Brake To Vacate
 

Hi there,

Brake To Vacate; is the deceleration linear or does it vary depending on factors such as velocity/lateral acceleration/runway conditions etc?

Thanks,

BD

If you achieve the required exit, does it matter?

BoeingDriver99 asked a simple question and you chose to be less than helpful. Why were you prepared to expend time being unhelpful, when writing nothing at all would have saved you some effort? 🙄

K-13, if you do operate an aircraft that has a brake-to-vacate system then please consider redeeming yourself and providing BoeingDriver99 a useful answer?

BoeingDriver99, I cannot help as I have never flown an aircraft with brake-to-vacate, my last type having been a Boeing with their standard Autobrake system.

For we who do not fly big jets ..

I will be checking with a man who does - for my own interest

Apparently not a linear deceleration.

"Brake-to-vacate combines satellite positioning with the aircraft's on-board airport database and flight-control systems to provide crews with braking distances to their preferred runway exit.
Once the pilot has selected a runway exit point, the system manages the deceleration - taking into account runway surface - in order to ensure the jet reaches the exit at the correct vacate speed.
… the system takes into account several elements - not just the aircraft's speed and position but also the temperature, wind and runway elevation."

https://www.flightglobal.com/easa-cl.../89832.article

https://skybrary.aero/sites/default/...helf/33873.pdf

Related: https://safetyfirst.airbus.com/app/t...ion-system.pdf

I assume ATC may also have some input

I actually think K13 was very helpful by reminding Boeingdriver99 to not worry too excessively about the trees lest he miss the forest. Especially if he (Boeingdriver99) does not fly a BTV-equipped aircraft.

Yes, we need lots more responses telling posters not to worry about stuff ...

I mean...there are ways of doing it, but yes, we do.

Yes, heavens forbid someone actually wants to expand their knowledge!

What has happened to the spirit of this forum?

Posted previously; the linked paper identifies one of the challenges which the industry faces.
https://thefederalist.com/2014/01/17...-of-expertise/

'Brake to Vacate' is an interesting subsystem interwoven with overrun prevention alerting (ROPS) and autobrake. The operation of the latter, often misunderstood, can contribute to a leading safety issue - overrun / hull loss.
https://www.dropbox.com/scl/fi/i788o...=j9em5bqn&dl=0

"If the goal is learning (and it should be) then using a method of retrospective learning should be confident in how it’s bringing to light data that can be turned into actionable information."
Start by asking 'How', and continue to do so.
https://www.oreilly.com/radar/the-infinite-hows/

Not to be too ignorant but isn’t this the million dollar ballpoint vs the lead pencil?
I’ve never flown Airbus and on Boeing it’s Autobrake 0-1-2-3-4-Max.
When you receive your landing performance select A/B setting based on LDA/brake temp prediction/desired exit.
When you desire another exit you use another brake setting.
Can someone explain why the million dollar solution is so much better then….pilotage?

From the links posted earlier:

So essentially it's about £€$, as are most things.

What spirit would that be?

"… the deceleration in the Brake to Vacate (BTV) system is not strictly linear."

See Comparison table with Autobrake

Advantages of the system in the text; even without considering the dated view of human performance and limitations favoured by Boeing, FAA, and USA.

https://chatgpt.com/share/67d5ba9a-4...9-1486611de0bf

Do we really need an AI answer? They're trash. Go ask if the 737 has a RAT, or which large aircraft don't.

It's pretty clear that BTV isn't going to be safer than using max autobrake/max manual every time, in much the same way that using TOGA thrust is going to give the best safety on takeoff (barring some exceptions around Vmc), or taking off with full fuel tanks even if not necessary.

However, acceptable safety can be achieved with a system that brings a bunch of maintenance, comfort, and/or throughput advantages.

The 'obvious' way to implement it would be to build a 'target' deceleration profile that gets you to the desired speed in the desired position, and modulate the brakes to achieve that profile, with feedback. That's not too different to what autobrake does, just the target is computer controlled rather than a fixed pre-set rate. Deceleration is obviously never going to perfectly match the target rate so there's going to be some hunting; even autobrake won't be perfectly linear. Achieving 'good enough' is the name of the game in these types of automation problems.

