Stopping! All about brakes and BTV – Brake To Vacate

I hope you enjoyed the first blog post. Thank you to all those who sent feedback!

Operating an aircraft isn’t all about what goes on in the air. Stopping is just as important as going. Now, before we start considering this, a warning…. This is a fairly long piece! So go and make a coffee, get comfy, and we will be fine. Alternatively, read this in bed if you can’t sleep and be prepared for the shock when whichever device you are reading it on hits you in the face when you doze off!

Contrary to a fairly common belief, it is the wheel braking system which provides most of the retardation for aircraft. What about reverse thrust? Well, the thrust reversers on the engines are there mostly to help the wheel braking systems but are not the primary source of braking. Indeed, Airbus originally planned for the A380 to not have any engine reverse thrust. In the final version of the aircraft which went into service, reverse thrust is only available on the inboard engines.

So our main source of braking is wheel braking. But which wheels are fitted with brakes? Not the nosewheels, they are used exclusively for steering. And what may surprise some people is that not all the main wheels on the A380 are fitted with brakes. The most rearward wheels on the main undercarriage are just rolling wheels. You will see in the photo below that the wheels on the right are much cleaner due to lack of brake dust.

Big wheels!
Big wheels! (Captain @Colin__Dick and I inspecting the undercarriage)

Braking is normally carried out using an autobrake function. This also includes a rejected takeoff (RTO) function which applies maximum braking if the thrust levers are closed once above 72 knots during the takeoff roll. Many aircraft have an autobrake system. These are normally armed during the approach and landing briefing, which is typically carried out just prior to starting the descent. Most autobrake systems are set using either a numbered system, where higher numbers giving a greater braking force, or a descriptive system, such as that used on the A320 series, where either LOW or MEDIUM braking will be selected for landing.

Airbus introduced a revolutionary (sorry!) new system on the A380 called Brake To Vacate (BTV). It is optional on the A380, but fitted as standard on the A350. This advanced system allows the landing pilot to pre-select the runway exit they wish to take, and the aircraft will apply automatic braking as appropriate to allow this to happen in the minimum time. The system is quite complex, but works exceptionally well. We use this system for virtually every landing in British Airways as it allows us to pre-plan our exit from the runway, reduces brake wear, and is very comfortable for our passengers.

So how do we use BTV? Firstly we set up the aircraft systems for our expected landing runway. In this example, we will use runway 24R at LAX. Initially we need to look up the airfield and the specific runway on our LIDO airfield charts. You will remember from the previous blog that we have to make sure we only use those runway exits and taxiways which are colour-coded green, ie. suitable for the A380. Here is the runway exit chart for LAX runway 24R.

FullSizeRender 4

So what is this showing us? Runway 24R is the runway at the top of the screen. You can actually only see the label for runway 06L here, but that is the same runway from the other end! Do you know how the numbering system works for runways? If you do, skip to the next paragraph. If not, it is actually quite simple. The two numbers are the first two numbers of the compass heading along which you will be pointing when looking down the runway. So in this case, 24 means the runway is set at about 240 degrees on a compass. Runway 18 would be pointing south, runway 09 pointing east etc. Runway 09 would also be runway 27 if you were using it in the opposite direction. If there is a letter after the numbers it means the airport has more than one runway pointing in the same direction so they are differentiated by R for right, L for left, and C for centre. I hope that is clear!

Just to the right of the 06L label on the chart you will see a green runway exit labelled AA. This is the usual exit we use at LAX when landing on 24R (The chart is orientated to the north, so when landing on runway 24R we will be moving from right to left down the runway as shown on this chart). The chart also clearly shows the green colour-coded taxiways the A380 is able to use. Anywhere not coloured green is out-of-bounds for the A380. So, for example, we could not use exit Z, the one to the right of AA. You will see on runway 24R itself there is some writing – 2721 G 46. This indicates the runway is 2721m long, has a grooved surface, and is 46m wide. 46m is a typical runway width. Some are 60m wide. Interesting to think the wingspan of the A380 is almost 80m and the distance between the outboard engines is 51.4m!

With runway 24R being 2721m long, a typical length, that should be more than sufficient for us to land on. But let’s make sure! For that we use the Landing Performance app installed on our Onboard Information Terminal (OIT). Below is the calculation performed for our flight. (Note that since I took the photos of the landing performance app and OANS displayed below, the runway length at LAX has increased by 1m! Thought I’d better get that in before the eagle-eyed among you did!)

Landing performance calculation
Landing performance calculation

Lots of numbers here! To summarise, on the left we enter the landing conditions and aircraft configuration. Top, centre, we select an available runway. We then press the ‘compute’ button, and after a few seconds the information in the ‘Results’ window appears. In this case the results show that for landing on runway 24R at 344.4 tonnes we should use FLAPS FULL, our landing distance LD will be 1656m. This is the minimum landing distance required using the autobrake setting we have selected, in this case, Lo braking. We use Lo braking as an indicator to start with as this is the most comfortable braking from a passenger point of view. This distance then has a 15% increment added to allow for handling and other variations on the day (the 1656m figure is that calculated as the best which would be achieved by the Airbus Test Pilots!), giving a Factored Landing Distance of 1984m.

