Carbon Brake Cooling Rates
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Carbon Brake Cooling Rates
Hi everyone, I've done a search of the pprune vaults and I can't find exactly what I am looking for.
My company has A320s without brake fans fitted. I am interested if anyone out there has any official documents that mention the cooling rate for carbon brakes (ideally with respect to the A320) and any variances with ambient temperature and wind.
For that matter I am also curious what anecdotal info or "rules of thumb" there might be used with various operators.
My company has A320s without brake fans fitted. I am interested if anyone out there has any official documents that mention the cooling rate for carbon brakes (ideally with respect to the A320) and any variances with ambient temperature and wind.
For that matter I am also curious what anecdotal info or "rules of thumb" there might be used with various operators.
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Have a look at the MEL "Brake Temp INOP" and you should have some nice tables indicating the cooling time starting from the energy dissipated during landing. It does take into account ambient temperature, but no mention of wind or braking during taxi...
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On the 744 there's no worries if the brake temps don't go into the yellow range (and they seldom do). If they do, the tables give the required cooldown time before attempting a takeoff (likely to ensure RTO performance).
In most cases, 70 minutes will do it, so that's the "rule of thumb" figure we keep in mind.
In most cases, 70 minutes will do it, so that's the "rule of thumb" figure we keep in mind.
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I'm on the lookout for exam questions, so thought I'd have a ramble.
On the effect of braking during taxying... stopping dead from 30knots only dumps 6% as much energy as stopping from 120knots, so the history after landing isn't likely to be crucial, unless there's been repeated hard braking.
Wind can only help cooling, by blowing more cool air over the ~500C brakes.
Even carbon brakes don't get hot enough to cool radiatively: at 500C, a half square meter of brake radiates ~1kW, but it's taken in ~50MJ on stopping, giving a radiative cooling time of ~10 hours. Convective cooling from 500C over a half square meter is going to lose much more: probably ~20kW, making an initial (then slowing) cooling rate right to lose the energy in ~40 minutes. An hour or so to cool would seem to be a decent rule of thumb.
Within reason, ambient temperature isn't too important either, as heat loss is quickest when the brakes are hottest. While there's more of a gap between -20C and 500C than there is between 30C and 500C, it's not by a huge amount. If the concrete is at 60C under a burning desert sun, then it's a bit more of an issue: the ambient air temperature might not be the right thing to put into the tables.
Whether the brakes are carbon or steel shouldn't make much difference to the cooling time, since they both absorb the same amount of energy; however, I'd expect that it could well affect the temperature to which they must cool before they are safe to be used full-on again.
In the heat and thin air at Calama in Chile, I know LAN turn around A318/19/20s in only 30 minutes. I pretty sure they need fans for this.
On the effect of braking during taxying... stopping dead from 30knots only dumps 6% as much energy as stopping from 120knots, so the history after landing isn't likely to be crucial, unless there's been repeated hard braking.
Wind can only help cooling, by blowing more cool air over the ~500C brakes.
Even carbon brakes don't get hot enough to cool radiatively: at 500C, a half square meter of brake radiates ~1kW, but it's taken in ~50MJ on stopping, giving a radiative cooling time of ~10 hours. Convective cooling from 500C over a half square meter is going to lose much more: probably ~20kW, making an initial (then slowing) cooling rate right to lose the energy in ~40 minutes. An hour or so to cool would seem to be a decent rule of thumb.
Within reason, ambient temperature isn't too important either, as heat loss is quickest when the brakes are hottest. While there's more of a gap between -20C and 500C than there is between 30C and 500C, it's not by a huge amount. If the concrete is at 60C under a burning desert sun, then it's a bit more of an issue: the ambient air temperature might not be the right thing to put into the tables.
Whether the brakes are carbon or steel shouldn't make much difference to the cooling time, since they both absorb the same amount of energy; however, I'd expect that it could well affect the temperature to which they must cool before they are safe to be used full-on again.
In the heat and thin air at Calama in Chile, I know LAN turn around A318/19/20s in only 30 minutes. I pretty sure they need fans for this.
Interesting discussion
I thought the MEL mentioned around 170deg per hour at 20deg OAT.
Through experience, that does seem about right.
Another discussion worth having is if they cool faster (without fans) with brakes on or off...
I believe it is faster with brakes on due to better heat sink. What exactly is the point of releasing carbon brakes when parked?
Brake fans are over-used by the many pilots who think brakes must be stone cold at push back. Whereas I understand carbon brakes are much more efficient once warmed up. So why does FCOM state switching brake fans on as soon as one reaches 100deg?
Through experience, that does seem about right.
Another discussion worth having is if they cool faster (without fans) with brakes on or off...
I believe it is faster with brakes on due to better heat sink. What exactly is the point of releasing carbon brakes when parked?
Brake fans are over-used by the many pilots who think brakes must be stone cold at push back. Whereas I understand carbon brakes are much more efficient once warmed up. So why does FCOM state switching brake fans on as soon as one reaches 100deg?
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Originally Posted by goeasy
Brake fans are over-used by the many pilots who think brakes must be stone cold at push back. Whereas I understand carbon brakes are much more efficient once warmed up. So why does FCOM state switching brake fans on as soon as one reaches 100deg?
Originally Posted by FCTM NO 040 p6/10
The FCOM limits brake temperature to 300 ˚C before takeoff is started. This limit ensures that, in the case of hydraulic fluid leakage, any hydraulic fluid, that may come into contact with the brake units, will not be ignited in the wheelwell. This limit does not ensure that, in the case of a high energy rejected takeoff, the maximum brake energy limitation will be respected.
