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The logic or illogic use of hard braking with carbon brakes on long dry runways

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The logic or illogic use of hard braking with carbon brakes on long dry runways

Old 5th May 2013, 07:06
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The logic or illogic use of hard braking with carbon brakes on long dry runways

Talking recently to a pilot with a Indonesian airline equipped with Boeing 737NG series that have carbon brakes. Company SOP is that autobrake 3 is used for all landings on dry hard runways regardless of length. However, if the runway surface is wet, it is SOP to select autobrake MAX regardless of runway length which could often be as much as 11,000 ft.

Whether or not this SOP is something to do with minimising brake wear with carbon brakes or perhaps a desire to mitigate against those pilots that deliberately float a long way to ensure a smooth touchdown, is anyone's guess.

An extract from an Airbus bulletin explains briefly about the principle of carbon brakes by saying: "Carbon brake wear and tear depends on the number of brake applications and on brake temperature. It does not depend on the applied pressure, or the duration of the braking. The temperature at which maximum brake wear occurs depends on the brake manufacturer. Therefore, the only way the pilot can minimize brake wear is to reduce the number of brake applications"


A further extract from a large SE Asia operator of A330 aircraft states in part: "Autobrake shall be armed for all landings. The use of autobrake is preferable because it minimizes the number of brake applications and thus reduces brake wear.

The use of LO autobrake should be preferred on long dry runways, whereas the use of MED autobrake should be preferred for short or contaminated runways.
The use of MAX autobrake is not recommended. On long dry runways where minimal braking is anticipated, the autobrake may be disengaged after touchdown provided the aircraft has landed in the Touchdown Zone."
............................................................ ....................................

A final point from the Boeing 737 Classic FCTM re use of autobrake. Again, in part it states: "Boeing recommends that whenever runway limited, using higher than normal approach speeds, landing on slippery runways or landing in a crosswind, the autobrake system be used".

This implies that there is no need to use the autobrake system if none of the above conditions apply. Despite this Boeing recommendation is it common for operators to use autobrakes for landing at all times notwith standing landing distances may be well in excess of performance requirements. In real life it is probably rare to be runway limited on most landings.

This gets around to my request for expert opinions () of the logic of the 737NG operator in Indonesia whose SOP is autobrake 3 be used for all landings regardless of runway length and autobrake MAX for wet runway landings also regardless of runway length. And what about the A330 operator whose SOP is autobrakes for all landings but then permits the crew to disengage the autobrake after touchdown where minimal braking is anticipated? What is the point of having the autobrake system selected when it will be disengaged after touch down?

In the example of the 737 NG operator with carbon brakes, does it mean that autobrake 3 (which by any standards is fairly heavy braking) reduces the wear on carbon brakes because they get hotter faster? If the aim is to save the cost of brake wear, then surely where excess runway is available then why use autobrakes at all? And to have an SOP requiring MAX autobrake when ever the runway is wet and which may be well in excess of performance limits in the FCOM/QRH, seems to me a massive overkill. Or an indictment of the standard of the crews....

To cap off the discussion on autobrake SOP, it is interesting to note that some of the Gulfstream Global Express series of corporate aircraft are not even fitted with autobrakes!

Last edited by Tee Emm; 5th May 2013 at 07:39.
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Old 5th May 2013, 10:06
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Would that be the Indonesian airline that had 7 (SEVEN!) runway overruns/excursions in the last three years?
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Old 5th May 2013, 10:50
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The engineering trick with carbon brakes seems to be to quickly get them warm, but not to cook them.

When they are warm (and that is below 500 oC), a smooth friction film is formed on the brakes which serves as a solid self-lubricant. This film protects the brakes, therefore the brakes wear less. The formation mechanism of the film is the subject of scientific debates, but it's known that it does not form at low energy braking, and it is destroyed at extremely high energy braking.

As temperature (and braking energy) rise very high (like in a rejected take-off), the silicon friction film breaks into chunks due to shear stresses and exposes the ceramic-carbon mix to higher friction and so the wear rate increases. As the carbon heats over 500 oC, it starts to oxidise. If the temperature of the total brake system exceeds 1000 oC, the thermal gradient is too steep, the ceramic can't offer enough protection, the carbon fibres get hotter, and the oxidation of the carbon fibres leads to a very rapid degradation of the brakes.

