In the 744, assuming you have auto brakes set to off. Can you enable them once the AC has touched down. Example: AC lands normally no auto brakes set, 50m past touchdown point can you command auto brakes 3 and achieve the desired effect?

Just curious.
Bobbi

Yes you can and lets say you have set autobrakes 2 fr landing and after touchdown you want a quicker exit or a better decelleration rate in wet coditions you can switch it up to 3 or 4 etc. etc.


Leo

Quoting Bobbidooley (Thread starter): In the 744, assuming you have auto brakes set to off. Can you enable them once the AC has touched down. Example: AC lands normally no auto brakes set, 50m past touchdown point can you command auto brakes 3 and achieve the desired effect?

This begs the question: Why??????

Quoting Qantas744ER (Reply 1): Yes you can and lets say you have set autobrakes 2 fr landing and after touchdown you want a quicker exit or a better decelleration rate in wet coditions you can switch it up to 3 or 4 etc. etc

Looking at your proflle, you might be able to do in in MSFS, but I'm not sure in the real aircraft. I can't find anything if Vol 2 but in all my time in the aircraft I've never seen nor heard of it being done. In fact, I've never seen it done in the sim as there is no need to ever do that. All it takes is a little more pedal pressure. Maximum braking is actually achieved by manual braking rather than max braking.

Quoting PhilSquares (Reply 2): but in all my time in the aircraft I've never seen nor heard of it being done

Why should it actually have to be done when this item are on your check list, and as you have got the weather info well in advance anyway. Correct?

Still,
Just to put some more fuel on the fire,
What would cause you to actually change this setting so close to touch down? If any.


Aim for the sky!
Sudden

Quoting PhilSquares (Reply 2): I can't find anything if Vol 2 but in all my time in the aircraft I've never seen nor heard of it being done. In fact,

I couldn't find anything in the MM that say's you couldn't, providing your throttles are at idle all the arming criteria are still met on the ground. I do not know why you would want to do it, why not just apply manual braking? Changing an autobrake setting like that is asking for trouble.

Quoting PhilSquares (Reply 2): Looking at your proflle, you might be able to do in in MSFS, but I'm not sure in the real aircraft. I can't find anything if Vol 2 but in all my time in the aircraft I've never seen nor heard of it being done. In fact, I've never seen it done in the sim as there is no need to ever do that. All it takes is a little more pedal pressure. Maximum braking is actually achieved by manual braking rather than max braking.

Hey,

If you check youtube, there is a video of a CX 744 landing in Kai Tak and with the camera filming in the cockpit. 5-6 seconds after touchdown in wet conditions the Captain orders Autobrakes 3 to the F/O who then switches the Autobrake from 2 to 3 because the Captain wants a quicker exit.

Cheers Leo

Quoting Qantas744ER (Reply 5): Hey, If you check youtube, there is a video of a CX 744 landing in Kai Tak and with the camera filming in the cockpit. 5-6 seconds after touchdown in wet conditions the Captain orders Autobrakes 3 to the F/O who then switches the Autobrake from 2 to 3 because the Captain wants a quicker exit. Cheers Leo

That is what prompted my question. Obviously I saw it done, albeit on YouTube; however, my brain could not connect the why to the video.

Thanks for the answers.
Bobbi

I Was searching for a thread relating to exactly how the brake assembly of large commercial aircraft looks. Ive been through 727 systems class and have been told how it is designed but I just cant seem to wrap my head around it.

That said I came across this thread and thought the question by the original poster to be interesting indeed; as Philsquares pointed out in MSFS you sure can but as for the real thing Im not sure.

The video mentioned above :


http://youtube.com/watch?v=V5GS3f0OfsY

Any further thoughts?

I don't know about the 400, but in the classic you can increase, decrease or select off your autobrake selection at any time. For example, minimum may be selected, but if you land a little long and need to make a particular turn off, you may move them up to medium. You may also land with medium due to expected runway conditions, but move them to minimum if you find conditions better than expected and want to keep brake temps down.

Speedbird2263

...but as for the real thing I'm not sure...

I am, and you can.

Typically done when the handling pilot has floated too far during the flare, and is now likely to miss the desired turn-off with the pre-selected autobrake setting.

