11 Traffic-alert and Collision Avoidance System (TCAS)

Transcription

1 11 Traffic-alert and Collision Avoidance System (TCAS) INSTRUMENTATION 11.1 Introduction In the early nineties the American FAA stated that civil aircraft flying in US airspace were equipped with a Traffic-alert and Collision Avoidance System (TCAS) to prevent mid-air collisions especially in congested airspace. TCAS is designed as a back-up to the ATC system and the see and avoid concept. At present, JAR-OPS states that turbine powered aeroplanes having a maximum certificated take-off mass in excess of 5700 kg or a maximum approved passenger seating configuration of more than 19 must be equipped with an Airborne Collision Avoidance System (ACAS) with a minimum performance level of at least ACAS II. ICAO Annex 10 describes the minimum performance level of ACAS II. ACAS I was only capable of showing the location of potential threats on a dedicated Traffic Advisory (TA) display. ACAS I was intended to support the see and avoid principle. ACAS II provides (vertical) evasive pitch commands as Resolution Advisories (RA) in addition to TA s. ACAS III is intended for future implementation and will, in addition to ACAS II, also provide (horizontal) evasive roll commands. ACAS II has the following functions: Surveillance, Generation of TA s, Threat detection, Generation of RA s, Coordination, Communication with ground stations. These functions will be dealt with when discussing the TCAS II. TCAS II Change 7 has modified software and is compliant with ACAS II General The TCAS II consists of a TCAS II computer and two antennas. TCAS II is controlled from the transponder panel. Furthermore TCAS II makes use of: a dedicated VSI or the EFIS displays to show the relative location of intruding aircraft and to give visual alerts and commands; the audio system to give aural alerts and commands; Traffic Collision Avoidance System (TCAS) 11-1 Instrumentation E5.indb :17:20

2 INSTRUMENTATION ATC antenna (directional) top antenna EFIS Pressure Altitude (ADC) Mode-S transporder TCAS computer Radio altitude Configuration Audio ATC/TCAS control panel (non)directional bottom antenna Fig. IN 11.1 TCAS II system overview the Mode-S transponder to allow airto-air, air-to-ground and ground-to-air communication; the radio altimeter to prevent descend commands at low altitudes The pressure altitude from the air data computer (ADC). (see Fig. IN 11.1) NM. The TCAS II computer interrogates other aircraft transponders in the format of a Mode-S all call message. This is compatible with a Mode-C interrogation. The interrogation cycle is once each second. 30 NM radius Surveillance volume 11.3 System description The TCAS II computer has directional top and bottom antennas. The bottom antenna becomes non-directional when the landing gear is down. The two antennas together scan a certain airspace volume, see figure IN The tracking range depends on number of aircraft. As an example, a maximum of 30 aircraft can be tracked until a range of 30 NM. The nominal tracking range is 14 TA volume RA volume Protective area Fig. IN 11.2 TCAS surveillance volume 11-2 Traffic Collision Avoidance System (TCAS) Instrumentation E5.indb :17:21

3 The directional antennas allows for the determination of a relative bearing with an accuracy of ±10 degrees. From the replies TCAS II determines the range, range rate, altitude, altitude rate and closure speed. These parameters in turn are used to calculate the so called time to closest point of approach (TCPA) to the protective area around the own aircraft. The protective area depends on altitude and is usually a few hundred feet. Based on range, relative altitude and time to closest point of approach TCAS II distinguishes be420000tween the following four alerting levels: Other traffic, Proximate traffic, Traffic Advisory, Resolution Advisory. (see Fig. IN 11.3) Other traffic When an intruder is at a range of more than 6 NM or at a relative altitude of more than DME 124 TAS / HDG Fig. IN 11.3 EFIS ND showing TCAS symbols with data tags M GS VOR ft, the intruder is classified as other traffic. A non threat intruder is shown on the EFIS ND (EHSI) as an open white or cyan diamond (lozenge). The symbol is shown with a range and bearing from the own aircraft symbol. The symbol may be accompanied with a data tag showing relative altitude in hundreds of feet above (+) or below (-) and an arrow showing a vertical speed greater than 500 fpm. Proximate traffic When the intruder is at a range of less than 6 NM and a relative altitude of less than 1200 ft, the intruder is classified as proximate traffic. Proximate traffic is shown as a filled white or cyan diamond (lozenge) on the same EFIS ND (EHSI). The symbol has a data tag showing relative altitude and an arrow showing excessive vertical speed. Traffic Advisory (TA) When the time to closest point of approach reduces to seconds, the intruder is considered a potential threat and a Traffic Advisory (TA) is issued. The TA is shown on the EFIS ND (EHSI) as a filled yellow or amber circle accompanied with a data tag and possibly an arrow. In addition the EFIS PFD (EADI) shows a yellow or amber text message TRAFFIC while the audio system issues an aural TRAFFIC TRAFFIC message. The TA is intended to assist the flight crew in sighting nearby aircraft only. No action is to be taken solely based on a TA. Resolution advisory (RA) When the TCPA has reduced to seconds, the intruder is considered a threat INSTRUMENTATION Traffic Collision Avoidance System (TCAS) 11-3 Instrumentation E5.indb :17:21

