Why is "heavy braking" recommended in an aircraft with carbon fiber brakes in order to prevent brake wear? That seems backwards!

Question

I get that standard brakes and carbon fiber brakes are made out of two different materials, but some of the procedures are different between them.

For instance, they recommend "heavy" braking on landing (even if it isn't required) in order to prevent brake wear. Why is that?

What other differences are there between them that a pilot needs to know about before flying an aircraft with carbon fiber brakes?

Answer 1

I know nothing about this topic but it's quite an interesting one so I did some Googling and found a detailed Airbus document called "Proper Operation of Carbon Brakes". It includes the following information:

All brake manufacturers highlight the fact that carbon wear is heavily affected by brake temperature

This is completely different from steel brakes:

...energy is theoretically not the primary parameter for carbon wear, whereas it is the most important one for steel brakes.

The document goes on to say this:

Generally speaking, to increase carbon brake life, brakes should either be operated cold or hot but not at intermediate warm temperatures

And this:

All recommended braking techniques should aim at reducing the number of brake applications and optimizing the carbon brake temperature

The specific temperatures mentioned in the document are different for different manufacturers, but generally the intermediate range to avoid seems to be roughly 100-300C. That would explain why 'heavy' braking might be recommended: it's better to use one single brake action that generates more heat and gets into the higher temperatures than to brake multiple times more lightly. Nevertheless, the document also says that overheating is still unsafe and the use of max autobrake is not recommended.

I found a related discussion of exactly this question elsewhere that goes into the physical explanation (although it doesn't quote any sources): at lower temperatures, the carbon becomes a powder which abrades the brake pads, lowering its life; as the temperature rises, the carbon starts behaving like a fluid so the wear is greatly reduced. Most discussions of carbon brakes that I found say that their physical properties and behavior are well documented, but not well understood.

Answer 2

The amount of time spent on the brakes is the most important thing which dictates the wear of carbon brakes, not the amount of energy put into them (which is the case with steel brakes).

By braking harder, you are spending less time on the brakes in total, which means that you will have less wear on the brakes. Let's just imaging you wear 1/10000th of the brake every second, no matter how hard or soft you brake. By braking lightly, taking 20 seconds to stop, you'd wear out 1/500th of the brake. But if you only took 10 seconds, you'd wear just 1/1000th of the brake. While it's not quite as simple as that, that's the general idea of it.

Another contribution factor is the non-linear and non-proportional correlation between brake wear and brake temperatures. While this is slightly counter-intuitive, brake wear increases until about 200°C disk temperature (note that indicated temperature is usually lower), then it actually decreases from there until it becomes much hotter (about 700-800°C). So, by braking harder, you will also heat up the brakes to the point where brake wear is less than by braking lightly.

Because of this, it's better to apply the brakes in one go, to reduce the total time on the brakes. Boeing's excellent article on the Operational Advantages of Carbon Brakes gives the following advice when taxiing:

Taxi braking recommendations for carbon and steel brakes

Because the wear mechanisms are different between carbon and steel brakes, different taxi braking techniques are recommended for carbon brakes in order to maximize brake life.

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.

These recommendations are intended as general taxi guidelines only. Safety and passenger comfort should remain the primary considerations.