What exactly is the purpose of Anti Skid??

[bunraku]

Hi

 

What exactly is the purpose of the Anti Skid function on the A-10C? Is it to do with landing or taking off or both and should it ever be off?

 

Thanks

[USSInchon]

Anti-skid does exactly what it sounds like it does. It prevent the wheels from locking up during braking. There really isn't a reason to have it disabled unless there is a systems failure that disables it.

[Wrench]

It's basically ABS. Pretty much the same as what's in modern cars.

[SkateZilla]

[cfarsr]

 

Hilarious

[Gladman]

^^^^ LMAO!!!!

[FSKRipper]

 

A simply great Explanation :megalol::megalol::megalol:

[bunraku]

Thanks guys and love the dog analogy.

[Sierra99]

 

Thanks skate! Now I have coffee in nose and on iPad!

 

Sierra

[AvgWhiteGuy]

There really isn't a reason to have it disabled unless there is a systems failure that disables it.

 

Not so fast here....let's review,

 

ABS works by keeping the tire rotating to maintain traction between the 2 surfaces; tire rubber and roadway surface. Once that rotation is lost and the wheel locks up an appreciable amount of braking is lost along with, more importantly, directional stability of the vehicle.

 

In an aircraft it can be complicated further because of the amount of weight sitting on a very small footprint of the skidding tires. The tire can quickly heat up and explode...that would be very bad for 20+ tons of A-10 sitting on 3 tires.

 

ABS works by monitoring and 'pumping' the braking system to avoid that loss of wheel rotation, much like us older drivers learned before the introduction of ABS. An ABS does this in a cycle much faster than humanly possible as it measures the rotation changes of 3-4 tires in milleseconds.

 

In gravel, sand and snow, ABS tends to INCREASE braking distances. On these surfaces, locked wheels dig in and stop the vehicle more quickly. ABS prevents this from occurring.

 

So it would be quite logical for a pilot to turn ABS off when landing on an unfinished dirt-gravel runway or one with heavy snow.

 

I'd be interested to hear from one of the experienced 'real' driver on this.

[Luzifer]

ABS works by monitoring and 'pumping' the braking system to avoid that loss of wheel rotation, much like us older drivers learned before the introduction of ABS. An ABS does this in a cycle much faster than humanly possible as it measures the rotation changes of 3-4 tires in milleseconds.

 

Additionally it handles every wheel (or wheels with a common brake) separately.

 

So it would be quite logical for a pilot to turn ABS off when landing on an unfinished dirt-gravel runway or one with heavy snow.

Blocking tires means you lose directional control. Spinning the plane and/or running off the sides of the runway are pretty undesirable outcomes. If you consider measures as desperate as this I'd say you made big mistakes long before landing.

[Cik]

it stops the skid

 

anti-skid

 

works against the skid

 

opposed to pro-skid, which i guess is default

 

man do i gotta answer everything? come on

[mvsgas]

Not so fast here....let's review,

 

ABS works by keeping the tire rotating to maintain traction between the 2 surfaces; tire rubber and roadway surface. Once that rotation is lost and the wheel locks up an appreciable amount of braking is lost along with, more importantly, directional stability of the vehicle.

 

In an aircraft it can be complicated further because of the amount of weight sitting on a very small footprint of the skidding tires. The tire can quickly heat up and explode...that would be very bad for 20+ tons of A-10 sitting on 3 tires.

 

ABS works by monitoring and 'pumping' the braking system to avoid that loss of wheel rotation, much like us older drivers learned before the introduction of ABS. An ABS does this in a cycle much faster than humanly possible as it measures the rotation changes of 3-4 tires in milleseconds.

 

In gravel, sand and snow, ABS tends to INCREASE braking distances. On these surfaces, locked wheels dig in and stop the vehicle more quickly. ABS prevents this from occurring.

 

So it would be quite logical for a pilot to turn ABS off when landing on an unfinished dirt-gravel runway or one with heavy snow.

 

I'd be interested to hear from one of the experienced 'real' driver on this.

I never worked on Mirages and I know very little on it, but on other aircraft you would never turn anti-skid unless there was a malfunction. Snow normally not a factor since most aircraft would not land in heavy snow covered runway, that is why the close airports if they can't keep the runway clean. Same for gravel, I do not see a jet fighter landing in gravel... I guess the A-10 could.

