acas overview jan09

8/9/2019 ACAS Overview Jan09

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1

Maastricht ATC 2006

Overview of ACAS II / TCAS II

Version 1.4 7 January 2009

DISCLAIMER

2009 The European Organisation for the Safety of Air Navigation (EUROCONTROL). This document is

published by EUROCONTROL for information purposes. It may be copied in whole or in part provided that

EUROCONTROL is mentioned as a source and to the extent justified by the non-commercial use (not for sale).

The information in this document may not be modified without prior written permission from EUROCONTROL.The use of this document is at user's sole risk and responsibility. EUROCONTROL expressly disclaims any andall warranties with respect to the document, expressed or implied.

Additionally, the disclaimer available under www.euroocontrol.int/acas applies to the information contained in

this presentation.


2/64

2Version 1.4 2009 EUROCONTROL

ACAS

ACAS: Airborne Collision Avoidance System

ACAS is a set of standards defined by ICAO:

an ACAS I issues Traffic Advisories (TAs) an ACAS II issues Resolution Advisories (RAs), in addition to TAs, in the

vertical plane only

an ACAS III issues Resolution Advisories (RAs), in addition to TAs, in both the

vertical and in the horizontal plane

Only ACAS II standards have been defined and approved

ACAS equipage automatically implies Mode S equipage


3/64


TCAS

TCAS: Traffic alert and Collision Avoidance System

TCAS II v7.0 is the current in-use system for Europe

An upgrade to TCAS II v7.1 is planned the implementation schedule is notknown yet

TCAS II v6.04a is still in-use in some countries, notably USA


4/64


ACAS II principles

Intended as a last resort protection against risk of collision

Designed forcollision avoidance only

Independent of ground-based systems and the auto-pilot and of

NAV systems

ACAS algorithms and parameters as far as possible chosen to be

compatible with separation standards - but:

Does not warn of loss of separation

Sometime generate unnecessary (nuisance) alerts

Sense of RAs coordinated between ACAS equipped aircraft

Complying with RAs can result in deviation from clearance


5/64


Situation in the world

Europe: The carriage of ACAS II has been mandatory from 1-1-2005 for all civilian

turbine-engined aircraft carrying more than 19 passengers or with the maximum

take-off mass more than 5700 kg

USA: TCAS II has been mandatory in US airspace since 1994, for all civilian turbine-

engined aircraft carrying more than 30 passengers

Three manufacturers (Rockwell-Collins, ACSS, Honeywell)

ICAO has recommended world-wide mandatory ACAS II carriage

(1-1-2003)


6/64


Equipment description

TCAS DIRECTIONAL

ANTENNA

MODE S

ANTENNA

MODE S

XPDRTCAS II

UNIT

GEARS &

FLAPS

AIR DATA

COMPUTER

(ADC)

RADAR ALTIMETER

LOWER TCAS

ANTENNA

MODE S

ANTENNA

TCAS + SSR

CONTROL

PANEL

EFIS or TCAS DISPLAY

AURAL

ANNUNCIATION

COORDINATION

& INHIBITION

RADAR ALTIMETER

ANTENNA


7/64


Functions

Surveillance of adjacent transponders (range = 14 NM)

Maximum number of aircraft processed = 30

Determination of the alerts: TA and RA

Co-ordination between two TCAS units (through the mode S data-link)

Aural annunciations

Display of information on Vertical Speed Indicators (VSI) or on EFIS (ND

+ PFD)

Processing cycle = 1 second


8/64


Example of IVSI-type TCAS display(classic instrumentation)

Other traffic

Proximate traffic

Intruding traffic

Threat

Resolution advisory

Intruder target

Relative altitude (ft x 100)

Vertical trend arrow

Vertical speed needleTCAS display centre

2-NM radius circle


9/64

The TCAS information are dispatched between the PFD (Primary Flight


Example of EFIS with RAs displayed onthe vertical speed scale

310 4020

315

305

320

300

340

360

280

.818 STD

AP1

A/THR

FL 310

MACH ALT HDG

2

The pilot must manoeuvre the aircraft to comply with the specified vertical

speed (here: -1500 fpm)


