Physics of Hot brakes

[YC]

So I was watching the A340 RTO fire video from our video Thread and I started thinking about how I've got this answer memorized: "In the T-37, maximum temperatures will occur in the wheel/brake assembly 5-15 minutes after maximum braking is applied."

But I don't know WHY max temps occur 5-15 minutes AFTER. That makes it sound like the brakes continue to heat up after you take away the friction and that doesn't make sense to me. Anyone got the answer?

[ 11. December 2006, 05:32: Message edited by: Toro ]

[Cooter]

It's the same in the RJ world. If we get hot breaks they take about 10-15 minutes to heat to their max temp. It's wierd standing there and righto n the minute they just flare up and start smoking. Don't know why they continue to heat just know that they do.

Cooter

[Guest Xtndr50boom]

Me thinks convective heating and heat transfer are the culprits.

[Guest Hydro130]

Methinks the lack o cooling airflow is the culprit, in addition to the rampant convective heating and heat transfer Xtendr50 mentioned. Even the little airflow from taxiing around with warm brakes helps cool them down significantly.

But, typically, what happens after a max braking scenario? You stop and don't go anywhere. When nothing is blowing the heat off those bad boys when they are just sitting there static, they heat UP like mugs. I've been on several Herks where we had to EGE for hot brakes (curses to you weakass E-model brakes!), and once had both mains on one side blow -- not fun. Hot brakes ain't nothing to play around with. When you hear/feel the "chatter", you best be getting airborne soonest and delaying the gear up call, foshizzle. When the "chatter" gets to be downright mushy & sticky, EGE ought to be on your brain. That and the LM/boom yelling "It smells like something is burning, yo!" (something is!).

"Run away, run away!...."

Cheers, Hydro

[Guest Xtndr50boom]

Hydro, your ebonics are cracking me up.

I'd also like to mention along the lines of hot brakes that the worst thing you can do is set the parking brake. It's like telling your PMS'ing wife she looks fat in the dress she's wearing. It only adds to the pain, and increases the possibility of the rotors melting to the gear.

EDITED: Cause I'm out of beer

[ 10. December 2006, 21:42: Message edited by: Xtndr50boom ]

[Cooter]

We've had an instance or two of folks setting the brake and the ground folks physically having break them loosed to get the jet moving again. It's never a good sign when you run off the plane turn around and seeing glowing red hot trucks holding your plane up.

Cooter

PS And I'll stick with PFM over all that convective science mumbo jumbo!

[Guest rotorhead]

Originally posted by YC:

In the T-37, maximum temperatures will occur in the wheel/brake assembly 5-15 minutes after maximum braking is applied."

Hmmm...I speculate that this is yet another instance of a poorly worded "technical manual."

I don't know how you can make more heat without more friction. I suggest that the wording might have "meant":

After maximum braking is applied, the maximum temperature will not be transferred to all wheel components until 5-15 minutes later. <OR>

After maximum braking is applied, the maximum temperature will not be SENSED until 5-15 minutes later, when the heat has been transferred to the sensing area (if there are sensors). <OR>

After maximum braking is applied, the maximum temperature will not be transferred to the fusible plugs until 5-15 minutes later. Therefore, tire deflation may be delayed by 5-15 minutes. <OR>

After maximum braking is applied, the maximum temperature will not be transferred to the combustable components (tires, seals, pucks, fluid, etc.) until 5-15 minutes later. <OR>

After maximum braking is applied, the maximum temperature will not be experienced until 5-15 minutes later, after the moron pilot, who thinks that high speed taxiing will cool the brakes down, forgets that to stop the high speed taxing, he has to apply more heavy brake pressure. <OR>

After maximum braking is applied, the damn brake/wheel assembly is going to remain Africa hot for a long, long, time.

The hot ain't getting hotter on its own, IMHO.

[Toasty]

Theory from a friend:

Maximum temps in the brakes are achieved during the RTO. After stopping the aircraft, the brakes begin to cool, and in doing so, convect heat outwards (or in this case, into the tires). The air in the tires slowly expands until the tires explode.

Unless we totally missed something.

[Chuck17]

Huh?

