Differential braking with anti-skid active

[tcasblue]

Never encountered the situation before. But I found this quote interesting. Has anybody actually been in this situation and used this technique effectively?

"It is important to emphasise that the anti-skid protection does not apply pressure on the brakes, but only relieves it. So, to perform a differential braking technique, the pilot should reduce pressure on the side opposite to the turn, instead of applying pressure to the desired side."

[MechEngr]

Never encountered the situation before. But I found this quote interesting. Has anybody actually been in this situation and used this technique effectively?

"It is important to emphasise that the anti-skid protection does not apply pressure on the brakes, but only relieves it. So, to perform a differential braking technique, the pilot should reduce pressure on the side opposite to the turn, instead of applying pressure to the desired side."

Considering this mechanically; supposing this is to be used during slick conditions, then applying enough pressure to the wheels opposite the desired turn direction should cause those wheels to lock up and the only response the anti-lock system has would be to completely relieve pressure until the wheel spins up to match the rate of the wheels on the other side, during which what ever braking is applied to the inside of the turn will still be available.

If the conditions are not so slick then the wheels do not lock up and the anti-skid system does not participate and there is no reversal.

[gearlever]

Never encountered the situation before. But I found this quote interesting. Has anybody actually been in this situation and used this technique effectively?

"It is important to emphasise that the anti-skid protection does not apply pressure on the brakes, but only relieves it. So, to perform a differential braking technique, the pilot should reduce pressure on the side opposite to the turn, instead of applying pressure to the desired side."
If you are talking about Airbus, no anti-skid below 20 kt.

[retired guy]

Never encountered the situation before. But I found this quote interesting. Has anybody actually been in this situation and used this technique effectively?

"It is important to emphasise that the anti-skid protection does not apply pressure on the brakes, but only relieves it. So, to perform a differential braking technique, the pilot should reduce pressure on the side opposite to the turn, instead of applying pressure to the desired side."

Dont try this at home folks. Airplane manual overrides at all times.
R Guy
ps it’s an interesting subject which might amuse us here but please remember that even when at full anti skid braking, it still provides max available braking. Far less than max of course. But it’s all you’ve got.
Best wishes
R Guy

[retired guy]

Never encountered the situation before. But I found this quote interesting. Has anybody actually been in this situation and used this technique effectively?

"It is important to emphasise that the anti-skid protection does not apply pressure on the brakes, but only relieves it. So, to perform a differential braking technique, the pilot should reduce pressure on the side opposite to the turn, instead of applying pressure to the desired side."
tcasblue

I would be interested where that quote came from. Do you have a source? At least two of us think it is not correct. But I would be worried that someone or organisation is suggesting this.
Cheers
R Guy

[tcasblue]

I don't know whose quote is that but pilots braking technique remains same differential or otherwise. Anti skid will release or apply the pressure but the result is maximum braking which cannot be bettered. When you want to turn right you apply right brake period! No flight manual suggest anything else.

One should not suggest that no manual suggest otherwise unless they have read all the manuals. Otherwise it is just an assumption.

This statement comes from the Embraer 135 AOM as quoted in an accident report(appendix C). If it is accurate, I wonder if it applies to all aircraft with anti-skid systems or just certain types of anti-skid systems. I suspect that it is an area that most of us have not experienced which is why I am curious if anyone here has experienced such a situation where differential braking was required and what your experience was. It seems to go against one's instinct.

https://reports.aviation-safety.net/2009/20091207-0_E-135_ZS-SJW.pdf

We should also clarify your statement that anti-skid will not only release pressure but will also apply pressure as I am not 100% sure it is correct. I suspect(to be confirmed by others) that it is the brake pedal application that applies the pressure while anti-skid only controls the amount of pressure release(with the amount of release starting at zero under normal conditions). Splitting hairs perhaps with an overall same effect.

[tcasblue]

I was talking about the five commercial jets I have flown. B707, 747, A300B4, A310, A320. They didn't recommend this. You imagine yourself in the situation of differential braking and how would you go about it. When you apply brake differential or otherwise you keep it pressed. The antiskid will release it if wheel is locking and then when it has speeded up again it will reapply. Your foot remains pressed unless you don't want to brake or want less braking. That's it.