Edited January 23, 2016 by mvsgas

[mwd2]

I never worked on Mirages and I know very little on it, but on other aircraft you would never turn anti-skid unless there was a malfunction. Snow normally not a factor since most aircraft would not land in heavy snow covered runway, that is why the close airports if they can't keep the runway clean. Same for gravel, I do not see a jet fighter landing in gravel... I guess the A-10 could.

 

It did

 

[Vitormouraa]

To understand how it works, you should watch this video:

 

[ame=

]

[/ame]

[tweet]

The anti-skid systems on aircraft are not anything like anti-lock brakes on cars. ABS on cars is far less sophisticated. It usually is active on paired wheels, has a limited ability to cycle the brakes due to minimal sensors and relatively primitive control hardware and software, and a solitary purpose of keeping the brakes from fully locking the wheel.

 

Anti-skid on modern aircraft works on each braked wheel independently,. There are sensors that measure wheel spin comparing it to ground speed to determine the amount of braking it will permit on each wheel. The target is usually something like an 80% rolling skid for fully depressed brakes. The control system modulates hydraulic pressure to each wheel's brake pad to keep each wheel's rotation at no less than 80% of the ground speed. (An 80% rolling skid has been determined to produce the shortest stopping distances on the usual surfaces that are used for runways.) Full pedal braking on a bare and dry surface will leave a rubber streak but the wheel continues to spin at a rate that produces the optimal braking force regardless of surface or tire conditions. Less wheel speed relative to ground speed, a lower percentage, damages tires quickly, has a higher probability of nasty events like reverted rubber (think rubber being melted to a liquid which lowers the friction between the tire and surface to near zero - like ice), and increases landing distances. Skids also greatly reduce the ability to steer the jet just like it does your car. Higher wheel speeds relative to ground speed, higher percentage, increases ground roll because the brakes are not being as efficiently applied as is possible.

 

Anti-skid greatly reduces the difficulty of getting a jet stopped. Every jet I've ever flown had at least a warning to avoid more than light brake use above 100 knots when anti-skid is inoperative. I've also seen prohibition of brake use above 100 knots if anti-skid is not working. At higher speeds aerodynamic braking is more effective. It is very easy to lock a wheel or wheels that can result in a skid with a loss of directional control. Remember that the lifting surfaces continue to generate significant amounts of lift at and above 100 KIAS which reduces the weight on the wheels, which lowers the friction between the ground and the tire, and results in less braking force necessary to lock a wheel. Anti-skid uses technology to nearly eliminate these issues.

 

Most fighter type aircraft have relatively small tires and wheels/brakes which generate far less braking force than larger tires like are on jet airliners. They are more susceptible to stopping distance problems. The shortest way to stop, other than say doing an arrested stop, is to aerobrake down to around 100 KIAS or wherever the jet loses elevator authority to keep the nose up. The nose is then lowered to the runway and brakes applied to a stop.

 

Jet airliners use a completely different strategy. They lower the nose to the runway shortly after touchdown and begin braking. This is equally efficient as they have high drag devices, spoilers that deploy on touchdown, which decreases lift increasing the weight on the wheels thus increasing the friction available when the brakes are applied. They also usually have engine thrust reversers which redirect some portion of the engine thrust forward to help slow the jet. Devices like ground spoilers and thrust reversers add significant amounts of weight that an aircraft designed for high performance must forego. It is uncommon to see them on fighters unless the user really wants to accept the performance penalty for some operational reason.

 

Anti-skid is pretty easy for the pilot to use. He/she just mashes on the brakes as hard as possible until reaching the desired speed. It doesn't matter what the condition of the runway might be - dry, snow, wet, or icy - as the maximum braking force is always delivered. Now, as you're probably thinking the problems begin when the runway condition is so bad it can't provide enough friction for the brakes to do their job. That's one of the reasons why pilots get paid the big bucks - to say no when conditions are too bad to operate on that runway.

[Crashmo]

The anti-skid systems on aircraft are not anything like anti-lock brakes on cars. ABS on cars is far less sophisticated. It usually is active on paired wheels, has a limited ability to cycle the brakes due to minimal sensors and relatively primitive control hardware and software, and a solitary purpose of keeping the brakes from fully locking the wheel.

