A few technical questions ... any input?

[Mach75]

Hey guys, I'm studying hard for my upcoming interview but I have a few questions that I'm having trouble answering. Any input would be greatly appreciated:

1) Can someone please outline the de-ice/anti-ice systems used on the 747?

2) I know that carbon brakes wear more when they're cold but can anyone tell me why?

[airamerica]

''Can someone please outline the de-ice/anti-ice systems used on the747? ''
If you are not rated on the 747 how are you supposed to know that?
On the other hand by speculating.

[NoseGear]

The de-ice/anti-ice is both electric, for windows and probes and bleed air for the leading edges and cowls.

Can't remember why they wear more cold, but they definately work better when warm, its the number of applications that is the main differing feature with the carbon brakes.

Good luck with the interview

Nosey

[extreme P]

Question
I am a 50 year old airline pilot. My dilemma:
I fly the B767 and B757 series aircraft, the
B767s are equipped with carbon fiber brakes, and the company manual says that the brakes wear less when they are hotter, therefore we should use more brake pressure rather than less. Specifically, we use a higher lever of autobrakes.
I would like to understand this phenomenon. I know that you work for free, but the more detail the better.
Thanks in advance.

Carbon fiber-based brakes are made out of so-called carbon-carbon composite material that consists of two components: weaved carbon cloth and solid carbon matrix. The carbon matrix is formed by Chemical Vapor Infiltration (CVI) method. According to this method, a vapors of gases (such as CCl4) are introduced to the weaved carbon cloth and react on the surface of these fiber with formation of solid carbon phase. This solid carbon phase is the one that plays important role in friction and wear performance of the brakes.
I read three articles on carbon brakes, and browsed few more. To my surprise, all of them agree that very little is understood about physics and mechanism that affect brake performance. I think that companies that make brakes have more knowledge, but tough competition on the market prevents them from publishing this information openly.
There are some facts and phenomena, however, that are discussed in the open literature. Among them 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 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.