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BRAKE-TO-VACATE SYSTEM - THE SMART AUTOMATIC BRAKING SYSTEM

Fabrice VILLAUMÉMulti-Programme Project Leader

Airbus Systems Engineering

The Brake-To-Vacate (BTV) is an Airbusinnovation in pilot aid to ease airport congestionand improve runway turnaround time. The BTVsystem, which will be available on the A380(2009) and A320 Family (2012/2013) as an optionand on A350XWB basically, helps reducingtaxiing time at busy airports by optimizing therunway occupancy time and lowering brakingenergy while maximizing passenger comfort.The BTV system, which is designed by a multi-disciplinary team (avionics, flight controls

and auto-flight, landing gear, flight tests, aircraftperformance and human factors) under the scopeof a multi-programme project, allows pilots toselect the appropriate runway exit during descentor approach preparation. The Airbus-patentedinnovative system uses the GPS (GlobalPositioning System), Airport Navigation, Auto-Flight and Auto-Brake Systems to regulatedeceleration, enabling the aircraft to reach anychosen exit at the correct speed in optimumconditions.

Brake-to-VacatesystemThe smart automatic braking systemfor enhanced surface operations

BRAKE-TO-VACATE SYSTEM - THE SMART AUTOMATIC BRAKING SYSTEMFA

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The project began in 1998 viaa PhD thesis. Following a feasi-bility phase until 2001, it com-pleted a prototype phase on anA340-600 prototype in 2006,with a first test flight in April 2004under various operational condi-tions. These preliminary demons-trations were performed within aresearch framework. A successfulflight test in March 2005 has beenperformed in real-time conditionsat Charles de Gaulle Airport inParis. The industrialization beganon A380 in October 2006 with afirst test flight in May 2008.

To better understand what is theBTV system, this article will high-light the current use of the alreadyexisting auto-brake system and itsimpact on aircraft operations, theevolving airport operation contextand will detail the Airbus BTVoperational answer.

The automaticbraking systemat landing:Historicalperspectiveof evolutionAutomatic braking system, alsocalled auto-brake system, is a typeof automatic wheel-based hydrau-lic brake system for advancedairplanes (Airbus, Boeing, etc.). Inorder to keep the pilot free toperform other tasks, the auto-brakehas been designed to control,robustly and rustically, aircraftlongitudinal deceleration duringrollout and down to full stop.The automatic deceleration controlroughly starts at the nose landinggear impact with an onset transitionramp for comfort.

Since the A300-600 model, auto-brake modes are selectable forlanding using either LO and MED,which provide low and mediumfixed deceleration control. In orderto give more operational flexibility,A340-500/600 models are fitted

Final approach on LFBO 32L runwaywith S10 exit selection for BTV

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with an auto-brake system enri-ched by five fixed decelerationmodes through a new rotaryswitch: LO, 2, 3, 4 and HI (seefigure 1). The A380 is fitted witha four-modes auto-brake system(LO, 2, 3 and HI).

In everyday operations, analysisshows that the auto-brake systemcannot be adapted to each landingsituation, which has specific touch-down characteristics (position andspeed) with respect to the desirableexit taxiway foreseen by the crew(type, position and speed).

The use of the auto-brake system isrecommended when the pilot’sworkload is high, and has becomesince the year 2000, Airbus recom-mended Standard Operating Proce-dures (SOP) at landing, but thereare some drawbacks:• Firstly, onset nose high on somemodels would be stronger thanthe pilot would wish,

• Secondly, brake pedals overrideincludes discomfort, as the pilothas frequently to brake more...to brake less, causing frequentassymetric braking.

Since the A380 models, a smoothand symetric system has beenintroduced by allowing auto-brake system disconnectionthrough the Auto-Thrust Ins-tinctive Disconnection buttonlocated on the thrust levers,

• Thirdly, associated to crosswindoperations, it may induceon some models assymetricbraking, which helps lateralcontrol, but has a negativeimpact on Turn Around Time(TAT): There is more brakingenergy on the more loadedwheels, so those brakes getto reach higher temperaturesand need more time to cooldown before the next departure,

• Finally, it is highlyrecommended to reduceas much as possible the numberof brake pedal applicationsduring landing roll to limitcarbon brake wear.

Additionally, current auto-brakesystem shortcomings at landing aremagnified with its systematic use.Everyday use leads to reach thedesired exit speed too far or tooshort from the desired exit.