Sometimes done by someone who, a few seconds earlier, used reverse thrust (with autobrake selected) and doesn't quite understand why that hasn't shortened his landing roll.  

As others have said, why not just use manual braking?

Regards

Bellerophon

Quoting Lowrider (Reply 8): For example, minimum may be selected, but if you land a little long and need to make a particular turn off, you may move them up to medium. You may also land with medium due to expected runway conditions, but move them to minimum if you find conditions better than expected and want to keep brake temps down.

So the Classics only allow you to alternate to and from minimum and medium? Presumably to avoid the drastic change to/from MAX, then?

2H4

Quoting Bellerophon (Reply 9): As others have said, why not just use manual braking?

SOP for our op is to use autobrakes, unless an abnormal, emergency, or MEL requires otherwise.

Quoting Bellerophon (Reply 9): Sometimes done by someone who, a few seconds earlier, used reverse thrust (with autobrake selected) and doesn't quite understand why that hasn't shortened his landing roll.

That is a fun little rabbit trail that some check airman try and lead candidates down during thier oral exams.

Quoting PhilSquares (Reply 2): Maximum braking is actually achieved by manual braking rather than max braking.

Really? I didn't know the 747 worked that way. I'm pretty certain that on the 737, MAX is 3000 psi brake pressure, which is max hydraulic pressure. You can't get higher than that.

Quoting Bellerophon (Reply 9): As others have said, why not just use manual braking?

Autobrakes can help reduce tire wear and increase brake life. Using a lower autobrake setting reduces brake pressure. On an airline level basis, consistently using autobrakes can make a difference instead of pilots modulating brake pressure on their own. However some airlines do not ever use the autobrakes (except RTO).

Quoting 2H4 (Reply 10): So the Classics only allow you to alternate to and from minimum and medium? Presumably to avoid the drastic change to/from MAX, then?

No, max is selectable as well, just rarely used. I have only used it in the sim. Min and med regulate rates of deceleration, 4 ft/sec for minimum and 6 ft/sec for medium. Reverser usage will decrease the energy the brakes must absorb, hence the lower brake temps. Max clamps on with a fixed pressure and only controls for antiskid.

An example of max autobrakes in action. Note, full pressure is only applied when the nose gear touches down, until then it provided medium level braking.
http://www.youtube.com/watch?v=f3NmLD5Z3EY

[Edited repeatedly for spelling and grammer 2008-03-11 12:29:25]

[Edited 2008-03-11 12:30:11]

Interesting read..a bit off topic, but is the anti-skid system on an airliner comparable with ABS on most modern cars?

Quoting LockstockNL777 (Reply 14): Interesting read..a bit off topic, but is the anti-skid system on an airliner comparable with ABS on most modern cars?

Yes it is! I remember reading a while back that the idea for ABS actually came off the Anti-Skid system that some aircraft were using at the time!

Please feel free to correct me if wrong  
leo

Quoting LockstockNL777 (Reply 14): Interesting read..a bit off topic, but is the anti-skid system on an airliner comparable with ABS on most modern cars?

What the anti skid system does is that it senses tire decelleration to modulate brake pressure. If tire speed suddenly decreases, then the anti-skid box will release unwanted hydraulic pressure to the specific wheel which to lower the brake pressure, which will decrease the braking force and allow the tire to continue to rotate. It does work essentially as ABS on a car works.

Quoting Bobbidooley (Thread starter): In the 744, assuming you have auto brakes set to off. Can you enable them once the AC has touched down. Example: AC lands normally no auto brakes set, 50m past touchdown point can you command auto brakes 3 and achieve the desired effect?

It sounds to me that some misunderstood your question, or maybe I do.

I've tested dozens of ABS(Auto Brake System) control boxes and as far as I know you can't select a land mode on the ground, like you can't select RTO ones in the air.
It simply doesn't arm as the right conditions are not met(weight on wheels, throttle levers, wheel rpm etc.)
For example:
Before Take Off you can arm RTO after which the ABS system will engage itself when all throttle levers are retarded, so in case of an engine problem and shutdown the a/c will proceed with a 3, 2 or single engine take off.
During approach you can arm one of the land modes(in some a/c this will be min, med or max and in others 1, 2, 3 etc) after which on touch down(weight on wheels) and all throttle levers retarded the system will engage it self and apply the selected brake pressure.