OP's question, to me, is about how the 'target' profile is constructed and whether it's updated continuously or only on touchdown. I have no clue, but I can throw some guesses out there:

1. Front-load the deceleration - decelerate early, then taper off as taxi speed is approached. This delivers the most margin for error but leads to the most brake wear and taxi time. It's basically what autobraking until the pilots cancel it delivers.
2. Linear deceleration - constant deceleration rate from touchdown to just before turn-off. You'll need to recalculate deceleration rates as the touchdown point moves. Basically just 'selecting the perfect autobrake setting'.
3. Back-load the deceleration - maintain a high speed relying mostly on reversers/spoilers until the last moment, then brake hard. This delivers the fastest turn-off time (because you're covering the distance at the highest speed) and puts the least energy into the brakes (because the reversers/spoilers have the most time at high speed). Highest risk of overrun because you're waiting until the last moment to brake.

I expect that #3 most closely matches reality.

There are some technical safeguards that could be added:

• Allow more margin for error (lower assumed deceleration rate or achieve taxi speed with more remaining distance) for full-length landings than intersection turn-offs. It doesn't really matter if you overshoot say 0.1% of landings by 100m/300ft if you're targetting a specific taxiway, but that's a major issue if you're approaching the runway end. IMHO it's a near certainty that this is included.
• Test the brakes/braking action on initial touchdown to check that the runway surface is good enough to achieve the required deceleration rate, then return to the target profile. If the surface isn't good enough (e.g. hydroplaning on what is selected as a dry runway, adjust braking profile to be earlier and lighter, perhaps ask pilots for full reverse.

On the A350, the deceleration varies according to the runway condition. The system has two modes: BTV on dry or wet runways, and BTV CONTAM on contaminated runways. BTV mode ramps up the braking after nose wheel touchdown, to achieve a maximum deceleration of 0.2 g (0.35 g in the event of a missed exit scenario). BTV mode aims to achieve a groundspeed of 10 knots about 60 m from the selected exit and may delay brake application if the exit is a long way down the runway, to minimise runway occupancy time. BTV CONTAM mode does not delay brake application and ramps up the braking to a maximum of 0.3 g (0.35 g for a missed exit). The maximum deceleration may not be reached if the antiskid activates. BTV CONTAM mode does not take into account the distance from the aircraft to the selected exit, and releases the brakes when the groundspeed is approx. 10 knots.

I'm slightly confused; if BTV CONTAM doesn't take into account the distance to the selected exit, how is it any different to standard autobrake?

The rest fits pretty well with what I surmised.

BTV CONTAM applies the brakes immediately, slows the aircraft to 10 knots, then releases the brakes. The basic auto brake mode (BRK MED) decelerates the aircraft at the same rate as BTV CONTAM (0.3 g), but it does not release the brakes at 10 knots and does not increase the braking to 0.35 g (subject to anti skid) in a missed exit scenario.

I previously said that braking occurs after nose wheel touchdown, but that's not quite right. The BTV or BTV CONTAM mode engages immediately after ground spoilers extension if the nose wheel is on the ground, or 5 sec after ground spoilers extension if the nose wheel is not on the ground. In BTV mode, brake application may be delayed, as previously described.

The main advantage for me is reduced runway occupancy time. Normal auto brake requires either an exit that matches the deceleration profile, or an acceptance of the occasional missed exit (or coming up dramatically short). BTV is weird to experience because you can go rocketing down the runway with no braking, and then the equivalent of about medium autobrake (probably equivalent to 4 on a Boeing) cutting in pretty late.

I like it, but I like the use of the instinctive disconnect on the thrust levers to knock our the auto brake even more.

The use and value of 'AI' can be debated elsewhere, but it is now opportune to engage with a thinking tool.
For subjects which can be fact checked - systems, books; a ChatGPT (Large Language Model) summary based on a wide source of information is more likely correct - law of large numbers, preferable to limited and variable human opinion. Consider the number of factually contributory posts vs the irrelevant ones in this thread.
Referencing ChatGPT is considerate, opposed to copy and paste without acknowledgement.

The glib conclusion of 'trash' lacks supporting evidence or reference.

Similarly 'the clear safety view', lacks reference. Airbus documents explain the purpose of BTV, not safety re landing overruns.

