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Is there an ICAO version 7.1 mandate?
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Amendment 85 to ICAO Annex 10 volume IV, published in October 2010, introduced a provision stating that all new ACAS installations after 1 January 2014 shall be compliant with version 7.1 and all ACAS units shall be compliant with version 7.1 after 1 January 2017.
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What does the European version 7.1 mandate apply to?
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On 20 December 2011, the European Commission published an Implementing Rule mandating the carriage of ACAS II version 7.1 within European Union airspace earlier than the dates stipulated in ICAO Annex 10 (see question above) by all aircraft with a maximum certified take-off mass exceeding 5,700 kg or authorised to carry more 19 passengers from 1 March 2012 with the exception of aircraft with an individual certificate of airworthiness issued before 1 March 2012 that must be equipped as of 1 December 2015. Aircraft not referred above but which will be equipped on a voluntary basis with ACAS II, must be equipped with version 7.1.
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Is version 7.1 compatible with earlier TCAS versions?
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Yes, version 7.1 is compatible with all existing versions being operated today; i.e. version 7.0 as well as version 6.04a (which is still in use by a small population of aircraft, mainly outside Europe).
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Why does it take so long to develop a new version of TCAS?
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Any change to the TCAS logic requires extensive studies to assess safety benefits and operational impact. Moreover, TCAS is a worldwide system, thus any change requires a consensus from all the parties involved in the design, manufacturing, certification and operations of TCAS. These are, by their nature, time consuming processes.
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Where can I find the full specification of version 7.1?
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TCAS II version 7.1 MOPS (Minimum Operational Performance Specification) has been simultaneously published by RTCA in the USA and EUROCAE (European Organisation for Civil Aviation Equipment).
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Where can I find the full text of European mandate?
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The European mandate has been published here:
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What validation was performed before the version 7.1 mandate?
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Both changes were subject of detailed review of standardization bodies (RTCA and EUROCAE) and certification authorities.
The replacement of the “Adjust vertical speed, adjust” RA with the new “Level off, level off” RA has been preceded by detailed analysis of events and radar data from several places in Europe and two busy TMAs in the USA. The radar data analyses indicate the new RA will not cause induced conflicts with third party aircraft more frequently than the current version of TCAS. Additionally, the new “Level off, level off” RA was positively assessed by pilots participating in flight simulator experiments.
The change to the reversal logic has been evaluated using mathematical modelling.
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What pilot training is required?
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Before the new version of TCAS is deployed to their fleets Aircraft Operators should ensure that crews are: (1) aware of the TCAS version upgrade; and (2) trained on the new “Level off, level off” RA and understand how to respond correctly to this RA.
There are no other differences (visible to pilots) between version 7.0 and version 7.1.
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What actions are required by aircraft operators?
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Aircraft Operators should ensure that TCAS version 7.1 is deployed on their fleet according to the mandate.
Aircraft Operators should also ensure that their flight crews understand the new features version 7.1 brings and are trained on correct responses to “Level off, level off” RAs.
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What controller training is required?
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Air Navigation Service Providers should ensure that air traffic controllers are: (1) aware of the TCAS version upgrade; and (2) understand the effect the new “Level off, level off” RA will have on ATC operations (i.e. there will be some instances of premature level-offs by aircraft following the RA).
Besides that there are no differences (visible to pilots and controllers) between version 7.0 and version 7.1.
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What is the difference between TCAS and ACAS?
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TCAS (Traffic Alert and Collision Avoidance System) is a specific implementation of the ACAS (Airborne Collision voidance System) concept. TCAS II version 7.0 and 7.1 are currently the only available equipment that is fully compliant with the ACAS II Standards and Recommended Practices (SARPs).
ACAS II provides "Resolution Advisories" (RA's) in the vertical sense (direction) telling the pilot how to regulate or adjust his vertical speed so as to avoid a collision.
TCAS II Minimum Operational Performance Specification (MOPS) have been published by RTCA (DO-185B) and EUROCAE (ED-143).
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How does ACAS II work if one aircraft is using the standard altimeter setting and the other QNH?
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ACAS II (TCAS II) always utilises pressure altitude information which relates to the standard pressure (altimeter setting 1013.25 hPa).
TCAS II operations are not affected if aircraft are flying Flight Levels on the standard pressure setting, altitude on QNH, or height on QFE as all always the same pressure source (i.e. standard) is used. The pressure selection by the flight crew does not affect the TCAS II system at all.
