Does the use of Autobrake save brake wear?

[Mork]

I was told that this is true but don´t know if the info is correct, and if it is, what is the explanation.

Mork

[scanscanscan]

I was told by the manufacturers representative it prevents Tromboneing which saves the brake units.
He loved airlines that did not use any auto brakes at all on landing as he sold more brake units this way.

[Devils Advocate]

By way of a simple explanation the AutoBrake system modulates the amount of hydraulic pressure to the brakes such that the aircraft slows at a predetermined rate of deceleration.

The amount of AutoBrake selected determines the level of that deceleration, and hence the stopping distance, e.g. (and roughly)

AB=1 (flaps 30), brake pressure (nominally 1250 PSI) is modulated to give a deceleration of 4' per sec sqrd and should bring the aircraft to a stop in 8000’ or just over.

AB=2 (flaps 30), brake pressure (nominally 1500 PSI) is modulated to give a deceleration of 5' per sec sqrd and should bring the aircraft to a stop in anything between 6001' to 7999'

AB=3 (flaps 40), brake pressure (nominally 2000 PSI) is modulated to give a deceleration of 7.2' per sec sqrd and should bring the aircraft to a stop in anything between 5001' to 6000'

AB=Max (flaps 40), brake pressure (nominally 3000 PSI) is modulated to give a deceleration of 12' per sec sqrd (below 80 Kts) or 14' per sec sqrd (above 80 Kts) and should bring the aircraft to a stop in 5000' or less.

NB. AutoBrake max does not maintain full brake pressure and as such, whilst it will put the brakes on for you when you touchdown, full toe brake pressure should be applied by the pilot just as soon as possible after touching down - whereby the aircrafts anti-skid system should stop the pilot from locking the wheels ( and accordingly there are big performance penalties associated with having the anti-skid system inoperative ).

Using engine reverse thrust in conjunction with AutoBrake has the effect of reducing / modulating the hydraulic pressure being applied to the brakes, i.e. as the reverse thrust is now helping to slow the aircraft (at the selected AutoBrake decelaration rate) the brakes are now getting some help - and so the autobrake system senses this and releases some of the hydraulic pressure being applied to them.

With normal AutoBrake (1,2,3,Max) you won’t stop any quicker or in any less distance by using reverse thrust – but you will save the brakes.

The other AutoBrake mode is RTO (Rejected TakeOff) - when this is selected (prior to take-off) should the pilots decide to reject a take-off the autobrake system (subject to certain provisos) will apply full unmodulated hydraulic pressure to the brakes and maintain this until the aircraft either comes to a halt and / or the pilots override it - and believe me, it's staggeringly effective - though a big problem after a high speed RTO is the heat in the brakes either melting the fuse plugs and / or starting a fire.

Nb. Of course the stopping distance for any AutoBrake mode is very much dependent upon the condition of the brakes and of the runway.

[Prof2MDA]

If you have carbon brakes, the wear can be more adversely affected by the number of applications than on the length of those application. Part of this is because carbon brakes wear less when they are warm, so a long and even application results in warmer brakes and overall lower brake wear. Autobrakes do a better job of that than a human can.

[18-Wheeler]

Another quick point - With the company I fly for, we often operate 747 Classics into places with very high ambient temperatures.
We've found that if you have a long enough runway we're often better off not using the autobrakes, as they kick in before you can get full reverse so they heat up quite a bit just for a couple of seconds after touchdown.
We start go get out of reverse at 80kts, so that's when the brakes would start to (effectively) come on.
With no autobrake, there's no heat-up on touchdown and we only have to gently touch the brakes at below 80kts, so they hardly heat up at all. They certainly seem to last longer if we do that.

[m&v]

The latest amendment of the Airbus Fcom's advocate(more so than before)the use of Auto brake on Landings.The 'one'brake application,whether it be immediate(medium)or modulated(low) does a better job that the human pilot in avoiding 'high' temps.
With runway to spare they suggest ,18wheeler's application,(manually) at 80knots..:)

[Mud Skipper]

18-Wheeler,

Good God Man - you're talking like a pilot who actually is allowed to think and control the aircraft using good judgement. No wonder you fly the Classic.

Unfortunatly in my company, after a senior pilot visited a golf course in Asia, we are required to use autobrakes for all landings. Can't be too carful now on 4000 m runways - I think the brake manufactureres are giving someone in ops a sling for rules like this to exist. Often after landing you see people immediately overide the aoutobrakes with a nasty jerk, especially in the 4K runway scenario with a vacate at the end.