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goeasy,
I understand that the cooling is mostly achieved by heating the air that flows through the brakes, so releasing the brakes - separating the "pads" and "discs" - would make sense both to allow more convecting air to flow between them, and to present a larger hot surface area to it. I'd expect this to hold whether the airflow is natural or by forced air.
I believe that carbon brakes do work better as brakes when the contact surfaces are warm/hot, but that happens fast when they're used. Cooler brakes should give you more total braking power if you have to reject, in addition to being able to stow brakes that won't burn oil if you do take off, as pointed out by Cough.
I understand that the cooling is mostly achieved by heating the air that flows through the brakes, so releasing the brakes - separating the "pads" and "discs" - would make sense both to allow more convecting air to flow between them, and to present a larger hot surface area to it. I'd expect this to hold whether the airflow is natural or by forced air.
I believe that carbon brakes do work better as brakes when the contact surfaces are warm/hot, but that happens fast when they're used. Cooler brakes should give you more total braking power if you have to reject, in addition to being able to stow brakes that won't burn oil if you do take off, as pointed out by Cough.
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Practical 101: Why release the brakes?
Here's a schoolboy practical explanation once demonstrated by a very respectable Boeing Flight Operations Engineer and never since forgotten:
- Pretend you are the aircraft and your hands are the brakes;
- On landing, you apply brakes, so clap your hands and push them together for 30 seconds;
- After landing you taxi to the parking position, so clap your hands twice to resemble stopping/slowing down on your way to there;
- Having reached the parking position you set brakes to park, so clap your hands one last time and keep them pressed together;
- Once chocks are in place, parking brake can be released, so after some 15 seconds of pressing hands together, move them apart and FEEL the effect this action has on your palms: THAT'S WHY we release brakes to cool them.
Clap & hold [30 seconds], clap, clap, clap & hold [15 seconds], release...
- Pretend you are the aircraft and your hands are the brakes;
- On landing, you apply brakes, so clap your hands and push them together for 30 seconds;
- After landing you taxi to the parking position, so clap your hands twice to resemble stopping/slowing down on your way to there;
- Having reached the parking position you set brakes to park, so clap your hands one last time and keep them pressed together;
- Once chocks are in place, parking brake can be released, so after some 15 seconds of pressing hands together, move them apart and FEEL the effect this action has on your palms: THAT'S WHY we release brakes to cool them.
Clap & hold [30 seconds], clap, clap, clap & hold [15 seconds], release...
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awblain... Yes that does make sense to me too, however, i have also had it explained that without brake fans, there is very little flow between discs, and that is why using the heat-synch of the whole brake unit can be more effective.
I have operated A330 without fans, and after engineering advice, found brakes do cool more quickly whilst still applied. Hard to prove I know, but curious to hear anyone else's experience.
Skyjob... that is also a great theory, however our hands are not made of carbon/metal, and so there is little heat absorption inwards.
Cough... I know the limitations well, but have also read guidance that 150-200 deg is best at commencement of takeoff, for maximum braking efficiency. And that is what AB may have used in certification.
I am no specialist on the subject, so happy to learn from anyone wiser in these matters!
I have operated A330 without fans, and after engineering advice, found brakes do cool more quickly whilst still applied. Hard to prove I know, but curious to hear anyone else's experience.
Skyjob... that is also a great theory, however our hands are not made of carbon/metal, and so there is little heat absorption inwards.
Cough... I know the limitations well, but have also read guidance that 150-200 deg is best at commencement of takeoff, for maximum braking efficiency. And that is what AB may have used in certification.
I am no specialist on the subject, so happy to learn from anyone wiser in these matters!
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Here is an excelente article on carbon brakes:
http://pt.scribd.com/doc/48490083/Airbus-Carbon-brakes
http://pt.scribd.com/doc/48490083/Airbus-Carbon-brakes
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outofsync,
I take that point - if the disks are isolated from airflow, then conduction would probably make for a better way to lose the heat to the air from the metal around that's now in better contact.
I buy the minimum temperature argument too - while that temperature will be reached and passed very quickly in a big stop, in F1 racing, where inertia is less, sharply increased bite is reported after about half a second, once a threshold temperature of about 500C is exceeded - although the brakes are working cooler, or they wouldn't heat so fast.
There's a A380 brake test dynamometer video that heats them up fast, presumably from cold over 25s or so. The bulk of the stopping is done with the brakes visibly yellow-carbon burning hot.
I take that point - if the disks are isolated from airflow, then conduction would probably make for a better way to lose the heat to the air from the metal around that's now in better contact.
I buy the minimum temperature argument too - while that temperature will be reached and passed very quickly in a big stop, in F1 racing, where inertia is less, sharply increased bite is reported after about half a second, once a threshold temperature of about 500C is exceeded - although the brakes are working cooler, or they wouldn't heat so fast.
There's a A380 brake test dynamometer video that heats them up fast, presumably from cold over 25s or so. The bulk of the stopping is done with the brakes visibly yellow-carbon burning hot.
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I have seen external aircondition being applied to hot brakes on the Airbus, is this a common practice, or there is any inconvenient regarding the brakes when applying AC to hot brakes.?
Only half a speed-brake
Here is an excelente article on carbon brakes:
http://pt.scribd.com/doc/48490083/Airbus-Carbon-brakes
http://pt.scribd.com/doc/48490083/Airbus-Carbon-brakes
