acas overview jan09
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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.
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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
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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
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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
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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)
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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
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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
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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
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The TCAS information are dispatched between the PFD (Primary Flight
9Version 1.4 2009 EUROCONTROL
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)
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On this PFD the RA is converted into an attitude value to avoid When the pilot
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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
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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
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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
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Illustration of the horizontal test
35seconds
Intruder
TCAS
The collision avoidance criterion is a generally a time parameter
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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
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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
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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
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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
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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
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Alerts: "Resolution Advisory"
Thresholds: horizontal: 15 to 35 s (or DMOD = 0.2 to 1.1 NM)
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
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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
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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
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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
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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%
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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
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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.
Doc. 8168, part VIII, para. 3.1.2
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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
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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
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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
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Sit ation in the orld
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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)
Equipment description
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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
Functions
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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
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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
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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
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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
Initiation of alerts
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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 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
Threshold values (TCAS II v7)
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Threshold values (TCAS II v7)
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
Illustration of the horizontal test
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Illustration of the horizontal test
35seconds
Intruder
TCAS
The collision avoidance criterion is a generally a time parameter
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RA without altitude crossing
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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 RA
issued
TCAS
Threat
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Miss Distance Filtering
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Miss Distance Filtering
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
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Alerts: "Resolution Advisory"
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y
Thresholds: horizontal: 15 to 35 s (or DMOD = 0.2 to 1.1 NM)
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
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TCAS Advantages
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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
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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
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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
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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
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Doc. 8168 (2/3)
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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, part VIII, para. 3.1.2
Doc. 8168 (3/3)
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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)
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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 & Aural Alerts (TCAS II version 7.0)
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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