10/64

On this PFD the RA is converted into an attitude value to avoid When the pilot


Example of EFIS with RAs displayed onthe artificial horizon

31040

2032

340

360

380

320

300

280

260

30800

30600

31200

31400

5

6

STD

M 0.82 31000

240CRS 110

SPD L-NAV ALT

0.81810 11 12

1314

09

08

The pilot acts on the aircraft attitude


11/64 The horizontal test is based upon a measurement of the slant range every


Initiation of alerts

Horizontal test: time to reach the Closest Point of Approach (CPA)

(or "DMOD" distance if the closing speed is low)

Vertical test: time to reach co-altitude (or "ZTHR" if the aircraft aresteady)

The TA/RA threshold values increase with the altitude

The bearing is not taken into account (only used for the display)

RA sense selection: objective of safe vertical distance "ALIM",

co-ordination

If the intruder does not transmit its altitude, there will be no RA


12/64


Threshold values (TCAS II v7)

Layer dependentSensitivity-level dependent

8503006001202-6000 AGL

8503006001250.215153+- 1001000 AGL

8503006002300.3520184+- 2002350 AGL

8503506003400.5525205+- 5005,000 MSL

8504006004450.830226+- 50010,000 MSL

8506007005481.135257+- 50020,000 MSL

12007008006481.135257+- 50042,000 MSL

ZTHR_TA(ft)

ALIM(ft)

ZTHR(ft)

LayerTAU_TA(s)

DMOD(nm)

TAU(s)

VTT(s)

Sensitivitylevel

Hysteresis(ft)

Above Altitude(ft)

TAU Time to CPA

CPA Closest Point of Approach

DMOD Distance Modif ier

Layer Altitude Layer

ZTHR Fixed Threshold (RA)ZTHR TA Fixed Threshold (TA)

ALIM Miss Distance

VTT Variable Threshold Test

1550 (+-100) AGL and below TCAS inhibits INCREASE DESCEND RAs

1100 (+-100) AGL and below TCAS inhibits DESCEND RAs

1000 (+-100) AGL and below TCAS inhibits all RAs500 (+-100) AGL and below TCAS inhibits all Aural Alerts


13/64 This figure shows the principle of TCAS operation The system knows neither


Illustration of the horizontal test

35seconds

Intruder

TCAS

The collision avoidance criterion is a generally a time parameter


14/64


RA sense selection

The 'downward' sense will achieve ALIM, the 'upward' sense will not.

The 'downward' option is therefore selected.

TCAS

Threat

CPA

'downward'

'upward'

ALIM

ALIM


15/64


RA without altitude crossing

Both 'downward' and 'upward' senses will achieve ALIM. The 'upward'

sense is selected because it prevents the TCAS aircraft from

crossing through the intruder's altitude

ALIM

CPA

ALIM

Climb RAissued

TCAS

Threat


16/64


Target on-the-ground determination

Radaraltimeter

value

1,700 feet above ground level

(Threshold below which TCAS checks for targets on the ground)

380-foot allowance

Barometricaltimeter

Ground level

Standard altimeter setting Estimated elevation of ground

Declaredairborne

Declared

on ground

Declared

on ground

TCAS uses the radar altimeter data to progressively inhibit some advisories


17/64


Miss Distance Filtering

TCAS

Intruder35seconds

HMD

TCAS A/C

ground speed

Intruderground speed

TCAS computes the Horizontal

Miss Distance (HMD).

If the HMD is "sufficient", no RA

TCAS II tries to estimate HMD as it triggers an RA


18/64


Alerts: "Traffic Advisory"

Thresholds: horizontal: 20 to 48 s (or DMOD = 0.3 to 1.3 NM)

vertical: 20 to 48 s (or ZTHR = 850 or 1,200 ft)

Aural annunciation: "Traffic, traffic

Intruder: solid amber circle

Helps to the visual acquisition of the intruder

Prepares the crew for a possible RA

No manoeuvres shall be made in response to TA

TA shall not be reported to ATC

Aircraft manoeuvres in response to TA are prohibited because of the high risk


19/64


Alerts: "Resolution Advisory"


vertical: 15 to 35 s (or ZTHR = 600 to 800 ft)

Aural annunciations: "climb", "descend", "increase climb", "increasedescent", "monitor vertical speed", etc.