In defence of autobrakes - they do, I believe, even out brake wear left & right. Yes, apart from X-wind, some of us are just naturally lefties or righties!

It's a tool, use it when appropriate.

[18-Wheeler]

We're supposed to also use autobrake for every landing as well, but often practicalities overide that.
We'll often do a quick turn-around in places like Riyadh - 2000' elevation I think, or a better example being Abha at 7,000', Sanaa at 7,000' - and to keep the brake temps down to a manageable level for the departure we have to put some effort into looking after them on arrival.

Also, on very slippery runways - Eg, Keflavic in Winter - we might not use autobrake if we're not too heavy. The reason being is that the local experience there is that using autobrake can make the plan move around on the runway almost uncontrollably, as one side hits ice & the other doesn't, causing the plane to yaw from side-to-side. By not braking at all and just using reverse thrust, the plane is kept straight and again down around 80kts you're pretty safe to use conventional braking.

And if in doubt, go around! ;)

Fortunately Chief Pilot is a terribly practical chap and is happy for us to look after the plane as best we can.

[Sniff]

The Boeing Aero Mag published a little article about Brake Temps during short-haul ops. Click Here (http://www.boeing.com/commercial/aeromagazine/aero_17/brake_story.html) to view it.

Sniff.

[druckmefunk]

Whilst i am a definite advocate of pilots discretion in this area, and fortunately for me, my company allows some discretion in the use of autobrake, mainly that it is only mandatory on slippery runways, however i take on board the comments about landing on ice at Kef. That is something i have not yet tried on an auto brake equipped A/C.
Having said all that, no matter how much money you save on brake wear, one visit to the golf course undoes all that good work, and then some. I know there was a lot of other factors in that small glitch in an otherwise perfect safety record, but it is likely that a logical autobrake selection (probably 3 or 4 given a wet runway) may and i repeat, may have saved the day. So it is fairly hard to argue against the use of autobrake on carbon fibre units. Steel brakes are a different matter as 18 wheeler points out. Turn around times and wear are can be significantly affected by judicious application of brakes around 80kts.

[Centaurus]

My experience real time and in the simulator (737) was that pilots used to using auto brakes for all landings - regardless of length of runway and dry conditions - were all at sea when carrying out a landing using manual braking from the word go. Usually they would jerk from side to side down the runway with uneven braking. What was clear was that they had got out of practice at applying normal braking without automatics. Their hand flying showed similar rusty skills. Quite worrying really when you know the pilots are so reliant on automatics that normal skills are eroded over time.

But I guess that's the way it will always be.

[javelin]

Problem with Airbus autobrakes is that they are:-

Not quite enough

Just too much

Oooh my f**king nose

Do the White Hatters still use brakes 3 and idle reverse - seems sensible to me on a Boing - on a 'bus I like med autobrake & idle reverse or full reverse and manual brakes - one application down to taxi.

[Brenoch]

m&v:

The Airbus Fcom suggests that the plane itself does a better job than the human pilot..

[mutt]

We have two different fleet types of similiar size. They have slightly different stopping procedures, in one, autobrakes are used until taxi speed is achieved, in the other, autobrakes are used only at the beginning of the landing. The brake life in the 1st type is almost double that of the 2nd.

Mutt. :):)

[Semaphore Sam]

In this case, Mutt, I must disagree. Best to use no brakes til 60, if able, and use most of the runway to slowdown. The taxiback time is minimal, and (unless at LHR, where runway exit-point MAY be critical), min braking is best. I've used auto-braking values of 2 and 3 (when F/O's have asked to use it), and had very hard jerking (in RUH) requiring logbook write-ups. I DO NOT like the system for normal use (due to experienced abnormal braking inputs). Has anyone else experienced this?

[Flight Detent]

I am with '18 - Wheeler' regarding this, in more ways than one!

Just an additional point, regarding auto-brake operation, at least on the B747 Classics, but I'm sure the 'other one' also has this, it's quite logical, really!

With "MAX" breaking selected, initially you will only get "MED" braking with wheel spin-up, with "MAX" being conditional on the nose gear (squat switches) detecting the NLG is on the ground.

Pity the -438 that went for a "hole-in-one" attempt, didn't have an FE to wake the Captain up to what was actually happening, maybe, just maybe, then that would not have happened!!