However ones in the air and ABS armed in what ever land mode you can switch to another land mode before actual touch down or even while ABS land mode is already engaged(and thus the brakes are applied).
The brake valve is current controlled and the control box will simply apply more current to open the valve a bit further without taking note of conditions like skidding.
Ones the Anti Skid Control box detects skidding it will control another valve to drain of the excess pressure to the brakes.

RoseFlyer

...Really? I didn't know the 747 worked that way...

Yes, on the B744, that is correct. On landing, max manual braking will produce a greater deceleration rate than max autobrakes.

Max autobrake is a pre-selected deceleration rate, which, whilst the highest deceleration rate that can be pre-selected, does not apply the maximum brake pressure which can be applied.

Not so on take-off, where the RTO autobrake system, when triggered, commands max brake pressure and produces max braking effort, which cannot be improved on by manual braking.


...Using a lower autobrake setting reduces brake pressure...

Probably, but not necessarily. Selecting a lower autobrake setting on the B744 only selects a reduced deceleration rate. Whilst this generally will result in reduced brake pressure, it does not, directly, select a reduced brake pressure.

If other retardation devices are used, such as reverse thrust or spoilers, then the applied brake pressure will be reduced, to maintain the selected deceleration rate. If the runway is slippery, the applied brake pressure will be increased, to achieve the selected deceleration rate.

Whatever brake pressure is required (up to the maximum landing autobrake limit) to achieve the commanded level of deceleration will be applied.


Lowrider

...SOP for our op is to use autobrakes, unless an abnormal, emergency, or MEL requires otherwise...

As is ours, and I am aware of their value and usefulness.

However, to clarify, I was referring to selecting a higher autobrake setting during the landing roll, when I said use manual brakes.

In my opinion, having one pilot looking (backwards and downwards) inside the flight deck, at speed during the landing roll, is not good practice. Turn the selector knob the wrong way, and you'll disarm the autobrakes, and as Aviopic has said, you will then not be able to re-select them. Turn the knob too far the right way, and you could well get a lot more braking than you bargained for.

In this context, I stand by my comments. If what you've selected on the autobrakes is now insufficient, use manual braking, and keep your eyes outside the flight deck during the landing roll.


...That is a fun little rabbit trail that some check airman try and lead candidates down during thier oral exams...

   

Why won't my lighter aircraft glide further than your heavier one, surely it stand to reason......  

Why is the temperature at altitude over the equator colder than over the poles, I'd have thought.....  


Best regards to both

Bellerophon

Quoting Bellerophon (Reply 18): However, to clarify,

Apologies. After the time to edit expired, I realized I may have misunderstood what you were saying. I thought you meant use manual brakes only for the entire landing. Upon review, I agree with your postion. I got lost in the translation of a.net.

[Edited 2008-03-11 20:34:16]

[Edited 2008-03-11 20:35:32]

Quoting Bellerophon (Reply 18): Max autobrake is a pre-selected deceleration rate, which, whilst the highest deceleration rate that can be pre-selected, does not apply the maximum brake pressure which can be applied.

Sorry about the confusion. I don't know the 747, but rather 737 and I always thought that MAX meant 3000 PSI brake pressure and that unlike settings 1, 2 and 3, it was not a deceleration rate. I guess I'll have to go look at the logic diagrams again.

If I get some free time at work, I'll look at the logic diagram for the 747 autobrake to see how switching the setting on the ground affects the autobrakes. I don't know if the on-ground mode affects whether or not the setting can be changed, but I'm sure I can find out.

About selecting a higher autobrake setting during landing roll;

Obviously I have no idea whether it is a good practice or not and many experts told their ideas about it, so it would be pointless for me to make a comment about it.

But you might want to be witness to a such application. It can be seen in the Justplanes CX 744 DVD. Towards the end, after they land to good old Kai Tak, captain commands "autobrake 3" to the F/O and F/O adjusts the brakes (increases the brake setting from 2 to 3).

