View Full Version : B757 autobrakes

21st Aug 2002, 22:25
Some help please, about hydraulic non-normals and use of autobrakes; or not as in this case. B757

1. The autobrakes work on the normal brake system, the Right. So, why when the Left or Left & Centre system pressure is gone does the QRH say "DO not to use autobrakes?"

It doesn't say they are U/S as in a Right system failure, just don't use them.

2. Further, in a Left & Right failure you'd select the Reserve Brakes system, and if it worked this would power up the Right (Normal)system for brakes. No mention in the QRH about status of autobrakes, so if they latch, use 'em.
However, in a Right system only failure you don't use reserve brakes system,(use alternate on the Left system) and so have no autobrakes. But using the reserve brakes system only isolates the right (u/s) system fluid from the useless user units so it can be used for normal brakes and hence autobrakes.
I've got two perfectly good size 11's to replace autobrakes, but I'm certainly curious about ? 1. No.2 would complete the big picture.

Many thanks, and I'll hold my peace before I fill some sponge like brains with utter drivel.

22nd Aug 2002, 01:06
To answer question number two:

The autobrakes "consumes" an extensive amount of hydraulic fluid to regulate the brakes to the desired level. Since the isolation/preservation of fluid only applies to the supply side of the brakes you might end up with a completely dry right system=no brakes if the leak is on the return side of the brakes..

If you are down to reserve brakes you really just want to do the one application, come to a full-stop and get towed away, unless you've just got pump failures and no leak, then I assume it's safe to use the autobrakes..

Bally Heck
22nd Aug 2002, 21:19
Interesting questions RAT. I think you need to get out more. :p

My theory for Q1 is that in the event of a left hydraulic system failure, the right system then has to drive the power transfer unit, which among other things powers the nose wheel steering. I suspect the poor little overworked right system does not have the cojones to do all this and power the autobrakes.

23rd Aug 2002, 23:57
Thanks B.H. & everybody else: and here's the BUT,

Does the autobrkae system consume more hydraulics than (I hate to use the term 'manual brakes' for feet) foot brakes? In the event of L. or L & C system failure, 2 x size 11's are going to do the job of autobrakes and cane the R system. Do I expect a problem? I doubt it, or rather I don't know; so does the auto-system use more power? That's a new one on me.

I must get out more as the handicap is soaring with the birds.

Bally Heck
24th Aug 2002, 02:53

Autobrake one will modulate the brakes on and off according to the FCTM. Therefore that setting will use hydraulic power. Could be that to simplify things Boeing specify no autobrake.

In general, if you have more than 8000ft, the thrust reversers will all but stop the a/c anyway, and one firm application will certainly do so.

24th Aug 2002, 04:55
I think the answer to question 1 is directional control. If the left system is u/s then only the right reverser is available and it is also posible that there is no nose wheel steering (if there has been a complete lose of fluid) therefore differential braking would be required. sayin that though I do remember a TC mumbling something about loss of the left system pressure and the possibility of not getting truck tilt so therefore no autobrakes but i'm still trying to figure that one out.

24th Aug 2002, 12:05
Hi Winston,

The truck tilt sounds a bit spurious. You must get truck tilt when planting 90.000kgs on a flat surface, I would have thought. It would severly dent the braking performance to use only the rear wheels and proberly necessitate a speedy visit to the pits for a tyre change.
Regarding the single reverser and differential braking; true, but I'd always had the idea that, when there were pedal steering problems it would be better to use autobrakes. That way you concentrate only on steering and not braking. If you have to do both, i.e. apply pedal steering and braking, it is very likely that you'll apply 80% of brakes to the same pedal that your pushing for steering, thus reducing the braking effect. Autobrakes will spread the load evenly and with greater effect.

I heartly agree with the 8000' and reversers idea. If it's not my lucky day and L & R are both useless, I'm off to one of these Space shuttle landing sites.

24th Aug 2002, 14:06
The thing with the autobrakes consuming hydraulic fluid is that the metering valve in the autobrake controller will continuously (spelling??) modulate the amount of hyd-fluid allowed to the brakes and the excess being sent back in the returnsystem. For the sake of argument, say you preselected autobrakes 2 , the maximum brake pressure allowed through to the brakes is 1750 psi and will be modulated so that the aircraft reaches a decelleration rate of 5.0 ft/sec/sec with all the means of stopping used (reversers and liftdump). That means almost half of the the hydraulic pressure=fluid is being sent back through the brake return pipes back to a potential leak in the R HYD SYS.

Whereas if you make the one application with yer size 11s, and you keep the pressure applied constant there will be no hydfluid going from the brakes via the return pipes back to the reservoir. It will all be locked on the supply side of the brakes.