OFFLO

MED

MAX

AUTO BRK

ONON

K A/SKID

Airbus innovation

A300-600 A320 Family A340-500/600 A380 (before BTV)

HI

43

2

LO

DISARM

AUTO/BRK LDGAUTO/BRK

L0 MED MAX

HI

32

LO

DISARM

AUTO/BRKLDGTO

HI

32

LO

BTV

LDG

DISARM

Figure 1

Automatic braking systemson Airbus aircraft


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i n f o r m a t i o n

Synthetically, currentauto-brake systemdrawbacks (induced by its usefulrusticity) are, in everyday avera-tions, an approximate optimizationachieved by the pilot (i.e. too highdeceleration followed by too longspeed taxi on the runway),a partial awareness of theestimated and measured brakingdistance, a lack of flexibilitywith respect to operational con-straints and an ‘Airport Navigation’unawareness.

GEO50XEXIT

XEXITGEO50

50 kts(0.3g)50 kts(0.2g)

50 kts(0.3g)50 kts(0.2g)

Dependingon exact geometry,taxiway exact angle,

turn radius and RWY width,aircraft can be at 50 KTSwith yellow line havingslightly started to turn

High speedturn off angleslightly steeper

than 30°,with 45m

wide runway(in Nice - France

exactly 30°)

The only way to improve thissituation is the pilot’s compen-sation by overriding the auto-brakesystem at the right time. Theoverride decision criterion thendepends on the pilot’s feelings,view and experience inducing aneveryday very limited brakeoptimization. In low visibilityconditions (crew blindness), thepilot cannot compensate the classicauto-brake system blindness, beingblind himself. In that operationalcase, auto-brake MED decele-ration level or equivalent is mostlyused, bringing the aircraft at lowspeed in the middle of nowhere.Then, the pilot taxies on therunway until...…an exit literally appears!

The auto-brake system controlprinciple is a closed-loop controlon deceleration. Hence, morereverse thrust results in lessbraking, but without shorteninglanding distance. At first sight, thisis really beneficial by reducingbrake energy. Nevertheless, thiseveryday situation increases therisk of runway end overrun in caseof long flare on short runways;even if the pilot selects, as com-mitted, the maximum reversethrust.

In terms of Human MachineInterface, the pilot, through theDECEL light extinction does a

simplistic auto-brake system moni-toring when current deceleration is20% below the target dece-leration:• The DECEL light might beextinguished at high speedon slippery runwayswhile auto-brake systemis operating normally,

• A partial improvement has beenintroduced on A340-500/600by adding an ACTIV lightin order to confirm the correctoperation of the auto-brakesystem. Nevertheless, it will nothelp the pilot in achievingan intended exit orin preventing a possiblerunway overrun.

An evolving airportoperation contextInnovative solutions are urgentlyrequired because congestion isalready a serious issue at someairports. A minority of airportsgenerates the majority of demands,and these airports are alreadyoperating at their maximumthroughput for sustained periods oftime. Among other topics, the on-going SESAR (Single EuropeanSky for Air Traffic ManagementResearch) project focuses onmaking best use of airport airsidecapacity based on the availableinfrastructure, because new cons-tructions are in many instancesstrictly limited by political andenvironmental constraints. Theairport's airside system capacity issignificantly influenced by therunway capacity, which should beconsidered as key determinant forthe overall intake. It has thepotential to significantly reducethe total amount of delays atairports. This will in turn reducethe requirements for the airport'sdevelopment, the impact on theenvironment and its resource use.

Among the wide spectrum ofrunway capacity elements, redu-cing the time spent by aircraft onthe runway is one of the mostimportant issues.

BTV is capable of highspeed turn off

2 Independent displayon existingNavigation Display(ND)

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This does not alter the fact thatRunway Occupancy Time (ROT) isinextricably linked to other issuessuch as wake vortex separationminima and minimum separationstandards for both arrival anddeparture. Minimizing the separa-tion between arriving and depar-ting traffic is equally crucial inreducing ROTs. Even environ-mental aspects, such as use ofpreferential runways, etc., may alsohave an impact on occupancy time.

Studies have shown that dependingon the traffic mix (various aircrafttypes), runway capacity can beincreased between 5% (in the caseof single-runway airports) and15% (multiple-runway airports) byreducing ROTs. A remarkableexample is the 19% capacityincrease achieved over a period ofthree years on the single runway atManchester, U.K.

It should be stressed that increa-sing runway capacity by mini-mizing runway occupancy is amatter of seconds per operation.Indeed, aircraft that unnecessarilyoccupy the runway for additionalseconds potentially provoke delaysof at least one order of magnitudegreater, (i.e. close to the minute orworse). If this develops into adomino effect, then overall systemcapacity will be reduced, causinglosses of slots. On the other hand,the saving of a few seconds permovement can represent animportant capacity increase.Enhancing runway capacity is notnecessarily a matter of seekingabsolute minimum occupancy timebut rather one of achieving consis-tent performance, thereby buildingup the confidence of pilots andcontrollers, which is necessary tooptimize runway capacity.

Finally, in case of low visibility, therunway capacity is drastically redu-ced due to lack of the operationalguarantee between the pilot and thecontroller, inducing an importantincrease of safety margins added toeveryday separation between twoconsecutive aircraft.