Display of a range of vertical speeds or attitudes to avoid

Objective: a safe vertical distance ALIM varying from 300 to 700 ft

An RA takes into account all existing threats

The RA intensity is checked and revised, if necessary, every second

The various thresholds relate to the altitude band in which the aircraft are


20/64


Alerts: Resolution Advisory

The pilot must react within 5 seconds

A vertical speed of +/-1,500 fpm (acceleration = 0.25 g) is generallyrequired, but may vary according to the event

Pilots shall nevermanoeuvre in the opposite sense to the TCAS RA(RAs are coordinated with other suitably equipped aircraft)

The pilot must inform ATC as soon as possible

The "Clear of Conflict" message is issued when the aircraft divergehorizontally

The pilot must then resume the ATC clearance

The vertical acceleration required to comply TCAS deviations should be hardly


21/64


TCAS Advantages

Independent system, which acts as a last resort safety barrier

Highly accurate distance measurements, with a one-second refresh

rate

All threats taken into account

TCAS-TCAS co-ordination

Detection of all transponding aircraft, including those which are not

displayed on the controllers screen

TCAS II reduced the risk of mid-air collision


22/64


Limitations: General

No detection of aircraft without or not operating transponders (e.g.

VFRs, military flights)

No knowledge of the pilots intentions and of the ATC separation

minima

Basic display: no identification, no past positions, no speed vector

The potential dangers of some normal controlling practices with the


23/64


Limitations: Unnecessary Alerts

Some alerts seen by the pilots and controllers as unnecessary

(nuisance) alerts, especially 1000-ft level offs

The alerts are unnecessary only in hindsight

Limited horizontal tracking unless ground tracks of both aircraft

remain very straight, an RA can be generated even if the aircraft are

3 NM apart

The RAs occurring in the 1 000 ft level off geometry situations make +/ 50%


24/64


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Operational impact: Controllers

Consequences of an RA:

Disturbance to the tactical control plan

Possibility of an induced conflict, although TCAS has a multiple threat

processing

Frequency overload, initial lack of understanding

In practice:

Controller will not know about the RA until notified by the pilot

If the RA event is known, the controller must cease issuing any

instructions, but should provide traffic information

The controllermust not considerthe TCAS equipment of the aircraft to

establish and maintain spacing (ICAO)

The concern of a conflict induced by an aircraft responding to an RA is


26/64


Doc. 8168 (1/3)

Airborne collision avoidance system (ACAS) indications shall be used by pilots in theavoidance of potential collisions, the enhancement of situational awareness, and the activesearch for, and visual acquisition of, conflicting traffic.

Doc. 8168, part VIII, para. 3.1.1

Pilots should use appropriate procedures by which an aeroplane climbing or descending toan assigned altitude or flight level, especially with an autopilot engaged, may do so at a rateless than 8 m/s (or 1 500 ft/min) throughout the last 300 m (or 1 000 ft) of climb or descent tothe assigned altitude or flight level when the pilot is made aware of another aircraft at orapproaching an adjacent altitude or flight level, unless otherwise instructed by ATC. Theseprocedures are intended to avoid unnecessary airborne collision avoidance system (ACAS II)resolution advisories in aircraft at or approaching adjacent altitudes or flight levels. Forcommercial operations, these procedures should be specified by the operator.

Doc. 8168, part VIII, para. 3.3


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Doc. 8168 (2/3)

Nothing in the procedures specified in 3.2 hereunder shall prevent pilots-in-command from exercising

their best judgement and full authority in the choice of the best course of action to resolve a traffic

conflict or avert a potential collision.

Note 1. The ability of ACAS to fulfil its role of assisting pilots in the avoidance of potential collisions is

dependent on the correct and timely response by pilots to ACAS indications. Operational experience

has shown that the correct response by pilots is dependent on the effectiveness of initial and recurrent

training in ACAS procedures.

Note 2. ACAS II Training Guidelines for Pilots are provided in Attachment A to Part VIII.