4 INSTRUMENTATION FMC SPD VNAV PTH G/S LNAV VOR/LOC Fig. IN 11.4 EFIS PFD showing RA pitch commands and a Resolution Advisory (RA) is issued. The intended RA manoeuvre is coordinated with the intruder by means of a Mode-S datalink. The coordination must prevent an evasive manoeuvre in the same direction. There are two types of RA: a preventive RA and a corrective RA. A preventive RA gives a pitch command to maintain the current flight path. A corrective RA gives a pitch command to deviate from CMD DH the current flight path. The manoeuvre must be executed within 5 s and with a 0.25 g manoeuvre. In addition a modification to a previously issued corrective RA can be issued. This modification can ask to increase or to reverse the manoeuvre. A modified corrective RA must be executed within 2.5 s and with a more aggressive 0.35 g manoeuvre. An RA is shown on the EFIS ND (EHSI) by means of a filled red square accompanied by a data tag and a possible arrow. Depending on aircraft type, the EFIS PFD (EADI) shows a forbidden pitch attitude and/or a forbidden vertical speed in the form of a red area, see figure IN The shape and limits of these areas depend on aircraft type and aircraft performance. The latter is determined by inputs from the flaps, gear and FMS. The visual command is accompanied with an aural message. The aural message depends on the evasive manoeuvre to be conducted. See the table IN 11.1 for the complete vocabulary. Alert Aural Visual Response TA TRAFFIC, TRAFFIC YELLOW or AMBER TRAFFIC message. Maintain vertical speed and try to visually locate the intruding aircraft. Preventive RA MONITOR VERTICAL SPEED Present vertical speed is not within the red restricted vertical speed shown on the vertical speed display. Keep the vertical speed out of the red segment. Corrective RA CLIMB, CLIMB Vertical speed display red from negative limit to +1,500 fpm and green from +1,500 fpm to +2,000 fpm. Climb with vertical speed shown by the green segment Traffic Collision Avoidance System (TCAS) Instrumentation E5.indb :17:21

5 Alert Aural DESCEND, DESCEND DESCEND, CROSSING DESCEND Visual Vertical speed display red from positive limit to -1,500 fpm and green from -1,500 fpm to -2,000 fpm. Same as DESCEND. Response Descend with vertical speed shown by the green segment. Descend with vertical speed shown by the green segment and be aware that your flight path will cross the altitude of the intruder. INSTRUMENTATION CLIMB, CROSSING CLIMB Same as CLIMB. Climb with vertical speed shown by the green segment and be aware that your flight path will cross the altitude of the intruder. ADJUST VERTICAL SPEED, ADJUST Vertical speed display indicates prohibited vertical speeds in red. Target vertical speed in green. Reduce vertical speed to that shown by the green segment, but do not deviate in the red segment. MAINTAIN VERTICAL SPEED, MAINTAIN Vertical speed display indicates prohibited vertical speeds in red. Keep the vertical speed out of the red segment. MAINTAIN VERTICAL SPEED CROSSING, MAINTAIN Vertical speed display indicates prohibited vertical speeds in red. Target vertical speed in green. Keep the vertical speed out of the red segment and be aware that your flight path will cross the altitude of the intruder. Modified corrective RA CLIMB, CLIMB NOW Vertical speed display red from negative limit to +1,500 fpm and green from +1,500 fpm to +2,000 fpm. Climb with vertical speed indicate by the green segment. This advisory follows a descend advisory when circumstances require a reversal of vertical direction to ensure adequate vertical separation. DESCEND, DESCEND NOW Vertical speed display red from positive limit to -1,500 fpm and green from -1,500 fpm to -2,000 fpm. Descend with vertical speed indicated by the green segment. This advisory follows a climb advisory when circumstances require a reversal of vertical direction to ensure adequate vertical separation. INCREASE CLIMB, INCREASE CLIMB Vertical speed display red from negative limit to +2,500 fpm and green from +2,500 fpm to +3,000 fpm. Increase climb to that shown by the green segment. Indicates the vertical speed must be increased to ensure adequate separation. Traffic Collision Avoidance System (TCAS) 11-5 Instrumentation E5.indb :17:22