Using Autobrake 3 as a standard makes sense in the context of the engineering (gets them warm but not overheated). And in Indonesia using autobrake max in the wet would be understandable in the context that there are a wide range of runways, and the length and/or engineering elements giving good wet weather braking can be variable. I am not a pilot, so my comments are made in the engineering sense only and there might well be operational considerations as well.
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Old 5th May 2013, 11:44
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I would say yes, that it is an overkill of using Autobrake 3 on all landings on dry runways and Autobrake Max on wet ones. My outfit has carbon brakes installed on the whole 737NG fleet and I'd say it of course all depends on runway length, the weather, is the terminal at the end of the runway or do you have to taxi all the way back, does the airport have "minimum runway occupancy time or not" etc.

To set autobrakes will ensure immediate braking upon touchdown and nice and even application of braking.

But if I am let's say landing on a 10-13000 foot dry runway and the terminal is for example at or close to the far end I would land with autobrakes OFF and just land and let it roll and vacate at the best exit to make the taxi short. Most likely one wouldn't have to touch the brakes at all.. as long as the landing was made in the touch down zone of course. It would seem a bit silly to land with autobrake 3 and then have to add thrust for the taxi down to the end then now wouldn't it? Simple logic. Unless of course it is a "minimum runway occupancy time airport" that is different..

So basically just plan the landing roll and the vacating depending on various factors.

Interesing points on the temperature though, nothing about that in our manuals, only that they wear per applications of braking not the pressure or for how long it is as above. And for the taxi to let the aircraft speed up a bit and then apply brakes and slow down to a fairly slow speed, e.g. not to wear the brakes and minimise the number of brake applications.
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Old 5th May 2013, 20:34
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'Carbon brake wear and tear depends on the number of brake applications and on brake temperature. It does not depend on the applied pressure, or the duration of the braking.'


So hypothetically, if the pilot drove his 747 down a long highway, only applying the brakes once, adding power to keep the airplane rolling, the brakes would never wear out. Never, as he only applied the brakes once.

This to me sounds like more manufacturer 'gaming' the airlines and thier pilots into doing SOPS, maneuvers that will keep the them out of hot water.

'Land, slam the brakes once, let autobraking do the job' (because chances are you were hired for your checklist reading skills not aviation skills)

Which is pretty much what we tell kids and our grandmothers that can't drive. Nascar doesn't use any autobraking for a reason.

Last edited by Teldorserious; 5th May 2013 at 20:35.
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Old 7th May 2013, 00:15
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google cut and paste...




Steel brake wear is directly proportional to the kinetic energy absorbed by the brakes. Maximum steel brake life can be achieved during taxi by using a large number of small, light brake applications, allowing some time for brake cooling between applications. High airplane gross weights and high brake application speeds tend to reduce steel brake life because they require the brakes to absorb a large amount of kinetic energy.


Carbon brake wear is primarily dependent on the total number of brake applications one firm brake application causes less wear than several light applications. Maximum carbon brake life can be achieved during taxi by using a small number of long, moderately firm brake applications instead of numerous light brake applications. This can be achieved by allowing taxi speed to increase from below target speed to above target speed, then using a single firm brake application to reduce speed below the target and repeating if required, rather than maintaining a constant taxi speed using numerous brake applications. Carbon brake wear is much less sensitive to airplane weight and speed than steel brake wear.
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Old 7th May 2013, 03:19
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This can be achieved by allowing taxi speed to increase from below target speed to above target speed, then using a single firm brake application to reduce speed below the target and repeating if required, rather than maintaining a constant taxi speed using numerous brake applications.
Isn't this the high-lighted statement above the same principle as espoused in the Boeing 737 Classic FCTM where it states:

"Avoid prolonged brake application to control taxi speed as this causes high brake temperatures and increased wear of brakes. If taxi speed is too high, reduce speed with a steady brake application and then release the brakes to allow them to cool. Braking to approximately 10 knots and subsequent release of the brakes results in less heat build up in the tires and brakes than when the brakes are constantly applied".

