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;

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.