Additionally, proposing design improvement 'without a clue and based on a guess', is a typical misunderstanding of complex systems - man, machine, environment.
Ackoff or Chapman have described the human aspect as 'those who propose solutions, are part of the issue'.
----

ChatGPT stated that BTV 'automatically controls deceleration using braking and reversers', 'considers reverser use dynamically', which implies thrust modulation; thus for clarity, further research:-
'Does the BTV system directly control the thrust reversers':
Response, 'No, the Brake to Vacate (BTV) system does not directly control the thrust reversers. However, it takes into account the reversers’ effect on deceleration and adapts braking accordingly.'

I do not agree with the ChatGPT conclusion inference ( my inference ) that the BTV (alone) reduces the risk of overrun, but if considered together with ROPS, then the latter is a valuable system. Possibly a weakness in language and context, but no more than in some posts.

This seems like one of the more straightforward and reported questions that can be asked:

https://cimg9.ibsrv.net/gimg/pprune....2066359422.png


Some of the statements are correct. In all but one case (MD-80/MD-90), the reference to specific aircraft types is incorrect.

It does not 'conclude' - it is not a thinking machine. It is just a text generator that fills in the most statistically likely word. That's why, when you ask it 'does [transport category aircraft] have a RAT' it shamelessly returns 'yes' because statistically the most likely answer to appear there is 'yes' because more aircraft have them than not. It's that simple. It seems very sophisticated but it takes only the slightest experimentation to realise that it's ultimately useless for anything connected to reality and facts because it just predicts words without having any understanding of the physical reality.

Do not be fooled by the marketing!

Please get this thread cleaned up...

An interesting discussion. I fly the 737 so was not aware that Airbus has a clever computer to calculate the best autobrake setting for a particular runway exit. In the age of criticism of pilots for being children of the magenta line, it seems counter-intuitive to keep adding automation that is either insufficiently thorough in terms of the factors it considers or takes so much thinking away from the pilot that, in the absence of an operating BTV system, they would struggle to make a sound brake to vacate decision.

We adhere to the brake to vacate mantra at my (very large) airline but it relies on a pilot considering flap, brake and brake cooling factors to achieve a safe, controlled exit at the speed appropriate for the RET or 90 degree exit in the prevailing conditions. A good decision should also consider passenger comfort in my opinion. The more we add automation the less pilots will understand how all these factors interact.

Just my two cents worth.

Given the number of aircraft including many many 737s that have had overruns, I'm not sure that the pre-BTV situation is that awesome.

Seems to me that the situation is ripe for automation: under normal conditions, it works fine. It's well-positioned to run effective self checks. If the self checks fail, it can either command maximum autobrake or hand over directly to the crew. If you get enough systems failures that it fails, there's nothing wrong with doing maximum braking to a stop if necessary.

ROWS/ROPS is intended to catch situations where pilots are landing long or not applying enough braking and thus predicted to be running out of runway, and tell them to go around or brake harder as the case may be. BTV is pretty minimal extension of that.

It's been in service more than a decade now. Anyone know of any relevant incidents on ROWS/ROPS/BTV aircraft?

Thanks for all the useful responses. As for the others; if it floats your boat then keep on floating…

I am capable of the following:

A) stopping any vehicle I operate comfortably and safely.

B) Learning more about technical things that interest me.

And… you may not believe this because you may not be capable of it… I can actually do BOTH at the same time! :ok:

The world wonders :ugh:

BD

Belief in safety
 

And … believe it or not, … experts in human behaviour and limitations would say that you are seriously mistaken.

Are you a pilot? Your responses seem more engineering focussed - which obviously has it’s place, however you seem quick to want to take the human out of the equation - which is unusual for a professional pilot.

I've used BTV on the A350-1000 and it works brilliantly. Slightly unnervingly it doesn't always start braking immediately but does it in a fantastically controlled way that brings you to 10kts at the point you have selected as your turn off. You can of course override it at any point. I fell in love with it very quickly, great for managing brake temps.

It's sole reason for existing is to reduce runway occupancy time. In fact on the 350 the ND will show you approx runway occupancy time when selecting the exit. As such the aircraft will brake as late as possible given it's programmed g load limits. Pilots new on fleet usually will be a little worried, disconnect early and brake manually, but once comfortable it's best to let it do it's job until reaching a speed that is suitable for the given exit. If you watch the brake pressure 'gauge' what you will notice in most cases is that after nose wheel touchdown one will see slight brake pressure (unless the exit is far in which case braking starts later), after which the pressure keeps increasing as one gets closer to the exit untill the aircraft reaches the BTV disconnect speed.