 

Anti-skid on modern aircraft works on each braked wheel independently,. There are sensors that measure wheel spin comparing it to ground speed to determine the amount of braking it will permit on each wheel. The target is usually something like an 80% rolling skid for fully depressed brakes. The control system modulates hydraulic pressure to each wheel's brake pad to keep each wheel's rotation at no less than 80% of the ground speed. (An 80% rolling skid has been determined to produce the shortest stopping distances on the usual surfaces that are used for runways.) Full pedal braking on a bare and dry surface will leave a rubber streak but the wheel continues to spin at a rate that produces the optimal braking force regardless of surface or tire conditions. Less wheel speed relative to ground speed, a lower percentage, damages tires quickly, has a higher probability of nasty events like reverted rubber (think rubber being melted to a liquid which lowers the friction between the tire and surface to near zero - like ice), and increases landing distances. Skids also greatly reduce the ability to steer the jet just like it does your car. Higher wheel speeds relative to ground speed, higher percentage, increases ground roll because the brakes are not being as efficiently applied as is possible.

 

Anti-skid greatly reduces the difficulty of getting a jet stopped. Every jet I've ever flown had at least a warning to avoid more than light brake use above 100 knots when anti-skid is inoperative. I've also seen prohibition of brake use above 100 knots if anti-skid is not working. At higher speeds aerodynamic braking is more effective. It is very easy to lock a wheel or wheels that can result in a skid with a loss of directional control. Remember that the lifting surfaces continue to generate significant amounts of lift at and above 100 KIAS which reduces the weight on the wheels, which lowers the friction between the ground and the tire, and results in less braking force necessary to lock a wheel. Anti-skid uses technology to nearly eliminate these issues.

 

Most fighter type aircraft have relatively small tires and wheels/brakes which generate far less braking force than larger tires like are on jet airliners. They are more susceptible to stopping distance problems. The shortest way to stop, other than say doing an arrested stop, is to aerobrake down to around 100 KIAS or wherever the jet loses elevator authority to keep the nose up. The nose is then lowered to the runway and brakes applied to a stop.

 

Jet airliners use a completely different strategy. They lower the nose to the runway shortly after touchdown and begin braking. This is equally efficient as they have high drag devices, spoilers that deploy on touchdown, which decreases lift increasing the weight on the wheels thus increasing the friction available when the brakes are applied. They also usually have engine thrust reversers which redirect some portion of the engine thrust forward to help slow the jet. Devices like ground spoilers and thrust reversers add significant amounts of weight that an aircraft designed for high performance must forego. It is uncommon to see them on fighters unless the user really wants to accept the performance penalty for some operational reason.

 

Anti-skid is pretty easy for the pilot to use. He/she just mashes on the brakes as hard as possible until reaching the desired speed. It doesn't matter what the condition of the runway might be - dry, snow, wet, or icy - as the maximum braking force is always delivered. Now, as you're probably thinking the problems begin when the runway condition is so bad it can't provide enough friction for the brakes to do their job. That's one of the reasons why pilots get paid the big bucks - to say no when conditions are too bad to operate on that runway.

 

Pretty thorough, Tweet.

 

I used to fly a CRJ. It had 2 degrees nose down sitting on the ramp ( for reason of getting the main cabin door low enough that it didn't require a slide) so an advantage of dropping the nose wheel down early was to dump all of the lift. It did have a nasty habit though. If you bounced a landing or even skimmed it nicely and became airborne again, all of the sensors would indicate a landing and the ground spoilers would deploy. This does not bode well if you're back in the air--say 5 to 10 feet. The nose pitches down abruptly and the pilot response is reactionary. We pitch up. The only effect that serves is to drive the main wheels onto the runway with a good deal of force. It was ugly and the resulting comments from passengers was enough to keep the door closed during deplaning. :music_whistling:

[TomCatMucDe]

The anti-skid systems on aircraft are not anything like anti-lock brakes on cars. ABS on cars is far less sophisticated. It usually is active on paired wheels, has a limited ability to cycle the brakes due to minimal sensors and relatively primitive control hardware and software, and a solitary purpose of keeping the brakes from fully locking the wheel.