Additionally, below 1750 feet, ACAS also uses radio altimeter data.
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Does TCAS II operate at low altitudes?
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INCREASE DESCENT RAs are inhibited below 1,450 feet radio altitude while descending and 1,650 feet while climbing.
DESCEND RAs are inhibited below 1,000 feet radio altitude when descending, and below 1,200 feet radio altitude when climbing.
All RAs are inhibited below 900 feet radio altitude when descending, and below 1,100 feet radio altitude when climbing.
TCAS voice alerts are inhibited below 500 feet radio altitude.
EGPWS (Enhanced Ground Proximity Warning System), wind-shear and stall warnings will take precedence over TCAS II RAs. TCAS II will automatically switch to the TA-only mode of operation once one of the above mentioned warnings has been generated. It will remain in TA-only mode for 10 seconds after the GPWS/TAWS or wind shear warning is removed.
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What is ACAS I (TCAS I)?
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ACAS I provides Traffic Advisories (TAs) but does not provide collision avoidance manoeuvre indications (RAs are not issued). Currently the only implementation of the ACAS I concept is TCAS I. ICAO Standards and Recommended Practices (SARPs) for ACAS I are published in ICAO Annex 10, volume IV and are limited to interoperability and interference issues with ACAS II. TCAS I Minimum Operational Performance Specification (MOPS) have been published by RTCA (DO-197A).
ACAS I is not mandated in Europe and there are no operational rules regarding the use of ACAS I. The main purpose of ACAS I is to aid pilots in acquiring threats visually; the collision avoidance manoeuvre direction is left to pilot’s discretion. ACAS I operations cannot be coordinated with ACAS II.
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What is the frequency of RAs?
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It is estimated, through various monitoring programmes and data obtained from operators, that an RA occurs every approximately 1000 flight hours on short and medium haul aircraft. The number increases to 3000 hours for long haul aircraft.
The results of monitoring in core European airspace (conducted within the EUROCONTROL PASS project) showed that in 80% of encounters only one aircraft involved in the encounter received an RA. It is because the threat aircraft was not TCAS II equipped, or the geometry of the conflict was such that the RA was not generated on the TCAS-equipped threat aircraft, or the threat’s TCAS was in TA-only mode. An RA encounter was observed every 2160 flight hours.
The most common observed RA was the Adjust Vertical Speed which amounted for 67% of all RAs, followed by the Climb (13%) and Descend (9%).
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Can an RA be generated against an aircraft that is not ACAS II equipped?
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An RA can be generated against all aircraft equipped with an altitude reporting transponder (Mode S or Mode A/C ). The intruder does not need to be fitted with TCAS II. However, RAs are coordinated only between TCAS II equipped aircraft.
Experience shows that in the majority of cases only one aircraft will receive an RA (regardless of whether the intruder is TCAS II aircraft or not).
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Can aircraft equipped only with Mode A transponders be detected by ACAS II?
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Aircraft equipped with Mode A transponders only are not tracked and detected by ACAS II because ACAS II does not use Mode A interrogations. However, if the aircraft is equipped with Mode A/C transponder but does not provide altitude information (Mode C) this aircraft will be tracked as a non-altitude reporting target using range and bearing information and it will shown on TCAS II traffic display. Neither a data tag nor a trend arrow will be shown for traffic symbol for an intruder that is not reporting altitude. TAs will be generated against non-altitude reporting aircraft when the range test for TA generation is satisfied. Non-altitude reporting aircraft are deemed to be at the same altitude as own aircraft.
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What are ACAS II range limitations?
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The required nominal tracking range of the ACAS II is 14 NM. However, when operating in high density, the interference limiting feature may reduce system range to 4.5 NM which is the range adequate to provide protection for a 500 kts encounter. Some ACAS implementations allow for the use of the Mode S extended squitter, the normal surveillance range may be increased beyond the nominal 14 NM. However, this information is not used for collision avoidance purposes. The equipment is not intended to interrogate a target unless the altitude information indicates that it is within 10,000 ft of own altitude. ACAS II will neither display nor give alerts against intruders with a vertical speed in excess of 10,000 ft/min. or closure speed over 1200 kt.
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Where can I find information regarding ACAS II operations on the ground?
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Best practices for ACAS II operations on the ground are outlined on the front page of ACAS II Bulletin no. 9.
