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ACAS II operations in the European RVSM environment

ACAS II operations in the European RVSM environment. Introduction. Training package on ACAS II performances in the RVSM operations : A brochure detailing the interaction between ACAS II and RVSM A quiz to verify that the basic knowledge is acquired

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ACAS II operations in the European RVSM environment

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  1. ACAS II operationsin the European RVSM environment

  2. Introduction • Training package on ACAS II performances in the RVSM operations: • A brochure detailing the interaction between ACAS II and RVSM • A quiz to verify that the basic knowledge is acquired • This set of slides as a support for academic training • Plan of the presentation: • Background • TCAS II description • Expected TCAS II alerts in RVSM airspace • Common encounters in RVSM operations in Europe • Operational implication

  3. Background

  4. ACAS II mandate in Europe (1/2) • Two steps: • 1st January 2000: all civil fixed-wing turbine-engine aircraft having a maximumtake-off mass exceeding 15,000 kg, or a maximum approved passenger seating configuration of more than 30 will be required to be equipped with ACAS II • 1st January 2005: all civil fixed-wing turbine-engine aircraft having a maximumtake-off mass exceeding 5,700 kg, or a maximum approved passenger seating configuration of more than 19 will be required to be equipped with ACAS II • ACAS II can trigger two types of advisories: • Traffic Advisories (TAs): • To prepare the crew for a possible RA and to help in the visual search • Resolution Advisories (RAs): • Avoidance manoeuvres in the vertical plane recommended to the pilot

  5. ACAS II mandate in Europe (2/2) • TCAS II version 6.04a: • Is not fully compliant with the ICAO ACAS II SARPs • Is not operationally compatible with RVSM • TCAS II version 7.0: • Was developed to improve the general TCAS II performances • Addresses the issue of TCAS II version 6.04a operational incompatibility with RVSM • Practical implementation issues (supply, installation, certification)  temporary exemptions until 30 September 2001

  6. RVSM implementation (1/2) • Principle: reduction of the separation to 1000 ft from FL290 to FL410 • Goal: increased airspace capacity and flight efficiency • Key dates: • March 1997: first implementation in the North Atlantic Region (NAT) • February 2000: implementation in the Pacific Region • January 2002: implementation in Europe • Early introduction in Germany/Austria (tactical), Ireland/UK (strategic) • ACAS II mandatory carriage and RVSM implementation are not linked • ACAS II is not, itself, a prerequisite for RVSM

  7. RVSM implementation (2/2)

  8. Experience of TCAS II in NAT RVSM • RVSM implemented in NAT since 27 March 1997 • Aircraft equipped with TCAS II version 6.04a have flown in this airspace since this date • A preliminary study had highlighted that TCAS II version 6.04a was not adapted to RVSM and anticipated some operational issues • Operational feedback: • Many undesirable long duration TAs (for aircraft flying on adjacent flight levels with similar speeds) • A few undesirable RAs (mainly due to turbulence or imperfect altitude keeping) • No approved operational procedure to avoid long duration advisories

  9. TCAS II description

  10. Principles (1/2) • System designed to prevent mid-air collisions and near mid-air collisions • Autonomous and independent of any aircraft navigation equipment and ground systems • TCAS II is based on two main concepts: • time-to-go to the Closest Point of Approach (CPA); modified according to • altitude bands (sensitivity level) • In a one-second cycle, TCAS II: • Monitors other aircraft by interrogating their transponder • Selects an advisory when a range testandan altitude test are satisfied • Presents the information to the pilot

  11. Principles (2/2) • RAs are co-ordinated between TCAS II equipped aircraft  Pilots should follow their RAs as accurately as possible • A manoeuvre in the opposite sense to the RA is hazardous

  12. Advisory triggering: range test (1/2) • TCAS II computes the time-to-go to the CPA • Based on the closure rate and the distance between aircraft • If this time is lower than the set threshold, the range test is satisfied TCAS Intruder < 48 s (TA) < 35 s (RA)

  13. Advisory triggering: range test (2/2) • Specific case of very low closure rates between aircraft: • If the distance between aircraft is lower than the set protection distance, the range test is satisfied Intruder (very low closure rate) TCAS 1.3 NM (TA) 1.1 NM (RA)

