Study Down Link of Traffic Collision Avoidance System Resolution Advisories

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Study Down Link of Traffic Collision Avoidance System Resolution Advisories

43RD ANNUAL CONFERENCE, Hong Kong, China (SAR), 22-26 March 2004

WP No. 96

Study Down Link of Traffic Collision Avoidance System Resolution Advisories

Presented by SC1


1.1 After one mid-air collision in 2002 and several near mid air collisions, it became evident that Traffic Collision Avoidance System (TCAS) procedures needed an urgent review. Especially, the interaction with ATC needed to be addressed to find suitable solutions for the highlighted problems. Contradictory ATC instructions during a TCAS Resolution Advisory (RA) became a top priority for aviation safety experts, not only in Europe, but worldwide.

1.2 Controllers are required not to attempt to modify the flight path of an aircraft following a TCAS RA as there is the potential that the ATC clearance is contradictory to the RA posted. In some of the cases, this instruction was sent out after the RA was (unknowingly to ATC) already posted in the cockpit. So how best to advise controllers in due time that an aircraft is subject to an RA? This has led to calls for urgent studies if down linking of TCAS RAs as a possible way forward.

1.3. These calls came from various groups of stakeholders, including IFALPA, the Japanese CAA and the European high-level action group on aviation safety (AGAS). SC 1 is of the opinion that a number of issues need to be addressed before the down linking of RAs could be introduced safely and successfully. These issues are generally broken down into three categories: technical issues, operational issues and finally legal implications (including controller liability). It is the aim of this working paper to study these issues and propose amended policy on the matter.


2.1. From a technical point of view, not only in relation to the proposed down linking of TCAS RAs to ATC, timing is of utmost importance for ATC. Air traffic controllers need to have correct data at their disposal to be able to provide safe and expeditious traffic handling. When it comes to situations where the separation management is entering a “collision avoidance mode”, and presentation of a TCAS RA is certainly one, the requirement for correct data has to be stressed even more. This exchange is not just between the aircraft and the ground receiver, but it is a performance requirement “endto-end”, in other words from the aircraft until it is displayed on the relevant controller’s radar screen.

2.2. Why do all the organisations mentioned believe it could be important and beneficial to send TCAS RAs to ATC? It is the risk that air traffic controllers might be transmitting contradictory ATC clearances to aircraft subject to a TCAS RA that had triggered this idea. As seen several times in the last five years, this possibility cannot be fully ruled out, and, in more than one of the occasions, the aircrew gave more priority to the ATC clearance going as far as ignoring the displayed RA-sense. Therefore, not only the timing and delay for a swift and correct RA-reception is very important, but also the requirement for reception at the correct Controller Working Position (CWP) must be addressed. It is a prerequisite that the controllers, that are in two-way radio communication (could be different sectors) with either one of the aircraft subject to TCAS RAs, are getting correctly informed, without delay, about a posting of RAs in the cockpit. Despite all these precautions it cannot be fully ruled out that simultaneous transmissions of contradictory RAs and ATC instructions continue to happen.

2.3. Wrong, irrelevant and nuisance messages are undesirable for any ATC unit. This is especially true when collision alert messages are received that don’t concern the units or sectors where it is received. This is more likely to happen in regions with high traffic complexity, having multilayered sector configurations being packed and intertwined. A general broadcast of an RA down link is unacceptable, if this results in the display of the RA to controllers not directly responsible for the aircraft affected by the RA. A selective addressing is needed to keep the number of irrelevant RA-postings to a strict minimum and avoid dangerous overload of air traffic controllers with emergency messages not relevant to them. Otherwise, the confidence of the operators would be degraded and this might have many undesirable side-effects, including late reaction, or non-reaction to real alarms.

2.4. Other problems are related to Operational problems of a possible TCAS RA down linking. They include, inter alia, the visualisation of the down linked message at the CWP concerned. There is no agreed standard for air situational displays for air traffic control, and there is no standardisation in the use of systems such as Short Term Conflict Alert (STCA), safety nets in general and controller tools. Down linking of RAs will add another level of alerts for the controllers to respond to. It is simple to assume that a controller team will receive an STCA and then later, if the problem is not corrected, an indication that an RA is occurring. However, due to the many different ways that STCA is being used in practise, this will not necessarily be true. It may also be the case that both are received simultaneous and the question of which takes precedence arises. There will be various requirements placed on controllers, for example to issue instructions if an STCA alert is received, but to not issue instructions if an RA is received, and this all in a short period of time.

2.5. The display of information and the interactions with it, the Human Machine Interface (HMI), will have to be carefully designed so that it supports the controller. It is at this moment that the possibility exists that a multitude of different alarms is presented to the controller team, all requiring different and disparate actions. The interaction between down linked TCAS RA messages and ground based safety nets, alerting in different and disparate moments, must be studied very carefully.