The Stop Margin is how much of the runway will be left when the aircraft comes to a halt. GA Gradient is concerned with aircraft performance in the event of a go-around, so is not something we are considering at the moment. Finally, the figure towards the bottom right, VAPP, is the final approach speed, 132 knots in this case. Despite the A380 being such a massive aircraft, the landing speeds are comparable to A320s and the like.

We have determined we need 1984m of runway to stop the aircraft. We know from our LIDO chart that runway 24R is 2721m long, so that is fine. Now we need to determine which exits we are able to make. So lets display our OANS – the Onboard Airport Navigation System. This is effectively a ground-based satnav for the aircraft. It can either display our actual position on an airfield, or we can look up any airfield in the database for planning purposes.

Select the landing runway
Select the landing runway

In this diagram we have selected runway 24R, as indicated by the numbers being shown in blue. The OANS then displays the information it contains about the runway. Remember the 2721m runway length shown on the LIDO chart? OANS shows 2720m. This is one of our crosschecks. If the LIDO chart and OANS disagree by more than 35m, we are not allowed to use BTV. But here we are fine.

You will see two labelled magenta lines drawn on runway 24R. WET and DRY. You will not be surprised to hear that these lines show the position BTV braking has calculated it can stop the aircraft on a wet or dry runway. If possible, even on a dry runway, we would select a runway exit Β beyond the wet line. Again, this is mainly due to passenger comfort, but also means the braking system isn’t working hard. Right in the centre of the above OANS display you will see a magenta up arrow with a magenta down arrow directly above it, and a small magenta dot in between. This is the trackball cursor. We would now move this over the top of exit AA, shown to the left of the screen, and beyond the wet line, and select this exit. This results in the display changing as shown below.

Exit AA now selected
Exit AA now selected

Exit AA is now shown in blue, and the display at Β the top left shows EXIT AA 2145m, ROT 80″, TURNAROUND 100’/120′. So, exit AA is 2145m along the runway. This is more than the 1984m we calculated we would need earlier using Lo autobrake, so that is fine, and reconfirms the calculated lines BTV drew on the OANS. ROT stands for Runway Occupancy Time. In this case, BTV has calculated this to be 80 seconds for us to vacate the runway at exit AA. TURNAROUND is the time in minutes it will take for the brakes to have cooled below 150C, which we would need before performing another takeoff. There are two numbers. The lower one is the time if we use maximum reverse thrust on landing, the higher number is for reverse idle.

The last thing to do is arm the system using the autobrake knob.

Now all we have to do is land the aircraft in the right place at the right speed and BTV will control the deceleration for us. It is a superb system. It can be a little unnerving the first few times you use it! This is because it constantly monitors the aircraft speed and position on the runway and, unless you have asked BTV to enable you to vacate at a limiting exit, only applies a noticeable amount of braking fairly late on in the landing roll. This allows the aircraft to naturally decelerate after landing using air braking and reverse thrust, so minimising the work the brake system has to do.

Once it has initiated braking, BTV targets a constant (passenger friendly) deceleration rate to achieve a speed of 10 knots, 65 metres from the selected runway exit. However, if the exit chosen is within 300 metres of the runway end, this changes to a target of 10 knots at 300 metres from the end.

All sounds nice and rosy so far doesn’t it? So what happens if we land further down the runway than we planned and BTV calculates we cannot stop by the selected exit? Firstly, don’t land too far down the runway in an A380! (or any aircraft for that matter). Better to throw away a poor approach and do it again than try to make the best of a bad job. However, if the landing is only slightly beyond the normal landing point and a limiting exit has been chosen, or conditions on the ground dictate that the original exit now cannot be achieved because the deceleration rate is not what was expected, what happens next? Firstly, remember the best piece of advice ever given in The Hitchhiker’s Guide to the Galaxy. Don’t Panic!

On landing, the wet and dry lines displayed on the OANS are replaced by a single green STOP line. This shows where the braking system believes the aircraft will stop. It is constantly updated during the landing roll. If the green stop line goes past the BTV selected runway exit it turns amber, as does the label for the selected exit, a ‘triple click’ sound is heard, and EXIT MISSED is displayed. This is not too much of a problem in this case, as there is still sufficient runway to stop the aircraft, just not to vacate it at the point initially selected. However, what happens if the green line goes past the end of the runway?

This would activate the ROW/ROP – Runway Overrun Warning / Runway Overrun Protection systems. These are quite brilliant systems which are worthy of a blog all of their own. And if you have made it this far through this one, you are probably ready for a rest now! So we will cover those at a later date when we have gone through how to operate the A380 when everything is working well, to how we deal with situations where things aren’t going quite so sweetly….!

I hope the above has helped you understand how the superb BTV function works and enables us to bring the aircraft down to taxi speed in the most comfortable and efficient way possible. If there is anything which is not clear, or you have any other questions, please let me know via @DaveWallsworth on twitter and I will do my best!

If you enjoyed reading this and the previous blog, please pass on a link to anyone you think may be interested, and let me know what other aspects of flying the A380 interest you.

Best wishes, and happy flying.

Dave