An operationalanswer: TheBrake-to-VacatefunctionIt becomes natural to imagine anenhancement of the existing classi-cal auto-brake system at landingwhich aims at reaching optimally adesired exit, by adding a new auto-brake mode (with the same acti-vation/disconnection principles).

Brake-to-Vacate system designobjectives can be expressed to:• Ensure the best possible brakingmanagement at landingfrom main landing gear impactto runway exit vacation,

• Develop a crew intuitiveselection, monitoringand terminationof the BTV system,

• Propose a seamless and naturalintegration with: In-flightlanding distances assessmentduring descent/approachpreparation and execution,Runway Overrun Preventionand Warning Systems (ROPand ROW) below 500ft untilaircraft runway vacation,Airport Navigation during taxi,

12 2

3 3

i n f o r m a t i o n

Synthetically, regardingpresent runway operations,current on-board systemscannot help improving runwaycapacity; therefore, runwaycapacity is not optimizedand is drastically reducedin low visibility conditions.

3 Selectionusing existingKey Board Controland Cursor Unit(KCCU)


1 Control using Auto-Brakeexisting selector.BTV instinctive disconnectionusing A/THR existing instinctivedisconnection button.

2

BTV configuration must be donein PLAN mode / ZOOM range.

The FMS runway is 14R.Runway selection with click on QFU.

Selection of airport navigationdisplay

BTV arming

Confirmation message

Selection of BTV runway

Exit selection

Exit selection with clickon exit label according to aircraftperformance, runway condition

and destination gate.

BTV armingby ABRK rotary switch.Runway LDA must be

cross-checked with charts.

1

1

2 3

HI3

2LO

BTV

LDG

DISARM

HI

32

LO

BTV

LDG

DISARM

HI

32

LO

BTV

LDG

DISARM

3


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• Ensure a safety improvementby increased crew situationawareness achieved withthe in-flight landing distancecomputation continued on finalapproach and ground roll, evenwith low visibility operations,

• Ensure a safety improvementwith the implementationof a brand new runway overrunprevention device coveringmost frequent caseson non-contaminated runways;feature which is also generalizedto all other classical auto-brakemodes. The description of BTVintegrated brand new RunwayOverrun Prevention (ROP)

system and display ofoperational landing distances,will be the object of twospecific articles in July andDecember 2009 in the editionsof Safety First magazine.

BRAKE-TO-VACATE GENERALOPTIMIZATION PRINCIPLES:AIRCRAFT SIDE

In more details, ‘the best possiblebraking management at landing’,targeted by the BTV system,considers the most robust andsimple compromize which guaran-tees to vacate at the assignedexit, optimizes the brake energyBTV configuration

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regarding the current operationalconstraints, minimizes the runwayoccupancy time and improves thepassenger comfort.

In an operation, the optimum exitselection depends on multiplecriteria and constraints that canonly be known in their fullcomplexity by the pilot and the airtraffic controller:• Optimum braking energy(complex as function of taxi,of requested Turn Around Time(TAT), of noise abatementprocedures preventing maximalreverse thrust usage out of safetyneeds),

• Minimum number of brakeapplications,

• Minimum runway occupancytime,

• Best exit for taxi duration.

A short exit selection can obviouslyproduce lower runway occupancytime than a far exit, but with higherbrake energy and Turn AroundTime. At the end, no automaticselection of the optimum exit ispossible.Then, for an exit selected by thepilot, the BTV system guaranteesthat simultaneously the lowestbrake energy and runway occupan-cy time are reached. For this,BTV system delays braking asmuch as possible, applies themaximum possible braking at thelatest possible time while reachingthe exit at suitable speed. But adesign compromize has been foundin order to respect the passengers’comfort constraints by fixingmaximum level of deceleration andvariation of deceleration over time,during the landing roll. Moreover,a special attention has been paidto the BTV deceleration profilecomputation regarding the visualperception of the pilot associatedto late braking in the case of aselected exit close to the runwayend.

To help the exit selection chosen bythe pilot, the BTV system proposesa dedicated interface providing

intuitive information to assist theselection of an optimum exit and tomonitor BTV operation. Thisdedicated interface provides also apredicted and guaranteed ROT(Runway Occupancy Time) and anestimated TAT in a sense of brakecooling time (assuming thrust idleor maximum reversers’ usage asper Standard Operating Proceduresindication).

Nevertheless, the selection of the‘optimum’ exit remains the pilot’sresponsibility. These indicationsthen help the crew on the optimalthrust reversers’ usage strategyduring the landing roll on dryrunway.