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What should a pilot do on receiving a TA?
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In accordance with PANS-OPS (ICAO Doc.8168), on receipt of a Traffic Advisory (TA) the pilot must use all available information to prepare for appropriate action if an RA occurs. Pilots must not manoeuvre their aircraft in response to TAs only.TAs are intended to alert pilots to the possibility of a resolution advisory (RA), to enhance situational awareness, and to assist in visual acquisition of conflicting traffic. However, visually acquired traffic may not be the same traffic causing a TA. Visual perception of an encounter may be misleading, particularly at night.
It must be noted that not all TAs are followed by an RA.
For more information see:
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What should a pilot do on receiving an RA?
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In accordance with PANS-OPS (ICAO Doc.8168), in the event of a Resolution Advisory (RA), the pilot must respond immediately by following the RA as indicated, even if there is a conflict between the RA and an air traffic control (ATC) instruction to manoeuvre. The pilot must not manoeuvre in the opposite direction to an RA.
For more information see:
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Is it required to follow an RA if the conflict aircraft is in sight?
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Provisions of ICAO PANS-OPS (Doc. 8168) are very clear - in the event of a Resolution Advisory (RA), the pilot must respond immediately by following the RA unless doing so would jeopardize the safety of the aircraft. That provision applies in all airspace classes and all meteorological conditions (i.e. VMC and IMC). If there is a conflict between the posted RA and an air traffic control (ATC) instruction, the RA must be followed and the ATC instruction ignored. The pilot must never manoeuvre in the opposite direction to an RA.
RAs will be triggered if TCAS calculates that there is a risk of collision between aircraft. The evaluation whether the RA was ‘nuisance’ can be done reliably in hindsight only. In real-time the pilot has little chance to assess whether the traffic acquired visually is in fact the one against which the RA has been generated.
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Are there any requirements to reduce the vertical speed in the last 1000 feet before level-off?
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Effective 22 November 2008, the following provision is applicable in ICAO PANS-OPS (Doc. 8168):
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.
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Shall I follow a "Climb RA" if I already operate at the maximum altitude?
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Even at the maximum certified flight level, aircraft should have the ability to climb, albeit maybe at less than 1500 fpm, for a short period of time – the RA duration is about 30 seconds at high altitudes. If unable to achieve the required climb rate, pilots should apply the best possible rate and must never manoeuvre opposite to a RA.
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Should RAs be ignored during closely spaced parallel approaches?
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TCAS II should be operated in the TA/RA mode at all times. An aircraft operating in the TA-only mode is denied the benefit of collision avoidance. When TCAS TA/RA mode is selected, then the crew should follow an RA generated. Not to do so would not only deny own aircraft the safety benefit provided by the RA, but would also compromise the effectiveness of a coordinated RA generated on the other aircraft.
Even if closely-spaced parallel approaches procedures are correctly applied ‘nuisance’ RAs may occasionally occur. However, the safety benefit provided by ACAS takes precedence over an occasional ‘nuisance’ RA. Additionally, there is always a possibility that another aircraft will penetrate the approach airspace causing a real threat.
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When can I use TCAS in TA only mode?
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The circumstances in which TCAS II can be operated in the TA-only mode should be stated in the pilot’s Flight Operations Manual, and are generally limited to certain in-flight failures (such as engine failure or emergency descent) and other specific conditions as specified by the appropriate regulatory authorities.
The aircraft operating TCAS II in the TA-only mode will not benefit from the TCAS II safety net, i.e. will not receive RAs.
If ACAS TA/RA mode is selected, then the crew should follow an RA generated. Not to do so not only would deny own aircraft the safety benefit provided by the RA, but would also compromise the effectiveness of a coordinated TCAS RA generated on the other aircraft. In other words, if a decision is taken not to follow any RA in a particular circumstance or procedure, then TCAS should be in TA-only mode.
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What is the correct pilot reaction if an RA is received while aircraft is in turn?
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ICAO ACAS Manual (Doc. 9863) states "if possible, comply with the controller’s clearance, e.g. turn to intercept an airway or localizer, at the same time as responding to an RA."
Flying the RA is the highest priority. Therefore, if the aircraft is in turn which makes achieving the required vertical rate difficult or impossible, the turn should be stopped. It may happen that stopping the turn will put your aircraft in the closer horizontal proximity to the threat aircraft but TCAS is evaluating the situation every second and it will change the RA if required. It is not required to comply with ATC instructions while responding to RAs.
