Review of Policy in Regards to TCAS RA Downlink

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Review of Policy in Regards to TCAS RA Downlink

53RD ANNUAL CONFERENCE, Gran Canaria, Spain, 5-9 May 2014

WP No. 94

Review of Policy in Regards to TCAS RA Downlink

Presented by PLC and TOC

Summary

The purpose of the paper is to examine the introduction of TCAS RA down link to controller working positions (CWP) and the impact on controllers’ workload and separation responsibilities. It will highlight safety and operational issues caused by TCAS-RA downlink, and review current IFATCA policy on TCAS RA down-linking to CWP.

Introduction

1.1  TCAS is a simple, robust and independent system that functions as a stand-alone airborne anti-collision safety net. Currently, TCAS procedures require that a crew, responding to a TCAS RA, shall disregard any contrary ATC clearance.

1.2  The near miss in 2001 over Japan and the mid-air collision which happened in 2002 over Lake Constance, Southern Germany, where during a TCAS RA event, a contrary (to the RA sense) ATC-instruction was given, which was followed by the crew of one of the aircraft involved. These events revealed the worst-case scenario and tragic consequences of crews following last minute ATC instructions rather than the TCAS RA presented in the cockpit.

1.3  The danger of crews following ATC-instructions that conflict with an active TCAS RA is identified as a significant safety issue. As a result, some states have required that TCAS RAs be down linked and displayed on ATCO radar displays. The aim was to provide ATCOs with enhanced situational awareness so that the controller would not issue a flight profile changing ATC clearance while an aircraft was reacting to a TCAS RA.

1.4  A working paper (AN-Conf/12-WP/01) on this topic, written by IFATCA, was presented at the 12th Air Navigation Conference. (Appendix A)

Discussion

2.1 Why TCAS-RA downlinking?

2.1.1 Flight crews have an Airborne Collision Avoidance System (ACAS, also commonly referred to as TCAS – Traffic Alert and Collision Avoidance System) which is designed as a last resort to help them to prevent midair collisions.

2.1.2  TCAS produces vertical collision avoidance advisories in Resolution Advisory (RA) messages which warn pilots with visual and/or aural alerts and by indicating vertical escape manoeuvres. TCAS RAs are coordinated automatically between suitable equipped aircraft (TCAS II equipped aircraft having Mode-S). All aircraft equipped with TCAS II must carry a Mode-S transponder in order to comply with the TCAS MOPS (Minimum Operational Performance Standards).

2.1.3  In the event of a conflict, Controllers may be provided with a Short Term Conflict Alert (STCA). STCA is a ground-based safety net that provides the controller with a warning of an imminent separation violation or potential collision, but does not provide any conflict resolution advice. STCA and TCAS systems operate completely independently of each other. Their warnings/alerts are delivered to separate users, being it the pilots (for TCAS II) or the ATCOs (for the STCA).

2.1.4  On occasions, Air Traffic Controllers may issue a vertical clearance while the aircraft is in receipt of a TCAS RA. The TCAS procedures mandate that a crew, being confronted with an incompatible ATC-clearance (when a TCAS RA is active in the cockpit), must refuse the ATC-instruction/clearance by using the “UNABLE TCAS RA” – phraseology. TCAS procedures are defined in PANS-ATM (Doc 4444) and in PANS-OPS (Doc 8168)

2.1.5  TCAS monitoring revealed that aircrews often are complying with ATC clearances during a TCAS RA event, even if the standard operating procedures (SOPs) require them to give absolute priority to the TCAS RA. The monitoring also revealed that the TCAS RA reporting of the crews to ATC (by voice) was far from systematic. As a result some states/ ANSPs have mandated that TCAS RAs be down linked and displayed on ATCO radar displays. The aim was to provide ATCOs with greater situational awareness so that the controller would not issue an ATC clearance while an aircraft was reacting to a TCAS RA.

2.1.6  TCAS RA Downlink to CWP could be used to automatically notify the controller about Resolution Advisories (RAs) generated in the cockpit by the Traffic Collision Avoidance System (TCAS).


2.2 Technology behind TCAS-RA downlinking

2.2.1 There are currently four known methods (or technologies) of how TCAS RA Information can be transmitted or exchanged, (including ground ATM-Systems). They are:

  • RA – Downlink or RADL (via Mode-S)
  • RA – Broadcast or RABC (via 1030 MHz)
  • Extended Squitter RA broadcast on 1090 MHz Extended squitter (ADS-B)
  • ACAS – TCAS-TCAS Coordination (split into RACM (TCAS Coordination Messages) and RACR TCAS-Coordination Reply messages)

Note: Although the data transmitted remains always the same (TCAS RAs, including TCAS Coordination messages), the method employed to transmit and receive them, and the timing and the latency of this data can be very different.