 

Anti-skid on modern aircraft works on each braked wheel independently,. There are sensors that measure wheel spin comparing it to ground speed to determine the amount of braking it will permit on each wheel. The target is usually something like an 80% rolling skid for fully depressed brakes. The control system modulates hydraulic pressure to each wheel's brake pad to keep each wheel's rotation at no less than 80% of the ground speed. (An 80% rolling skid has been determined to produce the shortest stopping distances on the usual surfaces that are used for runways.) Full pedal braking on a bare and dry surface will leave a rubber streak but the wheel continues to spin at a rate that produces the optimal braking force regardless of surface or tire conditions. Less wheel speed relative to ground speed, a lower percentage, damages tires quickly, has a higher probability of nasty events like reverted rubber (think rubber being melted to a liquid which lowers the friction between the tire and surface to near zero - like ice), and increases landing distances. Skids also greatly reduce the ability to steer the jet just like it does your car. Higher wheel speeds relative to ground speed, higher percentage, increases ground roll because the brakes are not being as efficiently applied as is possible.

 

Anti-skid greatly reduces the difficulty of getting a jet stopped. Every jet I've ever flown had at least a warning to avoid more than light brake use above 100 knots when anti-skid is inoperative. I've also seen prohibition of brake use above 100 knots if anti-skid is not working. At higher speeds aerodynamic braking is more effective. It is very easy to lock a wheel or wheels that can result in a skid with a loss of directional control. Remember that the lifting surfaces continue to generate significant amounts of lift at and above 100 KIAS which reduces the weight on the wheels, which lowers the friction between the ground and the tire, and results in less braking force necessary to lock a wheel. Anti-skid uses technology to nearly eliminate these issues.

 

Most fighter type aircraft have relatively small tires and wheels/brakes which generate far less braking force than larger tires like are on jet airliners. They are more susceptible to stopping distance problems. The shortest way to stop, other than say doing an arrested stop, is to aerobrake down to around 100 KIAS or wherever the jet loses elevator authority to keep the nose up. The nose is then lowered to the runway and brakes applied to a stop.

 

Jet airliners use a completely different strategy. They lower the nose to the runway shortly after touchdown and begin braking. This is equally efficient as they have high drag devices, spoilers that deploy on touchdown, which decreases lift increasing the weight on the wheels thus increasing the friction available when the brakes are applied. They also usually have engine thrust reversers which redirect some portion of the engine thrust forward to help slow the jet. Devices like ground spoilers and thrust reversers add significant amounts of weight that an aircraft designed for high performance must forego. It is uncommon to see them on fighters unless the user really wants to accept the performance penalty for some operational reason.

 

Anti-skid is pretty easy for the pilot to use. He/she just mashes on the brakes as hard as possible until reaching the desired speed. It doesn't matter what the condition of the runway might be - dry, snow, wet, or icy - as the maximum braking force is always delivered. Now, as you're probably thinking the problems begin when the runway condition is so bad it can't provide enough friction for the brakes to do their job. That's one of the reasons why pilots get paid the big bucks - to say no when conditions are too bad to operate on that runway.

 

I work in the automotive domain and I have to disagree, old ABS is more advanced than old anti skid systems in aviation. The modern automotive anti locking systems have little to do with old ABS systems, they rely on different principles and require powerful processors (32 bit microcontrollers). Every wheel is controlled individually. With a BMW 5 series you can drive quite smoothly on the snow and take tight turns without slipping much, and I am not even talking about X drive.

I think you compared old basic ABS with 8 bit controllers, or even without processors at all, to modern anti skid sys

Edited January 23, 2016 by TomCatMucDe

[TomCatMucDe]

To understand how it works, you should watch this video:

 

 

no, ABS DOESNT reduce the braking distance. It only prevent drifting sideways and losing control. It simply locks and releases, locks and releases... This video is misleading.

 

the red car appears to have been driving faster, and the rear wheels continued spinning as if the rear brakes were pretty bad.

Edited January 23, 2016 by TomCatMucDe

[tweet]

There have been some very nice advances in technology and design over the years. Too bad Canadair didn't bother to make take advantage of more robust logic for ground spoiler logic. Your story is something I would have expected out of the 707 or 727 generations of transports. But there isn't any manufacturer who doesn't use a design that saves that thing that actually makes airplanes fly, $$$, over the most robust features.

