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highflyer9790
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Autobrake Determination

Fri Dec 29, 2006 9:05 am

quite simply, how do the pilots determine what setting of autobrake they will use? i understand autobrake settings are something along the lines of RTO,OFF, 1, 2, 3, 4, MAX, but lets say that a 757 is landing on a 7,000 runway and plans on using reverse. is there a guide to what setting to use?

thanks in advance
highflyer
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AJ
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RE: Autobrake Determination

Fri Dec 29, 2006 9:31 am

Hi Highflyer,

Our Quick Reference Handbook (QRH) reproduces the Autobrake Stopping Chart from the performance manual. The chart shows the stopping distance required at various reference speeds and autobrake settings and is a guide for what to set for landing.

Other times it is just set from experience to exit via a certain taxiway.

More often than not if it is a tight runway the autobrakes will be set one unit higher incase of float (Mr. Boeing's test pilots do not float!). The autobrakes do not care if reverse is used or not as they provide a preprogrammed decelleration rate so will simply adjust brake pressure to achieve that. Not using reverse just works the brakes harder!

(Valid for the 767 and most modern Boeings)

Cheers.
 
highflyer9790
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RE: Autobrake Determination

Fri Dec 29, 2006 11:58 am

thanks aj!

highflyer
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fr8mech
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RE: Autobrake Determination

Fri Dec 29, 2006 12:04 pm

Quoting HighFlyer9790 (Thread starter):
but lets say that a 757 is landing on a 7,000 runway and plans on using reverse.

Just as a note, the use of reverse makes no difference. The autobrake system selection, selects a rate of deceleration. After the autobrakes engage, the aircraft will decelerate at a fixed rate, reverser use will lower the usage on the brakes, no reverser will increase usage.
When seconds count, the police are minutes away, or may not come at all.
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HAWK21M
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RE: Autobrake Determination

Fri Dec 29, 2006 12:54 pm

Quoting Fr8Mech (Reply 3):
reverser use will lower the usage on the brakes, no reverser will increase usage.

Apart from Analog & Digital types are there any other Autobrake systems these days.
regds
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barney captain
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RE: Autobrake Determination

Fri Dec 29, 2006 3:18 pm

Quoting HighFlyer9790 (Thread starter):
quite simply, how do the pilots determine what setting of autobrake they will use? i understand autobrake settings are something along the lines of RTO,OFF, 1, 2, 3, 4, MAX, but lets say that a 757 is landing on a 7,000 runway and plans on using reverse. is there a guide to what setting to use?

Here at WN (and I think Fedex as well) we use an onbord performace computer that tells us remaining stopping margin based on temp, weight, runway conditions etc. It tells us if the use of autobrakes are required or just optional.
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PhilSquares
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RE: Autobrake Determination

Fri Dec 29, 2006 3:22 pm

Quoting HighFlyer9790 (Thread starter):
quite simply, how do the pilots determine what setting of autobrake they will use? i understand autobrake settings are something along the lines of RTO,OFF, 1, 2, 3, 4, MAX, but lets say that a 757 is landing on a 7,000 runway and plans on using reverse. is there a guide to what setting to use?

At most airlines, it's specified in the SOP. With the cost of fuel and engine overhauls these days, more and more airlines are going to minimum reverse and some selection of auto brakes (2 or 3). The justification is increased service life on the engines and lower costs to replace the brakes compared to an engine.

At settings of 1 through 4 the autobrakes give you a rate of deceleration, so any increase in reverse thrust will only result in lower brake application. On the 744 min reverse and autobrakes 2 is more than sufficient to stop.
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Curmudgeon
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RE: Autobrake Determination

Fri Dec 29, 2006 7:51 pm

On the 737-300/400, autobrakes 2 yields the best results, and is easiest to disengage with light manual brake application. For some reason, autobrakes "1" is harder to kick off. On the 737-800 autobrakes 3 is the most commonly used setting.
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CosmicCruiser
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RE: Autobrake Determination

Fri Dec 29, 2006 10:36 pm

On the MD-11 we have RTO, Min, Med & Max. Med is used almost all the time with Max being used rarely except on say wet ungrooved runways and then length is a player.
 
highflyer9790
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RE: Autobrake Determination

Sat Dec 30, 2006 8:49 am

Quoting Fr8Mech (Reply 3):
Just as a note, the use of reverse makes no difference. The autobrake system selection, selects a rate of deceleration. After the autobrakes engage, the aircraft will decelerate at a fixed rate, reverser use will lower the usage on the brakes, no reverser will increase usage.

got ya. didnt know that.
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PhilSquares
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RE: Autobrake Determination

Sat Dec 30, 2006 9:02 am

Quoting Fr8Mech (Reply 3):
Just as a note, the use of reverse makes no difference. The autobrake system selection, selects a rate of deceleration. After the autobrakes engage, the aircraft will decelerate at a fixed rate, reverser use will lower the usage on the brakes, no reverser will increase usage.

