landing gear brakes. Crew inputs to the brake metering valve are mechanically transmitted via a series of cables from the toe brakes on the rudder pedals. The brake metering valve regulates hydraulic pressure to the brakes based on pilot or copilot input. An electronic anti-skid system monitors the main gear wheel speeds and
reduces brake pressure as necessary to optimize stopping distance and prevent wheel lock-up.

A parking brake valve is used to trap pressurized fluid in the brake lines and is controlled by a control knob in the cockpit. The pneumatic brake system is a back-up system used to supply pressure to the brake assemblies in the event of hydraulic brake system failure. It supplies pressurized nitrogen from a bottle in the nose directly to shuttle valves on the brakes and is controlled by a lever in the cockpit.

Brake Metering Valve & Cable System Operation
The brakes are operated by a separate, closed center hydraulic system with an independent reservoir, pump/electric motor, and accumulator. A pressure switch located near the fluid end of the accumulator senses brake system pressure and commands the pump on and off accordingly.
There is no cockpit switch for the brake pump. The pump is powered on any time the gear handle is in the down position and the accumulator pressure is below 1175 ± 75 psig.
When the accumulator pressure reaches 1500 ± 50 psig, the power is removed for the pump. A separate low pressure switch built into the pump monitors the system for low pressure. If the system pressure drops below (900 ± 50 psig) and the gear handle is down, the low pressure switch
causes the (PWR BRK LOW PRESS) annunciator to illuminate. During normal operation, the accumulator provides pressurized fluid to the brake metering valve which regulates pressure (0 to 1000 +50/-20 psi) to the brake assemblies in proportion to the brake pedal deflection of the pilot’s
or co-pilot’s brake pedals. Braking can be controlled independently from either cockpit position.

The cable system that transmits pedal deflection to the brake metering valve is designed such that pedal inputs by the pilot do not cause the copilots pedals to move, and vice versa.

If the pilots and copilots pedals are depressed simultaneously, the brake system accepts the highest input. Since the brake system is a cable controlled ‘power brake system’, the braking feel force at the pedals is created by the springs in the mixer, the springs within the brake metering
valve and a proportional hydraulic feedback force generated by the brake metering valve.

Anti-skid System Operation
The cockpit controls for the anti-skid system consist of a single ANTISKID switch which is a ON/OFF lever lock switch located just right of the gear handle. The primary function of the antiskid system is to provide maximum braking efficiency under all runway conditions. In addition, the anti-skid system provides touchdown protection, which prevents braking until adequate wheel spin-up has occurred and locked wheel crossover protection that prevents adverse differential braking.

Anti-skid Protection
Anti-skid protection is provided to allow maximum braking efficiency, which in turn minimizes landing distances. If the pilot applies enough brake pedal force to cause slippage between the tires and the runway, the wheel speed transducer data received by the control box will indicate a sudden deceleration for the slipping wheel. The control box will determine the severity of the impending skid and send the appropriate current signal to the anti-skid servo valve to reduce brake pressure accordingly. Dual servo valves reduce pressure for either brake independently.
Therefore, a single wheel skid will result in the reduction of brake pressure at the skidding wheel only. Anti-skid protection will be available unless the touchdown protection mode is active.

Touchdown Protection
Touchdown protection is provided to prevent the application of brake pressure prior to wheel spin up. During a landing, the wheels must be allowed to spin up to provide the anti-skid system a ‘reference’ velocity to which individual wheel speeds can be compared. Touchdown protection is
active only when an AIR signal is sensed by both main gear squat switches. In touchdown protection mode, the control box commands the anti-skid servo valves to dump all brake pressure. The full dump command will remain active for 3 seconds after WOW or until wheel spin up has occurred. Under normal circumstances, the wheels will spin up almost immediately after
touchdown. Therefore, the system incorporates a spin up override feature. When the velocity of a wheel exceeds 59+2 kts, touchdown protection is overridden and brake pressure application is allowed to that wheel. Each wheel is independent in regard to spin up override, specifically;
touchdown protection mode is overridden for each wheel independently only when the speed of a given wheel is in excess of 59+2 kts. The wheel spin up override will remain active until the wheel velocity falls below 15+2 kts.

