Autobrake on the 777

[Nightfire]

Could anybody explain to me the difference in time delay for the autobrake on the 777?
Apart from the set decelleration rate between 1,2,3,4 and MAX, there is also a difference about when the autobrake-system actually starts operating.

I couldn't find anything in the FCOM, except for the explanation that MAX is the same as 4 upon main gear wheel spin-up, until the pitch is less than 1°. Which is not the information that I'm looking for.

Does anybody know?

[lion-g]

Hi,

Autobrakes (with antiskid) on the normal only brake system provides auto-braking at selectable deceleration rates for landing and max braking for rejected takeoff.
• Autobrake 1………………………..4 FPS deceleration

• Autobrake 2………………………..5 FPS deceleration

• Autobrake 3………………………..6 FPS deceleration

• Autobrake 4………………………..7 FPS deceleration

• Autobrake MAX…………………..11 FPS deceleration

• Autobrake RTO……...Maximum possible deceleration
If RTO is selected for takeoff the system activates on the takeoff roll at speeds above 85 knots when both throttles are retarded to idle.
After landing, autobraking begins at wheel spinup and throttles at idle. Five levels of deceleration can be selected for autobraking on landing: I - 4 and MAX. If MAX is selected, deceleration level 4 is applied until the pitch angle is less than one degree.

As extracted from an unofficial manual.

Cheers

[Nightfire]

Thanks, Lion. However, I had all that information already.

All I'm wondering about is a time-delay in the system, from touchdown until autobrake engagement. Does it exist, and if so, how long is it?

[VNAV PATH]

do not have access to maintenance manuel from where I am , but time delay is 0.1 sec except in one mode : autobrake MAX
please correct me if my memory is wrong !

[gas path]

The autobrake function applies the brakes when these conditions occur:
· The autobrake selector is in 1,2,3,4, or MAX AUTO
· The thrust levers are in the idle position
· Ground mode (the two trucks are UNTILTED and the landing gear control lever is DOWN after a 0.1 second time delay)
· Wheel spin-up (average wheel speed of the 4th, 5th, 6th, and 7th fastest wheels is more than 60 knots).

[TopTup]

For the record, there is a greater deceleration that can be used other than RTO: max manual breaking. Check up on on RTO procedures in QRH.

[ACMS]

All you need to know is: A/B 1--- nice and gentle for longggg runways
A/B 2---still fairly gentle for medium to long runways
A/B 3---comes in fairly quickly for normal length runways ( most commonly used setting for day to day ops )
A/B 4---comes in quickly and sometimes savagely, can cause a nose drop if you're not ready for it. used on short runways
A/B max--- never used it, but it would be a step up from 4 no doubt
A/B RTO--- never experienced it thank god ( apart from the sim ) Comes in rapidly and savagely and is hard to disconnect using the toe brakes ( actually I think it can't be using them as the pressure required is too high for lil ol me )

What do I consider a short rwy? Well anything less than 7000' probably requires A/B 4 depending on your wt and rwy conditions.

Jfk 04R/22L.....Bom 27 at max ldg wt .........Fuk 16..........are those that need A/B 4

Bom 14/32 needs A/B max.

Most others in the CX network are long enough to only need A/B 3

disclaimer: the above depend on wt, wind rwy condition and 200 or 300 or 300ER type 777

[safetypee]

Nightfire, I don’t know the answer to your 777 auto-brake question, but I would like to understand the system better. I have found that in aviation, knowledge is a priceless commodity.

In some aircraft types a time delay is used with low auto-brake settings to maximise the effect of reverse at high speed by giving it priority.
If applicable to the 777, it might be important information when landing without reverse thrust or on a normal length runway which, due to runway conditions, becomes limiting i.e. ‘short’. As such this knowledge would be a vital component of judgement, adding to the process of experience.

Ref / info: FSF ALAR Tool Kit Briefing Notes Chap 8.4. Braking Devices. (www.flightsafety.org/alar/alar_bn8-4-braking.pdf)

For those who believe that they do not need to know this type of information, then consider that all of your experience might only be as good as the way in which you use it.

[ACMS]

yes exactly.............that judgement comes with exposure and experience on type. Knowing the exact delays in the A/B system might be "nice to know" but it's not necessary.