What you say is partially true. On the Boeings, in RTO or MAX you have full brake application without respect to thurst reverser useage.
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vikkyvik
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RE: Autobrake Determination

Sat Dec 30, 2006 9:09 am

Quoting PhilSquares (Reply 10):
What you say is partially true. On the Boeings, in RTO or MAX you have full brake application without respect to thurst reverser useage.

Phil (or others), how do the two settings (RTO vs. MAX) differ? Something about the way they're selected (in the event of a rejected takeoff)? Thanks...

~Vik
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Mr.BA
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RE: Autobrake Determination

Sat Dec 30, 2006 10:53 am

RTO is selected before takeoff. This one I'm not very sure but I believe the RTO setting cannot be selected in the air, i.e, prior to landing. In the event of of a rejected TO when the throttles are closed the brakes come in full force.

A tap on the brakes will release the autobrakes.
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Vref5
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RE: Autobrake Determination

Sat Dec 30, 2006 12:23 pm

Quoting Mr.BA (Reply 12):
This one I'm not very sure but I believe the RTO setting cannot be selected in the air, i.e, prior to landing. In the event of of a rejected TO when the throttles are closed the brakes come in full force.

Yup. For RTO to activate, you generally need at least these conditions to be true on the Boeings:

1) WOW (weight on wheels -- ie, on the ground)
2) Above a certain speed (85 knots is typical; high enough to avoid potential response during taxi)
3) Throttle levers at idle position
4) RTO being armed via switch selection.

If all conditions are met, the aircraft then immediately brakes since time is of the essence. However, once the aircraft thinks it is in the air (in event of a normal takeoff), RTO automatically goes to OFF in the Boeings. Spring loaded or something?

Particular details may differ slightly amongst various models, but that's the general gist.

It's a little different for the Airbus. I've taken some of the Airbus ground school training but never got to fly one, so my recollections are a little dim, and would defer to a current Airbus flight crew member.

On the Boeings, MAX is typically selected prior to landing if you think you'll be needing it. Not very common at all, but does happen. Particularly contaminated runway surface, really short (e.g. maybe emergency diversion to a less than preferred airport) runway, concerns about ability to stop, a dire emergency need to get people out as soon as possible (e.g. fire), etc.

The logic for MAX activation is somewhat different from the RTO activation logic, which makes sense because they're both designed for different phases of flight; each with their own slightly different needs. The end result is ultimately the same, though.
 
PhilSquares
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RE: Autobrake Determination

Sat Dec 30, 2006 12:26 pm

Quoting Vikkyvik (Reply 11):
Phil (or others), how do the two settings (RTO vs. MAX) differ? Something about the way they're selected (in the event of a rejected takeoff)? Thanks

RTO is similar to MAX in that it applies maximum brake pressure. However, system must be armed and ground speed is above 85 knots and all thrust levers are closed.

On landing MAX, if selected would engage if all thrust levers are closed, all main wheel spin up has occured and ground sensing has occured. Then MAX would apply max hydraulic pressure to the wheel brakes regardless of thrust reverser operation.

The autobrakes disarm under the following conditions:
• pedal braking applied
• any Thrust lever advanced after landing
• Speedbrake lever moved to DOWN detent after speedbrakes have
deployed on the ground
• DISARM or OFF position selected on Autobrake selector
• autobrake fault
• normal antiskid system fault
• loss of normal brake hydraulic pressure
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Max Q
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RE: Autobrake Determination

Sat Dec 30, 2006 12:45 pm

On the 757/767 'max autobrakes' is not maximum braking, it is the maximum landing rollout preprogrammed deceleration rate.

Rto braking on the other hand is maximum possible brake pressure that can be applied.

There IS a difference.