6 INSTRUMENTATION Alert Aural INCREASE DESCENT, INCREASE DESCENT Visual Vertical speed display red from positive limit to -2,500 fpm and green from -2,500 fpm to -3,000 fpm. Response Increase descent to that shown by the green segment. Indicates the vertical speed must be increased to ensure adequate separation. Clear of conflict CLEAR OF CONFLICT Vertical speed display red and green segments are removed. Intruder no longer appears as a red square. Return to and/or maintain the last applicable clearance unless otherwise directed by ATC Table IN 11.1 Aural and visual TCAS II alerts (aircraft typical) Control panel The combined ATC/TCAS control panel has a mode selector switch with the following modes: Standby (STBY) System in standby mode. ATC transponder and TCAS functions not active. ATC transponder code display active. Transponder (XPNDR) All basic transponder functions active. Activates ATC transponder when airborne and deactivates transponder when on ground. TCAS functions disabled. Traffic Advisory only (TA ONLY) ATC XPNDR and TCAS TA mode active but RA s disabled. An intruder will be displayed amber and the aural alert TRAFFIC, TRAFFIC will sound at the moment of an RA when TA/RA would have been selected. Traffic Advisories and Resolution Advisories (TA/RA) ATC XPNDR and TCAS functions TA/RA active. Altitude reporting off (ALT RPTG OFF) Selects ATC XPNDR altitude reporting function off within the coded reply. TCAS modes disabled. When the test button is operated a combined ATC/TCAS test is conducted. A voice message TCAS TEST OK is heard when the test result is satisfactory. With the Above-Normal-Below (ABV/N/ BLW)) switch, the flight crew can increase the airspace under surveillance. With the switch in the Normal position, traffic from 2700 ft above until 2700 ft below the own aircraft is indicated. With the switch in the Above or Below position the upper or lower surveillance area is extended to 7000 ft. (see Fig. IN 11.5) Sensitivity Level control (SLC) As stated before, the warning time of a TA varies between seconds and of an RA between seconds. The exact time depends on the so-called Sensitivity Level (SL). The higher the SL, the greater the warning time Traffic Collision Avoidance System (TCAS) Instrumentation E5.indb :17:22

7 T C A S ABV N BLW CLR TA ONLY TA/RA ALT RPTG XPNDR OFF 1 2 STBY IDENT XNPDR TEST FAIL A T C INSTRUMENTATION Fig. IN 11.5 ATC/TCAS control panel The SL is affected by the position of the mode selector switch on the ATC/TCAS control panel. As an example, when the switch is in Standby the SL is 1; when the switch is in TA Only, the SL is 2 and when the switch is in TA/RA the SL increases from 3 to 7 with increasing altitude. When the altitude increases, the SL also increases. In addition a ground based Mode-S station (ATC unit) may transmit an SL uplink message to influence the warning time of the TCAS in its airspace. Notes: Some aircraft only show TCAS traffic upon selection of a so-called TFC (Traffic) button. The TCAS display automatically pops-up in case of a TA or RA. Mode-S transponders that report on ground are not visible to TCAS. Non reporting aircraft are invisible to TCAS II. Non-altitude reporting aircraft cannot generate RA s. GPWS and windshear alerts have priority over the TCAS RA s. TA s and RA s can be shown regardless of the selected display range. For obvious reasons no descend RA s are issued below 1000 ft; no increase descend RA s are issued below 1450 ft. The generated RA can be sent to the onboard Mode-S transponder for downlink purposes to a ground based Mode-S station (ATC unit). TCAS does not show when in the EFIS Plan mode Non-EFIS equipped aircraft have a dedicated VSI display which combines the VSI and TCAS display, see figure IN In some other non-efis aircraft, the TCAS Fig. IN 11.6 Combined VSI/TCAS display Traffic Collision Avoidance System (TCAS) 11-7 Instrumentation E5.indb :17:22