2.3 TCAS RA Down-Linking using Mode S (RADL)

2.3.1  If a TCAS-unit triggers a TCAS Resolution Advisory (RA), this becomes an event-driven message, which is loaded (and so prepared for transmission) into the Mode-S Transponder (into Register, BDS-30). There are currently two different formats defined by the TCAS Minimum Operational Performance Standards (MOPS). They are:

  • DO-185A (for TCAS Version 7.0/ 7.1)
  • TSO-C119A (for TCAS 6.04)

Nowadays, about 95% (or more) of all aircraft use DO-185A, which is standard for TCAS 7.0/ 7.1, both standards being compatible.

2.3.2  When a Mode-S radar-head is interrogating this aircraft-transponder, to obtain the normal surveillance data, it will include in its reply a “flag”, specifying that a TCAS RA – report is available (ready for transmission).

2.3.3  The radar will come back and re-interrogate the transponder in order to obtain and download the flagged content of BDS-register 30. The reception of this data is heavily dependent on the antenna position (when the data can be received), as well as on the rotation speed of this radar head.

2.3.4  If the Tracker of the ATM-system has only one radar-head available the speed for receiving such down-links can be rather slow. In case of multi-radar heads systems, latency is much better. Radar update rate and RDPS tracker capabilities should also be taken into account. During subsequent radar-antenna sweeps the radar beam always down-links the content of register – BDS-30 (of all the flights involved with the relevant TCAS-RAs).

2.3.5  TCAS RA downlinking via the present Mode-S ground system has been identified as the most promising and most suitable technology for downlinking TCAS RAs to the controller working positions (CWPs). This technology is by far the most promising – in terms of speed, quality, but as well as in terms of fast and correct addressing of TCAS-RA data to the ground ATM-System.

2.3.6  The latency (delay) – meaning the time-frame required until the TCAS RA messages reach the Controller Working Positions, was seen as “better”, compared to the other technologies mentioned above (2.2.1).

2.3.7  The performance of this technology/system is heavily dependent on a well- established Mode-S ground network (with several rotating Mode-S radar antennas in range). All these radar-sensors feed the radar tracker, including the RDPS – Radar Data Processing System.


2.4 TCAS RA-Broadcast (RABC)

2.4.1  TCAS RA-Broadcast Is a method of downlinking TCAS RA information using a 1030 MHz message to a passive (non-interrogating) ground network.

2.4.2  TCAS RA-Broadcasts are transmitted from bottom antenna of the aircraft at full power on 1030 MHz. Information is repeated in a time-interval of 8 seconds or on any TCAS RA change in sense or strength- and this lasts for the whole duration of a TCAS RA (until clear of conflict).


2.5 Extended Squitter RA broadcast on 1090 MHz Extended Squitter (ADS-B)

2.5.1  This is the broadcast of an event driven Extended Squitter message using a 1090 MHz ADS-B. RAs are immediately triggered, and so transmitted (or “squitted) without any delay.

2.5.2  In both cases information can be received by a multitude of systems, beside antennas and receivers. 1090 MHz extended squitter (ES) is the best option to use in areas not so well equipped with Mode S ground infrastructure.


2.6 TCAS-Coordination (RACM and RACR)

2.6.1  TCAS-Coordination takes place before and during a TCAS RA (or maneuver), and it involves two or more aircraft, which are TCAS and Mode-S equipped (the Mode-S Transponder is used as communication device). Complementary TCAS evasion maneuvers are coordinated between these Mode-S units using the TCAS Coordination process. TCAS RA-Coordination messages can be sub-divided into TCAS Coordination Messages (RACM) and TCAS RA-Coordination Replies (RACR).

2.6.2  According to the Feasibility of ACAS RA Downlink Study (FARADS); a technical Study of RA Downlink Methods provided by Eurocontrol, TCAS-Coordination method is least appropriate for operational use as the target aircraft is not identified in the messages. They can only be used to detect less than 70% of TCAS conflicts and their detection on the ground has a low probability. It is not suitable as stand-alone TCAS RA downlink method but it can improve the latency, speed and certainty that TCAS RA is present in the cockpit.


2.7 Studies and workshops

2.7.1 There has already been a lot of work done in regard to TCAS-RA downlink both by individual states and by Eurocontrol inside the SPIN (Safety Nets Performance Improvement Network) and the ATM Procedures Development Sub-Group (APDSG). They have made some proposals to possibly be inserted into ICAO document 4444 (PANS-ATM, although none have been produced thus far. The FAA has currently no plans to work on and to implement TCAS RA Down-Linking to CWPs.