28/64


Doc. 8168 (3/3)

In the event of an RA, pilots shall:

1) respond immediately by following the RA as indicated, unless doing so would jeopardize the safety of the aeroplane;Note 1. Stall warning, wind shear, and ground proximity warning system alerts have precedence over ACAS

Note 2. Visually acquired traffic may not be the same traffic causing an RA. Visual perception of an encounter may be misleading, particularlyat night.

2) follow the RA even if there is a conflict between the RA and an air traffic control (ATC) instruction to manoeuvre;

3) not manoeuvre in the opposite sense to an RA;Note. In the case of an ACAS-ACAS coordinated encounter, the RAs complement each other in order to reduce the potential for collision.Manoeuvres, or lack of manoeuvres, that result in vertical rates opposite to the sense of an RA could result in a collision with the threat aircraft.

4) as soon as possible, as permitted by aircrew workload, notify the appropriate ATC unit of any RA which requires a deviationfrom the current air traffic control instruction or clearance;Note. Unless informed by the pilot, ATC does not know when ACAS issues RAs. It is possible for ATC to issue instructions that areunknowingly contrary to ACAS RA indications. Therefore, it is important that ATC be notified when an ATC instruction or clearance is not beingfollowed because it conflicts with an RA.

5) promptly comply with any modified RAs;

6) limit the alterations of the flight path to the minimum extent necessary to comply with the RAs;

7) promptly return to the terms of the ATC instruction or clearance when the conflict is resolved; and

8) notify ATC when returning to the current clearance.Note. Procedures in regard to ACAS-equipped aircraft and the phraseology to be used for the notification of manoeuvres in response to aresolution advisory are contained in the PANS-ATM (Doc 4444), Chapters 15 and 12, respectively.

Doc. 8168, part VIII para. 3.2.c


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Doc. 4444 (1/2)

When a pilot reports an ACAS resolution advisory (RA), the controller shall not attempt

to modify the aircraft flight path until the pilot reports Clear of Conflict.

Doc. 4444, para. 15.6.3.2


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RA Phraseology (Doc. 4444)

... after a flight crew starts to deviate from any ATC clearance or instruction to comply with an ACAS resolution advisory (RA)(Pilot and controller interchange):

PILOT: [callsign] TCAS RA;

ATC: [callsign] ROGER;

... after the response to an ACAS RA is completed and a return to the ATC clearance or instruction is initiated (Pilot and

controller interchange):

PILOT: [callsign] CLEAR OF CONFLICT, RETURNING TO (assigned clearance);

ATC: [callsign] ROGER (or alternative instructions);

after the response to an ACAS RA is completed and the assigned ATC clearance or instruction has been resumed (Pilot and

controller interchange):

PILOT: [callsign] CLEAR OF CONFLICT, RETURNING TO (assigned clearance);

ATC: [callsign] ROGER (or alternative instructions);

after an ATC clearance or instruction contradictory to the ACAS RA is received, the flight crew will follow the RA and inform

ATC directly (Pilot and controller interchange):

PILOT: [callsign] CLEAR OF CONFLICT (assigned clearance) RESUMED;

ATC: [callsign] ROGER;

Doc. 4444, para. 12.3.1.2 r- y


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RA & Aural Alerts (TCAS II version 7.0)

Monitor vertical speedNo changePreventive RAMonitor vertical speedNo changePreventive RA

Clear of conflictRA RemovedClear of conflictRA Removed

Maintain vertical speed, crossing

maintain 1500 to 4400

Maintain crossing

descend

Maintain vertical speed,

crossing maintain1500 to 4400

Maintain crossing

climb

Increase descent, increase

descent2500Increase descentIncrease climb, increase climb2500Increase climb

Descend, descend NOW;

Descend, descend NOW 1500Reversal descent

Climb, climb NOW; Climb, climb

NOW1500Reversal climb

Adjust vertical speed, adjust

< 0

< +500

< +1000

< +2000

Reduce descentAdjust vertical speed, adjust

> 0

> 500

> 1000

> 2000

Reduce climb

Maintain vertical speed, maintain 1500 to 4400Maintain descendMaintain vertical speed,

maintain1500 to 4400Maintain climb

Descend, crossing descend;

