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Differential braking with anti-skid active

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Differential braking with anti-skid active

Old 30th Jan 2020, 00:48
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Differential braking with anti-skid active

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."
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Old 30th Jan 2020, 02:49
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Originally Posted by tcasblue View Post
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.
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Old 30th Jan 2020, 15:03
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Originally Posted by tcasblue View Post
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.
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Old 31st Jan 2020, 08:08
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Originally Posted by tcasblue View Post
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
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Old 31st Jan 2020, 12:47
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Originally Posted by tcasblue View Post
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
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Old 31st Jan 2020, 14:16
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Originally Posted by vilas View Post
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/...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.

Last edited by tcasblue; 31st Jan 2020 at 14:27.
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Old 31st Jan 2020, 16:22
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Originally Posted by vilas View Post
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.

Last edited by tcasblue; 31st Jan 2020 at 16:34.
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Old 31st Jan 2020, 16:55
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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.
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Old 31st Jan 2020, 17:14
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Originally Posted by vilas View Post
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."
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Old 31st Jan 2020, 17:15
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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.
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Old 1st Feb 2020, 00:13
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Originally Posted by hans brinker View Post
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."
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Old 1st Feb 2020, 05:52
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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.
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Old 1st Feb 2020, 11:59
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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...
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Old 1st Feb 2020, 12:31
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Originally Posted by FullWings View Post
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.
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Old 1st Feb 2020, 12:57
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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...
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Old 1st Feb 2020, 13:21
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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.
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Old 1st Feb 2020, 13:27
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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.
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Old 1st Feb 2020, 14:32
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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.
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Old 1st Feb 2020, 15:27
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Originally Posted by FullWings View Post
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-con...psWithALAR.pdf

It is not "one neat trick that icy runways hate!!!” It is simply physics - as valid as avoiding a wing stall.
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Old 1st Feb 2020, 18:54
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Originally Posted by Goldenrivett View Post
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-con...psWithALAR.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
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