2.7.2  Following a number of safety recommendations to address an issue of improving a controller’s situational awareness when pilot reports of RA are delayed, missing or incomplete, Eurocontrol launched the FARADS project in 2003. Two RA downlink experiments (RADE-1 & 2) were carried out in Eurocontrol experimental centre in Bretigny, France in 2003 and 2006.

2.7.3  As a follow-up to FARADS, monitoring activities have been undertaken to assess the number and severity of TCAS encounters in Europe. Furthermore, interactions between airborne (i.e. TCAS) and ground-based safety nets (i.e. STCA) were analysed.

2.7.4  Results received from the analysis were:

  • TCAS-RA downlink is technically feasible. Within the Mode S coverage area, Mode S RA reports are the best solution. They are already specified in ICAO Annex 10. Outside mentioned area ADS-B (extended squitter) could be used.
  • Any potential benefit of RA downlink is highly dependent on the delay with which a message is delivered (latency). The overall delay is made up of technical (speed of detection and data transmission) and human (perception and interpretation of data received) elements. The point in time at which the controller could become aware of a RA is most important. Today on average, according the Study of latency of RA downlink by Eurocontrol from 2007, en-route controllers will become aware of an RA, 30 seconds after the RA is displayed to the pilot in the cockpit if transmitted by pilot via frequency. With Mode-S RA reports, the controller will be aware of the RAs in 95% of cases within 9 seconds of their occurrence which is consistent with a 10 seconds requirement set as a target goal by Eurocontrol FARADS.

2.8 TCAS RA downlink principle

2.8.1  The TCAS-logic (or software) tracks the current flight path (horizontally and vertically), by scouting surrounding airspace via active radar-like interrogations – calculating ahead if there are any conflicts or “intruders” in the vicinity.

2.8.2  This radar-scanning and forward calculation takes into account the current speeds, being horizontal and vertical, as well as forward speed. The TCAS-software however does not know whether a change in the vertical or horizontal plan is planned and expected to occur in the next few seconds (e.g. consistent with a level-off at the cleared altitude or Flight Level, most of time being 1000 feet apart from the “intruder”).

2.8.3  The TSO (MOPS) format of TCAS 6.04 uses 10 bits (bits 41 to 50) from the Active Resolution Advisory (ARA) field in order to communicate and transmit details of the TCAS-RAs present in the cockpit. This ARA-data will provide information about the vertical speed limit (VSL), but it does not indicate the TCAS RA type.

2.8.4  The newer standard, DO-185A (TCAS Version 7.0/ 7.1) uses only 7 bits for the same transmissions (bits 41 to 47), and in these particular bits the TCAS RAs are coded via the ARA field. There is an additional bit present, which is called the “MTE bit” that provides the information about the RA type.

2.8.5  Data coded in both formats need to be interpreted (in post-analysis) in order to have a precise idea about the TCAS RAs that were displayed in the cockpit. This cannot be done immediately and “live” – when the active TCAS RAs are instantly and directly downlinked to the ground ATM-System.


2.9  Quality, latency and operational issues with TCAS-RA downlinking

2.10  Monitoring in the European (core area) has shown that many aircraft indicate incorrect TCAS equipage status. A recent study has shown that about 95% of all TCAS-RAs monitored were so called “ghosts” or empty TCAS RA messages. It is TCAS RAs triggered on-board without a real threat being present. It can be caused by self- tracking, garbling, an RA on the ground or by a transponder malfunction

2.10.1  Many of these were coming from the same aircraft (transponders). Once these are all correctly identified and tracked-down, most of the problems can be solved.

2.10.2  Modern ground ATC systems are able to filter-out these empty RA messages so these false warnings will never be seen at CWPs.

2.10.3  One of the main requirements is to have only fresh and updated TCAS RAs shown at the relevant CWPs. The operational “value” of old TCAS RAs (several seconds old) is very limited – almost close to zero after a delay of over 5 seconds.

2.10.4  German TCAS RA-monitoring studies indicate that almost 95% of all monitored TCAS RAs triggered above FL 100 are received within 4 seconds. And in 72% of all the cases, the TCAS RAs are received within 2 seconds. One advantage of the TCAS RA monitoring system of DFS, the German ANSP, is that this system listens simultaneously on 1030 and 1090 MHz, which can also detect TCAS-TCAS co- ordinations. This improves not only the latency, but also gives a higher credibility (certainty) and as an additional confirmation that a TCAS RA is really present.

2.10.5  Despite their short duration (between 25 and 35 seconds), TCAS RAs can either change in strength, and in direction (TCAS Reversals). It is therefore of utmost importance that if TCAS RAs are down-linked to CWPs, that this occurs with the best possible latency (keeping the delay as short of possible).