Descend, crossing descend 1500Crossing descend

Climb, crossing climb; Climb,

crossing climb1500Crossing climb

Descend, descend 1500DescendClimb, climb1500Climb

AuralRequired vertical

rate (fpm)RAAural

Required vertical

rate (fpm)RA

Downward senseUpward sense


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1

Maastricht ATC 2006

Overview of ACAS II / TCAS II

Version 1.4 7 January 2009

DISCLAIMER

2009 The European Organisation for the Safety of Air Navigation (EUROCONTROL). This document is

published by EUROCONTROL for information purposes. It may be copied in whole or in part provided that

EUROCONTROL is mentioned as a source and to the extent justified by the non-commercial use (not for sale).

The information in this document may not be modified without prior written permission from EUROCONTROL.

The use of this document is at user's sole risk and responsibility. EUROCONTROL expressly disclaims any and

all warranties with respect to the document, expressed or implied.

Additionally, the disclaimer available under www.euroocontrol.int/acas applies to the information contained in

this presentation.

ACAS


34/64


ACAS

ACAS: Airborne Collision Avoidance System

ACAS is a set of standards defined by ICAO:

an ACAS I issues Traffic Advisories (TAs)

an ACAS II issues Resolution Advisories (RAs), in addition to TAs, in the

vertical plane only an ACAS III issues Resolution Advisories (RAs), in addition to TAs, in both the

vertical and in the horizontal plane

Only ACAS II standards have been defined and approved

ACAS equipage automatically implies Mode S equipage

TCAS


35/64


TCAS

TCAS: Traffic alert and Collision Avoidance System

TCAS II v7.0 is the current in-use system for Europe

An upgrade to TCAS II v7.1 is planned the implementation schedule is not

known yet

TCAS II v6.04a is still in-use in some countries, notably USA


36/64

Sit ation in the orld


37/64


Situation in the world

Europe: The carriage of ACAS II has been mandatory from 1-1-2005 for all civilian

turbine-engined aircraft carrying more than 19 passengers or with the maximum

take-off mass more than 5700 kg

USA: TCAS II has been mandatory in US airspace since 1994, for all civilian turbine-

engined aircraft carrying more than 30 passengers

Three manufacturers (Rockwell-Collins, ACSS, Honeywell)

ICAO has recommended world-wide mandatory ACAS II carriage

(1-1-2003)



38/64



TCAS DIRECTIONAL

ANTENNA

MODE S

ANTENNA

MODE S

XPDRTCAS II

UNIT

GEARS &

FLAPS

AIR DATA

COMPUTER

(ADC)

RADAR ALTIMETER

LOWER TCAS

ANTENNA

MODE S

ANTENNA

TCAS + SSR

CONTROL

PANEL

EFIS or TCAS DISPLAY

AURAL

ANNUNCIATION

COORDINATION

& INHIBITION

RADAR ALTIMETER

ANTENNA

Functions


39/64


Functions

Surveillance of adjacent transponders (range = 14 NM)

Maximum number of aircraft processed = 30

Determination of the alerts: TA and RA

Co-ordination between two TCAS units (through the mode S data-link)

Aural annunciations

Display of information on Vertical Speed Indicators (VSI) or on EFIS (ND

+ PFD)

Processing cycle = 1 second

Example of IVSI-type TCAS display


40/64


p yp p y

(classic instrumentation)

Other traffic

Proximate traffic

Intruding traffic

Threat

Resolution advisory

Intruder target

Relative altitude (ft x 100)

Vertical trend arrow

Vertical speed needleTCAS display centre

2-NM radius circle

Example of EFIS with RAs displayed on


41/64


the vertical speed scale

31040

20

315

305

320

300

340

360

280

.818 STD

AP1

A/THR

FL 310

MACH ALT HDG

2

The pilot must manoeuvre the aircraft to comply with the specified vertical

speed (here: -1500 fpm)

Example of EFIS with RAs displayed on


42/64


the artificial horizon

31040

2032

340

360

380

320

300

280

260

30800

30600

31200

31400

5

6

STD

M 0.82 31000

240CRS 110

SPD L-NAV ALT

0.81810 11 12

13

14

09

08

The pilot acts on the aircraft attitude



43/64



Horizontal test: time to reach the Closest Point of Approach (CPA)(or "DMOD" distance if the closing speed is low)

