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Whose fault is it that the brakes are grabby

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Whose fault is it that the brakes are grabby

Old 15th Jan 2016, 17:27
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Whose fault is it that the brakes are grabby

The quote below is from an article about transitioning to the left seat. According to the writer, if the brake application you are using is resulting in them being "grabby" with the result of too much being applied and therefore not having smooth operation, it is likely the pilots fault.

I have flown on several types and one of them seemed to be particularly "grabby" but the last couple of large Boeing products I have flown do not have this tendency at all. So is it poor pilot technique or poor design technique? Perhaps the design should so that the pilot does not even have to think about how to operate the brake smoothly, it is just done easily while concentrating on more important things at a critical time.
Do you have a preferred type or disliked type for brake operation?

Taking Left Seat - Even With Two-Person Crew - Important Career Step | Business Aviation content from Aviation Week

"Two other left seat challenges are also improved with practice: tiller steering and braking smoothness. Both issues are a result of larger aircraft size and weight, which translates into more inertia. You have a lot of mass behind and underneath you and it takes a lot of force to get it moving, to change its direction once moving, and to get it stopped again.

A nose wheel tiller translates a small movement in your wrist into a large movement of a lever positioned on the outside of a very large arc. You will have a few embarrassing moments until you realize you should not be moving that tiller with an eye towards making it move by certain amounts, say a few degrees or inches. Your aim should be to apply pressure to the tiller, not deflection. Practice on an obstacle-free ramp by smoothly applying pressure, evaluating the aircraft’s reaction, and then adjusting. Some captains figure this out on day one; others take longer.

The first time you use the brakes on a large aircraft you are likely to think they are “grabby.” They might be, but it is more likely a problem with technique. Here again you should be thinking about pressures, not brake pedal deflection. When in doubt, press the brakes as needed. But try braking earlier with just a little pressure and see how it goes. When braking after landing the brakes become more effective as they heat up. You may find that your initial brake pedal pressure results in increased braking as the landing roll continues."
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Old 15th Jan 2016, 20:26
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Well, I've never flown other jet if not the B737 and one thing I can say...

The acfts equipped with carbon brakes are waaaaaaay more grabby than the ones with steel brakes.

Yes... I've read the FCTM section regarding proper brake usage. But as far as I've talked to my colleagues, they have the exact same opinion.

On the other hand, when flying as a passenger in the Airbus (319/20/21) I don't feel the grabby brakes.

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Old 16th Jan 2016, 05:06
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Carbon-carbon brake materials have unique friction characteristics. The coefficient of friction increases fairly abruptly as the temperature at the sliding contact interface rises when the brakes are initially applied. Almost like flipping an off-on switch, and very hard to modulate. While C-C brake systems might not perform super smoothly, they have other advantages over conventional brake materials, such as significantly lower weight and much greater braking capacity.
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Old 16th Jan 2016, 05:23
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The MD-80 has to have the worst brakes of all.

When they get hot, they chatter and shake the whole cabin with each application. Releasing and gently reapplying them, more often than not, has no effect on reducing the chatter.
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Old 16th Jan 2016, 09:42
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Both the 744 and 777 have carbon brakes and they apply very smoothly in my experience. So I am not sure about the carbon brakes theory.
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Old 16th Jan 2016, 10:15
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When it comes to undesired braking behaviour, the F70/100 with its taxi brake system has to be rather high up the list.

Presumably as a measure to save brake lifetime, under a certain taxi speed (may have been 30kts, but it has been a while) and below a certain brake pedal deflection, only one of the two wheel brakes on each main wheel is actually delivered pressure. However, if that deflection threshold is crossed, the valves are opened and the measured pressure is applied not only to half the brake units but to all of them. A smooth (ish) braking process will thus be turned into the equivalent of hitting a brick wall, loose stuff tumbles forward through the cabin etc.