In the discussion on use of carbon brakes (they perform better when hot) it should not be forgotten that hotter brakes inevitably cause hotter tyres; which is undesirable, whichever way you look at it.
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Old 7th May 2013, 03:36
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And remember that with carbon brakes, you can modulated the brake pressure through, for example, an S bend/turn on taxiing. This will mean just one application and warms the brakes up nicely, should they be cool in the first place. Most pilots I fly with add an extra two brake applications in this particular circumstance.
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Old 7th May 2013, 05:18
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Interesting discussion. I've never operated the large end of town, only the smaller brethren, but from this discussion I seem to be reading: if on Steel brakes, dab at 'em; Carbon brakes, dragging them is best... or am I completely missing something in there?
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Old 7th May 2013, 06:56
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In the discussion on use of carbon brakes (they perform better when hot) it should not be forgotten that hotter brakes inevitably cause hotter tyres; which is undesirable, whichever way you look at it.
Only in the case of a thermal, usually caused by a heavy hot aircraft that the PIC is determined to make the first turn off on well after the aircraft has made it to the gate. Hell hot tyres perform better, ever watched a drag race or NASCAR?

Last edited by grounded27; 7th May 2013 at 06:57.
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Old 7th May 2013, 07:13
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AERO - Operational Advantages of Carbon Brakes
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Old 7th May 2013, 07:22
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The standard taxi practice where I work is to speed up to 20 kts, then brake to 10 kts, speed up to 20, etc., etc.

There are no standard landing brake setting procedures. I have seen everything from no autobrakes to max.
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Old 7th May 2013, 07:27
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I spent some years in a Middle Eastern country with high ambient temps and several times per week taxied steel braked 747s several miles to run up areas.The aircraft were empty with low fuel loads and as a result tended to keep accelerating due to high residual thrust - company procedures insisted on 4 engines running.Trying to keep to a constant speed overheated the brakes rapidly.It was found that a single hard application to bring the aircraft almost to a standstill then allowing it to accelerate prior to a single application before turning kept the brake temperatures to an acceptable level.Whether this technique would be applicable to carbon brakes I do not know but it appeared to permit the least heat input and the longest cooling period.
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Old 7th May 2013, 09:11
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.The aircraft were empty with low fuel loads and as a result tended to keep accelerating due to high residual thrust - company procedures insisted on 4 engines running

First aircraft I taxied (from remote to the gate) was an empty 742, she does quite well on 1&4 alone. Turns on a dime....
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Old 7th May 2013, 10:42
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Carbon brakes have been kept somewhat mysterious, and I can only guess there are commercial reasons for doing so. I was interested in them a while back so I decided to find out more about how they work. That proved pretty hard, so I just looked harder using academic research tools until eventually I pieced together the following (which I suspect no manufacturer will confirm or deny).

Carbon fibre-based brakes are made out of so-called carbon-carbon composite material that consists of two components: weaved carbon cloth and a solid carbon (and other things) matrix. The carbon matrix is formed by Chemical Vapour Infiltration (CVI) method. According to this method, a vapour of gases (such as CCl4) are introduced across the [heated] weaved carbon cloth and react on the surface of these fibres with formation of solid carbon phase which has ceramic and silicon in it. This solid carbon phase is the one that plays important role in friction and wear performance of the brakes. The CVI coating is a few microns thick and is fine grained and harder than similar materials produced using conventional ceramic fabrication processes.

There is a general wear mechanism for carbon-carbon aircraft brakes proposed in 1988. According to this mechanism, there are two types of wear:

Type I. This type of wear happens at low energy conditions, such as aircraft taxiing, or when low pressure is applied during braking. At these conditions, a particulate powdery wear debris are formed. The worn particles cause abrasive wear which is the most damaging mode in terms of brake wear - it's like applying a sand paper over the brakes. The particles are mostly formed by carbon matrix, not carbon fibres.
Keep 'em cool and grind away the brake discs

Type II. This type of wear is at high energy conditions, such as in aircraft landing, or when high pressure is applied during braking. The difference is that at these conditions, a smooth friction film is formed on the brakes which serves as a solid self-lubricant. This film protects the brakes, therefore the brakes wear less. Of coarse, the braking efficiency suffers, meaning that the friction coefficient is lower for brakes that have formed such a film.
Keep 'em warm, they self-lubricate and last longest

The mechanism of formation of this film is not completely clear, even though its existence was proved many times by many researchers. Usually, the following explanation is offered: under higher braking energy condition, higher pressure and temperature assist deformation of wear particles to form a debris film. The particles do not melt though, but plastically deform (carbon does not melt). Nobody will say anything more definitive about this film formation, although there have been a lot of research done on density, crystalline structure, porosity, microscopy and X-Ray diffraction of these films. However Malhotra did work on silicon barriers on carbon-carbon and also on ceramics, which makes me think that these are important parts of it. Murdie, Don, Wright at the Materials Technology Center (MTC) at Southern Illinois University, USA (later CAFS / Centre for Friction Studies) did most of the work on the aircraft braking systems, thanks initially to those kind folks from BF Goodrich and Aircraft Braking Systems (and thereafter the sponsors look like a who's who of industry).