Vertical test: time to reach co-altitude (or "ZTHR" if the aircraft are

steady)

The TA/RA threshold values increase with the altitude

The bearing is not taken into account (only used for the display)

RA sense selection: objective of safe vertical distance "ALIM",

co-ordination

If the intruder does not transmit its altitude, there will be no RA



44/64



Sensitivity-level dependent Layer dependent

Above Altitude(ft)

Hysteresis(ft)

Sensitivitylevel

VTT(s)

TAU(s)

DMOD(nm)

TAU_TA(s)

Layer ZTHR(ft)

ALIM(ft)

ZTHR_TA(ft)

42,000 MSL +- 500 7 25 35 1.1 48 6 800 700 1200

20,000 MSL +- 500 7 25 35 1.1 48 5 700 600 850

10,000 MSL +- 500 6 22 30 0.8 45 4 600 400 850

5,000 MSL +- 500 5 20 25 0.55 40 3 600 350 850

2350 AGL +- 200 4 18 20 0.35 30 2 600 300 850

1000 AGL +- 100 3 15 15 0.2 25 1 600 300 850

-6000 AGL 2 20 1 600 300 850

TAU Time to CPA

CPA Closest Point of Approach

DMOD Distance Modifier Layer Altitude Layer

ZTHR Fixed Threshold (RA)

ZTHR TA Fixed Threshold (TA)

ALIM Miss Distance

VTT Variable Threshold Test

1550 (+-100) AGL and below TCAS inhibits INCREASE DESCEND RAs

1100 (+-100) AGL and below TCAS inhibits DESCEND RAs

1000 (+-100) AGL and below TCAS inhibits all RAs500 (+-100) AGL and below TCAS inhibits all Aural Alerts



45/64



35seconds

Intruder

TCAS

The collision avoidance criterion is a generally a time parameter


46/64



47/64



Both 'downward' and 'upward' senses will achieve ALIM. The 'upward'sense is selected because it prevents the TCAS aircraft from

crossing through the intruder's altitude

ALIM

CPA

ALIM

Climb RA

issued

TCAS

Threat


48/64



49/64



TCAS

Intruder35seconds

HMD

TCAS A/C

ground speed

Intruder

ground speed

TCAS computes the Horizontal

Miss Distance (HMD).If the HMD is "sufficient", no RA


50/64

Alerts: "Resolution Advisory"


51/64


y


vertical: 15 to 35 s (or ZTHR = 600 to 800 ft)

Aural annunciations: "climb", "descend", "increase climb", "increasedescent", "monitor vertical speed", etc.

Display of a range of vertical speeds or attitudes to avoid

Objective: a safe vertical distance ALIM varying from 300 to 700 ft

An RA takes into account all existing threats

The RA intensity is checked and revised, if necessary, every second


52/64

TCAS Advantages


53/64


g

Independent system, which acts as a last resort safety barrier

Highly accurate distance measurements, with a one-second refresh

rate

All threats taken into account

TCAS-TCAS co-ordination

Detection of all transponding aircraft, including those which are not

displayed on the controllers screen

TCAS II reduced the risk of mid-air collision

Limitations: General


54/64


No detection of aircraft without or not operating transponders (e.g.

VFRs, military flights)

No knowledge of the pilots intentions and of the ATC separation

minima

Basic display: no identification, no past positions, no speed vector

Limitations: Unnecessary Alerts


55/64


Some alerts seen by the pilots and controllers as unnecessary(nuisance) alerts, especially 1000-ft level offs

The alerts are unnecessary only in hindsight

Limited horizontal tracking unless ground tracks of both aircraft

remain very straight, an RA can be generated even if the aircraft are3 NM apart

Operational impact: Pilots


56/64


An RA is usually stressful experience: surprise, very rapiddevelopment (around 30 s)

Consequences:

frequently excessive deviations (observed average: 650 ft)

slow returns to the ATC clearance (brief loss of confidence) discussions on the frequency and correct phraseology not always used

Crews often use the TCAS display as a surveillance tool: high risk of

misinterpretation

Air crew training is essential


57/64


58/64

Doc. 8168 (2/3)


59/64


Nothing in the procedures specified in 3.2 hereunder shall prevent pilots-in-command from exercisingtheir best judgement and full authority in the choice of the best course of action to resolve a traffic

conflict or avert a potential collision.