2.10.6  Latency is the delay between the issuance of the TCAS RA in the cockpit, and the RA notification being successfully delivered to the CWP where the aircraft is in two-way contact. The latency is dependent on several factors such as the number of ground stations within range, the probability of detection dependent on the interference present in the spectrum, as well as from the position of the transponder (in relation to the ATC-antenna) and the delay in transmission through the ground infrastructure.

2.10.7  The ground antenna position in relation to the aircraft transmitting a TCAS RA is a very relevant factor in the latency of such RA downlinks to the ground.

2.10.8  DFS specifies that “Latency” has two main factors:

  • The delay between the TCAS RA being triggered in the aircraft until the reception of the RA-Down-Link at the ground station (normally the technical system of an ATC-unit)
  • The delay for “handling” the data (extraction, addressing), followed by the transmission of the TCAS RA-data to the relevant controller working positions.

2.10.9  DFS is also working actively within the SESAR-project on the same issue (TCAS RA Down-Linking to CWPs, under SESAR project 4.8.3) and has defined the following technical and operational requirement:

“The monitoring system shall provide an RA message to the ATM-System within 2 seconds from the annunciation of the RA in the cockpit in 95% of the cases.”

Note 1: The overall “event-to-display” shall be less than 4 seconds.

Note 2: SESAR (so far) is only talking of a “monitoring system”, and so avoiding the term TCAS RA Down-Linking to CWPs.

2.10.10 Eurocontrol has attempted to establish a precise list of all TCAS-RAs types and proposals on how they are supposed to be presented to ATCOs on CWPs (standard HMI-presentation on the screens):

  • Indication of all initial TCAS RAs (preventative and corrective) including the identity of the aircraft generating the RA and the intruder aircraft;
  • All follow-up or strengthening RAs shall be indicated;
  • All follow-up TCAS RA reversals shall be indicated – but all follow-up weakening RAs will not be indicated;
  • The climb/descend, increase climb/increase descend, crossing/climb/descend, reversal climb/reversal descend RA information shall be displayed in a graphical form representing the vertical movement;
  • For Vertical Speed Limit RAs, information is presented in graphical form indicating that a reduction in vertical speed is required (towards a level off – preventive RA);
  • For all other RAs, (Monitor vertical speed, or RAs against multiple intruders) information is presented without a graphical vertical direction symbol.

2.10.11 Eurocontrol proposes in the same document that there shall be no indication of ‘Clear of Conflict’ – However, it is specified that the TCAS RA indication shall be removed from the screen once the TCAS-RA has stopped in the cockpit (a clear of conflict is given).


2.11 Handling of TCAS RA DL

2.11.1  Currently Controllers will only become aware of a TCAS-RA if pilots report it via R/T. There is a chance that if controllers are unaware of an aircraft following a TCAS-RA they might issue a clearance to that aircraft, contradictory to the TCAS RA manoeuvre. If the crew followed this instruction, the effectiveness of TCAS is reduced, and the risk of collision is increased.

2.11.2  Eurocontrol proposed that TCAS-RAs are downlinked and displayed at the CWP (on the radar-screen) “as a possible method of increasing controller awareness of ACAS events, thereby reducing the probability of the controller intervening during an RA”.

2.11.3  The ideal operational concept discussed by Eurocontrol (by the various ECTL working groups and task forces) was heading into the direction where the TCAS RA D/L visual alert is to be handled in exactly the same way as a TCAS-RA report made by a crew (via R/T). So, from the moment a TCAS-RA is observed on the screen, ATCOs shall not transmit any ATC-clearances to any of the flights involved in the TCAS-RA event. This suggestion was withdrawn, as the implementation was judged to be too challenging, as the performance of the technology employed does not currently offer the stability and performance that is required. It would also require ICAO SARPS changes and extensive safety activities.

2.11.4  A key point of the ‘TCAS RA’ statement is that this is the point at which the ATCO ceases to be responsible for separation of that aircraft pair. However, the technical performance and the lack of standardization of the technical performance of the TCAS RA D/L function didn’t allow for this TCAS RA DL presentation at the CWP – from the regulatory perspective – to become to have the same value and the same meaning as the pilot’s verbal RA-report.

2.11.5  So far, IFATCA is aware of only one operational solution that has been implemented operationally in order to down-link TCAS-RAs to CWPs – it is the concept of “for information only” meaning no particular/special procedures are attached to this data display (except, that it is specified that ATCOs must use their best judgment about what to do with the displayed data);

2.11.6  In some countries ANSPs have decided to display TCAS-RAs on CWPs “for information only”. Feedback from the controllers filling in the IFATCA questionnaire indicates that they “appreciate” this kind of information being displayed to them, despite the fact that no clear procedures exist as what to do on receipt of these RA indications. We should take into consideration that a majority of the controllers making comments to the questionnaire of IFATCA mentioned however that they never had experienced any “live” TCAS RA down-link event in operations. It is suggested that in some cases the latency/delay at which ATCOs would become aware of an active TCAS-RA present in their sector could be improved significantly compared to voice reports from the crews.

