anti-skid

[colubridae]

I know this is a dumb question...

 

 

but...

 

 

what is the point of switching anti-skid off? (on any aircraft)

 

:helpsmilie::)

[bear.is.flying]

Anti-skid can malfunction and cause the brakes to automatically release. Not a good thing when taxiing around the boat.

[Kang]

Even if it's working as intended it might reduce your braking power. It reduces wear on the tyres and brakes, but 'stop NOW!' is sometimes more important, for example as Darkmage pointed out, when you are headed for an abrupt end of a taxiway.

[Flagrum]

Even if it's working as intended it might reduce your braking power. It reduces wear on the tyres and brakes, but 'stop NOW!' is sometimes more important, for example as Darkmage pointed out, when you are headed for an abrupt end of a taxiway.

 

I can't think of any situation where sliding/gliding on melted rubber would be more effective than a few percent less braking effectiveness compared to a "perfect braking" (which neither ABS nor humans could probably archive anyways).

[colubridae]

in my last working years I was a driving instructor.

 

We were taught (and had to teach) that skidding was bad. In a skid deceleration is not maximised, it also strongly diminishes steering control.

 

Ergo anti-lock braking (which i assume is the same as anti-skid) maximises deceleration whilst maintaining steering capability.

 

I cannot think of any reason for switching off anti-skid braking, yet there must be a reason since all the aircraft seem able to deactivate it.

 

:helpsmilie::)

[Kang]

I'm fairly certain that, as far as cars are concerned, the whole reasoning of anti-lock brakes making you decelerate faster is, quite frankly, bogus from the early days of these systems; the whole point there is that they make sure you retain steering control.

[QuiGon]

I'm fairly certain that, as far as cars are concerned, the whole reasoning of anti-lock brakes making you decelerate faster is, quite frankly, bogus from the early days of these systems; the whole point there is that they make sure you retain steering control.

If you're applying full brake on a slippery surface, your wheels will lock up and you will just slide on. Anti skid prevents that by releasing braking pressure, so that the wheels can still turn and therefore will still have grip, which leads to faster deceleration.

[Kang]

I don't want to argue, especially since I'm not an expert either.

After a little research I found that anti-lock brakes do indeed reduce your stopping distance on wet roads, so I was wrong in that scenario, have hardly an influence on it at all on dry roads and are in fact making it longer on loose surfaces, but that's probably not important to airplanes anyway (but the reason there is an 'off-road' switch in some cars).

[colubridae]

maybe the answer could be found in aircraft manuals that contain protocols for when anti-skid should be used and when it shouldn't be used.

 

 

at least it would show the thinking behind the switchable ability.

 

 

perhaps.

 

 

(I wouldn't know where to look) :cry:

[Joker328]

Maximum braking performance is not achieved by either skidding or by activating the ABS or anti-skid system. You will decelerate more quickly by braking just hard enough to keep the wheels from locking up. The reason ABS / anti-skid is needed is that this is sometimes difficult to judge, and on wet surfaces, you can hit a puddle or wet spot and suddenly lock up one wheel before you can react. The electronics can react faster than you and help maintain control.

 

As for why you would turn it off on the boat, for one thing, it's not needed. The wire is what decelerates you, not the brakes. It's also possible the rapid acceleration or deceleration somehow trips up the system.

 

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[Nodak]

Ever driven up to an icy intersection sloped slightly down with anti skid, it don't work. There's no braking action at all, I'd rather have direct control, at least I have the ability to get the wheel stopped, ABS won't stop it with low traction.

 

 

I'd imagine those carrier decks get awful slick with all the fluid leaks and petrol products about. That and some occasional sudden angle changes from wave action, not a good mix with taxi inertia. Don't see how you'd do it without direct control, someone would get their toes crushed.

[colubridae]

Maximum braking performance is not achieved by either skidding or by activating the ABS or anti-skid system. You will decelerate more quickly by braking just hard enough to keep the wheels from locking up. The reason ABS / anti-skid is needed is that this is sometimes difficult to judge, and on wet surfaces, you can hit a puddle or wet spot and suddenly lock up one wheel before you can react. The electronics can react faster than you and help maintain control.

 

As for why you would turn it off on the boat, for one thing, it's not needed. The wire is what decelerates you, not the brakes. It's also possible the rapid acceleration or deceleration somehow trips up the system.

 

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thanks joker.

 

your analysis of braking/ABS is more detailed (so 'more' correct than mine) and explains why it is a common feature in aircraft.

 

 

but it doesn't explain why it's switchable in a purely land based aircraft such as the A-10.

 

:(

[Nodak]

RCR 0, Alaska, nuff said.

[Midnightzulu]

Anti skid is usually disabled only as a result of a hydraulic failure. Anti skid systems can actuate the brakes many times a second. If you have a reduced fluid level, and/or a leak in the system, antiskid will very quickly deplete what you have left, leaving you with zero braking. Obviously bad. Conversely, even a small amount of remaining fluid can allow you to actuate the brakes once or twice. The procedure braking with a fluid leak (anti skid turned off) usually calls for one continuous brake actuation, coming on the brakes gingery at first (avoiding lock up) and gradually increasing pressure until the airplane is stopped.

[colubridae]

Anti skid is usually disabled only as a result of a hydraulic failure. Anti skid systems can actuate the brakes many times a second. If you have a reduced fluid level, and/or a leak in the system, antiskid will very quickly deplete what you have left, leaving you with zero braking. Obviously bad. Conversely, even a small amount of remaining fluid can allow you to actuate the brakes once or twice. The procedure braking with a fluid leak (anti skid turned off) usually calls for one continuous brake actuation, coming on the brakes gingery at first (avoiding lock up) and gradually increasing pressure until the airplane is stopped.

 

 

 

sounds good!!! do you have any documentation? :)

[mkiii]

https://www.flight-mechanic.com/brake-anti-skid-systems-part-one/

https://www.flight-mechanic.com/brake-anti-skid-systems-part-two/

https://forums.eagle.ru/showthread.php?t=224862

[colubridae]

https://www.flight-mechanic.com/brake-anti-skid-systems-part-one/

https://www.flight-mechanic.com/brake-anti-skid-systems-part-two/

https://forums.eagle.ru/showthread.php?t=224862

 

 

 

Thanks for these pointers. But I must say I saw nothing in them that suggests that disabling the anti-skid gives the pilot better braking ability.

 

 

excellent descriptions of anti-skid systems.

 

 

 

 

But all that i saw simply suggests that maximum braking effect is only obtained by using anti-skid - which keeps the wheels right on the skid margin. unless the pilot is imbued with near magical braking skills he cannot match this capability.

 

 

sorry to be unconvinced, perhaps best to agree to disagree.

 

 

If I ever come across anything with (IMHO) a valid reason I promise to 'fess up.

 

:)

[Deano87]

In an aircraft most automated systems need the ability to be turned off for unexpected circumstances, a combat aircraft only moreso. System malfunction, aircraft damage etc etc, makes sense to have the switch.

 

The post above about depleting hydraulic pressure is spot on. If you had a hydraulic pump failure but still had pressure in the system one of the first things you’d turn off would be the anti-skid.

 

It’s turned off on the carrier because at the speeds you move around the carrier at you shouldn’t be near the limit of adhesion but you definitely don’t want a system between your foot and the brake system that could malfunction and lead to you either rolling off the edge of the ship or into other aircraft or worse the people around you on the deck.