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What is the correct way to report an RA to ATC?
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Effective 22 November 2007 ICAO PANS-OPS and PANS-ATM (Doc. 8168 and 4444) have been changed with regard to TCAS RA reporting.
In summary, pilots are now supposed to report only those RAs that cause them to depart from their current ATC clearance and the phraseology is "TCAS RA".
Note: The correct pronunciation of the phrase "TCAS RA" is "TEE-CAS-AR-AY" (per ICAO Doc. 8400 - ICAO Abbreviations and Codes).
For more information see:
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What is a 'nuisance' RA?
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ICAO Annex 10 states that an RA shall be considered a 'nuisance' unless, at some point in the encounter in the absence of ACAS, the horizontal separation and the vertical separation would have been simultaneously less than the following values:
Above FL100: horizontal separation: 2.0 NM; vertical separation 750 ft Below FL100: horizontal separation: 1.2 NM; vertical separation 750 ft
It needs to be noted that the evaluation whether the RA was ‘nuisance’ or ‘unnecessary’ can be done only in hindsight. In real-time the pilot has little chance to correctly evaluate the threat (e.g. assess whether the threat aircraft is levelling off at its cleared level).
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Shall I inform ATC if my TCAS II fails in flight?
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In Europe, there is no requirement to notify ATC of TCAS II in flight failures.
Similarly, if the aircraft is dispatched with TCAS II inoperative (under Minimum Equipment List), there is no requirement to notify ATC or to make a remark in the flight plan about that.
However, some States outside Europe, e.g. India, require a remark in the flight plan confirming ACAS equipage. Please refer to the individual States authorities for more information.
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Can ACAS II downlink RA information to the controller?
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ACAS II has the capability to downlink RAs to the ground-based air traffic control system via Mode S (if the necessary complementary features in the ground system have been installed). An automatic downlink of RAs to controller working position is under investigation.
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What are the controller actions once the pilot has reported an RA?
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In accordance with PANS-ATM (ICAO Doc. 4444) when a pilot reports an ACAS resolution advisory (RA), the controller must not attempt to modify the aircraft flight path until the pilot reports “Clear of Conflict”.
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As a controller, how do I know which version of TCAS the aircraft operates?
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There is no need for controllers to know which version of TCAS the aircraft operates. In Europe there is no requirement to indicate in the flight plan which version of TCAS the aircraft operates or whether the aircraft is equipped.
TCAS versions are compatible, so proper TCAS-TCAS coordination will take place in coordinated encounters.
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Do I need to apply different procedures or separation minima to the aircraft which are not TCAS equipped?
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As stipulated in ICAO Doc. 4444, the provision of air traffic services to aircraft equipped with TCAS shall be identical to those that are not equipped.
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Can an aircraft operate without serviceable TCAS?
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In Europe an aircraft may operate under the Minimum Equipment List (MEL) provisions with TCAS II inoperative for up to 10 calendar days.
In German airspace the time period during which TCAS II may be inoperative is reduced to 3 days. National regulators may impose more restrictive deadlines for some operators or parts of airspace.
In Europe, there is no requirement to notify ATC or to make a remark in the flight plan about TCAS II being inoperative.
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What are TCAS II equipage requirements in Europe?
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As of 1 January 2005, ACAS II version 7.0 must be carried and operated in the European Region by all civil, fixed-wing turbine-engined aircraft having a maximum take-off mass exceeding 5,700 kg or a maximum approved passenger seating configuration of more than 19.
On 20 December 2011, the European Commission published an Implementing Rule mandating the carriage of ACAS II version 7.1 within European Union airspace earlier than the dates stipulated in ICAO Annex 10 (see question above) by all aircraft with a maximum certified take-off mass exceeding 5,700 kg or authorised to carry more 19 passengers from 1 March 2012 with the exception of aircraft with an individual certificate of airworthiness issued before 1 March 2012 that must be equipped as of 1 December 2015. Aircraft not referred above but which will be equipped on a voluntary basis with ACAS II, must be equipped with version 7.1.
Other State or regions may apply different requirements. Please refer to the individual States authorities for more information.
For TCAS II MEL (Minimum Equipment List) provisions see the following question.
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What are the equipage requirements for military (state) aircraft?
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For information regarding State (military) equipage requirements, please refer to the web page of the EUROCONTROL Directorate if Civil-Military ATM Coordination:
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What are TCAS II MEL (Minimum Equipment List) requirements?