Locked Wheel Crossover Protection
Locked wheel crossover protection prevents inadvertent turning of the aircraft due to differential braking caused by adverse runway conditions. The velocities of the two wheels are compared to each other to determine if one wheel is locked. If the velocity of one wheel falls to less than 30%
of the velocity of the other wheel, the control box will send a full dump command to the anti-skid servo valve controlling the slower wheel. The full dump will remain in effect until the velocity of the slow wheel increases above the 30% threshold. The locked wheel crossover feature is
inactive at wheel speeds below 25 kts to allow for low speed taxiing maneuvers.

Anti-skid System Self Monitoring
The anti-skid system performs continuous integrity checks on the wheel speed transducer circuits, the anti-skid servo valve circuit and the regulated power to the control box. If a fault is detected during the continuous monitoring the ANTISKID INOP annunciator is illuminated and a
signal is sent to the anti-skid fault display unit. The fault display unit is located in the RH nose compartment on the forward side of the forward pressure bulkhead. The fault display unit consists of 5 rotary flags to aid in the troubleshooting of an ANTISKID INOP indication. There is one rotary
flag for each of the following conditions: LH transducer fault, RH transducer fault, servo valve fault, control box fault and a squat switch disagree. Both main gear squat switch signals are monitored and compared. If the signals disagree for more than 12.8+1 seconds, the squat switch disagree flag is tripped. However, this fault does not cause the ANTISKID INOP annunciator to illuminate.
There is also one condition, low supply voltage, which causes the ANTISKID INOP annunciator to illuminate but does not result in a tripped flag on the fault display unit. The continuous monitor function evaluates the voltage supplied to the control box. Anytime the input voltage is less than
7.0+1 volts, the ANTISKID INOP annunciator is illuminated but the control box flag will not be tripped. This feature is intended to alert the crew that the anti-skid system is either switched off or is unavailable due to insufficient power. In addition to continuous monitoring, the anti-skid control
box performs a dynamic self test which is initiated by any of the following events:
- initial power-up of the anti-skid system
- transition of the gear handle to the down position
- selection of (ANTISKID) on the rotary test switch
During a dynamic self test, a signal is sent to illuminate the ANTISKID INOP annunciator. Upon successful completion of the test, the ANTISKID INOP annunciator will extinguish. If a fault is detected during the self test, the annunciator will remain illuminated. A dynamic self test performed in the air takes approximately 3 seconds, while a dynamic self test performed on the
ground takes approximately 6 seconds. The dynamic self test routine is inhibited if wheel speed is
greater than 15+5 kts.

[SNS3Guppy]

Anti-skid works to prevent a tire skidding by sensing the condition of each individual wheel (or sets of wheels, in some cases), and modifying the brake pressure being applied to that wheel.

The theory as bullethead explained is to prevent the wheel from actually skidding. If skidding occurs, directional control and braking efficiency is lost. A sensor in each axle of each wheel is connected to an anti-skid computer. If the computer senses that the change in wheel speed is incorrect (it stops turning, or turns too slowly), the computer directs a bypass valve in the brake line or system to divert brake fluid from the brakes on that particular wheel. This has the effect of decreasing brake pressure, and preventing the skid.

In theory, no matter how hard the pilot attempts to apply braking, the anti-skid system won't let the pilot skid the wheels. Some systems are fully modulating, which means they do all the work. Other systems, particularly on some smaller airplanes, do not fully modulate. They will work to prevent a skid, but aren't pilot proof.