8 INSTRUMENTATION display is integrated in the weather radar display In case of an RA, the FD does not show the evasive manoeuvre. It is possible to use the AP to carry out the R/A manoeuvre. However it is common practise to disengage the AP before carrying out the evasive manoeuvre Traffic Collision Avoidance System (TCAS) Instrumentation E5.indb :17:22

9 QUESTIONS INSTRUMENTATION 1 Upon interrogation, the TCAS II computer receives a reply signal containing: a) transponder code and pressure altitude. b) range and bearing. c) altitude and (vertical) speed. d) aircraft type and position. 2 The ACAS II concept asks for three Mode-S communication links: air-to-air, groundto-air and air-to-ground. Give examples of all three communication links. a) Air-air: coordination link between RA s, Ground-air: uplink of SLC, Air-ground: downlink of RA. b) Air-air: TCAS interrogation, Ground-air: uplink of RA s, Air-ground: RA downlink. c) Air-air: coordination between RA s, Ground-air: coordination of RA s, Air-ground: coordination of RA s. d) Air-air: all TCAS communication: interrogation, reply and coordination, Ground-air: Standard ATC interrogation at 1030 MHz, Air-ground: Standard ATC reply at 1090 MHz. 3 The coloured band along the scale of the VSI shown in figure IN 11.6 shows a: a) TA. b) Preventive RA. c) Corrective RA. d) Modified corrective RA. Traffic Collision Avoidance System (TCAS) 11-9 Instrumentation E5.indb :17:22

10 INSTRUMENTATION 4 When the mode selector switch on the control panel is in the TA/RA position while the own aircraft is on the ground TCAS does not show: a) Intruders. b) Aircraft that are on the ground. c) Proximity traffic. d) Other aircraft. 5 The arrow appearing near the TCAS symbol shows: a) a vector representing the direction and speed of the intruder (so-called trail ). b) a vector representing the bearing of the intruder. c) a vertical speed greater than 500 fpm. d) a climbing or descending intruder; the number represents the vertical speed in hundreds of feet. e) the direction of the evasive manoeuvre to be conducted. 6 In response to the TCAS II audio message TRAFFIC TRAFFIC, the crew should: a) Manoeuvre the aircraft in the vertical plane to increase the vertical separation. b) Attempt to visually locate the intruder and to be prepared to manoeuvre the aircraft. c) Manoeuvre the aircraft in the horizontal plane to increase the horizontal separation. d) Push the TCAS test button to ensure the system is fully operational. 7 When responding to a TCAS II Resolution Advisory, the crew must respond within: a) 2.5 seconds using g-forces of ± 0.25g. b) 5 seconds using g-forces of ± 0.25g. c) 5 second using g-forces of ± 0.5 g. d) 5 seconds and by selecting Level Change on the MCP Traffic Collision Avoidance System (TCAS) Instrumentation E5.indb :17:23

11 8 A TCAS II equipped intruder is at a relative altitude of 900 ft and at a range of 5 NM. The indication on your TCAS indicator will be: a) A filled white diamond. b) An open cyan lozenge. c) A filled red square. d) An open yellow circle. INSTRUMENTATION 9 If an intruder is SSR equipped but is non-altitude reporting, a TCAS II equipped aircraft would give the following alerting message in case the TCPA goes below 30 seconds: a) TRAFFIC TRAFFIC. b) CLIMB or DESCEND. c) MONITOR VERTICAL SPEED. d) No aural alert will be given. 10 With reference to TCAS II, a Resolution Advisory will appear as: a) an open yellow circle. b) a filled yellow circle. c) an open red square. d) a filled red square. The answers can be found at the end of the book. Traffic Collision Avoidance System (TCAS) Instrumentation E5.indb :17:23

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