Hm, that sounds like the opposite of what I would do for pax comfort... In my agricultural Boeing I try to keep a constant deceleration until reaching 40-50 at the beginning of the high speed exit - when deadheading I hate when people clearly set an autobrake setting that's too low to make it, and then step on it hard to try and vacate using the first exit.

Braking as late as possible gets you:

• Highest average speed and therefore shortest time on runway, clearing the runway for the next arrival/departure ASAP (as noted above)
• Maximum energy lost to reverse thrust, drag, spoilers etc.
  • Therefore, minimum energy absorbed by the brakes and hopefully minimum brake wear - carbon makes that a bit complex.
• Minimum stopping margin, because you're assuming you have full expected braking performance right when you need it, and any shortage will cause an overrun (past desired exit, not necessarily off the runway)

My understanding is this is one of the main ways that automatic train operation and moving-block signalling lets you fit more trains in: computers are better at doing the braking maths than humans, so can operate with less safety margin but still be safer overall.

I find that surprising. Airlines don't care much about runway occupancy. They do, however, care a lot about brake wear and money. As for brake cooling, how often does that matter in the slightest with longhaul flying with their lengthy turnarounds? Even with shorthaul airlines with 25 minute turnarounds brake cooling is rarely of practical consequence.

Incidentally, a competent pilot can achieve the same thing (brake later and then at a moderate, comfortable rate, to achieve the exit at the chosen point, thereby remaining in the loop throughout.

BTV is perfectly compatible with minimising brake wear, as previous posts have explained.

Until the day that they can't.
Like every competent pilot can stop within the expected landing distance, but some don't.

ATC and the airport operator are interested in runway occupancy, traffic flow, density, revenue; thence back to the operator.

Yes and no. Reducing ROT means getting able to fit more arrivals and/or departures in, leading to increased runway utilization and possibly the availability of more slots into crowded airports. Airlines and the industry on a whole prefer this.

It's not necessarily the case the BTV increases brake wear. Carbon brakes do not wear based on pressure applied but on the number of applications, so a single constant application of increasing force shouldn't be any different to any other application.

Usually with a regular autobrake pilots will disengage during the landing role and then reapply brakes a second time manually, so that's two applications per landing vs one with BTV.

The benefit with BTV is that there is less braking during the high speed phase of the landing role so the reversers and drag do the majority of the work as the deceleration rate increases towards the end of the landing. This actually leads to lower brake temperatures in most cases especially if using a high speed exit as BTV can be disconnected early and the highest planned brake pressure is not even reached.

What puzzles me is the punters who select a deceleration rate about 45 minutes before and almost immediately on touch down stamp the foot things resulting in a jerk, immediately release, wait for a bit and then smash both clompers in some sort of uncoordinated way to achieve a “safe” but ##### exit from the runway…

Still learning what’s new; still stopping where I should…

BD

I didn't say they don't care. I said they don't care much.

I rarely use autobrakes at all, make the desired exit, manage the brake temp and company says we do not have any brake issues. It is possible! Aircraft have become an engineering playground. Auto TCAS, Auto Emergency Descent, everything Auto...

SLF and retired Software Engineer.

... and also consider the spate of "daft - but sounds authoritative" responses to some questions.

"You Can’t Lick a Badger Twice": Google's AI Is Making Up Explanations for Nonexistent Folksy Sayings
https://futurism.com/google-ai-overviews-fake-idioms

... and a host of follow-up examples involving non-existent idioms. I think that the AI's interface has been changed to prevent answering similar questions, but IMHO it exposes fundamental flaws in large-language model applications.

PS I don't like the term Artificial Intelligence , mainly because it covers too wide a range of algorithms. But some "AI" software -- e.g. screening medical scans for cancer, etc -- seems significantly better than a human (let alone a human reviewing scan after scan).

I still fondly remember the occasional use of "Artificial Intelligentsia" in some technical journals back in the 60's and 70s. Referring to those producing the overblown marketing hype around the term AI.