2.11.7  The information gained by TCAS-RA downlink only has additional value if it prevents controllers from issuing a flight path modifying clearance to an aircraft that receives simultaneously a TCAS RA. In particular if opposite direction ATC-instructions could be avoided. Currently ICAO procedures instruct aircrews to explicitly refuse a conflicting ATC clearance while a TCAS-RA is active.

2.11.8  At first sight this seems to be an advantage as earlier notification to the ATCO of the TCAS RA could reduce the risk of a contrary ATC instruction being passed to the aircraft. Additionally, this procedure would help prevent pilots following an ATC instruction contrary to TCAS RA, but also help in situations when the pilot’s RA- reports are missing and/or are very late. However, the display of this new RA-data at CWPs brings up new problems and different safety issues that must be considered.

2.11.9  Several ATSUs have removed the display of TCAS RA Downlink from the CWPs. The UK Civil Aviation Authority issued a safety notice SN2013/017, which states;

“CAA Policy is that ACAS RA downlink data shall not be displayed to controllers on the surveillance display, for the following reasons:

  • All RAs are downlinked without distinction between their type and nature, not just those that are required to be announced by the pilot on RT. In accordance with ICAO procedures contained within Doc 8168 (PANS-OPS), RAs which do not require a deviation from current ATC instructions or clearances (e.g. Monitor Vertical Speed, Maintain Vertical Speed, Maintain or Maintain Vertical Speed, Crossing Maintain) are not announced on RT.
  • A downlinked RA without adequate discrimination may lead the ATCO to inappropriately cease the provision of ATC instructions. However, under current ICAO procedures, the controller will continue to provide ATC instructions during RA events that do not deviate from the clearance or instruction unless such a clearance is at variance with the RA, at which point the pilot will report ‘UNABLE, TCAS RA’.
  • There is no assurance as to the integrity of the RA downlink and absent or false downlink data could be a possibility.
  • There are currently no ICAO, European or UK pilot procedures, ATC procedures or legal responsibilities for the use of ACAS RA downlink. However, this subject is under consideration by Eurocontrol”.

2.12 Safety issues and problems created by TCAS-RA downlink

2.12.1  TCAS is basically a very simple and robust system, as it functions as a stand-alone airborne safety net. TCAS-software is far from simple but, the operational concept of the anti-collision system is very robust, and also rather simple – which is part of its strength.

2.12.2  The addition of RA downlinking to CWPs will add new safety concerns and operational consequences. Current ICAO PANS and SARPS do not contain provision for operational use of RA Downlink. Clear delineation of controller responsibilities needs to be addressed before stakeholders might start to think about any operational implementation.

2.12.3  Normally when new elements are introduced to the system, operations become more complicated and complex. And the more different elements that are added the more complexity it brings into the system. The simplicity of the aircrew reacting to a TCAS RA has been thoughtfully studied and clearly defined in ICAO SARPS.

2.12.4  As seen several times in the past the interface between TCAS and ATC – meaning safe, correct and independent functioning of the safety net TCAS within the current ATM and aviation system is a serious safety issue. On repeated occasions the danger of opposite-sense last minute ATC-instructions during active TCAS RAs was revealed as one of the most significant safety issues.

2.12.5  Pilot training (in particular operational and “live” training) was seen as one of the weak points of the current TCAS-operations. Up-to-date TCAS-monitoring has shown that air crews only followed displayed TCAS RAs in about 50% of all cases – which leads to the conclusion that the need for intense and practical (simulator) TCAS- training for pilots cannot be over-emphasized.

2.12.6  While downlinking the TCAS-RAs to the CWP increases controller awareness, and may reduce the likelihood of a controller issuing a clearance contrary to a TCAS RA, it would increase the complexity of procedures and interactions of the system and its vital parts such as pilots, controllers, tools and machines. This may also lead to an increase in controller workload.

2.12.7  Eurocontrol states in its documents:

“The RA Downlink Safety Assessment concluded that the existing ICAO procedures are inconsistent and should be reviewed. The issue of unclear controller responsibilities before and – even more – after the potential implementation of RA Downlink was also discussed during the RA Downlink Workshop… Current ICAO procedures do not contain provision for operational use of RA downlink.

It has been recommended that current ICAO procedures are reviewed as soon as possible, and, if required, action is taken to rectify any existing inconsistencies. The review should also identify how the existing procedures could be changed to accommodate operational use of RA downlink”.


