Brake Temperature
 

Upon arrival at the gate and after coming to a complete stop if you see the brake temperature rising would this indicate that the airplane has been chocked incorrectly?

What would a rise in brake temperature and pressure indicate in such a scenario?

I wouldn't have thought so. The chocks, whether positioned correctly or not shouldn't affect the brake temps at all.

The brake temps will generally rise a bit after you arrive on stand, due to the lack of airflow over the brake unit. If you have brake fans a la BAe146, then you will see the temps decreasing, hopefully!

Hope that helps.

Brake Temps...
 

Brakes are devices that convert kinetic energy to heat energy. I don't have the reference with me (I'm in Asia, and my manuals are back in the U.S.), but, generally, the complete transfer of heat energy throughout the brake and wheel assemblies occurs approximately (on average) 30 minutes after landing.

Thus, if you have an aircraft equipped with brake temp indicators, you'll see the temp peak out about 30 minutes after landing. Brake fans, stiff winds, etc. all contribute to the reduction of that actual peak temperature, but the peak still occurs about this point in time...whatever that specific temperature may be.
Chocks in place, correctly or incorrectly placed, is an irrelevent issue.

Further, if you have brake fans, and you're using them, take note not to switch off the fans too soon. A common mistake is to (say 20 minutes after landing and maybe 15 minutes after brake fan activation) turn off the fans, thinking the temps have decreased to an acceptable value...only to see the temps rise, again, and peak at that 30 minute point.

Thus, it is perfectly normal to see the temps rise...chocked or not, fans or not...after gate arrival.

Regards,

Tom

Thanks for your replies.

On the flightdeck what would indicate -- if anything -- that the airplane is chocked incorrectly?

I am not clear what you mean by chocked incorrectly

airplane chocked incorectly: release brakes and it will start moving,especially if not on a perfect horizontal apron.
Chockes have nothing to do with brake temperature...you have to use them to be able ro release brakes,so cooling them faster.

alexban

Who says that parked hot brake units cool faster if the brake pressure is released?

What do you expect to happen when you release the park brakes to chocked wheels? How about - nothing, except for brake pressure going to zero and perhaps a miniscule realignment of the brake discs.

Wouldn't released brakes expose the brake surface to air, allowing for slightly more heat dispersion than with applied brakes. It's probably a minor effect, but would theoretically let the brakes cool a little faster.

Adding to TomConard's post, the additional factor unspoken in his post is that there is the position of the thermal sensor to consider. Obviously it can't be on the business side of the brakes, and therefore it is not directly exposed to the heating. So the sensor takes time (due to the lag he describes) to actually see the brake temperature. Which means that the sensed peak may be lower than the real peak AND occur after it.

Yep. In fact it's an SOP in our company if the brakes get above a certain temperature, or the brake fans are u/s.

By definition an Aircraft is incorrectly chocked when upon release of the parking brake the view out the window begins to move longitudinally about the axis of the eye, with or without engines running but usually in an uncommanded fashion. :)

Brake Cooling..
 

It's a well known fact that brakes cool better when they are released. If the brakes are set, the disks are squeezed together as a single unit. If the brakes are released, air is allowed to circulate between the disks and will allow for a quicker dissipation of heat.
As for properly putting chocks in place....there is no way of seeing it from the cockpit. If the airplane begins to roll...common sense says they werent put in correctly.

How can brake release assist hot disk brake unit cooling?

Without return springs or wheel movement how do the brake disks magically move apart when you release the park brake?

Same on your car having disk brakes. It is disk rotation which creates the microscopic gap between the disks or the pads.

Methinks we have a phurphy here which may be a carry over from drum braking systems.

Milt - chock the jet - stand at one of the brakes and have someone release the brakes for you. All will be revealed :rolleyes:

Not [U]necessarily[U] true in fact.

If there is little or no airflow over the brakes then releasing the park brake can, in fact, reduce the rate of cooling, as the gap between the discs is tiny and heat is simply radiated across the gap from disc to pad (and v.v)

By leaving the park brake set the heat can be transfered and radiated (away) through the whole assembly.

BUT......

If you have a suitable breeze and especially if you are using brake fans then the better option is to have the park brake off. The airflow being sufficient to pass between the pads/disc and draw heat away (and cooler air in).

The actually difference in cooling times (no fans) with the park brake on or off is actually quite minimal anyway.
Having said that it may still be a significant factor for LCCs with their rapid turnarounds...I dont know?

Chocks? Complete red-herring.
The brakes will continue to heat up for several minutes after landing...its normal as TC notes above.

Might the SOP about releasing hot brakes be related to the risk of the brakes seizing on if hot, rather than for cooling?

Milt, Boeing says that in case of hot brakes,for ex high speed RTo, use of parking brakes should be avoided.What would be the reason for this,in your oppinion?

Mad Scientist of Flying

It's going back a while now but as far as I can remember the reason for leaving the parking brake off was solely for cooling. There was never a mention of the brakes seizing on.

:ok:

Organic brakes (Heat packs) in the days prior to carbon heat packs and temp sensors, were certainly prone to seizing if the brakes were left applied. This usually occurred after hard use or after numerous short hops. Changed a few for that very reason.

There used to be a great training video called 'Time to stop again' about a 737 (If I recall correctly), doing 20-30 min multi-sectors, showing the cumulative temp rise at each point. I remember showing it to some drivers who had absolutely no idea and were quite shocked!

Don't agree, small gap it may be, but even without a breeze it will still set up a convection current, same way the radiator / heater bolted to your wall does, and for that matter the same reason why said heating device has fins and isn't just one big lump of metal, greater dissipation / transfer of heat.