  14. Advisory triggering: altitude test (1/2) • TCAS II computes the time to reach co-altitude • Based on the relative altitude between aircraft and the relative vertical speed • If this time is lower than the set threshold, the altitude test is satisfied TCAS < 48 s (TA) < 35 s (RA)< 25 s (RA for level aircraft) Intruder

  15. 850 ft (TA) 700 ft (RA) TCAS 850 ft (TA) 700 ft (RA) Advisory triggering: altitude test (2/2) • Specific case of level aircraft: • If the relative altitude between aircraft is lower than the set threshold, the altitude test is satisfied  No advisory  Advisory triggering

  16. Improvements of TCAS II version 7.0 • Modification of altitude band boundaries: FL200-FL300 FL200-FL420 • Additional improvements: • Decrease of the reduced time threshold for level aircraft (30 s  25 s) • Introduction of a Miss-Distance Filtering to filter RAs in the horizontal plane • Fewer RAs that go against the aircraft vertical trajectory • Triggering of weakening RAs to reduce deviations in very slow divergence

  17. Expected TCAS II advisoriesin RVSM airspace

  18. The ACASA project • Airborne Collision Avoidance System Analysis • EUROCONTROL co-ordinated project • Investigates several areas related to TCAS II operations in Europe • Includes a study of ACAS/RVSM interaction in Europe: • Objective: investigate technical and potential operational issues • Method: use of multiple sources of data • Modified radar data • Real-time and fast-time simulations • Automatic and non-automatic artificial encounters • Comparison between current situation in the FL250-FL290 altitude band and the forecast situation above FL290

  19. Traffic Advisories • Increased number of TAs in comparison with CVSM but lower number than in the FL250-FL290 altitude band • Expected TA rate with TCAS II version 7.0: • 1 TA every 20 flight hours in RVSM • 1 TA every 60 flight hours in CVSM • 1 TA every 5 flight hours in the FL250-FL290 altitude band • Expected TA rate with TCAS II version 6.04a: • 1 TA every flight hour in RVSM (operational issue for the pilots) • Repetitive TAs are a potential operational issue for pilots • Long duration TAs are not expected to be an issue

  20. Resolution Advisories • Increased number of RAs in comparison with CVSM but lower number than in the FL250-FL290 altitude band • Expected RA rate with TCAS II version 7.0: • 1 RA every 330 flight hours in RVSM • 1 RA every 1000 flight hours in CVSM • 1 RA every 70 flight hours in the FL250-FL290 altitude band • Expected RA rate with TCAS II version 6.04a: • 1 RA every 165 flight hours in RVSM (high number of TAs without RAs) • Undesirable RAs mainly triggered during 1000 ft level-off encounters but in similar proportion than in the FL250-FL290 altitude band

  21. Concluding Remarks • TCAS II version 7.0 is compatible with RVSM: • Increased number of advisories in comparison in CVSM • But lower number than between FL250 and FL290 • TCAS II version 6.04a is not operationally compatible with RVSM • Mixed equipage (low percentage of TCAS II version 6.04a): • No expected significant implication for controllers • Operational issue for the pilots of TCAS II version 6.04a equipped aircraft • No TCAS II safety issues related to the implementation of RVSM • TCAS II efficiency is maintained in RVSM airspace

  22. Common encounters for RVSM operations in Europe

  23. Standard RVSM vertical separation (1/2) • Both aircraft are level and separated by 1000 ft • TCAS II version 7.0: • Altitude thresholds: 850 ft for TAs and 700 ft for RAs • The altitude test is not satisfied  No advisory

  24. Standard RVSM vertical separation (2/2) • TCAS II version 6.04a: • Altitude thresholds: 1200 ft for TAs and 800 ft for RAs • The altitude test for TAs is satisfied  Triggering of TAs • Concluding remarks: • TCAS II version 7.0 is compatible with 1000 ft vertical separation • Pilots of TCAS II version 6.04a equipped aircraft will experience a large number of TAs • If a TA is triggered, • Pilots shall comply with the airline operational instructions • Standard pilots’ behaviour is expected • No controllers’ action since TAs should not be notified by pilots

  25. Vertical offset (1/3) • Two level aircraft at adjacent flight levels have a vertical offset • Considerations about altitude: • RVSM approved aircraft can have a 65 ft maximum vertical offset • Aircraft report their altitude by either 25 ft or 100 ft quanta • TCAS II can overestimate vertical offsets (e.g. 75 ft or 100 ft instead of 65 ft)