[Edited 2008-03-12 10:48:55]

Ahh.....I think the video needs a closer look(and listen). While watching - and after you here "autobrakes 3" - you never see the FO move his hand anywhere near the AB switch (which is near the gear handle). I think what you are actually hearing is "Autobrakes FREE" a call out (I assume) that they make as the AB's are over ridden by manual braking. We have a similar call out and suspect that is what is actually being said. I can't imagine ever increasing braking after landing by selecting a higher setting on the AB's.

Quoting Speedbird2263 (Reply 7): I Was searching for a thread relating to exactly how the brake assembly of large commercial aircraft looks. Ive been through 727 systems class and have been told how it is designed but I just cant seem to wrap my head around it.

The best way to picture the workings of an aircraft brake is to imagine how a multi-plate clutch works on a motorcycle. Both of these mechanisms are exactly the same in principle.


http://www.freepatentsonline.com/6532637-0-large.jpg


http://www.freepatentsonline.com/6702068-0-large.jpg

As you can see from the diagrams, there is a series of annular disc and pad surfaces alternatively arranged in a stack. With respect to the second diagram, you can see that every alternate disc has splines on it's inside surface (items 38-41) that locate onto matching surfaces (item 35) on the torque tube (item 32) of the brake unit. This torque tube is firmly attached to the rest of the brake unit (item 27). The brake unit is free to revolve around the axle, the brake torque loads being taken by the brake torque rod which is shown in part at the very bottom left of the second diagram.

The other end of the torque tube (item 32) flares out to form the mushroom shaped structural element (item 33). This flared structure reacts the clamping forces produce by the brake pistons, of which one is shown as item 53. This squeezes the entire stack of disks and pads together when the brakes are applied.

The pads have splines on their outside circumference (items 44-47), which match up to corresponding surfaces (item 37) in the inside of the wheel rim (items 14-19), and are thus driven by the wheel. You should be able to now see, that when the wheel spins, there is an alternate pattern of stationary discs (attached to the brake unit torque tube) and rotating pads (attached to inside of wheel rim). Squeezing the whole lot between the brake pistons (item 53 ) and torque tube end flare (item 33) creates friction to slow the spinning pads and hence the wheel rim. The brake torque is then reacted by the brake torque rod, which you can see in the following photo of a 744.


View Large View Medium

Photo © Stephen Fox

Out of interest, you can also make out the sensors that measure the axial force in the brake rods. This can be used along with the diameter of the brake unit to work out the brake torque being generated, which can be then displayed on EICAS and the CMC.

Quoting LockstockNL777 (Reply 14): but is the anti-skid system on an airliner comparable with ABS on most modern cars?

Quoting Qantas744ER (Reply 15): I remember reading a while back that the idea for ABS actually came off the Anti-Skid system that some aircraft were using at the time!

I remember seeing ABS on the 707, when I was working on them, so ABS on airliners has been around at least since the early 1950's. I think it was quite a few decades after this before ABS systems started appearing on cars.

Regards, JetMech

Quoting Barney Captain (Reply 22):

Oh, thanks for that! I've seen the video long time ago (F/O leaning to the autobrake switch is probably the work of my imagination) and now your explanation makes great sense. I vaguely remember they vacate the runway after a short time "autobrakes free" callout.

I would be very surprised (actually I was until I read your reply) to see a practice in that video, which is not approved by the experts here, so now all the pieces of the puzzle are complete.

Cheers,
Emre

[Edited 2008-03-12 11:28:24]

Quoting Barney Captain (Reply 22): Ahh.....I think the video needs a closer look(and listen). While watching - and after you here "autobrakes 3" - you never see the FO move his hand anywhere near the AB switch (which is near the gear handle). I think what you are actually hearing is "Autobrakes FREE" a call out (I assume) that they make as the AB's are over ridden by manual braking. We have a similar call out and suspect that is what is actually being said. I can't imagine ever increasing braking after landing by selecting a higher setting on the AB's

He's calling "Autobrake 3". At the end of the roll he calls "Manual Braking" which indicates he has started using the brake pedals and so the auto-brake has disarmed. "Autobrake FREE" means nothing.