BRAKE-TO-VACATE GENERALOPTIMIZATION PRINCIPLES: ATM(AIR TRAFFIC MANAGEMENT) SIDE

The main principle is to useefficiently the runway occupancytime reduction allowed by BTV-fitted aircraft in the whole trafficconverging on the considered air-port; particularly, it takes benefitfrom the knowledge of the effectiveand guaranteed runway occupancyof the BTV-fitted aircraft using the‘runway resource’.

Then, the followed dedicatedarrival procedure is today imagined(still under study with the ATMcommunity):• Since the ‘approach’ controllermanages the BTV-fitted aircraft,the pilot and the controller agreeon the in-service landing runwayand exit taxiway, which dependson several points as the airportlayout configuration, aircraftlanding performances, airlineoperational procedures and allother current landing conditions,

• The pilot (or in the future theaircraft itself) communicatesthe predictive runway occupancytime, which will be guaranteed,

• The ‘approach’ controllermanages arrivals consideringseparations to be respectedand the forecasted arrival timeon the landing runway threshold.


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The results of this sequencingtask is the location of all aircraftin final approach in orderto optimize the arrival flow withrespect to the runway occupancytime, but also allowable minimalseparations (radar and wakevortex). Also, it results inmanaging forecasted separationin time on a given futureposition on the approachtrajectory (runway threshold),

• Once the aircraft is establishedin final segment, the ‘tower’controller monitors approachesso that the forecasted timingis respected (in the future,the aircraft itself will be ableto manage these constraints),

• The ‘tower’ controller gives thelanding clearance with respectto its own conviction that therunway will be vacated on time.

The operational management ofmixed traffic (BTV-fitted and non-BTV-fitted aircraft) is obviouslymuch more complex. Nevertheless,it remains consistent. In this case,conservative forecasted runwayoccupancy time would overestimate

known mean values with a sufficientmargin to take into account uncer-tainties (like it is practised today).For non-specialized runways (usedsimultaneously for takeoff andlanding), the arrival timing has tobe also optimized; a takeoff canimmediately follow the vacation ofthe previous just-landed aircraftand then take benefit of the timesaved.

In addition to the operational andsafety gains foreseen above, theimmediate consequence is theminimization of strong constraints,which allow the improvement ofadmissible cadences. The inducedATM operational gain is based onthe increase of runway technicalcapacity. This gain is particularlyremarkable in case of low visibilityconditions because standard sepa-ration can be reached thanks toBTV system (within the limits ofsensitive radio electric protectionzone constraints). The runwayoccupancy time can be then thesame as in case of good visibilityoperations because low speedevolution on the runway is reduced

Auto-brake activation(Nose Landing Gear down or five

seconds after Main Landing Gear down)

Symbology for allAuto-brake modes(classical auto-brake and BTV)

Symbologyonly forBTV mode

WET

DRY

Nominal glide slope (3°)

500 ft

50 ft

ROP(Runway Overrun Protection)

Stop the aircraft

ROW(Runway Overrun Warning)

Go around

{

BTV system

25

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Brake-To-Vacate (BTV) is an Airbusdevelopment effort for improving the pilot’smanagement of the approach and landingphases. The well known GPS and the newon-board airport map databasehas permitted this innovation. Tangiblevalue will be brought to our customers by:• Reducing brake wear and temperature,• Using less and even removingbrake fans,

• Relieving maximum thrust reversers’usage on dry runways,

• Reducing noise level on ground,fuel consumption and gas emission,

• Controlling Turn Around Time beforelanding (guarantee for the nextdeparture slot),

• Improving passengers’ comfortduring landing roll,

• Avoiding missed exit situations,• And, minimizing runwayoccupancy time.BTV system is coupledto a Runway Overrun Prevention system,also called ROW/ROP. This Airbuspatented solution offers a comprehensiveand efficient answer to the runwayexcursion risk at landing.It can then be seen as a major safetyenhancement feature. Through theminimization of the runway occupancytime, BTV helps also to reduce significantlythe exposure time to a runwayincursion risk.

Conclusion

to its minimum. Moreover, asthe runway occupancy time of theprevious aircraft is known andrespected, a certain number of go-around manoeuvres (due to non-cleared runway) will be avoided.

The reduction of the time spent onthe runway can be translated by anoperation time gain for the airline.It could be negligible for anisolated aircraft but important for afleet. The most visible gains will beobtained on the delays reductionoccurring during airport saturationperiods. Operation gains can thenbe magnified by a network effectwhen considering sets of platformsthat are interconnected with localBTV induced gains. They will bealso sensitive to ‘a hub effect’ dueto an important part of fittedaircraft.

Eventually, the airport managerwill benefit from a declaredimproved operational capacitywithout doing expensive invest-ments (additional and/or exit run-way building, etc.).

CONTACT DETAILS

Fabrice VILLAUMÉMulti-ProgrammeProject LeaderAirbus Systems EngineeringTel: +33 (0)5 61 18 63 50Fax: +33 (0)5 61 18 25 [email protected]


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