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Willit Run
27th Jun 2005, 02:26
In regards to Auto brakes and heat build-up on the Classic, does anyone have any documentation whether minimum or mediem auto brakes stores more heat in the wheels and tires and brakes? We have two schools of thought here, and I am just curious. Would like to put this to bed!

Phil Squares
27th Jun 2005, 05:58
It will really depend on the use of reverse thrust. Without reverse thrust, both settings will result in the same amount of energy being absorbed. However, the min brake setting will absorb the energy over a slightly longer period of time. The end result will be the min setting will result in slightly cooler brakes.

If "judicious" reverse thrust is used, at higher speeds, the reverse thrust will be more effective than min brakes, so no braking will be applied. With med, you will probably get some braking at higher speeds. Net result is higher brake temps using med auto brakes.

IIRC, there is a chart in the performance manual that talks about auto brakes and max reverse thrust.

zerozero
27th Jun 2005, 09:07
...just a clarification:

With MIN brakes and reverse thrust there's a "modulation" of braking commensurate with the amount of reverse.

With MED brakes, there's a constant amount of brake pressure applied no matter what amount of reverse.

Therefore, I'd say cooler brakes with MIN everything else being equal.

At least that's my understanding without diving into the books. Standing by for further clarification...

;)

GlueBall
27th Jun 2005, 19:27
Only "MAX" autobrake setting applies constant 3000psi pressure irrespective of reverse thrust inputs.

gas path
27th Jun 2005, 22:29
zerozero
With MIN or MED selected the autobrakes are modulated to achieve a constant deceleration rate in the order of 4ft/sec/sec and 6ft/sec/sec. at about 1800psi
MAX auto and RTO give the max. rate of deceleration with hyd. press of 3000psi.
As glueball says with the application of rev. thrust the autobrake will still aim for a constant rate of decel as selected except when MAX auto is selected.

stilton
27th Jun 2005, 23:25
I can't speak for the classic 747 but on the 757/767 only the 'RTO'
setting provides the maximum 3000 psi pressure to the brakes, and not the 'max' setting, if you want maximum braking on landing it has to be applied manually.

Curious to know if this is the case on the 747.

gas path
27th Jun 2005, 23:44
747 MAX auto and RTO give the full 3000psi to the brake system.
IIRC MAX auto not often used unless for example runway length dictated it. Still it excersizes the antiskid valves nicely.:ooh:

Willit Run
28th Jun 2005, 01:00
I understand how the system works, but i wondered if there was some data or info somewhere that stated whether or not, more heat was generated with min. or med. brakes. The amount of energy disipated is the same, but how its done, affects the amount of heat stored in the brakes. Some argue it's kind of like; which came first, the chicken or the egg?
one school of thought is; because using mediem brakes, they are applied for a shorter amount of time, thus creating less heat. The other school of thought is, min brakes create less friction. But from basic physics, and i mean basic, the total energy is the same dissipated.
I think using min brakes causes less wear but creates more heat energy stored in the brakes; and that mediem brakes wears more , but creates less heat!
Hell, this is why i'm asking? I can't seem to come up with a solid answer.
Just wondering, don't want this to become another US Air/Aer Lingus, ATC disaster.

18-Wheeler
28th Jun 2005, 11:04
Simple -> more brakes = more heat.

Hawker-rider
29th Jun 2005, 17:57
exactly!

there will not be the same amount of energy absorbed in the brakes or the wheels.

With MIN autobrakes you will not need to dissipate as much energy simply because you will use more runway with the MIN setting as compared to MED with all other things being the same. Therefore the extra roll will decrease the energy needed to stop.

Intruder
29th Jun 2005, 21:42
one school of thought is; because using mediem brakes, they are applied for a shorter amount of time, thus creating less heat. The other school of thought is, min brakes create less friction. But from basic physics, and i mean basic, the total energy is the same dissipated.

Not quite...

More brake pressure means more energy dissipated in a shorter amount of time, or more braking power used. It also means less time for thrust reversers to work and less time for aerodynamic and other drag to do their work. So, the brakes will dissipate more of the total kinetic energy of the airplane.

catch 22
1st Jul 2005, 07:29
As posted earlier the brake setting of Min and Med are based on decelerations of 4 Ft/sec/sec and 6 ft/sec/sec. Due to form drag on landing and spoilers the airplane will usually decelerate at least 4ft/sec/sec until about 100 knots. Then, with Min brakes the brakes will start to retard the aircraft. With Med the brake will start applying almost immediately. If you recall that the energy absorbed is related to the SQUARE of the speed you can see that at 140 knots much more energy is produced than at 100 knots. I will wager that under ALL circumstances Min braking produces less heat than Med. In practice that is also my experience.