Also part of the anti-skid system are autobrake systems. These apply brakes automatically to achieve a predetermined rate of acceleration (or "deceleration," if you prefer). There are typically several levels or degrees of antiskid, depending on how much braking should be done. Different aircraft and different operators have their own ways of utilizing these systems. With autobrakes, the system only cares how fast the aircraft slows down...not how much brake pressure is applied. If the pilot uses reverse thrust, the brake system won't work so hard, won't heat up so much, and won't use as much brake energy to slow the aircraft...but it will still slow at the predetermined rate...the slowing effect being a combination of braking and other inputs such as the reverse thrust. In the maximum setting, full braking energy is used, just shy of skidding the wheels, and any outside decelerating force such as reverse thrust is added to the braking effect, or increases the rate at which the aircraft slows down. Autobrakes are a part of the anti-skid system, and are used for landing.

Another feature many aircraft use are takeoff autobrakes, sometimes called RTO brakes (rejected takeoff brakes). These are also part of the antiskid system, and serve in a similiar fashion to the landing autobrakes. Braking is automatically applied if a takeoff is rejected. Various parameters arm them: a certain amount of power lever travel, a certain velocity (80 knots, for example) on certain sets of wheels, etc, tell the system the aircraft is taking off. If other triggers occur, such as retarding certain power levers, the takeoff autobrakes kick into effect, and apply maximum braking to the aircraft.

Some systems use the brake valves to accomplish automatic braking, some use a separate system to do this, and each modulates the autobraking using the antiskid system.

Antiskid on an aircraft works very much like antiskid on a car.

[mitchnvic]

Many thanks to all 3 of you guys for your explanations - they also answered questions I had about autobrake systems! :ok:

[myw627]

Hi inbalance, can you please check your PM inbox? Thanks :ok:

w627

[FE Hoppy]

Not bad but I would tend to regard Auto Brake as a separate function to Anti-skid. Many aircraft have anti skid as standard and auto brake as an option. Generally the RTO function of an auto brake system will deliver maximum brake pressure and rely on the anti skid system to modulate the individual wheel pressure.

I would also add that the old maxaret system was a mechanical version of anti skid.

maxarets 1953 (http://www.flightglobal.com/pdfarchive/view/1953/1953%20-%201433.html)

[tcasblue]

FAA AC 25-7C, Flight Test Guide for Certification of Transport Category Airplanes, dated
10/16/2012, describes the three types of anti-skid braking systems
identified in §25.109 as follows:

(aa) The efficiency values specified in §25.109(c)(2) are a function of the type of anti-skid system
installed on the airplane. Three broad system types are identified in the rule: on/off, quasi-modulating,
and fully modulating. These classifications represent evolving levels of technology and
differing performance capabilities on dry and wet runways. The classification of anti-skid system
types and the assigned efficiency values are based on information contained in Society of
Automotive Engineers (SAE) Aerospace Information Report (AIR) 1739, titled “Information on Anti-
Skid Systems.”

(bb) On/off systems are the simplest of the three types of anti-skid systems. For these systems,
full-metered brake pressure (as commanded by the pilot) is applied until wheel locking is sensed.
Brake pressure is then released to allow the wheel to spin back up. When the system senses that
the wheel is accelerating back to synchronous speed (i.e., ground speed), full-metered pressure is
again applied. The cycle of full pressure application/complete pressure release is repeated
throughout the stop (or until the wheel ceases to skid with pressure applied).

(cc) Quasi-modulating systems attempt to continuously regulate brake pressure as a function of
wheel speed. Typically, brake pressure is released when the wheel deceleration rate exceeds a
preselected value. Brake pressure is re-applied at a lower level after a length of time appropriate
to the depth of the skid. Brake pressure is then gradually increased until another incipient skid
condition is sensed. In general, the corrective actions taken by these systems to exit the skid
condition are based on a pre-programmed sequence rather than the wheel speed time history.

(dd) Fully modulating systems are a further refinement of the quasi-modulating systems. The
major difference between these two types of anti-skid systems is in the implementation of the skid
control logic. During a skid, corrective action is based on the sensed wheel speed signal, rather
than a pre-programmed response. Specifically, the amount of pressure reduction or reapplication
is based on the rate at which the wheel is going into or recovering from a skid.