Blackened
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Can Anybody Explain "Autobrake" - Quick!

Mon Mar 25, 2002 8:44 am

I know this might be in the wrong forum but the tech forum isn't that busy right now but I need to know it fast.
What is the autobrake function of an aircraft and does it work exactly. I'm sure some of you know.
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Spitfire
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RE: Can Anybody Explain "Autobrake" - Quick!

Mon Mar 25, 2002 8:55 am

As soon as there are wheels spinup on landing, hydraulic pressure (around 1000psi) is applied to the brakes, in order to maintain a constant deceleration choosen by the pilot (minimum, medium and maximum) without pushing on the brakes pedals (on the rudder pedals). This is of course done through the antiskid ( antilock or ABS) system in order to keep the wheels spinning. It's very usefull especially with very strong cross-wind, slippery runways or during a CAT III landing, in order to stop the plane in a minimum of time.
Hope it helps.
Sptfire
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TG992
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RE: Can Anybody Explain "Autobrake" - Quick!

Mon Mar 25, 2002 8:55 am

Surely the name would give you a clue?
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777236ER
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RE: Can Anybody Explain "Autobrake" - Quick!

Mon Mar 25, 2002 8:57 am

It's a brake, that's automatic. It brakes the aircraft......automatically. Hence the name, "Auto..." um "..brake"

Want me to simplify?
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Rick767
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RE: Can Anybody Explain "Autobrake" - Quick!

Mon Mar 25, 2002 9:12 am

"in order to stop the plane in a minimum of time."

A common misconception about the Autobrake, it does not stop the aircraft in the minimum time. Even maximum autobrake does not provide the level of retardation supplied by full, manual braking.

The benefits of the autobrake system are that it provides optimum anti-skid operation, it allows smooth, timely and even braking irrespective of pilot workload and it maintains uniform braking even during rudder inputs (i.e. crosswind landing benefit).

It is never used to stop very quickly, the best way to do this is the good old-fashioned "slam on the brakes"!
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Spitfire
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RE: Can Anybody Explain "Autobrake" - Quick!

Mon Mar 25, 2002 8:44 pm

Rick767,

You are right. To prove that we just have to look at the braking charts in our different AOM. With autobrake the stop distances are longer.
I only tried to make it quick and simple..."Autobrake for dummies..."  Wink/being sarcastic
Thanks for the correction.
Spitfire
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FDXmech
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RE: Can Anybody Explain "Autobrake" - Quick!

Mon Mar 25, 2002 9:35 pm

What's your thoughts on autobrake stopping capability in regard to "MAX", "RTO" or other selectable modes depending on manufacturer or a/c type designed to send maximum hydraulic pressure to brake system (modulated by antiskid of course). This I thought was akin to "slamming the brakes".
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FDXmech
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RE: Can Anybody Explain "Autobrake" - Quick!

Mon Mar 25, 2002 11:11 pm

Also, the stopping power (in land mode) in "MAX" and stopping distance might be deceiving.

During landing using "MAX" ABS, there is normally a time delay before the pressure is sent to the brake system which would increase stopping distance.

But the hydraulic pressure IS at its maximum with the antiskid actually controlling the deceleration rate.
You're only as good as your last departure.
 
Spitfire
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RE: Can Anybody Explain "Autobrake" - Quick!

Mon Mar 25, 2002 11:19 pm

Actually I 've never used autobrake in MAX (except in simulator) and never had a "live" RTO at V1,only one or two "interrupted" TO before 80 kts...so braking cool...
By the way, as I was initialy trained on a B737-200 without RTO autobraking possibility, I continue on "bigger" airplane to use the same reflex: throttles idles, MAXIMUM "manual" braking (let's say pedals trough the "firewall"!!!), and spoilers (even if already deployed).
I've seen, some years ago, the rubber marks of a DC10 real RTO at V1 in Bujumbura...ended very very close to the end of the runway (15 feet or so...)
Rgds.
Spitfire
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Blackened
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RE: Can Anybody Explain "Autobrake" - Quick!

Mon Mar 25, 2002 11:44 pm

What does RTO mean? Something about aborted take-offs I guess.
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Jan Mogren
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RE: Can Anybody Explain "Autobrake" - Quick!

Mon Mar 25, 2002 11:47 pm

RTO=Rejected Take Off
/JM
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lmml 14/32
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RE: Can Anybody Explain "Autobrake" - Quick!

Mon Mar 25, 2002 11:57 pm

Does the autobrake work in conjuction with reverse thrust? What I mean is does the system relax the brakes in inverse proportion to the reverse thrust applied?
 
Jan Mogren
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RE: Can Anybody Explain "Autobrake" - Quick!

Tue Mar 26, 2002 12:05 am

I don't think so. The wheel brakes are much more effective than reversing.
/JM
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Spitfire
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RE: Can Anybody Explain "Autobrake" - Quick!

Tue Mar 26, 2002 12:35 am

On A340 or A330, when during the TO roll, the trust levers are put in "idle", ground spoilers will deploy (above 72 kts) and max autobrake will occur (everything set "as in the book" !!).

In case of RTO (that's a REAL EMERGENCY): it is always FULL reverse, FULL spoilers, FULL braking. We do NOT "modulate" any of this until a FULL stop (except for the reverse put in idle around 70-60 kts for the risk of durt ingestions and engine surge due hot air ingestion). By the way, reverse "braking" in much more efficient at high speed.
Anyway, there is no link between reverse lever position (and so the "power" of the reverse thrust) and the amount of braking: always MAXIMUM braking till the end of the roll.
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FredT
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RE: Can Anybody Explain "Autobrake" - Quick!

Tue Mar 26, 2002 12:47 am

What you select (talking B744 here) with the autobrake selector is effectively a deceleration value. The system will try to hold this deceleration by modulating the brake pressure. If you use reverse, the braking power needed to achieve this deceleration will be less, leaving you with cooler brakes.

RTO mode would probably work as above though, maximum braking with anti-skid. If you have an RTO on your hands, a brake fire is probably not your biggest concern...

Cheers,
Fred
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Spitfire
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RE: Can Anybody Explain "Autobrake" - Quick!

Tue Mar 26, 2002 1:44 am

That's right FredT, it's a preselected decelaration rate , always the same with or without reverse. That's the idea for the normal use of the auto brake system.

And that's right also, I prefer to stop 150 yards BEFORE the end of the rwy (even with a brake fire --> emergency evac) than 10 yards AFTER, in the mud and with a broken landing gear (well, the mud could be a good brake cooler, but....  Laugh out loud )
Spitfire
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FDXmech
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RE: Can Anybody Explain "Autobrake" - Quick!

Tue Mar 26, 2002 4:27 am

Normally, the ABS in RTO or MAX will not slow the aircraft at a preselected decel such as LO or MED but rather send full hydraulic pressure to the brakes.

At this point the max decel rate is under the jurisdiction of the antiskid system who, dare I say, modulates the pressure (or more properly said, vents excess pressure).
You're only as good as your last departure.

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