  26. Vertical offset (2/3) • With a 100 ft quantization: • The intruder is seen with a 100 ft offset if the real offset  51 ft • The relative altitude seen by TCAS II can be as low as 835 ft • Combined offset: 100 ft (intruder) + 65 ft (own) = 165 ft • A combined offset of 150 ft is needed to trigger a TA (threshold = 850 ft) A TA will be triggered if both aircraft have an offset  51 ft • A combined offset of 300 ft is needed to trigger a RA (threshold = 700 ft) No RA can be triggered • With a 25 ft quantization: • The intruder is seen with a 75 ft offset if the real offset  63 ft • The relative altitude seen by TCAS II cannot be lower than 860 ft No advisory can be triggered

  27. Vertical offset (3/3) • TCAS II version 6.04a: • Altitude thresholds: 1200 ft for TAs and 800 ft for RAs • Triggering of TAs between level aircraft separated by 1000 ft with or without vertical offsets • Concluding remarks: • TAs triggered by TCAS II version 7.0 because of vertical offsets are rare events: • Two offsets  51 ft and • 100 ft altitude report quantization (a small proportion of the fleet) • If a TA is triggered, • Pilots shall comply with the airline operational instructions • Standard pilots’ behaviour is expected • No controllers’ action since TAs should not be notified by pilots

  28. Oscillation (1/2) • Two level aircraft at adjacent flight levels oscillate in the altitude keeping • RVSM approved aircraft can oscillate with a 65 ftmaximum amplitude • This configuration is very similar to the vertical offset: • All calculations for vertical offsets are valid for oscillations

  29. Oscillation (2/2) • Concluding remarks: • TAs triggered by TCAS II version 7.0 because of oscillations are rare events: • Two simultaneous opposite oscillations  51 ft and • 100 ft altitude report quantization (a small proportion of the fleet) • TCAS II version 6.04a triggers TAs between level aircraft separated by 1000 ft with or without oscillations • When a TA is triggered, • Pilots shall comply with the airline operational instructions • Standard pilots’ behaviour is expected • No controllers’ action since TAs should not be notified by pilots

  30. Turbulence (1/3) • Due to turbulence, an aircraft deviates towards another aircraft level at an adjacent flight level • TCAS II detects a high vertical closure rate configuration

  31. Turbulence (2/3) • Similar behaviour of both aircraft’s TCAS II • If the predicted time to co-altitude is • Lower than the time thresholds for TAs and RAsA pop-up RA is triggered • Lower than the time threshold for TAs onlyA TA is triggered • TCAS II version 6.04a: • Triggering of TAs between level aircraft separated by 1000 ft with or without turbulence • Slightly greater number of pop-up RAs for level aircraft

  32. Turbulence (3/3) • Concluding remarks: • These events should be rare in the European airspace • More likely in NAT • When a TA or an RA is triggered, • Pilots shall comply with the airline operational instructions • Normal pilots’ response is expected • When a controller is informed of a manoeuvre in response to an RA, the standard procedure shall apply

  33. 1000 ft level-off encounter (1/4) • One aircraft is levelling off at 1000 ft from another aircraft • TCAS II has no knowledge of the intent of own and intruder aircraft • Assumes that the current flight profiles will be maintained

  34. 1000 ft level-off encounter (2/4) • Altitude test based on the time to reach co-altitude • Example: • TCAS II of the descending aircraft triggers a TA and an RA • TCAS II of the level aircraft triggers only a TA

  35. 1000 ft level-off encounter (3/4) • RAs are more likely if both aircraft are manoeuvring • TCAS II version 7.0 tries to avoid “Climb” RAs for descending aircraft (and vice-versa) • A 1000 fpm vertical rate by 1000 ft from the cleared altitude should: • Prevent RAs • Minimise the number of TAs • TCAS II version 6.04a is expected to trigger more RAs than version 7.0: • Larger time threshold for level aircraft (30 s) • More RAs requesting a vertical trajectory change • Many improvements provided by TCAS II version 7.0