2.6. Initial indications from recent studies indicate that approximately 30% of the received downlink messages of TCAS RAs are false, garbled, or not completely representing what is shown in the relevant cockpits. This percentage is unacceptable, especially when taking into account that RAs result in ATC and aircrew to enter a “collision avoidance mode”, while this is not needed. It is therefore of utmost importance that nuisance and false alerts are kept to the absolute minimum.

2.7. Operational controllers are concerned about the legal implications and consequences of a possible down linking of TCAS RAs to their CWP. It is feared that this information will result in additional liabilities. Controllers need guidance on how to react when only one RA message is received from an aircraft pair seen in conflict? Is it safe to assume that the other is logically also subject to an RA? Or is it only the reception of the RA that is a problem, or is there another issue? Could it be that the TCAS is unserviceable, either as reported, or unexpectedly? Does the controller have any responsibility to inform the second aircraft that an RA is in progress?

2.8. What is the reaction if, in the middle of a flight path modifying ATC message, the RA is coming in and displayed at the sector? Should the ATCO continue to transmit its message, or maybe stop it and just continue giving traffic information to the aircraft involved? Or, as others have suggested, say something such as “follow your TCAS”? This, when it comes to legal considerations, may have serious consequences for persons not actually performing the avoidance manoeuvres, especially in case of an incident or an accident.

2.9. The performance requirements will be determined by the way the down linked RA information will be used. These requirements must also be matched with requirements of the airborne equipment. It is crucial that ground and airborne systems do not present conflicting information to controllers and/or flight crews.


3.1. Various technical issues and several unsolved problems result in the fact that IFATCA continues to be opposed to the down linking of TCAS to ground ATC stations. However, should the down linking of TCAS RAs be mandated (or introduced), a number of issues and problems must be addressed and resolved before a TCAS RA down linking could be accepted by IFATCA.

3.1.1 Timing, meaning delay in the posting of the RA-messages on ground displays, is of utmost importance and should be minimized.

3.1.2 Timing for the reception on ground must be seen as “end to end” performance where the delay must be considered between the very moment an RA is posted in the cockpit and the display at the relevant controller working positions where the aircraft subject to an RA are in radio contact with ATC. This time frame must be measured and kept as short as possible so that TCAS RA messages presented to the controller are as closely as possible to the reality shown to the crews in the cockpits. The reception and presentation at a CWP of an RA message must be as rapid as possible.

3.1.3 “Broadcast type” reception of TCAS RAs, meaning that RA messages are not addressed selectively to the relevant controllers working stations where the aircraft subject to RAs are handled, are not acceptable. Firstly as it could lead to an overload of the ATCOs working, having to read and understand alarm messages. Secondly, their sharpness and alertness will be degraded quickly by too many alarm messages not concerning their sector. This problem will have a larger impact for airspaces with high traffic density, complex traffic patterns, multilayered sector layouts and complex configurations. Only a correct and speedy addressing of the RAs messages, in a selective way to the correct CWPs, is acceptable.

3.1.4 Controllers at their CWP receive many other warnings, such as from STCA. Should RA-messages be added, controllers would have to react in different ways to the reception of different warning messages. Either act positively (such as with STCA) or act passively, when it concerns an RA-message. Therefore, careful studies are required to address how it would be feasible to post such warning messages on ground at the relevant CWPs.

3.2. Legal considerations, including liability issues, for air traffic controllers receiving down linked ACAS RAs will have to be addressed. Controllers’ responsibilities should in such cases not be considered in isolation from other safety net responsibilities.

3.3. Studies for situations such as where only one RA is received from one aircraft in a conflict pair as the TCAS has either failed or has been declared out of operations are needed. Or when a TCAS RA is received on the ground at the very moment when the air traffic controllers are in the process of sending up flight path modifying clearances to the aircraft subject to a TCAS RA. Also in these situations, it is crucial that the legal liability and also the operational requirements of ATCOS must be clearly defined.


It is recommended that:

4.1 Current IFATCA policy on page 3 2 1 2:

“Due to the fact that there is currently a lack of compatibility between ACAS and the ATC system, and the risk that controllers workload therefore could be increased by unnecessary information, IFATCA is opposed to the down linking of any advisories generated by ACAS.”

be deleted.

4.2 IFATCA policy is:

“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.”

and should be included in the IFATCA manual on page 3 2 1 2.

4.3 SC4 is tasked to review professional policy in the light of this new policy.

Last Update: September 29, 2020  

March 24, 2020   657   Jean-Francois Lepage    2004    

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