2.13 New Technology

2.13.1  Auto Pilot/Flight Director (AP/FD) TCAS mode essentially completes the existing TCAS functionality by implementing a TCAS vertical guidance feature into the Auto Flight computer. The result is that now the Auto Flight computer can control the vertical speed of the aircraft which is adapted to each resolution advisory acquired from TCAS.

2.13.2  A promising technology is currently installed (and certified) in the new A380 aircraft. TCAS is connected to the Flight Director and thus the Autopilot. This solution permits an automatic and correct execution of the aircraft to all TCAS RAs posted in the cockpit.

2.13.3  It is worth noting that not all airlines which are currently operating the Airbus 380 have this new feature installed (it is an option only).

2.13.4  Pilots can switch this feature off (either disable it totally – or just for one particular manoeuvre (even during the TCAS RA – a manual switch-off can be made anytime). Such disabling will not be “seen” on Radar, at the CWP – or be reported via the transponder (Mode-S). The EASA certification of this Airbus 380 feature is for the safe linkage of TCAS-RA with the FD/Autopilot. This linkage is reported to be flawless.

2.13.5  “Correct execution” means that all TCAS parameters and – assumptions of the TCAS system are fully matched and respected. So far the pilot feedback has been very positive, but of course further study and analyses is essential before considering this a potential solution to pilot inaction to RA’s and pilots following an ATC instruction during an RA event.

2.13.6  TCAS Alert Prevention (TCAP) is a system Airbus has introduced to reduce a number of undesired TCAS RA in level-off situations by “softening” the aircrafts’ arrival at the intended altitude by reducing the vertical speed of these flights.

2.13.7  TCAP function is introducing a new altitude capture law to improve TCAS compatibility with ATM. SESAR results on TCAP are related to equipage: with a theoretical 100% equipage in Europe, the likelihood to receive an RA during a 1000 foot level-off encounter is divided by up to 32; for 50% equipage, the likelihood is already divided by 2 which confirms the improved compatibility with ATM.

2.13.8  IFATCA is of the opinion that care has to be taken when choosing to use (new) technology to solve problems related to safety issues. However, based on the data, automated TCAS RA’s appear promising, as:

  • It increases the likelihood that the TCAS RA’s are correctly followed;
  • It keeps the stress level lower for aircrews facing such an event;
  • It gives the crew much more time to visually acquire the intruder;
  • It reduces controller stress level by reducing a large number of undesired TCAS RA.

2.14 Current IFATCA policy on TCAS RA downlinking

2.14.1 IFATCA TPM 2013 page 3212/3213, 49/50 of 356 states;

IFATCA is opposed to down linking of any advisories generated by ACAS.

If down linking of ACAS Resolution Advisories becomes mandated, then IFATCA can only accept this provided that the following criteria are met:

  • Clear and unambiguous controller legal responsibilities;
  • Downlink without delay;
  • ATC system to be able to receive, process and display the down link to the appropriate control positions;
  • Compatibility with all ground based safety nets;
  • Nuisance and false alerts must be kept to an absolute minimum;
  • ACAS should only be considered as a ‘safety net’.

See: WP 94 – Buenos Aires 2003 and WP 84 – Kaohsiung 2006 and WP 93 – Arusha 2008, WP 78 – Estoril 1984, WP 79 – Christchurch 1993 and WP 88 – Jerusalem 1995

 

2.14.2 There is currently no evidence to suggest that this policy requires amendment. However, following discussions by the Technical and Operations committee (TOC), and the Professional and Legal committee (PLC), it was considered that there is general ambiguity regarding controller responsibility for separation after an aircraft has reported clear of conflict, and not just in cases involving TCAS RA DL. This topic was considered outside the scope of this paper; however a follow up paper reviewing all IFATCA TCAS policy has been prepared and will be presented at this conference.


2.15 ANC working paper recommendations

2.15.1 The ANC paper urged ICAO to take the following steps before considering SARPs for ACAS RA down link;

  • Complete a comprehensive study and analysis on the impact of ACAS RAs on controller work positions, specifically human factors studies and the consequences to operational efficiency and safety including those unrelated to the aircraft involved in the RA.
  • Emphasize increased pilot training on the responsibilities and requirements associated with communicating and correctly reacting to ACAS RA events.
  • Complete a comprehensive study and analysis on the benefits that automated execution of ACAS RA manoeuvres provides to operational efficiency and safety.

Conclusions

3.1  There is a history of aircrews following ATC clearances contrary to an ACAS RA. Controllers are not aware of a TCAS RA event unless notified by the crew; TCAS RA down linking to the controller work position may provide an additional level of awareness to possibly preclude ATCO’s from issuing conflicting instructions.

3.2  Downlinking ACAS RA DL to CWP also creates unintended Human factors issues such as operational problems associated with multiple alerts and radar display congestion, as well as safety issues relating to new procedures and uncertain responsibilities for separation provision incorporated in ICAO SARP’s.