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The MEL for TCAS II through out Europe is Class A - 10 days (excluding the day of failure discovery). Operation under the terms of the EASA-OPS 1 TCAS II MEL has been agreed and accepted by the ECAC Member States. JAA TGL 26 (which is still applicable) states that TCAS II "may be inoperative provided the system is deactivated and secured, and repairs or replacements are carried out within 10 calendar days. Note: Local Authorities may impose a more restrictive rectification interval days."
Additional MEL requirements concerning partial failures are also listed in the TGL 26. Note: the actual MEL period applicable to an aircraft is set by the National Authority of the aircraft operator, but if longer than Class A it would cause operational issues for flight in European airspace.
However, one State in Europe applies a more restrictive requirement: in German airspace the time period during which TCAS II may be inoperative is reduced to 3 days (refer to German AIP GEN 1.5 para. 5). This applies to all aircraft.
In Europe, there is no requirement to notify ATC or to make a remark in the flight plan about TCAS II being inoperative.
Finally, if flying with an unserviceable ACAS II, then the altitude reporting transponder must be serviceable.
NOTE:
1. Equipage and flight planning requirements are subject to change at any time. Please refer to the individual States authorities for more information.
2. Equipage and flight planning requirements outside Europe, e.g. India, China, Japan, Australia are different. Please refer to the individual States authorities for more information.
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In my country my aircraft is not required to be fitted with ACAS II. I am coming to Europe only for a short trip. Can I get an exemption from the ACAS II equipage requirements?
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ACAS II equipage exemptions requests must be requested directly from the national regulatory authorities of all countries whose airspace the flight will enter.
From 1 January 2010 EUROCONTROL will no longer process any ACAS II exemption requests, including those for delivery or maintenance flights.
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Do I require ACAS/TCAS to operate in Reduced Vertical Separation Minima (RVSM) airspace?
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The carriage and operation of ACAS/TCAS is not an RVSM requirement in itself. For ACAS Equipage requirements and MEL provisions see questions above.
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Will VLJ (Very Light Jets) be required to be fitted with ACAS II in European airspace?
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Under current regulations, VLJs are outside the ACAS II carriage requirement. In October 2009, EUROCONTROL completed the ACAS on VLJs and LJs – Assessment of safety Level (AVAL) study to assess whether VLJ operations, as currently forecast, will affect the performance of the ACAS safety net.
The project findings support the conclusion that modifying the criteria for ACAS II equipage in Europe so as to include at least the mainstream VLJs, and preferably all light jets under 5700 kg, is the best option for safe and effective VLJ operations in Europe. The project also concluded that equipping VLJs and other LJs with TCAS I is the less preferred option and no ACAS equipage of these aircraft is better.
The project recommended to extend the European ACAS II mandate to include all civil fixed-wing turbine-engined aircraft with a maximum cruising speed of over 250 kt.
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Will UAS (Unmanned Aircraft Systems) be equipped with ACAS II?
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At this time, it has not yet been determined whether UAS (Unmanned Aircraft Systems) will be required to be equipped with ACAS II or any other UAS dedicated collision avoidance system. ACAS II has been designed for use by the flight crew of manned aircraft and its use by UAS needs to be carefully examined.
In order to ensure safety benefit to all airspace users, any UAS collision avoidance must be interoperable with ACAS II. The role of Sense-and-Avoid in UAS collision avoidance needs also to be examined.
A CAUSE study that was completed in December 2009 examined the subject of UAS collision avoidance. The results from previous EUROCONTROL safety studies of ACAS were used to demonstrate that there is a need for UAS to have a collision avoidance capability comparable to that delivered by ACAS on manned aircraft. The study also investigated to what extent carriage of ACAS by UAS might deliver this capability and the issues involved.
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What is the status of the development of ACAS that will issue both horizontal and vertical Resolution Advisories?
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Although ACAS III, a system that provides horizontal and vertical collision avoidance manoeuvres, is mentioned as a future system in ICAO Annex 10, ACAS III is unlikely to materialize due to difficulties the current surveillance systems have with horizontal tracking. Currently, research is being conducted to develop a future collision avoidance system (under the working name of ACAS X). If developed and certified, ACAS X will not be commercially available before mid 2020s. It is unclear at this stage whether ACAS X would provide any horizontal resolutions.
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