I have never understood why MAX is not the same as RTO, if I selected MAX that is what i would want but that is Boeings philosophy.

If you want Max you need to apply the brakes manually on landing (tripping off the autobrakes if they are in use)

Maybe the 744 is different.

Also, reverse thrust does make a difference, if you have selected brakes 3 for example and runway conditions do not allow that deceleration rate you have no way of knowing that, if you use reverse to 'help' you stand a much better chance of achieving your desired deceleration rate.
The best contribution to safety is a competent Pilot.


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fr8mech
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RE: Autobrake Determination

Sat Dec 30, 2006 1:24 pm

Quoting Max Q (Reply 15):
Also, reverse thrust does make a difference, if you have selected brakes 3 for example and runway conditions do not allow that deceleration rate you have no way of knowing that, if you use reverse to 'help' you stand a much better chance of achieving your desired deceleration rate

Let me re-phrase: The A/B system will seek to maintain a specified deceleration rate. If you're on a cluttered runway, your brakes may not be able to attain or sustain the selected deceleration rate, this is where the reversers will 'add' to the braking action.
When seconds count, the police are minutes away, or may not come at all.
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PhilSquares
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RE: Autobrake Determination

Sat Dec 30, 2006 4:10 pm

Quoting Max Q (Reply 15):
Maybe the 744 is different.

It's a little more than maybe....what I quoted was a cut and paste right out of Vol 2!
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AJ
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RE: Autobrake Determination

Sat Dec 30, 2006 5:09 pm

Quoting PhilSquares (Reply 14):
• Speedbrake lever moved to DOWN detent after speedbrakes have
deployed on the ground

That's another difference to the 767....our vol 2 states:
• speedbrake lever is moved from the full up position after the speedbrakes have deployed on the ground

A subtle difference but one which was practiced to smoothly disengage autobraking by nudging the speedbrake lever out of up then back to up. Since a certain overrun we no longer use this practice!
 
vikkyvik
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RE: Autobrake Determination

Sun Dec 31, 2006 3:34 am

Thanks everyone for your responses.

Quoting Max Q (Reply 15):

If you want Max you need to apply the brakes manually on landing (tripping off the autobrakes if they are in use)

That's why I was asking. I remember Slamclick saying something to the effect of to achieve the minimum stopping distance, he'd smash the brake pedals with his feet while selecting max reverse. But apparently it differs between types.

~Vik
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Max Q
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RE: Autobrake Determination

Sun Dec 31, 2006 4:31 am

That is a great feature on the 744 to get real maximum braking with autobrakes on landing in the maximum position.

If we are quoting flight manuals though..!

(from our 757 / 767 flight manual)

'The RTO auto brake setting commands maximum braking pressure if:'

The aircraft is on the ground.

Groundspeed is above 85 knots.

Both thrust levers are retarted to idle.

Maximum braking is obtained in this mode. If an RTO is initiated below 85 knots, the RTO brake function does not operate.

Five levels of deceleration can be selected for landing. However on dry runways, the maximum auto brake decleration rate in the landing mode is less than that produced by full pedal braking.
The best contribution to safety is a competent Pilot.


GGg
 
CosmicCruiser
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RE: Autobrake Determination

Sun Dec 31, 2006 12:46 pm

Quoting Max Q (Reply 20):
That is a great feature on the 744 to get real maximum braking with autobrakes on landing in the maximum position

That's the same with most big jets as has been said..Max is Max; RTO is Max
 
PhilSquares
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RE: Autobrake Determination

Sun Dec 31, 2006 2:51 pm

Quoting Max Q (Reply 20):
Five levels of deceleration can be selected for landing. However on dry runways, the maximum auto brake decleration rate in the landing mode is less than that produced by full pedal braking.

As Cosmicruiser pointed out, it's like that on every aircraft I've flown with autobrakes. You can always do a better job standing on the pedals than the autobrakes at Max can give you.
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Max Q
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RE: Autobrake Determination

Mon Jan 01, 2007 2:56 am

Sorry Phil but you contradict yourself a little there.

Your earlier post states that Max autobrakes supplys maximum hydraulic pressure to the brakes and now you state that one can always do a better job standing on the pedals than autobrakes at Max can give you.

It seems a trifling difference, but that was my point all along!

Rto braking is the maximum braking the system can give you, and probably superior to full pedal braking in a rejected takeoff situation (considering reaction time, technique etc..)