Because carbon is oxidised in air at temperatures as low as 500 degC, the extensive research aimed to improve the oxidation resistance of carbon-carbon composites. CAFS researchers studied the oxidation of carbon-carbon materials.

There were two commonly accepted ways to protect these materials against oxidation. The first method makes use of oxidation-resistant coatings, such as SiC (silicon carbide). The major problem with this method is the fact that coatings usually induce stresses and often lead to crack formation. The other method of protecting C-C composites was by using matrix inhibitors, such as boron or boron carbide. They reduce the carbon oxidation by spreading a sealant borate glass within the composite. However, due to their low melting point, such inhibitors introduce temperature limitations for composite applications and are effective only after an appreciable fraction of carbon has been gasified. In the braking process and at high humidity, a carbon composite loses much of its friction property and becomes greasy--more like a lubricant. This could be a problem operationally [he says tongue-in-cheek].

What MTC did, but won't tell anyone, was to develop a third method of protecting against oxidisation by making sure the carbon didn't get too hot. The nano-composite material uses ceramic particles to protect carbon from high heat in an oxidising environment. I reckon that there is silicon in the coating as well. The silicon forms the film and the ceramic retards the heat transfer to the carbon fibres. Their dynamometer testing (because they had one) showed that the ceramic-enhanced carbon composite had about a 20-fold higher coefficient of friction than a standard carbon composite. For certain friction applications, ceramic doped carbon materials exhibit more braking capability.

The upper limit comes because as temperature and braking energy rise even higher (like in a rejected take-off), the silicon friction film would break into chunks due to shear stresses and thus expose the ceramic-carbon mix to higher friction and so the wear rate would increase again. The other bad thing that could happen at extremely high energy braking is the ceramic cannot sufficiently slow the heat transfer to the carbon fibres and the carbon heats over 500 degC and starts to oxidise. This is especially critical if the temperature of the total brake system exceeds 1000 degC because that means that the thermal gradient is too steep, the ceramic can't offer enough protection, the carbon fibres get hotter, and the oxidation of the carbon fibres leads to a very rapid degradation of the brakes.
Overheat 'em and they rapidly burn up

In conclusion, high braking pressure leads to a lower brake wear, but only up to a limit. The lower brake wear is due to the formation of the silicon film at high energy braking which serves as a lubricant and protector of the ceramic/carbon brake material. The formation mechanism of the film is the subject of scientific debates, but it's known that it does not form at low energy braking, and it is destroyed at extremely high energy braking.

Hope that gives some more understanding,
Cheers
Overrun

Last edited by OverRun; 7th May 2013 at 13:42. Reason: Added the summary points in bold
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Old 7th May 2013, 13:04
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Over-Run,

Thank you for your erudite technical explanation of the mysterious operations of carbon brakes. Despite that, I must admit I find it hard to get my head around the subject and I suspect a lot of pilots have the same trouble.

Meanwhile, seems to me that brake technique during taxiing is practically identical whether using steel brakes or carbon brakes. That is, allow the speed to build up to a certain safe figure while keeping feet clear of the brakes to avoid inadvertant `dragging'. Then apply brakes normally to reduce speed.

That done, take feet of the brakes again(momentary cooling period) while the aircraft once more accelerates under idle thrust. Repeat exercise. It probably works out that once a safe taxi speed is reached (20-30 knots), the brakes are applied for about 5-10 seconds once every minute to slow to 10 knots depending on aircraft gross weight and other factors.
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Old 7th May 2013, 15:08
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Curious in all this discussion no-one has mentioned the use of Thrust Reversers in combination with autobrakes. I assume the airbus types do as Boeings and reduce the braking with T/R activation. In my various airlines there was a mixture of steel & carbon brakes. There was also a mixture of auto-brakes and idle T/R, and auto-brake for the landing distance planned and use of 2nd detent. Thus I think there is more to the topic than just brakes.
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