And I've never come across anyone in 25 years of Airline ops on the 737, 747, 777 and indeed the Airbus family of jets that can tell you these numbers. We know what our Jets are capable of and we've seen it under all sorts of different conditions.

Sorry to be direct but you need to get a life.

Save your brain cells for something more important, like remembering which bar has the nicest birds on ur overnights.:ok:

[Spooky 2]

One more little item worth consideration. Autobrakes 2 or 3 optimizes brake wear, passenger comfort and stopping perfromance.

[Green-dot]

Nightfire:

All I'm wondering about is a time-delay in the system, from touchdown until autobrake engagement. Does it exist, and if so, how long is it?

For autobrake application, the brake system control unit (BSCU) uses a 0.1 second time delay after ground signal when switch position 1 through MAX AUTO is selected.

Regards,
Green-dot

[fullforward]

A simple receipt for stopping a 777:
if the runways is long and dry, use NO AUTOBRAKES AT ALL!
You´d be surprised how effective reverse thrust is on this acft, you go on full reverse down to 90 kt when you step lightly/firmly on the brakes, keep rev till 60 kt.
You´ll save tires and brakes (you´ll see how cold they remain) and fuel spent is worth less than 30 USD...
Otherwise, BRK 2 or 3 do the job.
Just my two cents. (of course there are stupid sops from the geniuses telling otherwise)

[BuzzBox]

You´ll save tires and brakes (you´ll see how cold they remain) and fuel spent is worth less than 30 USD...

If my understanding is correct, the temperature has stuff all to do with the brake wear on aircraft with carbon brakes. It's related to the number of brake applications, not the braking force, and they are actually more effective at higher temperatures.

I dare say the extra maintenance costs incurred by using max reverse on every landing would very quickly outweigh any savings gained by going easy on the brakes.

[FullWings]

For the record, there is a greater deceleration that can be used other than RTO: max manual braking. Check up on on RTO procedures in QRH.
I think you might be confusing landing with takeoff... The QRH figures show that MAX MAN gives a shorter landing run than MAX AUTO. This is backed up by: "...on dry runways, the maximum autobrake deceleration rate in the landing mode is less than that produced by full pedal braking...".

The only reference I can find to RTO performance says: "Maximum braking is obtained in this mode", i.e. you won't get anything better by standing on the pedals.

From a purely practical point-of-view, I find it much easier to let the RTO autobrake do its thing which gives me more of a chance of keeping the aircraft on the runway. In a limiting crosswind, keeping full pressure on both pedals (that's quite a bit...) and significantly varying the rudder deflection is no easy task.

[Nightfire]

Thankyou, guys! :ok: The value of 0.1 sec was what I wanted to know! The rest to be known about the system is written in the FCOM.

A simple receipt for stopping a 777:
if the runways is long and dry, use NO AUTOBRAKES AT ALL!
You´d be surprised how effective reverse thrust is on this acft, you go on full reverse down to 90 kt when you step lightly/firmly on the brakes, keep rev till 60 kt.
You´ll save tires and brakes (you´ll see how cold they remain) and fuel spent is worth less than 30 USD...
Otherwise, BRK 2 or 3 do the job.
Just my two cents. (of course there are stupid sops from the geniuses telling otherwise)

I disagree to that, fullforward. It is not just the fuel you're concerned about, but also the maintenance and engine life.
The carbon-brakes of the 777 don't need to stay cool. The best way to use them is by using an autobrake-setting which is not too low or too high, and using idle-reverse or just a little bit more than idle.

But that's a different discussion.

[kijangnim]

Greeting,
as far as I am concerned even if the runway is 18000 ft long I still use autobrakes to minimum (Low of airbus) considering that if brakes are faulty I will know at an early stage, rather when it is too late :}

[safetypee]

kijangnim, I agree with the principle of using brakes, but the logic in detecting brake faults might be flawed.
One of the weaknesses of auto-brake is that the lack of “feet-to-seat” (brake application to deceleration) feedback; this degrades the ability to assess a system’s effectiveness in comparison to manual brake application.
With auto-brake, judgement of satisfactory performance is made on deceleration alone, but in many circumstances the brake system is not providing the majority, or any deceleration at all. This is particularly important on slippery runways where for other reasons the brakes may not very effective; this has been discussed in the thread http://www.pprune.org/forums/rumours-news/329140-taca-aircraft-crashed-honduras.html.