Cheers

[birdofprey]

Airbus are apprently catching on to the limited levels of Autobrake by introducing 5 rates selectable by rotary swicth on the good old A340-500/600. :)

[Leslie]

In my company, autobrakes are recommended. In practice I tend to look at the plate, RW condition and A/C weight and pick the turn-off I'd like to take. With a little experience I find it seems to pull me up nicely!
Once down to the turn-off speed or taxi speed I simply squeeze the pedals and the autobrakes (normally) smoothly drop out with minimal heat build up. Certainly for PAX comfort I find it works well. I have no piloting experience of the John Wayne but as a passenger and speaking to the crews I understand that the Airbus low setting is not enough and the med setting is slightly too much. When positioning on them normally one feels the brakes followed a distance down the runway by a jolt as the brakes are dropped out and then manual braking with your nose up against the front seat again. Tactically it may be necessary but from a PAX comfort view I try to avoid that if possible.
Certainly the 757 brakes (steel) can be kaned and rarely go above 3 (out of 9) - they are as good as gold! Have used max braking a few times and it is very good - in fact it stopped the jet so quick, we needed to put power on to make the only turnoff about 5-600 metres into the runway! (we were empty before you ask!)
The 767 however (Carbon) can be difficult as although they slow you quickly, they do heat up and in our company we can not use the first setting due to resonance encountered when the type was introduced. If there is a short turn around, we are vacating at the end of the RW, no ATC pressures, etc I suspect it is wise to use the reversers to scrub most of the speed. However I have watched someone go for a greaser with the ensueing float and end up having to throw out the anchors to stop our pulses rising despite the RW being over 3000m - however that is of course a matter of landing technique rather than tactical use or otherwise of an automatic system.
We recently had an interesting little situation involving landing rollout and basically I do not hang about with the brakes.I select what I think I need and try to ensure I put it down in the right place on speed.After flying the nose wheel on to the deck I find the brakes spool up very smoothly (noticable on even high settings). The brakes kick in on passing 1 degree attitude on the derotation.
As an aside I understand that since it became policy to use idle reverse at our base and let the brakes do the work, the brake wear has actually decreased. Certainly the longhaul types are all carbon that work better/erode less when hot so by putting the load on them has apparently cut our wear. Just a thought...
So as you may guess, I think that autobrakes are a great little invention, along with auto speedbrake and I only wish I could have had them on the C152 when I learnt!!! ( I jest!)

Have fun chaps and chapettes,
yours aye,
Leslie!

[chapter32]

Having done some work with the braking system on the Airbus I can confirm that the A340-600 will have 5 levels of autobrake with rotary switch, exactly for the reasons given by javelin and Leslie.

On the ‘bus each autobrake setting corresponds to a deceleration rate using feedback from the ADIRUs. I think that the system that controls brake pressure described by devils advocate is found on the Boeing classics.

During flight testing of the A340 a lot of effort was put into trying to obtain smooth onset characteristics for autobrake. When this was achieved and the aircraft was subsequently used by a crew who had not been involved in the autobrake testing it was thought that this would be a good test of the improvements. On landing they used LO autobrake and found the onset so smooth that they thought the system had failed and snagged it in the logbook. I think that Airbus really got the message that LO was TOO LO. I’m not so sure what it is like now.

As for brake wear there are so many factors as has been pointed out that it is difficult to draw more than general conclusions. Basically carbon brakes do like being hot and don’t like repeated applications. One brake application down to taxi speed would appear to offer the best brake wear as indicated by mutt and as far as the carbon is concerned it does not matter if it is controlled by the pilot or autobrake.

I hope that there are some brake system designers looking at this thread since they can learn a lot about what you get up to with the system when it gets into the real world.

Keep on stopping !

[birdofprey]

The Airbus policy on Autobrake is one which is becoming more and more reliant on the automatic system. In fact, the FCOM is due to be updated to recommend Autobrake for all landings unless the pilo thinks otherwise.

The A340-500/600 representeed a major step in the Autobrake philosophy for Airbus with a brilliant idea to add more than 2 landing modes (which are incidently pin-programmable so those of you with a squashewd nose in MED can always do something about it!).

:)

Mode LO was increased, mainly because the pilots were concerned about the onset. This is partyly due to the fact that the MLG are so far behind the pilot!!:cool:

Disengagement of Autobrake is computer controlledwith a nice smooooth pressure drop to avoid spillage of coffee. Brake wear levels are impossible to judge between pedal and autobraking. At least with Aurobrake, you know what your supposed to get!:)