View Large View Medium

Photo © Craig Murray


The autobrake selector on the CX 744 is on the aisle stand, not near the landing gear handle, see photo above. You'll notice the F/O has his left hand out and behind him reaching for the selector even before the captain makes the command, so he's clearly expecting it might happen. It's as if it was procedure for CX, maybe due to the apparently wet runway.

To sum up, on a 744 you can change the autobrake setting during the roll-out, but you can't activate autobrakes after touchdown.

[Edited 2008-03-12 11:31:17]

[Edited 2008-03-12 11:33:10]

Quoting Jetlagged (Reply 25): The autobrake selector on the CX 744 is on the aisle stand, not near the landing gear handle, see photo above.

My bad! I ASS U ME'd that all boeings had them near the LG lever (every one I've been in did).  

Repeat after me "all Boeings are not created equal, all Boeing are not created equal....."   

Quoting Barney Captain (Reply 22): Ahh.....I think the video needs a closer look(and listen). While watching - and after you here "autobrakes 3" - you never see the FO move his hand anywhere near the AB switch (which is near the gear handle). I think what you are actually hearing is "Autobrakes FREE" a call out (I assume) that they make as the AB's are over ridden by manual braking. We have a similar call out and suspect that is what is actually being said. I can't imagine ever increasing braking after landing by selecting a higher setting on the AB's.

The autobrake control on a 744 is on the pedestal towards the rear, you can see the f/o adjust it.

At around 2.29 on the video you can hear the call "Manual braking"

Quoting Jetlagged (Reply 25):

Oh, I am not a native English speaker so I thought mistaking three with free was fairly possible but now somehow I am happy that I understood correctly what the captain said.

To be certain, I just refound the video and watched it again. It is clear that captain orders "autobrake 3" and F/O does what he says. They certainly change the brake setting. So it can be done, but I understand that "not doing it" might be a better idea or at least, you can find a few good reasons in this topic for not doing it.

Just an earthquake hit here while I was typing, I hope it isn't serious.

Thanks Jetlagger.

Emre

[Edited 2008-03-12 11:55:40]

Quoting Jetlagged (Reply 25): The autobrake selector on the CX 744 is on the aisle stand, not near the landing gear handle, see photo above. You'll notice the F/O has his left hand out and behind him reaching for the selector even before the captain makes the command, so he's clearly expecting it might happen. It's as if it was procedure for CX, maybe due to the apparently wet runway. To sum up, on a 744 you can change the autobrake setting during the roll-out, but you can't activate autobrakes after touchdown.

Talking to a CX 744 Captain.

He said the usual AB setting at CX is AB3, idle reverse. Thrust reverse will go full if required by Captain. Lower setting is rarely used as it will only be used if a rollout is necessary on 07R at HKG when nobody is behind them. The other day with wet runway and 5kts tail, they landed with AB4 and Full reverse at HKG.

Regarding the video i linked to him, he said it is choice of the Captain to order a higher AB setting, and that he has done this a couple of times when landing at Kai Tak a while back. This was nothing out of the usual as flares could end up being a little longer etc. and so a higher setting could then be commanded during the landing roll if the Captain thought it was necessary.

Hope this helps!

Leo  

Hi!

What is the situation with the reverse AND the autobrakes, as far as affecting landing distance, etc. I have never flown a plane with autobrakes, so I am not familiar with how those two systems interact. And, I am asking for the 747-400...I assume that was the airplane for the situation that was mentioned in this thread.

Thank you!

cliff
LFW

Quoting atpcliff (Reply 30): What is the situation with the reverse AND the autobrakes, as far as affecting landing distance, etc.

The autobrake system on the 400 will slow the aircraft at a predetermined rate. Using reverse thrust will not affect the rate, but it will reduce the amount of energy to be absorbed by the brakes. The exception to this is the "RTO" setting, which applies max available braking regardless of deceleration rate, once certain conditions are satisfied.

Quoting JetMech (Reply 23): I remember seeing ABS on the 707, when I was working on them, so ABS on airliners has been around at least since the early 1950's. I think it was quite a few decades after this before ABS systems started appearing on cars

In 1966 the dH Trident had Dunlop Maxaret antiskid fitted.
It was totally mechanical, consisting of a wheel about 4in diameter rolling on the rim of the aircraft wheel, with a system inside which sensed change of decelleration. The hyd supply to the individual brake was routed through the unit. It was an on/off device, but it worked.
At the time, the exact same unit was fitted to a top of the range car, I think it was a Bentley.

