Overview of ACAS II / TCAS II - E-ATPLe-atpl.com/pdf/ACAS_overview_Jan09.pdf · TCAS zTCAS: Traffic...

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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.

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 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


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


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


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)


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


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(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 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)


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


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)

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


Illustration of the horizontal test

35 secondsIntruder

TCAS

The collision avoidance criterion is a generally a time parameter


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


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


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


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


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


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


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


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


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


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


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


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)


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


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


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


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


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


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


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

Overview of ACAS II / TCAS II - E-ATPLe-atpl.com/pdf/ACAS_overview_Jan09.pdf · TCAS zTCAS: Traffic...

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