Thanks, I haven't seen it any manuals on aircraft I have flown either with anti-skid, a few of which are different from your types(includes Boeing, Airbus and Lockheed). So it brings up the question. Is there a reason why Embraer has written this the way they have. Is their anti-skid different, or is there some other reason.

Now that I think about it, do any of the other manufacturers actually give detail about differential braking technique while anti-skid is operating. Perhaps Embraer is correct and the others just don't discuss it. If I remember correctly, Boeing states something along the lines of….it is assumed that the pilot already knows how to operate an aircraft and the manuals are not a training aid on how to fly/operate an aircraft in general. If so, perhaps that includes anti-skid operating techniques.

[vilas]

tcasblue
I went through the report. Now I understood what it means. Actually anti skid principally remains same so it can be applicable to all. What it means is when brakes are applied equally depending on the locking runway condition the anti skid will cycle. Now if you want to turn to one side then either you increase the pressure on that side or decrease the pressure on the opposite side. If the braking is moderate the anti skid may not be cycling yet and increasing brake pressure on one side will bring more braking that side but may in bring the release decreasing the differential effect. However once wheel has slowed down the brake will apply again. This is accident specific information you won't find it in manual.

[tcasblue]

tcasblue
I went through the report. Now I understood what it means. Actually anti skid principally remains same so it can be applicable to all. What it means is when brakes are applied equally depending on the locking runway condition the anti skid will cycle. Now if you want to turn to one side then either you increase the pressure on that side or decrease the pressure on the opposite side. If the braking is moderate the anti skid may not be cycling yet and increasing brake pressure on one side will bring more braking that side but may in bring the release decreasing the differential effect. However once wheel has slowed down the brake will apply again. This is accident specific information you won't find it in manual.


Thanks,

I do appreciate your efforts. However, the quote does appear to come from the E135 AOM. I don't know if it was published before the accident or as a result of it but is quoted on page 142 of the report as being in the E-135 AOM.

"So, to perform a differential braking technique, the pilot should reduce pressure on the side opposite to the turn, instead of applying pressure to the desired side."

[hans brinker]

If you need max available braking, and you need to use differential braking at the same time (be it for a turn or directional control) you are not having a great day. Having said that, the only way to achieve differential braking while using max braking effort is by reducing braking on the outside, as there is no way to increase braking on the inside because you are at max braking effort on the inside already. This might not be in a manual, but it is basic physics.

[tcasblue]

Having said that, the only way to achieve differential braking while using max braking effort is by reducing braking on the outside, as there is no way to increase braking on the inside because you are at max braking effort on the inside already. This might not be in a manual, but it is basic physics.

I believe what they are saying is that......whether you are using max braking(pedals fully depressed) or partial braking(pedals not fully depressed) the proper technique is to "reduce pressure on the side opposite to the turn, instead of applying pressure to the desired side."

[vilas]

tcasblue
An apology is due to you from me. You had quoted an authentic statement and before refuting that I should have at least checked any available FCTM. The only thing right in what I said was all anti skid systems are same and what you quoted should be applicable to all. Below is from A320 FCTM:
. Differential braking is to be used if necessary. On wet and contaminated runways, the same braking effect may be reached with full or half deflection of the pedals; additionally the anti skid system releases the brake pressure on both sides very early when the pilot presses on the pedals. Thus if differential braking is to be used, the crew will totally release the pedal on the opposite side to the expected turn direction
and with that I am deleting the previous post.

[FullWings]

Interesting. I’m not sure how much practical benefit there is here because, as mentioned earlier, if you need/think differential braking is required to keep the aircraft on the runway, you’re having a very bad day. I suspect that by then you’ll be at maximum rudder deflection (as that’s the initial method of directional control) which makes differential braking much harder.

If the runway coefficients are low enough that you’re losing control through aerodynamic effects, then the amount of restoring asymmetric braking force available is likely to be correspondingly low...

[Goldenrivett]

If the runway coefficients are low enough that you’re losing control through aerodynamic effects, then the amount of restoring asymmetric braking force available is likely to be correspondingly low...
I disagree. At low speeds (say <60 kts) the aerodynamic effects are low and differential braking on a slick surface will certainly help more than nose wheel steering alone.