RTO can produce brake overtemp, I'd agree. However, AFTER the RTO the brakes do not begin to cool. They heat up, reaching their max temp 10-15 mins after application due to the amount of energy absorbed by the brakes. Any idea how much energy ceramic brakes absorb to bring a 500,000 pound jet from 120 kts to a dead stop? Thats why you dont apply the parking brakes - they will fuse to the brake assembly - they are WHITE HOT.

The tires (on heavies) are equipped with fuse plugs that will automatically blow and deflate the tires to avoid tire failure from overtemp.

The jet in this case would RTO, exit the runway, crew would emergency ground egress for anticipation of hot brakes, the mains would then blow when the temp gets hot enough, all 14 firetrucks on base would respond to the incident since there is nothing else for them to do, the crew would be piss-tested and youd have to fill out some paperwork.

Chuck

[ 11. December 2006, 22:38: Message edited by: Chuck17 ]

[Guest Xtndr50boom]

^^ Air medals/DFCs to be awarded to the C-17 folks, Q-3s for all other heavies

[Guest rotorhead]

Originally posted by Chuck17:

AFTER the RTO the brakes do not begin to cool. They heat up, reaching their max temp 10-15 mins after application due to the amount of energy absorbed by the brakes. Any idea how much energy ceramic brakes absorb to bring a 500,000 pound jet from 120 kts to a dead stop? Thats why you dont apply the parking brakes - they will fuse to the brake assembly - they are WHITE HOT.

I totally agree that one would be ill-advised to apply parking brakes in this situation due to the possibility of fusing components.

I cannot agree with "...the brakes do not begin to cool. They heat up..." If no more energy (e.g., friction) is added to the system, the total heat in the system will not increase. The heat absorbed by a component (e.g., brake pad) will be TRANSFERRED to other components, overheating them...but there is not a increase in total heat. If you can put a little heat in, and with no more energy added, get more heat out, you have solved the world's energy needs, made a few dollars, and rocked the world of physics. I stand by my post above...the message in the dash-1 may be important, but the wording is poor.

[FourFans]

Originally posted by Xtndr50boom:

^^ Air medals/DFCs to be awarded to the C-17 folks, Q-3s for all other heavies

True. True.

[Guest cguarino]

Rotorhead is right. You can't make energy. All you can do is convert it or transfer it. The issue here has to be the conductive and/or the convective heating of the wheel. When you stop, all of the kinetic energy of the moving aircraft goes into the brake disc as heat. When you stop the heat of the brake disk is transfered to the rest of the wheel/brake assembly,tire and axle. That takes time. Maybe 5-15 minutes. Simple physics.

[Guest cobaltmetallic]

I think that rotorhead explained it best. You can't add energy to a system without taking it from somewhere else (the brakes won't build up heat unless they are being used to stop the aircraft). It's the First Law of Thermodynamics.

"A perpetual motion machine?? In this house we obey the laws of thermodynamics!!!" -Homer J. Simpson

[Cooter]

All I know is one I watched a set of brakes flare up 15 minutes after I landed. And the plane was just sitting there, thermodynamic be damned. Those brakes continue to heat. On a side note I've heard of ceramic, magnesium and carbon brakes now. Anybody had experience with all?

Cooter

[Coasta]

WOW! A urinary olympiad about hot brakes!!! It's a good thing everyone on this board is a physicist. Don't forget to chalk the nose gear with the co-pilot.

[Coasta]

Better yet...make it a raven.

[Guest Hydro130]

Originally posted by Coasta:

Better yet...make it a raven.

That checks!

Cheers, Hydro

[EvilEagle]

What is this "parking brake" that you speak of?

[Guest Hoser]

Sounds like the Albino's don't have a parking brake. I learned something new today.

The F-15E does have a parking brake.

Cap-10

[Guest Hydro130]

Yep, a nifty little steering wheel, parking brake, cup holders, and reclining seats, and even some muddin' tires. Some of the things that help make the ground time waiting on ATOC a little more comfy, I guess :rolleyes:

Cheers, Hydro

[Smokin]

Could it be that the brake pads themselves are ceramic and capable of absorbing a lot of heat (kinda like the tiles on the space shuttle)? Then they transfer their heat to the larger brake assembly, which is metal and not as capable of withstanding the energy being given off by the brake pads? The energy must always be there, so the cause of the delay must be in the transfer from the brake pads to the wheel assembly (kinda like rotorhead and Chuck17 said)