Post on 31-Jan-2018
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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 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.
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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 onlyan 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 approvedACAS 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 EuropeAn 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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ACAS II principles
Intended as a last resort protection against risk of collisionDesigned for collision avoidance onlyIndependent 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 separationSometime generate unnecessary (nuisance) alerts
Sense of RAs coordinated between ACAS equipped aircraftComplying 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 passengersThree manufacturers (Rockwell-Collins, ACSS, Honeywell)
ICAO has recommended world-wide mandatory ACAS II carriage (1-1-2003)
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Equipment description
TCAS DIRECTIONALANTENNA
MODE SANTENNA
MODE SXPDR
TCAS IIUNIT
GEARS & FLAPS
AIR DATACOMPUTER
(ADC)
RADAR ALTIMETER
LOWER TCAS ANTENNA
MODE SANTENNA
TCAS + SSRCONTROL
PANEL
EFIS or TCAS DISPLAY
AURALANNUNCIATION
COORDINATION& INHIBITION
RADAR ALTIMETERANTENNA
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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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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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Example of EFIS with RAs displayed onthe artificial horizon
310402032
340
360
380
320
300
280
260
30800
30600
31200
31400
56
STD
M 0.82 31000
240CRS 110
SPD L-NAV ALT
0.81810 11 12 13 14
0908
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 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
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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 120020,000 MSL +- 500 7 25 35 1.1 48 5 700 600 85010,000 MSL +- 500 6 22 30 0.8 45 4 600 400 8505,000 MSL +- 500 5 20 25 0.55 40 3 600 350 8502350 AGL +- 200 4 18 20 0.35 30 2 600 300 8501000 AGL +- 100 3 15 15 0.2 25 1 600 300 850
-6000 AGL 2 20 1 600 300 850
TAU Time to CPACPA Closest Point of ApproachDMOD Distance ModifierLayer Altitude LayerZTHR Fixed Threshold (RA)ZTHR TA Fixed Threshold (TA)ALIM Miss DistanceVTT Variable Threshold Test
1550’ (+-100’) AGL and below TCAS inhibits INCREASE DESCEND RAs1100’ (+-100’) AGL and below TCAS inhibits DESCEND RAs1000’ (+-100’) AGL and below TCAS inhibits all RAs
500’ (+-100’) AGL and below TCAS inhibits all Aural Alerts
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Illustration of the horizontal test
35 secondsIntruder
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
Declaredon ground
Declaredon ground
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Miss Distance Filtering
TCAS
Intruder35 seconds
HMD
TCAS A/Cground speed
Intruderground speed
TCAS computes the HorizontalMiss Distance (HMD).
If the HMD is "sufficient", no 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
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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", "increase descent", "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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Alerts: “Resolution Advisory”
The pilot must react within 5 seconds
A vertical speed of +/-1,500 fpm (acceleration = 0.25 g) is generally required, but may vary according to the event
Pilots shall never manoeuvre 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 diverge horizontally
The pilot must then resume the ATC clearance
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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 controller’s 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 pilot’s intentions and of the ATC separationminima
Basic display: no identification, no past positions, no speed vector
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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
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Operational impact: Pilots
An RA is usually stressful experience: surprise, very rapid development (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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Operational impact: Controllers
Consequences of an RA: Disturbance to the tactical control planPossibility of an induced conflict, although TCAS has a multiple threat processingFrequency overload, initial lack of understanding
In practice:Controller will not know about the RA until notified by the pilotIf the RA event is known, the controller must cease issuing any instructions, but should provide traffic informationThe controller must not consider the TCAS equipment of the aircraft to establish and maintain spacing (ICAO)
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Doc. 8168 (1/3)
Airborne collision avoidance system (ACAS) indications shall be used by pilots in the avoidance of potential collisions, the enhancement of situational awareness, and the active search 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 to an assigned altitude or flight level, especially with an autopilot engaged, may do so at a rate less than 8 m/s (or 1 500 ft/min) throughout the last 300 m (or 1 000 ft) of climb or descent to the assigned altitude or flight level when the pilot is made aware of another aircraft at or approaching an adjacent altitude or flight level, unless otherwise instructed by ATC. These procedures are intended to avoid unnecessary airborne collision avoidance system (ACAS II) resolution advisories in aircraft at or approaching adjacent altitudes or flight levels. For commercial 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 ACASNote 2.— Visually acquired traffic may not be the same traffic causing an RA. Visual perception of an encounter may be misleading, particularly at 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 are unknowingly contrary to ACAS RA indications. Therefore, it is important that ATC be notified when an ATC instruction or clearance is not being followed 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 a resolution 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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Doc. 4444 (2/2)
Once an aircraft departs from its ATC clearance or instruction in compliance with an RA, or a pilot reports an RA, the controller ceases to be responsible for providing separation between that aircraft and any other aircraft affected as a direct consequence of the manoeuvre induced by the RA. The controller shall resume responsibility for providing separation for all the affected aircraft when:
a) the controller acknowledges a report from the flight crew that the aircraft has resumed the current clearance; orb) the controller acknowledges a report from the flight crew that the aircraft is resuming the current clearance and issues an alternative clearance which is acknowledged by the flight crew.
Note.— Pilots are required to report RAs which require a deviation from the current ATC clearance or instruction (see PANS-OPS, Volume I, Part III, Section 3, Chapter 3, 3.2 c) 4)). This report informs the controller that a deviation from clearance or instruction is taking place in response to an ACAS RA.
Doc. 4444, para. 15.6.3.3
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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)
Upward sense Downward sense
RA Required vertical rate (fpm) Aural RA Required vertical
rate (fpm) Aural
Climb 1500 Climb, climb Descend – 1500 Descend, descend
Crossing climb 1500 Climb, crossing climb; Climb, crossing climb Crossing descend – 1500 Descend, crossing descend;
Descend, crossing descend
Maintain climb 1500 to 4400 Maintain vertical speed, maintain Maintain descend – 1500 to – 4400 Maintain vertical speed, maintain
Maintain crossing climb 1500 to 4400 Maintain vertical speed,
crossing maintainMaintain 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 1500 Climb, climb NOW; Climb, climb NOW Reversal descent – 1500 Descend, descend NOW;
Descend, descend NOW
Increase climb 2500 Increase climb, increase climb Increase descent 2500 Increase descent, increase descent