During the initial landing roll at high speed with low auto-brake setting and reverse thrust, and providing runway conditions permit, the auto-brake desired deceleration can be fulfilled by reverse and aerodynamic drag alone, i.e. no brakes applied. As speed decreases so do the effects of reverse and aerodynamic braking, then the auto-brake applies brakes to maintain the desired level of deceleration. This is illustrated in ‘Braking Devices’ - Fig 3 (www.flightsafety.org/alar/alar_bn8-4-braking.pdf). The brakes are only applied as the reverse decreases below 80 kts, which might be too late to take remedial action.

I don’t believe that this is a big issue for brake faults, particularly as brake system’s reliability (and back up) are far superior when compared to the problems of human performance in detecting less than adequate deceleration (from any means) and the judgement of runway conditions leading to the choice of the auto-brake setting or manual brakes in adverse conditions prior to the landing.

The above suggests that if there are doubts about the level deceleration which can be achieved on the runway – possible low friction but you decide to land, then manual braking (auto-brake off) might be a better option. Good judgement might choose to use ‘half power’ manual brakes, assess the situation and then apply maximum brakes as required. However, if you have already suffered poor judgement in runway assessment, then all this might do is minimise the speed during the overrun.

[fullforward]

If you read what Mr Boeing say on FCTM you´d realize they strongly suggest to use max rev thrust (wich is actually less than 70% of max TO) as the most efficient way to stop our bird...and they are the most interested in show the best way to operate the B777!
No discussion as well that the best way to save brakes is not using it, or using at the minimum.
Just a thought!:ok:

[BuzzBox]

If you read what Mr Boeing say on FCTM you´d realize they strongly suggest to use max rev thrust (wich is actually less than 70% of max TO) as the most efficient way to stop our bird...and they are the most interested in show the best way to operate the B777!

No discussion as well that the best way to save brakes is not using it, or using at the minimum.

Well, my copy of the 777 FCTM has the following to say wrt to carbon brake life:

"Brake wear is primarily dependent upon the number of brake applications. For example, one firm brake application causes less wear than several light applications. Continuous light applications of the brakes to keep the aircraft from accelerating over a long period of time (riding the brakes) to maintain a constant taxi speed produces more wear than proper brake application.

During taxi, proper braking should involve applying brakes to decelerate the aircraft, releasing the brakes when the lower speed is attained and allowing the aircraft to accelerate, then repeating.

During landing, one hard, high energy, long-duration brake application produces the same amount of wear as a light, low-energy, short application.This is in contrast to steel brakes which wear as a function of the energy input during the stop.

For normal landing conditions, autobrakes 2 or 3 optimizes brake wear, passenger comfort, and stopping performance. Since autobrake settings apply the brakes dependant upon the deceleration rate, an autobrake setting of 1 will result in a higher probability that the autobrakes will modulate, especially when the reversers are used. Autobrakes 2 or 3 results in a continuous brake application, which can increase carbon brake life."

[fullforward]

Mine says exactly the same.
Now go to what the manufacurer says about reverse thrust usage...read all.

[BuzzBox]

Now go to what the manufacurer says about reverse thrust usage...read all.

I have 'read all' thanks very much. The FCTM states the following:

Full reverse thrust should be used when stopping distance is limited or when runway conditions significantly affect braking action. Idle reverse thrust may be used when conditions permit.


The FCTM makes lots of other statements regarding brake temperature, brake life, etc, some of which are contradictory. However, nowhere does it recommend the technique you described.

[kijangnim]

Greetings,

The Airbus FCTM, adresses the brakes wear, efficiency, and temperature :ok: stating that max brake temp before takeoff must be max 300 deg C (explaining that it is a protection against igniting hydraulic fluid that might leak) then explains that brake wear is minimum and efficiency is best when brakes are hot 9above 150 deg C), but further Airbus specifies that if the brakes are 300 deg C, then a reject takeoff may not be successfull if high energy is required :confused:
Furthermore loss of braking is a memory item meaning he brakes are not that flawless :E
BTW I know that the subject was B777 :}