Max braking is the maximum autobrake deceleration rate available for landing, it is not maximum available braking.
The best contribution to safety is a competent Pilot.


GGg
 
Bellerophon
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RE: Autobrake Determination

Mon Jan 01, 2007 4:10 am

Max Q

I'm going to join in and say that I think you may be correct.

...On the 757/767 'max autobrakes' is not maximum braking, it is the maximum landing rollout preprogrammed deceleration rate...

That is also my understanding of the system on the B747-400.


...RTO braking on the other hand is maximum possible brake pressure that can be applied...

That is also my understanding of the system on the B747-400.

In view of what has been posted by a knowledgeable poster, I have carefully reviewed the system description in our B747-436 Technical Manual, which is quite clear about the following points:

  • MAX autobrake is a pre-selected deceleration rate.
  • MAX autobrake, whilst the highest deceleration rate that can be pre-selected, does not apply the maximum brake pressure which can be applied.
  • MAX autobrake produces a deceleration rate which is less than that which can be produced by full pedal manual braking.
  • RTO autobrake commands maximum brake pressure and produces a maximum braking effort.
  • RTO autobrake deceleration can not be improved by manual braking.

So my understanding, at least, accords with yours.


PhilSquares

...On the Boeings, in RTO or MAX you have full brake application without respect to thurst reverser useage....

I agree with regards to the RTO autobrake setting on take-off, but that doesn't appear to be the case with the MAX autobrake setting on landing, at least not on the B747-436.

I understood that, firstly, MAX was still a deceleration rate, and, secondly, that even at MAX, the system would/could never apply maximum brake pressure.

I don't have any other technical information beyond the Tech Manual, and haven't ever (yet) used MAX on landing, so I would be interested in your comments.

Could there be airline specific differences in the functioning of autobrake system on the B747-400?

Could our Tech Manuals be wrong?

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PhilSquares
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RE: Autobrake Determination

Mon Jan 01, 2007 11:45 am

Quoting Bellerophon (Reply 24):
On the Boeings, in RTO or MAX you have full brake application without respect to thurst reverser useage....

I agree with regards to the RTO autobrake setting on take-off, but that doesn't appear to be the case with the MAX autobrake setting on landing, at least not on the B747-436.

I understood that, firstly, MAX was still a deceleration rate, and, secondly, that even at MAX, the system would/could never apply maximum brake pressure.

I don't have any other technical information beyond the Tech Manual, and haven't ever (yet) used MAX on landing, so I would be interested in your comments.

Could there be airline specific differences in the functioning of autobrake system on the B747-400?

Could our Tech Manuals be wrong?

You are correct. In RTO, assuming you're above 85 knots and reject the takeoff, the brakes will apply maximum pressure not maximum rate. In Autobrakes MAX the autobrakes will apply a deceleration rate almost equal to maximum pressure braking. In the Max setting thrust reversers do not have any effect on the wheel braking, it's still the max setting, unlike the other landing settings.

My reference was to the landing part of the RTO. If I wrote or inferred the same was true for RTO I was mistaken. However, in the FCTM there is a little section on braking techniques that says max manual braking will always result in slightly shorter stopping distances when compared to the autobrakes. Perhaps that is a little type by Mr. Boeing. I haven't really gone into the sim to try that, I have just taken him at his word.
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aviopic
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RE: Autobrake Determination

Mon Jan 01, 2007 11:54 pm

More info can be found here: RTO Procedures (by Speedracer1407 Oct 3 2006 in Tech Ops)

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CosmicCruiser
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RE: Autobrake Determination

Tue Jan 02, 2007 12:52 am

Here's what it says about the MD-11

In the MAX position, maximum pressure from brake
system 1 and 2 is applied to the brakes by both of the
IBCVs. The maximum pressure is reduced only by
ant i-skid operation. Maximum deceleration is
achieved.

In the rejected takeoff (RTO) mode above 100 knots
the ABS provides maximum brake pressure from both
brake system 1 (hyd sys 1) and brake system 2 (hyd
sys 3). Maximum brake pressure is reduced only by
anti-skid operation. Maximum deceleration is
achieved.
RTO initiation must occur at speeds greater than 100
knots for maximum dual system braking. If RTO is initiated
below 100 knots the ABS reverts to landing
mode operation and MIN braking is applied.

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