3.3  Other solutions to mitigate and even alleviate aircrews from following a conflicting ATC clearance during an ACAS RA event range from increasing and improving simulation training of pilots, to automating the ACAS RA manoeuvres through the Flight Director and autopilot.

3.4  This paper endorses the recommendations contained within the ANC working paper (AN-Conf/12-WP/01).

Recommendations

4.1 It is recommended that this paper is accepted as information.

References

12th Air Navigation Conference working paper (AN-Conf/12-WP/01).

UK Civil Aviation Authority safety notice SN2013/017.

IFATCA TPM2013 page 3212/3213.

PANS-ATM (Doc 4444) and in PANS-OPS (Doc 8168).

Appendix A

TWELFTH AIR NAVIGATION CONFERENCE

Montréal, 19 to 30 November 2012

 

Agenda Item 3: Interoperability and Data – through globally interoperable information management

3.1: Performance improvement through the application of system-wide information management (SWIM)

 

ACAS Resolution Advisory Down Link to Controller Work Position

(Presented by IFATCA)

 

SUMMARY

This paper will examine the introduction of ACAS RA down link to controller working positions (CWP), highlighting the need for close examination of the consequences to safety and the impact on current ICAO provisions for collision avoidance and separation responsibility.

The danger of crews following conflicting ATC-instructions during an active ACAS RA is accepted as a significant safety issue. Pilot training is seen as one of the weak points of the current ACAS-operations, resulting in inconsistent responses to displayed ACAS RAs.

It is therefore necessary that more complete testing and safety analysis be conducted surrounding ACAS RA-DL and to evaluate other less intrusive solutions to determine if they are more suitable to eliminating potential airborne collisions before ICAO SARP revisions are proposed.

Action: The Conference is invited to agree to the recommendations and conclusions in paragraph 5.


1. INTRODUCTION

1.1 ACAS is a simple, robust and independent system that functions as a stand-alone airborne anti-collision safety net. It is collision avoidance technology, not a separation tool. Currently, ACAS procedures require that a crew, being confronted with an ATC-clearance during an ACAS RA, should disregard the clearance and when possible advise ATC of the RA and explicitly refuse the A TC-clearance.

1.2 In 2001 over Japan and in 2002 over Southern Germany (Lake Constance), the interface between ACAS and ATC exposed a serious safety issue. The Lake Constance incident revealed the worst-case scenario and tragic consequence of a crew following conflicting ATC- instructions during an active ACAS RA.

1.3 Pilot training has been identified as one of the weak points of current ACAS- operations. A presentation from the German DFS to Eurocontrol revealed the results of their 3-month survey of crew compliance to ACAS RA’s over the whole of Germany (March-May 2012). Of the 772 RA’s downloaded and analysed (4.3/day; 63% IFR/IFR, 27% IFR/VFR) Crew Compliance was only 72%.


2. BACKGROUND – ACAS RA DL TO CWP

2.1 As a reaction to the fact that aircrews were complying with conflicting ATC clearances during a ACAS RA event, some states have required that ACAS RA’s be down linked and displayed on ATCO radar displays to provide ATCO’s with greater situational awareness to prevent the controller from providing conflicting instructions. However, this proved to be problematic.

2.2 Of the seven States and ANSP’s that have implemented ACAS RA DL, three are currently disabled (Tokyo, Belfast, and Cardiff). Initial information indicates one of the reasons for disabling ACAS RA DL can be explained by the addition of another “alert” to a system of numerous alerts and an already clustered radar display. Furthermore, while independent consultants study and survey the issue of multiple alerts distraction, the RA display can create even more confusion for the ATCO when considering the likelihood of pilot compliance and if and when the ATCO is responsible again for separation. The result is a tremendous amount of ambiguity about what to do and what is expected from the ATCO when such a situation occurs.

2.3 Currently, there are three possible operational solutions of how ACAS-RA’s down- linked to CWP’s could be handled:

a) No ACAS RA DL displayed on CWP;

b) ACAS RA’s displayed at CWP’s “for information only”, meaning no particular/special procedures are attached to this data, and no controller action is expected; and,

c) Eurocontrol is proposing an operational scenario where ATCO’s shall stop transmitting flight-path modifying clearances to these aircraft once they are showing a ACAS-RA on their radar display. ATCO’s would be required to consider ACAS RA’s displayed as if receiving a voice report by the crew.

Note: This will require amendment to ICAO-SARPs.