  36. 1000 ft level-off encounter (4/4) • Concluding remarks: • TAs and RAs can be triggered during 1000 ft level-off encounters • These RAs can be necessary if an aircraft does not level-off as required • With a 1000 fpm vertical rate by 1000 ft from the cleared altitude : • no advisory is expected to be triggered with one manoeuvring aircraft • only TAs are expected to be triggered with two manoeuvring aircraft • When a TA or an RA is triggered, • Pilots shall comply with the airline operational instructions • Normal pilots’ response is expected • When a controller is informed of a manoeuvre in response to an RA, the standard procedure shall apply

  37. Low closure rate encounter (1/3) • Two level aircraft at adjacent flight levels follow the same route. The trailing aircraft has a slow overtake • Slow overtake: • By itself does not trigger any advisory, but • Induces longer durationadvisories when triggered by the previous configurations

  38. Low closure rate encounter (2/3) • For low closure rates, the range test is based on the protection distance: • 1.3 NM for TAs • 1.1 NM for RAs • Advisories last about the time that the aircraft remains within the protection distance • TCAS II includes mechanisms for an early termination of advisories: • TAs and RAs end: • When the aircraft are diverging • Even if they are still slightly within the protection distance • Possible secondary short duration advisories

  39. Low closure rate encounter (3/3) • TCAS II version 6.04a: • Higher frequency because of the greater number of TAs • No weakening RAs  larger deviations • Concluding remarks: • These events should be rare in the European airspace • More likely in NAT • When a TA or an RA is triggered, • Pilots shall comply with the airline operational instructions • Normal pilots’ response is expected • When a controller is informed of a manoeuvre in response to an RA, the standard procedure shall apply

  40. Operational implication

  41. Controllers (1/2) • Increased number of TCAS II advisories in comparison with CVSM • But lower number than between FL250 and FL290 • No effect on RVSM operations • TAs should not be notified by pilots to controllers • No standardised phraseology to report TAs • When a pilot reports a manoeuvre induced by an RA, the controller: • Shall not attempt to modify the aircraft flight path but • Shall provide traffic information as appropriate (PANS-RAC Doc. 4444) • The controller is not responsible for the provision of separation for an aircraft, which is deviating from a clearance in response to an RA

  42. Controllers (2/2) • Increased number of TAs in comparison with CVSM : • Possible increase of the number of traffic information to provide because of pilots’ requests • Increased number of RAs in comparison with CVSM : • Increased number of pilots’ response to RAs, but: • Only a subset inducing a deviation from the clearance with TCAS II version 7.0 • Lower number than in the FL250-290 altitude band • Deviations should be limited when pilots follow properly the RAs • Induced encounters with a third aircraft expected to be unlikely • TCAS II includes a Multiple Threat Encounter logic • Provision of traffic information if practical during encounters that can result in RAs (e.g. 1000 ft level-off)

  43. Lower than in the FL250-FL290 altitude band Pilots (1/2) • Increased number of TCAS II advisories: • Expected advisories with TCAS II version 7.0: • 1 TA every 20 flight hours • 1 RA every 330 flight hours • Expected advisories with TCAS II version 6.04a: • 1 TA per flight hour • 1 RA every 165 flight hours • Recommendation for 1000 ft level-off encounters: • 1000 fpm vertical rate by 1000 ft from the cleared altitude: • No advisory when one aircraft is manoeuvring • Only TAs if both aircraft are manoeuvring

  44. Pilots (2/2) • Standard behaviour (cf. PANS OPS Doc. 8168 and JAA TGL 11) • Pilot’s response is the same whatever the TCAS II version and the airspace • Pilots shall comply with their airline operational instructions • When a TA is triggered, pilots: • Should not request for a traffic information • Shall not manoeuvre in response to TAs only • When an RA is triggered, pilots: • Should follow the RA as accurately as possible • Shall not manoeuvre in the opposite sense of the RA • Shall limit the deviation to the minimum extent necessary to comply with the RAs • Shall promptly return to the initial clearance after the “Clear of Conflict” • To take a lateral offset to stop a long duration advisory is not an approved procedure

  45. Conclusion • TCAS II is a last resort safety net designed to prevent mid-air collisions • Valid in RVSM as in any other airspace • TCAS II version 7.0 is compatible with RVSM • TCAS II version 6.04a is not operationally compatible with RVSM • The implementation of RVSM shall not modify pilots’ and controllers’ behaviours • Pilots shall not manoeuvre in the opposite sense of the RAs • Maximum safety is obtained through compliance with RAs

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