Note 1. The ability of ACAS to fulfil its role of assisting pilots in the avoidance of potential collisions is

dependent on the correct and timely response by pilots to ACAS indications. Operational experience

has shown that the correct response by pilots is dependent on the effectiveness of initial and recurrenttraining in ACAS procedures.

Note 2. ACAS II Training Guidelines for Pilots are provided in Attachment A to Part VIII.


Doc. 8168 (3/3)


60/64


In the event of an RA, pilots shall:

1) respond immediately by following the RA as indicated, unless doing so would jeopardize the safety of the aeroplane;Note 1. Stall warning, wind shear, and ground proximity warning system alerts have precedence over ACAS

Note 2. Visually acquired traffic may not be the same traffic causing an RA. Visual perception of an encounter may be misleading, particularlyat night.

2) follow the RA even if there is a conflict between the RA and an air traffic control (ATC) instruction to manoeuvre;

3) not manoeuvre in the opposite sense to an RA;Note. In the case of an ACAS-ACAS coordinated encounter, the RAs complement each other in order to reduce the potential for collision.Manoeuvres, or lack of manoeuvres, that result in vertical rates opposite to the sense of an RA could result in a collision with the threat aircraft.

4) as soon as possible, as permitted by aircrew workload, notify the appropriate ATC unit of any RA which requires a deviation

from the current air traffic control instruction or clearance;Note. Unless informed by the pilot, ATC does not know when ACAS issues RAs. It is possible for ATC to issue instructions that areunknowingly contrary to ACAS RA indications. Therefore, it is important that ATC be notified when an ATC instruction or clearance is not beingfollowed because it conflicts with an RA.

5) promptly comply with any modified RAs;

6) limit the alterations of the flight path to the minimum extent necessary to comply with the RAs;

7) promptly return to the terms of the ATC instruction or clearance when the conflict is resolved; and

8) notify ATC when returning to the current clearance.

Note. Procedures in regard to ACAS-equipped aircraft and the phraseology to be used for the notification of manoeuvres in response to aresolution advisory are contained in the PANS-ATM (Doc 4444), Chapters 15 and 12, respectively.

Doc. 8168, part VIII para. 3.2.c

Doc. 4444 (1/2)


61/64


When a pilot reports an ACAS resolution advisory (RA), the controller shall not attempt

to modify the aircraft flight path until the pilot reports Clear of Conflict.

Doc. 4444, para. 15.6.3.2


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63/64

RA & Aural Alerts (TCAS II version 7.0)


64/64


Upward sense Downward sense

RA Required verticalrate (fpm) Aural RA Required verticalrate (fpm) Aural

Climb 1500 Climb, climb Descend 1500 Descend, descend

Crossing climb 1500Climb, crossing climb; Climb,

crossing climbCrossing descend 1500

Descend, crossing descend;

Descend, crossing descend

Maintain climb 1500 to 4400Maintain vertical speed,

maintainMaintain descend 1500 to 4400 Maintain vertical speed, maintain

Maintain crossing

climb1500 to 4400

Maintain vertical speed,

crossing maintain

Maintain crossing

descend 1500 to 4400

Maintain vertical speed, crossing

maintain

Preventive RA No change Monitor vertical speed Preventive RA No change Monitor vertical speed

RA Removed Clear of conflict RA Removed Clear of conflict

Reduce climb

> 0

> 500

> 1000

> 2000

Adjust vertical speed, adjust Reduce descent

< 0

< +500

< +1000

< +2000

Adjust vertical speed, adjust

Reversal climb 1500Climb, climb NOW; Climb, climb

NOWReversal descent 1500

Descend, descend NOW;

Descend, descend NOW

Increase climb 2500 Increase climb, increase climb Increase descent 2500Increase descent, increase

descent

acas overview jan09

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