Overview of ACAS II / TCAS II

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1 Maastricht ATC 2006 Overview of ACAS II / TCAS II 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 applies to the information contained in this presentation. Version January

2 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 2

3 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 3

4 ACAS II principles Intended as a last resort protection against risk of collision Designed for collision 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 4

5 Situation in the world Europe: The carriage of ACAS II has been mandatory from 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 turbineengined aircraft carrying more than 30 passengers Three manufacturers (Rockwell-Collins, ACSS, Honeywell) ICAO has recommended world-wide mandatory ACAS II carriage ( ) 5

6 Equipment description TCAS DIRECTIONAL ANTENNA MODE S ANTENNA AURAL ANNUNCIATION EFIS or TCAS DISPLAY COORDINATION & INHIBITION TCAS II UNIT MODE S XPDR RADAR ALTIMETER AIR DATA COMPUTER (ADC) GEARS & FLAPS TCAS + SSR CONTROL PANEL RADAR ALTIMETER ANTENNA LOWER TCAS ANTENNA MODE S ANTENNA 6

7 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 7

8 Example of IVSI-type TCAS display (classic instrumentation) Other traffic Proximate traffic Intruding traffic Threat Intruder target Relative altitude (ft x 100) Vertical trend arrow TCAS display centre Vertical speed needle 2-NM radius circle Resolution advisory 8

9 Example of EFIS with RAs displayed on the vertical speed scale MACH ALT HDG AP1 A/THR 360 FL STD 2 The pilot must manoeuvre the aircraft to comply with the specified vertical speed (here: fpm) 9

Example of EFIS with RAs displayed on the artificial horizon SPD L-NAV ALT M 0.

10 Example of EFIS with RAs displayed on the artificial horizon SPD L-NAV ALT M CRS STD The pilot acts on the aircraft attitude

11 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 11

12 Threshold values (TCAS II v7) Sensitivity-level dependent Layer dependent Above Altitude (ft) Hysteresis (ft) Sensitivity level VTT (s) TAU (s) DMOD (nm) TAU_TA (s) Layer ZTHR (ft) ALIM (ft) ZTHR_TA (ft) 42,000 MSL ,000 MSL ,000 MSL ,000 MSL AGL AGL AGL TAU CPA DMOD Layer ZTHR ZTHR TA ALIM VTT 12 Time to CPA Closest Point of Approach Distance Modifier Altitude Layer Fixed Threshold (RA) Fixed Threshold (TA) Miss Distance 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 RAs 500 (+-100 ) AGL and below TCAS inhibits all Aural Alerts

13 Illustration of the horizontal test TCAS 35 seconds Intruder The collision avoidance criterion is a generally a time parameter 13

14 RA sense selection Threat TCAS 'upward' CPA ALIM 'downward' ALIM The 'downward' sense will achieve ALIM, the 'upward' sense will not. The 'downward' option is therefore selected. 14

15 RA without altitude crossing TCAS Climb RA issued CPA ALIM ALIM Threat 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 15

16 Target on-the-ground determination 1,700 feet above ground level (Threshold below which TCAS checks for targets on the ground) Barometric altimeter Declared airborne Radar altimeter value Declared on ground Ground level Standard altimeter setting 380-foot allowance Declared on ground Estimated elevation of ground 16

17 Miss Distance Filtering TCAS 35 seconds Intruder TCAS A/C ground speed HMD Intruder ground speed TCAS computes the Horizontal Miss Distance (HMD). If the HMD is "sufficient", no RA 17

18 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 18

19 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 19

20 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 20

21 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 21

22 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 separation minima Basic display: no identification, no past positions, no speed vector 22

23 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 23

24 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 24

25 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 controller must not consider the TCAS equipment of the aircraft to establish and maintain spacing (ICAO) 25

26 Doc (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 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 ft/min) throughout the last 300 m (or 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

27 Doc (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

28 Doc (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, 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 28

29 Doc (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

30 Doc (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; or b) 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

31 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: ATC: [callsign] TCAS RA; [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: ATC: [callsign] CLEAR OF CONFLICT, RETURNING TO (assigned clearance); [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: ATC: [callsign] CLEAR OF CONFLICT, RETURNING TO (assigned clearance); [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: ATC: [callsign] CLEAR OF CONFLICT (assigned clearance) RESUMED; [callsign] ROGER; 31 Doc. 4444, para r- y

32 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 maintain Maintain crossing descend 1500 to 4400 Maintain vertical speed, crossing maintain Reduce climb > 0 > 500 > 1000 > 2000 Adjust vertical speed, adjust Reduce descent < 0 < +500 < < 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 Preventive RA No change Monitor vertical speed Preventive RA No change Monitor vertical speed RA Removed Clear of conflict RA Removed Clear of conflict 32

OPERATIONS MANUAL PART A

OPERATIONS MANUAL PART A PAGE: 1 Table of Contents A.GENERAL /CHAPTER 32. -...3 32. OF THE AIRBORNE COLLISION AVOIDANCE... 3 32.1 ACAS Training Requirements... 3 32.2 Policy and Procedures for the use of ACAS or TCAS (as applicable)...