 

I flew the A330 for the last eight years of my flying career before retiring. The ground spoiler, autobrake, and thrust reverser systems were very thoroughly integrated and designed. The thrust reversrers were unlocked at main tire spinup while the ground spoiler didn't deploy until weight on all main wheels was senesed and the auto brakes engaged immediately or two seconds afterwards depending on the selection made prior to landing. The system made for a very smooth efficient start of the landing roll.

 

However, as is the story for almost anything man made there are still traps and idiosyncrasies. The airplane landed very well from a pilot's perspective. It mostly fit my idea of how an airplane should work. One of the obvious features of the design included three distinct touchdowns when you landed the beast. The first set of main tires was the easiest to make smooth and if you gave up flying the second set of tires would announce the fact quite clearly. The nose had to be told to land as well unless you didn't do it before about 100 knots when it would thunk down. Like the CRJ the picture out the window at nose gear touchdown looked nose low. Most new guys, and a few old hands who weren't paying full attention, could get the first straight away, learn the second fairly soon, and continue to hunt for the third for some time. Everyone learned to start the nose down right after touchdown with a cross between a small bunt and relaxing back pressure. Eventually.

 

During all of this the spoilers deploy with hardly any noticeable nose down tendency. Even popping the thrust reversers as soon as you touchdown and the finger lifts unlock doesn't do anything. The autobrakes a bit different. Autobrakes Low is like the rest of the system, smooth with only a slight nose down feel. The activate two seconds after both trucks are on the ground and modulate braking for a fixed deceleration rate. They are completely smooth and consistent right up to a lurching stop if forgotten. Medium is completely different. First the target deceleration rate is somewhat higher and the initiation of braking is immediate upon the second touch down. There is a noticeable nose down move that has to be counteracted with a bit of aft stick or it bangs down the nose firmly. The first use of medium autobrakes by a new pilot almost always surprises him/her. They're thinking push the nose down when the need to be thinking about not letting it come down with too much authority. Autobrakes High is a takeoff feature only although there have been a couple knuckleheads who selected it for landing with very bad results. The last little oddity of autobraking occurred when disconnecting them. We'd just push the thrust levers up a bit in the A320 to make it all go away when we got down to a safe taxi speed. Doing that in the A330 cause the brakes to release with a thunking lurch. Actually, releasing them with the brake pedals did the same if you weren't easy on the pedals. I always taught to think about curling you toes and to not wait until the airplane got much below 40 knots unless stopping might be a problem.

 

Advancing the thrust levers to just above idle disconnected the brakes and stowed the thrust reversers which was very nice in the event of an aborted landing after touchdown which though rare is often a huge surprise. Stowing the thrust reversers, spoilers, and disconnecting the brakes takes coordination and time which is usually in short supply while also trying to fly out of a bad situation. I seem to remember something odd about when the spoilers were automatically stowed during this sequence but I'm not sure what. I know a go around, or the rare touch and go for training or maintenance testing, only required advancing the thrust levers and a quick check to be sure everything worked as advertised.

 

That's all more than most anyone would want to know about Airbus stuff but some of you might find it interesting even if you came to this thread only to learn something about anti-skid.

[tweet]

Thanks for that TomCat. I have exactly zero expertise in automotive ABS beyond a long ago explanation of early systems and my own experiences. I defer to your knowledge. I can surely appreciate how modern electronics would make ABS far more beneficial than it was originally invented to do - aid maintaining steering control under heavy braking. Anti-skid has the same functions while it's original design target was probably reducing stopping distance. I'll also agree that early generation anti-skid was OK when it worked but had many, many flaws. The differences between early generation A/S and what I used at the end of my flying career is astonishing. Almost all of the improvements were brought about by the introduction of electronics and digital systems.

 

My only objection is the statement you made to Vitormouraa that claims ABS does not reduce stopping distance. I totally agree that the design purpose of ABS is to ensure directional controlability under braking of a car or truck. However, a secondary benefit is that anything that prevents wheel locking during braking will have the additional benefit of reducing stopping distance. Not all conditions and on all surfaces for sure but under most. I think that is probably correct just from looking at what it does from an engineering standpoint. I certainly could be wrong.

Edited January 23, 2016 by tweet

[jubus]

no, ABS DOESNT reduce the braking distance. It only prevent drifting sideways and losing control. It simply locks and releases, locks and releases... This video is misleading.

 

the red car appears to have been driving faster, and the rear wheels continued spinning as if the rear brakes were pretty bad.