3. DISCUSSION – OPERATIONAL ISSUES OF DISPLAYING ACAS RA’S ON CWP’S

3.1 Excluding option (a) above, despite certain benefits, both scenarios (b) and (c) have significant operational problems, as well as technical drawbacks. In some cases the delay at which ATCO’s would become aware of an active ACAS RA present in their sector could be improved over current R/T procedures. This advance information has value only if the ATCO can be assured that the pilot will execute the maneuver or during this lapse of time – gained by the ACAS RA-D/L – an identical clearance would be transmitted by ATC to the same aircraft involved in an RA-maneuver.

3.2 Air traffic controllers are only aware that an RA has been issued once notified by the pilot via radio. Being unaware of an RA, the controller might instruct the aircraft to maneuver in a manner contrary to the RA. While the controller, unaware of the RA, is required to resolve an imminent conflict and assure safe separation, it must be stressed that the current ACAS procedures state that a crew confronted with an incompatible ATC-clearance during an active ACAS RA, should explicitly refuse the ATC-clearance, using the “UNABLE, TCAS RA” phraseology. Continual recurrent training of pilots should reinforce this requirement.

3.3 Although contrary to the requirement, pilots in some cases follow the ATC clearance rather than an ACAS RA. Compliance with the contradictory ATC clearance severely degrades the effectiveness of ACAS. RA Downlink has been proposed as a possible method of improving controller awareness of ACAS events, thereby reducing the probability of the controller intervening during an RA. It is important that any safety assessment evaluate the possibility of “ghost RAs”, that is an RA displayed to the controller that is not present in the cockpit, and any other spurious alerts not present.

3.4 If enhanced controller situational awareness is the goal, robust pilot training to ensure notification of ATC of the RA maneuver as soon as practical and to refuse to act on any ATC clearances contrary to the RA solution, could achieve the same result without introducing new system risks.


4. DISCUSSION – SAFETY ISSUES AND PROBLEMS CREATED FROM ACAS RA DL

4.1 From a conceptual point of view it must be accepted that down linked RA’s would bring new parties and stakeholders into the “system”. Down linking RA’s to the CWP may improve awareness to the controller, it would likewise increase the complexity of the different procedures and interactions of the system and its actors (pilots or controllers – including the tools and machines). These new complexities beget the likelihood that something can either go wrong or not perform as planned or expected. The simplicity of the crew reacting to a ACAS RA has been thoughtfully studied and clearly defined in ICAO SARP’s.

4.2 The addition of RA down linking will add new safety concerns and operational consequences. Current ICAO procedures do not contain provision for operational use of RA Downlink, and clear delineation of controller and pilot responsibilities needs to be addressed before the potential implementation of RA downlink.

4.3 A more promising and robust technology to study is currently installed (and certified) in the new A380 aircraft. ACAS is connected to the Flight Director and thus the Autopilot. This solution permits an automatic and correct execution of the aircraft to all ACAS RA’s posted in the cockpit. “Correct execution” means that all ACAS parameters and – assumptions of the ACAS system are fully matched and respected. Further study and analyses is essential before considering this potential solution. IFATCA believes we must always be careful when we chose to use technology to solve problems and safety issues. However, based on the data, automated execution of ACAS RA’s appear promising, as:

1) It increases the likelihood that the ACAS RA’s are correctly followed;

2) It keeps the stress level down for the crews facing such a event; and,

3) It gives the crew much more time to visually acquire the intruder.


5. CONCLUSION AND RECOMMENDATIONS

5.1 There is a history of aircrews following ATC clearances contrary to an ACAS RA.

5.2 Controllers are not aware of an ACAS event until notified by the crew, which has been problematic.

5.3 ACAS RA down linking to the controller work position may provide an additional level of awareness to preclude ATCO’s from issuing conflicting instructions.

5.4 ACAS RA DL to CWP also creates unintended consequences such as operational problems associated with multiple alerts and radar display congestion, as well as safety issues relating to new procedures and uncertain responsibilities not incorporated in ICAO SARP’s.

5.5 Other solutions to mitigate and even alleviate aircrews from following a conflicting ATC clearance during an ACAS RA event range from increasing and improving simulation training of pilots, to automating the ACAS RA manoeuvres through the Flight Director and autopilot.

5.6 We invite the Conference to:

a)  Note the content of the paper; and,

b)  Urge ICAO take the following steps before considering SARPs for ACAS Down Link:

1)  Complete a comprehensive study and analysis on the impact of ACAS RAs on controller work positions, specifically human factors studies and the consequences to operational efficiency and safety including those unrelated to the aircraft involved in the RA.

2)  Emphasize increased pilot training on the responsibilities and requirements associated with communicating and correctly reacting to ACAS RA events.

3)  Complete comprehensive study and analysis on the benefits that automated execution of ACAS RA manoeuvres provides to operational efficiency and safety.

 

— END —

Last Update: October 1, 2020  

June 8, 2020   377   Jean-Francois Lepage    2014    

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