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.