 

Incorrect. ABS = anti-lock braking system.

 

ABS can reduce the braking distance on concrete and other high-traction surfaces. Why? For the same reason why Formula 1 drivers avoid locking their brakes: when the wheels lock, you lose grip. They use a technique called threshold braking.

 

The physical explanation is, that the friction between the wheels and the surface is at maximum just before the wheels start to slip. You can see this peak in the following graph just before static friction turns into kinetic friction.

 

 

On surfaces, where the average braking force applied by the ABS is higher than what we would get by locking the brakes (kinetic friction + other forces), then naturally the braking distance would also be shorter with ABS.

 

More info:

https://en.wikipedia.org/wiki/Anti-lock_braking_system

https://en.wikipedia.org/wiki/Threshold_braking

https://en.wikipedia.org/wiki/Friction

Edited January 24, 2016 by jubus

[TomCatMucDe]

Incorrect. ABS = anti-lock braking system.

 

ABS can reduce the braking distance on concrete and other high-traction surfaces. Why? For the same reason why Formula 1 drivers avoid locking their brakes: when the wheels lock, you lose grip. They use a technique called threshold braking.

 

The physical explanation is, that the friction between the wheels and the surface is at maximum just before the wheels start to slip. You can see this peak in the following graph just before static friction turns into kinetic friction.

 

 

On surfaces, where the average braking force applied by the ABS is higher than what we would get by locking the brakes (kinetic friction + other forces), then naturally the braking distance would also be shorter with ABS.

 

More info:

https://en.wikipedia.org/wiki/Anti-lock_braking_system

https://en.wikipedia.org/wiki/Threshold_braking

https://en.wikipedia.org/wiki/Friction

 

 

If you read my post you would see that I talked about lock.

 

As I said, I work in an automotive company and we sell processors for automative application ac including braking. OEMs stopped claiming that ABS itself reduces braking distance. It improves controllability and steering while braking and therefore you can it improve braking distance.

 

Here is what Toyota claims on its website:

 

http://www.toyota-global.com/innovation/safety_technology/safety_technology/technology_file/active/

 

The ABS (Anti-lock Brake System) monitors the speed of each wheel to detect locking. When it detects sudden braking, it will release braking pressure for a moment and then provide optimum braking pressure to each wheel. By repeating this process in a short period of time, it enhances steering control during sudden stops. As a result, it will also help improve the ability of stopping the vehicle.

 

If you read it on BMW website you have similar claims.

 

Here is what drivingfast.net says:

 

http://www.drivingfast.net/technology/abs.htm

 

You may have heard that it's possible to slow down quicker in a car without ABS. This has elements of truth, but in practical terms the benefits of ABS massively outweigh the slightly longer braking distances. For road use, ABS is an absolute must as it will allow you to steer out of the way of unexpected hazards. Diagrams 2 and 3 below explain the practical reasons for ABS.

 

In the Wikipedia article you linked it also says to that ABS increases braking distance on soft surfaces.

 

Wikipedia is a good starting point but it contains a lot of mistakes as well. It is not to be taken as a trusted source.

[jubus]

@TomCatMucDe, I did read your post, did you read mine?

 

We may have misunderstood each other. I argued that ABS can reduce the braking distance compared to locking the brakes when braking in a straight line, which was the context of that video you responded to.

 

1) I agree you can slow down faster without ABS using threshold braking.

 

2) If the average braking force applied by the ABS is higher than we would get by locking the brakes, then we can slow down faster with ABS than by locking the brakes.

 

For example (pulling the numbers out of my hat), let the maximum braking force be 3 Newtons and the kinetic friction we get from locking the brakes be 1 Newton. If the ABS spends half of the time braking with the optimum braking force and half of the time with the brakes released, then the average braking force is 1.5 N.

 

3) Otherwise the braking distance with ABS is equal or higher than we get by locking the brakes.

 

My viewpoint is purely physical and I have no knowledge of how the auto companies actually implement their ABS. My point is that it is possible to implement an ABS which gives shorter braking distances compared to locking the brakes on certain surfaces.

[TomCatMucDe]

Then we do agree. The video is though misleading because it compares not only ABS to non ABS but bad brakes to better ones as well as cars driving at different speed. The braking difference in that video is huge. It can't be because of the ABS in my opinion