24th Aug 2002, 22:09
Umn fruitloop, are you sure you are not mixing it up with the 767.. The C hyd sys has got nothing to do with the brakes on the 757 as far as I know..


25th Aug 2002, 21:02
Brenoch and all:

Many thanks for all the info'. I think we've caned this one. Learnt a lot from you guys, but what was the message from fruitloop and the CTR system. Never saw that one.

Bye till next time.

26th Aug 2002, 01:51
"but I'd always had the idea that, when there were pedal steering problems it would be better to use autobrakes. That way you concentrate only on steering and not braking."

Not sure I understand this... If there were (rudder) pedal steering problems, Rat, wouldn't you need to use differential braking? In which case, you would be automatically tripping off the autobrakes anyway(?).

"Whereas if you make the one application with yer size 11s, and you keep the pressure applied constant there will be no hydfluid going from the brakes via the return pipes back to the reservoir. It will all be locked on the supply side of the brakes."

Not familiar with the 757, but with size 11's, on your average Boeing, wouldn't the antiskid system be ejecting a considerable portion of the fluid back into the return lines, Brenoch? :D


26th Aug 2002, 02:42
Sorry Brenoch
You are entirely correct (didn't read the question correctly,757 as opposed to 767 !!)
With regards to tilt pressure coming into the landing characteristics I very much doubt it .From memory(not to be trusted due to intake of red wine)the only time that the tilt comes into pressure is when retract is selected.(to allow gear to fit into hole due to swing angle etc.etc. so when the gears are down (selected by either means)will not consume any more fluid.


Question for Brenoch
What system powers the In-flight braking ??

Thank you in advance

27th Aug 2002, 02:40
QAVION: Steady application, not slamming on them.. :D

Right you are but I for one would be careful with the brakes if I where to find myself down to reserve brakes..

Fruitloop: I'll look into it tomorrow but my best guess is that whatever brakesystem is the master-sys on ground is also the one responsible for stopping the wheels from spinning when you retract the gear.. However I'd like to see the idiot dispatching a 757 with one hyd-sys inop.. :D:eek: :D

28th Aug 2002, 23:45
On The B757 you must not use autobrakes if the left sys is u/s as there will be no nose wheel steering, and posiblly asymetric reverse thrust.

Bally Heck
29th Aug 2002, 00:44
78deg, check above.

If the left hydraulic system is u/s, the power transfer unit, powered by the right system supplies hydraulic power to the nose wheel steering!

29th Aug 2002, 10:48

You're quite right; on re-reading it I can understand your confusion.
What I was referring to was a situation that would cause problems in keeping it down the middle, e.g. single thrust reverser or severe cross wind. If there is a need for strong pedal input to keep it straight, it is likely that most of the braking (non-autobrakes size 11's) will come from that same side. This will be less effective than allowing the A/B's to do the job for you, evenly.


31st Aug 2002, 13:29
Howdy Brenoch,

The alternate brake system supplies the un-spinning braking on main gear retraction.

31st Aug 2002, 13:41
Cheers shlittlenellie..

Thanx, never bothered to look it up in the AMM. Haven't seen you around for ages.. How are things in your neck of the woods??

Send me a PM with the latest news..


Bally Heck
31st Aug 2002, 18:48
I don't know why my mind works this way, but as I was trying to get to sleep last night and thinking completely unrelated thoughts it occurred to me that the truck tilt actuators are operated by the left system and truck untilt is one of the inputs required to enable the autobrakes.

I am thinking of taking classes in realising the patently bleeding obvoius! :(

1st Sep 2002, 20:59
Bally Heck

you might be right, about the truck tilt, but the QRH doesn't say the autobrakes are U/S, which would be the case. It says don't use them. The test of the autobrakes being OK is if they latch in the armed position. The QRH tells us not to even try. In the T.T. case they wouldn't work; that's not the same thing.
It would seem that the return line feedback loop, in earlier posts, might be the answer.

I wish you a good night's sleep.


Bally Heck
2nd Sep 2002, 03:55

Because one of the parameters required by Boeing to enable the autobrakes is no longer reliable, they say you should not use the autobrakes. They may well still latch in because the gear is tilted, but when the gear is extended, without the tilt actuators, they will do their own thing.

2nd Sep 2002, 08:05

AH Ha! This is now getting way beyond me. The amount of info that Boeing presents to the simple pilot is so brief and scant, that we work in the realms of "just do what it says, don't think about it too much, and DO NOT touch anything else."

Shame really. In the old days I used to enjoy having a rough idea of what I was doing and WHY. :)