I would advise as seldomfixit suggests, one never measures the amount of brake wear with brakes off, unless you are minutes from finishing your shift and feel the next shift should change the worn brake unit:D :ok:

I would suggest that the issue of releasing the parking brake from a hot disc assembly is to aid even cooling - thus avoiding the possibility of distorting the discs. The pads & associated equipment would probably provide a greater heat path than say still air, for the section of disc that is clamped.

I would suggest that some of you need to read your manuals!:bored:
Boeing has an expanation for all of this in it's books, I am sure Airbus would as well!;)

Ok, so my knowledge of this comes from racing cars - but the brake tech's pretty similar, so I'm extrapolating.. You don't leave your foot on the pedal when you've stopped with (very) hot brakes because you get a hot spot where the pads are in contact, that leads to differential cooling, which is quite likely to warp iron discs. It's not just about airflow, but thermal inertia (I admit I don't clearly understand *how* the thermal inertia effects things). But the small airgap makes a difference. Maybe a red herring in this case...

Don't we still need someone to tell us what it is that may cause the brake pads to move away from the disks when, parked against the chocks, we let the brakes off? Same for a car's brakes. Could it be the relaxation of the O ring seals (usually square) slightly retracting their pistons?

Once did a max brake stop with a heavy on Boscombe Down's wetted runway from a speed calculated to cause the brake units to reach maximum designed energy absorption. Antiskids were Maxarets. An exciting ride!

A couple of tyres blew at low speed and much smoke as we ground to a stop. Disks were thick copper plated with hard chrome. Disassembled brake units looked terrible but they passed the test. Don't recall the final disk temperature nor whether the brake units were reuseable.

mark,I don't know about racing cars,but brakes on the 737 are nothing like brakes on my car.There is small chance of getting a 'hot spot' cause the brake pad is actually a brake disk,not like a car brake pad.
milt,there are some springs on brake assembly,which could probably move away the brake pad.First time I'll see the mechs changing a tyre I'll go take a closer look at the brake...I have the designs but I can't get a clue from them..

B737 AMM:
"Each main wheel is provided with a brake unit bolted to a flange on the axle. The brakes are multiple disc type, with stationary carrier and divided lining discs, and segmented rotating brake discs. Each brake is provided with pistons which actuate the brakes when hydraulic pressure is applied. The brakes are also provided with combination return springs and automatic adjusters. The automatic adjusters compensate for brake wear."

and A320:(d)

Do not set the parking brake ON when "BRAKES HOT" warning is shown on the upper ECAM DU.



12 twists per inch is right, it's a bit of a hangover from the old days of brake siezures. Some airlines release them on blocks and some dont.

Another aspect to consider, perhaps the most important, is the heat flow and heat soak into the tyres. Tyres probably suffer more from overheat than the brake packs.
This was one of the reasons in the recommendation in the BAe146 / Avro RJ for brakes off; it also was a major part of the ‘brakes’ limitation on early aircraft for the number of short sectors within a given time.
__________________
Unless specifically authorized everything else is forbidden.

Ok, so it was a red herring then... Sorry!

Don't recall which flight manual now but had a cautionary note re hot brakes with the advise to leave them off so that they did not weld. (steel disks)

Does this help shon7?



"Increasingly, people in industry and academia are exploring the benefits of of fuzzy logic and its related technologies. Fuzzy logic can be used for situations in which conventional logic technologies are not effective, such as systems and devices that cannot be precisely described by mathematical models, those that have significant uncertainties or contradictory conditions, and linguistically controlled devices or systems. As Lotfi Zadeh once stated, fuzzy logic is not going to replace conventional logic (computers) or methodologies, rather it will supplement them in circumstances where conventional approaches fail to solve a problem effectively.
"In recent years, there has been a growing interest in fuzzy logic, both in industry and academia. Current applications include modeling, evaluation, optimization, decision making, control, diagnosis and information. In particular, fuzzy logic is best suited for control-systems fields. For instance, fuzzy logic has been applied in areas such as breakdown prediction of nuclear reactors in Europe, earthquake forecasting in China, and subway control in Japan.
"One prominent application of fuzzy logic is in the anti-lock braking system found in many modern automobiles. The control rules that describe an anti-lock braking system may consist of parameters such as the car's speed, the brake pressure, the brake temperature, the interval between applications of the brakes and the angle of the car's lateral motion to its forward motion. The range of values of these parameters are all continuous, open to interpretation by a design engineer. One such rule in an anti-lock braking system could be:
IF brake temperature is 'warm' AND speed is 'not very fast,' then brake pressure is 'slightly decreased.'

"The temperature might have a range of states such as cold, cool, warm and hot; the range of these linguistic terms can be precisely determined by defining membership functions by an expert. "There are many consumer products that use fuzzy logic in their operation. There are also many fuzzy logic chips (processors) that are built to do special tasks without using conventional computers. The outlook for fuzzy logic is therefore very promising."


:p

Brake cooling

I thought there was a much-earlier thread on brake cooling and release/not-release, but I couldn't find it because the search facilities of this latest generation of [PPRUNE's] vBulletin software had defeated me and I couldn't make the search or advanced search functions search more than one keyword.

I might have typed in several keywords but it searched as keyword1 OR keyword2 OR keyword3 etc. This potentially returned thousands of posts but since there is a limit of 500 posts in the search response, many were not actually listed. The results were often not much use.

No longer – the earlier brake cooling thread is thread=10891

For other searchers who have been struggling, let me share with you the results of umpteen trawls through arcane geek forums to find out how to search this bulletin board with more than one keyword. It's probably something that everyone else knows but here it is:

RIGHT WAY
brakes+cooling+release

ERRATIC WAY
brakes +cooling +release

WRONG WAY
brakes AND cooling AND release

WRONG WAY
brakes AND cooling AND release

ANOTHER RIGHT WAY
(brakes AND cooling) AND release)

Boolean (from George Boole)