And to make life extra interesting, this system was not installed in all the aircraft of the fleet - when it reared its head, it was occasionally a little bit unexpected and resulted in the application of strong language.
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Old 16th Jan 2016, 10:25
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The Md80 brakes are terrible, as is the lack of airflow from the packs, ice making wing, crappy handling, leaky cockpit windows and a cramped cabin to name a few.

Time to turn them all to scrap.
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Old 16th Jan 2016, 11:17
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I would echo the comment about the 777 brakes - I’ve never flown a “grabby” one. I can’t find it in the manuals but I dimly remember from the conversion course that the system didn’t use all the brake units during normal taxying and cycled through them instead, to prolong life.

It is possible to operate the 777 in a jerky manner if you’re a “dabber” or “binary braker”... I’ve never understood why some people find it necessary to increase the braking effort to maximum just before coming to a halt - you wouldn’t do that in a car, why do it in an aeroplane?
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Old 16th Jan 2016, 22:28
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Back in the 1980s, I was doing a lot of flight testing on the re-engined 767 (CF6-80C2) - which was also the same time they were making carbon brakes an option. The test pilots had considerable complaints about the 'On-Off' characteristics of the carbon brakes and far preferred the 'feel' of the steel brakes.
Educated guess here (I'm an engine guy), but I suspect that newer models - such as the 777 - which were originally designed for carbon brakes have changes in the brake system to give better feel.
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Old 17th Jan 2016, 00:20
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Is the sheer mass of the aircraft a factor ?

Have complaints about F70 & MD-80, good reports on B777 & B747.

MTOWs (ballpark tonnes):
F70: 40
MD-8x: 65-70
B777: 250-350
B747: 350-450

B767: 150-200
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Old 20th Jan 2016, 06:59
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I would imagine that most modern commercial aircraft have ABS. So to maximize braking performance, the brakes should be fully applied and allow the ABS to modulate their function. However, with C-C materials even ABS may not be able to smoothly modulate initial performance from a cold condition.

ABS has much better ability to modulate braking than any human does.
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Old 23rd Jan 2016, 18:08
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BAe 146/RJ brakes, which I think were carbon? - long time ago now - didn't do much until they warmed up. I got used to applying gentle braking early in the landing roll to heat them, then just as the braking coefficient rapidly increased, you could reduce the pedal pressure but the braking force would still increase. With practice you could modulate the brakes quite well to give smooth braking and turn off at the exit you wanted.

And as someone has said; reduce the pedal pressure as the aircraft speed approaches zero, so that by the time you have stopped, you have virtually no brake pressure. Much smoother
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Old 23rd Jan 2016, 18:29
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@ riff_raff:

You say:

I would imagine that most modern commercial aircraft have ABS. So to maximize braking performance, the brakes should be fully applied and allow the ABS to modulate their function. However, with C-C materials even ABS may not be able to smoothly modulate initial performance from a cold condition.

ABS has much better ability to modulate braking than any human does.

Whilst what you say is true it has little practical relevance for most landings. Pilots have safety in mind but also passenger comfort. Maximum effort braking on a dry runway is not at all comfortable. In fact it is quite brutal.

Do you usually hit the brakes hard (max) in your car when you approach a red light or arrive in your driveway? Didn't think so
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Old 24th Jan 2016, 18:20
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I seem to remember some old 747-200s that were tricky to get onto parking smoothly. No carbon brakes there.
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Old 25th Jan 2016, 02:03
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We operate a mixed fleet of steel and carbon brakes. Old 767-200s with the steel brakes are grabby and difficult to apply evenly, 767-300s with carbon brakes are smooth as silk, taxiing and landing.
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Old 25th Jan 2016, 02:12
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I used to fly a mix of old and new 320s. The old ones had spongy pedals with no apparent effect when you pushed them a little and suddenly becoming grabby if you pushed more.

The new ones had smooth, linear response.

Both had the same carbon brakes, so I think it's not a matter of brake type, but rather the brake control system.

Also - weight matters too. A lightly loaded a/c has a tendency to be more grabby than one near MTOW - inertia smooths things out
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