PPRuNe Forums - Differential braking with anti-skid active

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- - Differential braking with anti-skid active (https://www.pprune.org/tech-log/629298-differential-braking-anti-skid-active.html)

Quote:

Originally Posted by FullWings (Post 10677135)

Virtually none of us have any formal training on this is given in the sim and it is unknown how accurate it would be in any of the variety of sims out there. Which is why I am discussing this. For our knowledge.

Quote:

Originally Posted by FullWings (Post 10677135)

All good advice and SOP on many types, however in the situation where you are sliding off the runway at <60kts due wind, it is because the side loading on the airframe exceeds the lateral grip you can generate. As this indicates a *very* low friction surface, I would not hold out for a “huge change” under those circumstances, no matter what you do with the brakes. .

Maybe you wouldn't hold out for a huge change but maybe it will be enough to save the day. Each situation is different. And it is certainly better to NOT make the brake input that actually makes things worse which appears to be not only the instinctive input, but what appears to be the belief by many...of being the correct input.

Quote:

Originally Posted by FullWings (Post 10677135)

The real answer is to do everything possible not to expose yourself to these conditions in the first place and thinking you have “this one neat trick that icy runways hate!!!” up your sleeve may encourage riskier behaviour.

This is not the "Real" answer. It is a good preventative measure that is of no use once the situation is encountered. One could say that the real answer to a windshear encounter from a thunderstorm is to not operate near one and therefore, no more escape maneuvers need be in the training and therefore, you "have this one neat trick up your sleeve" called the windshear escape maneuver.

The use of reverse thrust was mentioned a couple of times here but the aircraft in the accident report that I provided a link to, like many jets...has no reverse thrust capability. So perhaps best to focus on the proper differential brake input in terms of brake use to try and salvage a situation.

Hey, hold my beer and watch this!

Having spent 25 years on 737-200's on short and gravel ops and 15 years on wide-bodies into a large variety of places (including PHOG in 787's ) don't count on anything being constant throughout the event. Use your brakes until they are ineffective (they will be in a reject from overheating when at 30 kts you think they might slow you so you let up) and use reverse to it's limit if in doubt.
Always have my hands on the reversers when approaching the gate.
Love to see the look on the opposite side breaking groups face when they use it to invoke a turn and when they hit a dry patch and it wasn't perfectly at max pedal.
Never hit a blade of grass yet.
9 days to retirement.

Quote:

Originally Posted by Busserday (Post 10677524)

Love to see the look on the opposite side breaking groups face when they use it to invoke a turn and when they hit a dry patch and it wasn't perfectly at max pedal.
.

Thanks,

While dry patches are somewhat rare during moderate to heavy rain events(which is where most hydroplaning events happen), I suppose post rain event on a runway with pools of water or during spring conditions of melting snow/ice can lead to varying conditions on runways including dry patches.

Below freezing winter conditions involving partial coverage by contamination could lead to multiple dry patches among slippery areas.

Quote:

Originally Posted by FullWings (Post 10677135)

Oh yes, that we do. But how much formal training (as in hands on in the sim) is given on aircraft handling on icy/slippery runways? Have you ever tried differential braking whilst holding full rudder deflection on your type?

All good advice and SOP on many types, however in the situation where you are sliding off the runway at <60kts due wind, it is because the side loading on the airframe exceeds the lateral grip you can generate. As this indicates a *very* low friction surface, I would not hold out for a “huge change” under those circumstances, no matter what you do with the brakes. It’s like encountering an icy corner in a car at too fast a speed: brakes on/off, steering left/right, power on/off... you’re going to leave the road and the only choice is the direction you’ll be facing when it happens.

The real answer is to do everything possible not to expose yourself to these conditions in the first place and thinking you have “this one neat trick that icy runways hate!!!” up your sleeve may encourage riskier behaviour. Yes, you might save the day having been encouraged by inaccurate reporting into using an inappropriate runway but it would be foolish to expect that the above instructions are a 100% guaranteed get out from ground-based LOC. If you find yourself deploying any of these techniques, which are perfectly valid, I agree, you are in the last stage of incident/accident mitigation.

I think the real answer is that you do what needs to be done. If both feet are applying max pressure (whether regulated by anti-skid or not) then the ONLY way to get differential braking is by releasing the brakes on the outside of the desired turn. On the other hand if you aren’t apply any brake at all then you will need to apply brake to the inside of the desired turn. At intermediate positions then you can use one or other or a combination, whatever gives you the desired effect.

Quote:

Originally Posted by AerocatS2A (Post 10677664)

Quote:

Originally Posted by AerocatS2A (Post 10677664)

At intermediate positions then you can use one or other or a combination, whatever gives you the desired effect.

So if anti-skid is releasing brake pressure(ie overriding pedal input), how will more pedal input apply more brake pressure?

Antiskid is intermittent action. As the wheel speeds up brake will apply again. The FCTM.write up could have been better. It means with steady brake pressure on both the brakes to increase differential effect the opposite brake release will provide immediate response. If the required side brake is applied more it will still produce the result but not instantly.

Quote:

Originally Posted by vilas (Post 10677706)

Antiskid is intermittent action. As the wheel speeds up brake will apply again. .... If the required side brake is applied more it will still produce the result but not instantly.

Please explain how a braked wheel / tyre, which is at the limit of frictional grip (hence antiskid operation) can possibly grip more if extra brake pressure is applied.

Quote:

Originally Posted by Goldenrivett (Post 10677744)

Please explain how a braked wheel / tyre, which is at the limit of frictional grip (hence antiskid operation) can possibly grip more if extra brake pressure is applied.

During release period off course.nothing will happen but as the wheel speeds up brake will be applied again with more intensity . This delay may not be there if instead other side is released. That is what I meant.

Hi vilas,

I hope this link explains tyre friction https://suspensionsecrets.co.uk/tyre...a-deeper-look/ and how once F= uN is maximum, you can't increase it further by applying more brake pressure.

The "Friction Circle" helps explain why steering is improved with brakes released.

Quote:

Originally Posted by Goldenrivett (Post 10677794)

Thanks Goldenrivett
I think this is an example of this concept which is in The Boeing training manuals.Drifting off the side of the runway

A tyre can apply a force to runway laterally and longitudinally, and normally does both, but when you reach max autobrake or full anti skid, you are using all the available friction fore and aft to try and stop. None left for lateral forces. And in a cross wind you can drift downwind and off the side.
To regain traction you have to do the almost unthinkable and release the brakes, cancel reverse which by now is vectoring you downwind too, and point the ship towards the wind using rudder and yes, maybe diff braking.
Once back on C/L or even a bit upwind of it, back to full reverse and max braking. This is from memory but it is in the manual which I am trying to find.
A 747 did this at LHR a few years back and I think still blocked 27R for a while but no damage. Anyone got a pic?
Cheers
R Guy

Quote:

Originally Posted by tcasblue (Post 10677697)

So if anti-skid is releasing brake pressure(ie overriding pedal input), how will more pedal input apply more brake pressure?

I was unclear. If your braking is not yet limited by anti-skid, then you have the option of increasing pressure on the inside, decreasing pressure on the outside, or both, depending on how much differential you wish to achieve.