Quoting Tristarsteve (Reply 32): It was an on/off device, but it worked

I still think many ABS systems work like this. I recently read a book about ABS systems fitted to cars, and they all seemed to work with an "on-off" type of operation. Perhaps the response time of such an operational mode is quicker than using smooth modulation?

Regards, JetMech

[Edited 2010-07-18 19:31:49]

If I may jump in here -

Quoting Bellerophon (Reply 18): Max autobrake is a pre-selected deceleration rate, which, whilst the highest deceleration rate that can be pre-selected, does not apply the maximum brake pressure which can be applied.

Why is it that Max Autobrake commands a pressure lower than the maximum brake pressure you can apply when using the pedals?

Quoting Tristarsteve (Reply 32): At the time, the exact same unit was fitted to a top of the range car, I think it was a Bentley.

Actually, the jensen FF sport sedan
see here a nice history of car brakes

Quoting Schienenflieger (Reply 34): Why is it that Max Autobrake commands a pressure lower than the maximum brake pressure you can apply when using the pedals?

Because maximum autobrake is still a set deceleration...it works like all the other landing autobrake modes, it just has the highest rate.

Max pressure will give you whatever the runway/gear combination can do up to the anti-skid system kicking in.

Tom.

Hi!
"The autobrake system on the 400 will slow the aircraft at a predetermined rate. Using reverse thrust will not affect the rate, but it will reduce the amount of energy to be absorbed by the brakes. The exception to this is the "RTO" setting, which applies max available braking regardless of deceleration rate, once certain conditions are satisfied."

So, under normal conditions, the length of runway required to stop would be the same, with or without reversers, BUT the brakes would absorb less energy, and their temps would rise less, if you used reversers.

AND, if I understand this correctly:
If the runway condiions were poor, then using reversers MAY shorten the landing distance, if the autobrakes were at max pressure trying to, and unable to, achieve the desired deceleration rate. If no reversers, then the runway distance would be "X". If you USED the reversers, the runway distance required to land would be "X-___", because the brakes would be at max pressure supplied by the autobrakes, and the deceleration rate called for would not be achieved without the reversers.

Is the above correct?

Thanx 4 the info!

cliff
LFW

atpcliff

...So, under normal conditions, the length of runway required to stop would be the same, with or without reversers, BUT the brakes would absorb less energy, and their temps would rise less, if you used reversers....

Yes.

Under normal conditions, when using autobrake, the use of reverse thrust will keep the brake temperatures lower than they would otherwise have been, will burn more fuel and will create more noise, but will not shorten the landing roll.


...If the runway condiions were poor, then using reversers MAY shorten the landing distance, if the autobrakes were at max pressure trying to, and unable to, achieve the desired deceleration rate....

Yes.

If, when landing on an icy runway where MAX autobrake cannot achieve the selected deceleration rate because the Anti-Skid system is operating, reverse thrust will assist in shortening the landing roll.

Nice to see an Airliners.Net reader paying attention!   


However, and this is something of a new topic, having landed on a slippery runway, especially in a cross-wind, reverse thrust must be used with care, as it can de-stabilise the landing roll by pushing you towards the side of the runway.

It may become necessary to cancel reverse thrust, whilst the runway centre-line is regained, and whilst doing this there may not be a lot of retardation being supplied by the autobrakes, meaning the end of the runway may be coming towards you much faster that desired!

Flying training is all about instilling best practice to guard against the worst situations.

On the B747, we trained that daylight landings in summer, on long, dry runways in calm winds, were, nevertheless, always in the touchdown zone, on the runway centreline, with the aircraft tracking down the centreline.

This was to ensure, as far as possible, that when you had to make a night landing, in winter, on a short snow covered runway, in a strong cross wind, then, immediately after touchdown, you were in the optimum position from which to start bringing the aircraft safely to a halt on the paved surface.

The B747 is a great aircraft in cross-winds on slippery runways, but all aircraft bite fools!

Best regards,

Bellerophon