[FullWings]

If you’re in a crosswind and being blown off the runway at <60kts, you’re in trouble! Yes, differential braking will help more than NWS but this scenario is telling you that there is virtually no grip from the tyres, so don’t expect much to change...

[Goldenrivett]

but this scenario is telling you that there is virtually no grip from the tyres, so don’t expect much to change...
On the contrary. Expect a huge change if you:
1) "In all cases, brakes and reverse should be applied smoothly. If there is any concern with directional controllability then reduce or cancel reverse as necessary and reduce braking until control is regained. Then smoothly re-apply brakes and reverse if necessary." (Safety First Magazine - issue 12 page 11)
2) "As required, or when taking over from autobrake, applying brakes normally with a steady pressure; • For directional control, using rudder pedals and differential braking, as required (i.e., not using nose-wheel-steering tiller); • If differential braking is necessary, applying pedal braking on the required side and releasing completely the pedal action on the opposite side; and, • After reaching taxi speed, using nose-wheel steering with care." (Getting to Grips with Approach and Landings accidents reduction - Page 19)

Or carry on with full reverse and full brakes whilst you push yourself off the side of the runway.

[tcasblue]

Guys, it is very nice to say something along the lines of.....if you are in this situation, you are having a bad day or you are in trouble, in what almost seems like a sort of should never happen so don’t worry about it(even if that was not the intention of the statement).

But that is exactly what happens on rare occasion. Bad days. It appears that that there are a lot of very experienced pilots who are not aware of this technique(including myself) and would react opposite to what is required.

That could be problematic, especially because even if you are aware of this little detail, instinctive reaction may result in an incorrect input anyways.

We train/discuss for a lot of things where if it happens, you are having a bad day.

[FullWings]

We train/discuss for a lot of things where if it happens, you are having a bad day.
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?

On the contrary. Expect a huge change if you:
1) "In all cases, brakes and reverse should be applied smoothly. If there is any concern with directional controllability then reduce or cancel reverse as necessary and reduce braking until control is regained. Then smoothly re-apply brakes and reverse if necessary." (Safety First Magazine - issue 12 page 11)
2) "As required, or when taking over from autobrake, applying brakes normally with a steady pressure; • For directional control, using rudder pedals and differential braking, as required (i.e., not using nose-wheel-steering tiller); • If differential braking is necessary, applying pedal braking on the required side and releasing completely the pedal action on the opposite side; and, • After reaching taxi speed, using nose-wheel steering with care." (Getting to Grips with Approach and Landings accidents reduction - Page 19)

Or carry on with full reverse and full brakes whilst you push yourself off the side of the runway.
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.

[Goldenrivett]

Have you ever tried differential braking whilst holding full rudder deflection on your type?
Yes.

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.

The side load is only part of the problem. There is a large force due to the direction of the reverse thrust vector with crab angle towards the centreline. See Page 255 (Fig 6 on page 7 of Crosswind Landings section)
https://www.cockpitseeker.com/wp-content/uploads/goodies/ac/a320/pdf/data/GettingToGripsWithALAR.pdf

It is not "one neat trick that icy runways hate!!!” It is simply physics - as valid as avoiding a wing stall.

[gearlever]

Yes.



The side load is only part of the problem. There is a large force due to the direction of the reverse thrust vector with crab angle towards the centreline. See Page 255 (Fig 6 on page 7 of Crosswind Landings section)
https://www.cockpitseeker.com/wp-content/uploads/goodies/ac/a320/pdf/data/GettingToGripsWithALAR.pdf

It is not "one neat trick that icy runways hate!!!” It is simply physics - as valid as avoiding a wing stall.
Thanks GR, I was looking for that article today and you came up with it:ok:

[tcasblue]

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?.

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.


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.


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.

[Busserday]

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.

[tcasblue]

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.

[AerocatS2A]

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.

[tcasblue]

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.


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?

[vilas]

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.

[Goldenrivett]

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.

[vilas]

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.

[Goldenrivett]

Hi vilas,

I hope this link explains tyre friction https://suspensionsecrets.co.uk/tyre-performance-and-grip-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.

[retired guy]

Hi vilas,

I hope this link explains tyre friction https://suspensionsecrets.co.uk/tyre-performance-and-grip-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.

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

[AerocatS2A]

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.