Carbon brakes, questions

[KristianNorway]

Can anyone tell me about carbon brakes?
Why is there no linear relation between brake temperature and brake wear? And why is it only the number of brake applications that matter?

I've read up somewhat on the subject, but would appreciate an educated and clarifying answer. :ok:

[bearfoil]

No, I only have more questions. Why does the 380 burn through brake packs like they are cheese?? Why does braking cause a vibrating flight deck (380) ?? Why is brake performance such a crap shoot for this aircraft ??

anyone??

[divinehover]

As far as I understand a brake application and release causes oxidization on the surface of the brake pad. The next application 'shaves' the oxidized layer off the pad. This could be nonsense though. I've also read that brake temp effects the alignment of the molecules in carbon brakes. At cold temps the molecules stand straight up and therefore wear easier. At warmer temps they tend to lie flat reducing wear.

I'm sure a more educated individual will provide a more suitable answer.

DH

[mototopo]

I found that on the net, maybe can help!

http://chihchin.sg1002.myweb.hinet.net/A330/Carbon%20brakes.pdf


Bye! :ok:

Moto

[Cardinal]

Great link!

[lion-g]

Hi,

From my guess, the 380 is a huge aircraft with MLW of 391 T. However due to weight issue, only 16 of out of the 20 body gears are fitted with brakes. On top of that, only the inboard engines have reversers.

Imagine the amount of energy required to counter the Kinetic and potential energy during landing.

ECAM msg "BRAKES BRAKES HOT" always pops up after shutdown at the gate. A good rule of thumb for the brakes to cool at an acceptable temperature for dispatch is 3 hours without brake fan !

Any other points to add ?

Happy new year to all, safe landing always !!!

cheers,
lion-g

[KristianNorway]

Thanks for the link, but it really doesn't answer my question. WHY does it wear more on carbon breaks when you have shorter and more applications compared to one long?

[cwatters]

As I understand it carbon brakes wear more quickly when they are cold than when they are hot. In which case you want to avoid a lot of short braking actions as that will average out cooler than one long one.

Elsewhere someone posted this in response to your post. See the box on taxi recommendatiions..

AERO - Operational Advantages of Carbon Brakes (http://www.boeing.com/commercial/aeromagazine/articles/qtr_03_09/article_05_1.html)

PS Each landing obviously starts with them cold so you might expect more wear on a lighter aircraft doing lots of short flights than on a heavier long distance.

Perhaps one day someone will figure out how to preheat the brakes. Perhaps use a RAT to generate electricity during the descent and pump that into the brakes? Probably too much weight.

[mustafagander]

My mob has carbon brakes on the B744 - like everyone else I expect. We are told that it is the number of brake APPLICATIONS that matters WRT wear. Hence let the aircraft speed up on taxi, then one application to slow it, not little nibbles.

I know nothing about the "blunderbus", thank God!!

[KristianNorway]

Again.. I thank you guys for answers, but the question stands. I get a lot of information from everyone on the fact that it's the number of applications that wear on carbon brakes and not the brake energy, but no one tells me why.

Why is this a fact?

[Canuckbirdstrike]

There are some subtle distinctions in semantics in the referenced articles. Energy absorption is not the primary driver of carbon break wear, the number of applications is. My understanding that this is directly due to the differences in material properties of the two brake systems.

Steel brakes use steel disks and a different material pad, much like car brakes. The material that wears the most is the pad material not the steel rotors. The pads abrade to dust and the rotors turn the rotational energy into heat.

On carbon brakes both the rotor and the "pad" are the same material. Carbon brakes will produce some dust as they wear, but as temperature increases the material will sublimate.

So the short answer is different material have different wear properties and primary wear factors.

[Turbine D]

KristianNorway

As to "why", I don't think you are going to get a really good answer to this question as much of the details are kept under wraps by leading carbon brake manufacturers. But here may be a simplistic explanation:

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 brakes wear - it's like applying a sand paper over the brakes. The particles are mostly formed by carbon matrix, not carbon fibers.

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 film.

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 can 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.

Other types of wear. If temperature and braking energy rise even higher (like in rejected take-off), the friction film would break into chunks due to shear stresses and the wear rate would increase again. Other bad thing that could happen at extremely high energy braking is oxidation of the brakes due to their reaction with oxygen from the air. This is especially critical if the temperature of the brakes exceeds 1000°C (1832°F) - oxidation at these temperatures would lead to a very rapid degradation of the brakes.

In conclusion, to answer your question - yes, higher braking pressure would lead to a lower brakes wear, but only to a certain level. This effect is due to the formation of carbon film at high energy braking which serves as a lubricant and protector of the 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. So there exists some kind of "sweet spot" where the wear is small.

This comes from some aeronautical engineering experts.

Turbine D

[KristianNorway]

This is great feedback! Thanks.
This helps one to somewhat understand what's happening. :ok:

[low n' slow]

Forgive me if this is stupid question, but when a brake is referred to as a carbon brake, is it the disc (rotor) or the brake pad (stator) that is made of carbon or both? Perhaps I missed a vital part in my training. I asked a friend who works as a technician and he said that the disc is always made of steel. This is my belief aswell, but in the airbus document mentioned above (great info btw, thanks!) they refer to the disc as a carbon disc. Am I missing something? I teach in Airframes and Systems so it's embarrasing to not have a solid answer for these questions...

(Sorry if I stole the thread...)

/LnS

[Checkboard]

The disk is carbon. The choices for aviation are steel or carbon brakes. Cars get a ceramic brake option as well.

Some racing cars and airplanes use brakes with carbon fiber discs and carbon fiber pads to reduce weight. Wear rates tend to be high, and braking may be poor or grabby until the brake is hot.

Disc brake - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Disc_brake)

AMG Mercedes Carbon-Ceramic brake disk.

http://upload.wikimedia.org/wikipedia/commons/thumb/4/4f/AMG_Carbon_Ceramic_brake..JPG/800px-AMG_Carbon_Ceramic_brake..JPG

[low n' slow]

Thanks checkboard. I appreciate the fact that the picture shows a carbon-ceramic disc, but isn't it a disc made for a car? I don't want to discredit your post or anything, but the fact that discs are available in a multitude of materials for applications other than aviation ones doesn't really answer the question in an undisputable way... :8

Can anyone come up with a picture of a real aircraft specific disc made of carbon or point me to any technical document where this is stated as a fact?

Regards / LnS

[crippen]

Some racing cars and airplanes use brakes with carbon fiber discs and carbon fiber pads to reduce weight. Disc brake - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Disc_brake)

[18-Wheeler]

The discs and brake pads are made from carbon filament, the disc hats/mounts are made from steel.

[Turbine D]

low n' slow

Here is a site that may answer what you asked (Boeing 757).

Academy of Aerospace Solid Propulsion Technology (http://ensunvalorcasc.w1.xacnnic.com/products_show.aspx?classid=80)

[Cardinal]

The discs and brake pads are made from carbon filament, the disc hats/mounts are made from steel.

The most straight-forward, accurate answer yet. Stator, rotor, stator, rotor, stator, rotor, stator, rotor, stator. When the wheel assembly slides over the stack, it engages those hats on the rotors. Picture of such, A320:

http://www.avbuyer.com.cn/editer/UploadFile/2007920161544263.jpg
.