G200/Galaxy brakes-WTF?

[kf4amu]

P.S. Take it easy.

Sorry if it came across the wrong way, I meant it literally...I don't know everything either but I'm always willing to listen.

 

[svcta]

The ever-present dangers of internet discussion. No sweat!

 

[svcta]

Glazing is do to long application of brakes, making them too hot. Which would mean big brakes are less likely to glaze. Not sure how this gets reversed on airplanes or carbon brakes but I'm all ears.

.

Okay, this conversation lead me to our DOM, who is one of those very smart people when it comes to stuff like this. And he even showed me a video that was quite informative.

First of all, carbon brakes as installed on airplanes do not have ceramic in them. So, they don't "glaze" in the way that auto brakes do (which was cited in this conversation).

When carbon brakes are used, carbon bits are worn off of the brake components and create friction between each other (rotors and stators). At first these carbon particles are very big and consequently cause A LOT of wear at lower temperatures. As the temperature in the brakes increases these particles get smaller and smaller and ultimately become almost like a film between the rotors and stators. As the brakes are heated toward 150 deg C the level of wear increases rapidly, until the 150 degree point is crossed, at which point the wear rate decreases to its lowest level. So the hotter they are, the lower the rate of brake wear.

The point being that when you use a set of carbon brakes (on airplanes) you want to get them as hot as possible as quickly as possible. And once they're hot, keep them that way. Long brake applications during taxi are still taboo, as we agreed. The preference being short bursts of braking from around 20 knots to a near stop. Let it speed up, then slow it down, never just drag them.

The manufacturer even suggested that turboprop operators use the brakes instead of reversing the props when possible, for example. The main thrust of the thing being this: If you're going to use the brakes, then light those suckers up. Don't be gentle with them. They must get hot to work their best.

 

[SansPlane]

Found the information below on the Boeing site at the bottom of the page: svcta - is that video accessible somewhere on the Internet?


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.

 

[svcta]

It is, i'll find the link. Realize that what you posted is concurrent with what I said regarding taxi technique. Slow the airplane down with one brake action from a relatively fast taxi speed, and then let the speed build back up. Don't drag the brakes, or slow down and speed up over and over and over. We're in agreement on that.

Edit: the video in question:
http://www.google.com/url?sa=t&rct=...1N2jBw&usg=AFQjCNGZqw0KyF2mUVxJC6XCw_3bQIoiTw

 

[kf4amu]

.

Okay, this conversation lead me to our DOM, who is one of those very smart people when it comes to stuff like this. And he even showed me a video that was quite informative.

First of all, carbon brakes as installed on airplanes do not have ceramic in them. So, they don't "glaze" in the way that auto brakes do (which was cited in this conversation).

When carbon brakes are used, carbon bits are worn off of the brake components and create friction between each other (rotors and stators). At first these carbon particles are very big and consequently cause A LOT of wear at lower temperatures. As the temperature in the brakes increases these particles get smaller and smaller and ultimately become almost like a film between the rotors and stators. As the brakes are heated toward 150 deg C the level of wear increases rapidly, until the 150 degree point is crossed, at which point the wear rate decreases to its lowest level. So the hotter they are, the lower the rate of brake wear.

The point being that when you use a set of carbon brakes (on airplanes) you want to get them as hot as possible as quickly as possible. And once they're hot, keep them that way. Long brake applications during taxi are still taboo, as we agreed. The preference being short bursts of braking from around 20 knots to a near stop. Let it speed up, then slow it down, never just drag them.

The manufacturer even suggested that turboprop operators use the brakes instead of reversing the props when possible, for example. The main thrust of the thing being this: If you're going to use the brakes, then light those suckers up. Don't be gentle with them. They must get hot to work their best.

Interesting. I guess in ceramic brakes, the "glazing" is actually a good thing?

This is one of those things that Embry Riddle teaches you but doesn't teach you the WHY or HOW it works that way. I always knew carbon brakes wore quicker when cold and when used sparingly, but it didn't make any sense to me. Thanks for the explanation.

 

[SansPlane]

Realize that what you posted is concurrent with what I said regarding taxi technique.

Yes - note to self ;-)

and then let the speed build back up.

landing downhill?

 

[ttflyer]

landing downhill?

Looks like he was referring to taxiing (where most wear occurs), not landing... Lots of jets have enough residual thrust to build up speed on level ground during taxi at idle power.