Professional and Legal Aspects of Downlinking TCAS RAs (TCAS Resolution Advisories)

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Professional and Legal Aspects of Downlinking TCAS RAs (TCAS Resolution Advisories)

44TH ANNUAL CONFERENCE, Melbourne, Australia, 17-22 April 2005

WP No. 166

Professional and Legal Aspects of Downlinking TCAS RAs (TCAS Resolution Advisories)

Presented by PLC

Introduction

1.1. At the 43rd annual IFATCA conference in Hong Kong, SC1 (now Technical and Operations Committee, TOC) amended IFATCA technical policy about the downlinking of TCAS RA’s direct to ATCO’s screens.

1.2. TOC tasked the PLC with reviewing IFATCA professional policy in light of this new technical policy.

Discussion

2.1. Working paper 96 of Committee B at the Hong Kong Conference comprehensively covered the technical issues of downlinking TCAS Resolution Advisories to ATC positions. As a result of this paper, the IFATCA manual was amended on page 3 2 1 2 by replacing:

“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 downlinking of any advisories generated by ACAS.”

with:

“IFATCA is opposed to downlinking of any advisories generated by ACAS. If downlinking 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 downlink to the appropriate control positions;
  • Compatibility with all ground based safety nets;
  • Nuisance and false alerts must be kept to an absolute minimum”

 

2.2. This policy is quite clear in stating that IFATCA is opposed to the downlinking of advisories from ACAS systems. But should such downlinking become mandated, various conditions have to be met. These will be discussed in detail.

2.2.1. “Downlink without delay”

There could be a number of different methods of TCAS RA downlink transmission (e.g. mode S radar, RA receivers etc. These are detailed in the Eurocontrol TCAS RA downlink experiments). The IFATCA technical policy adopted in Hong Kong requires “downlink without delay”. This is a crucial requirement from a professional perspective in order to ensure that the information displayed to the ATCO is valid and timely. This is to prevent the ATCO from transmitting information or instructions contradictory to the TCAS RA.

2.2.2. “ATC system to be able to receive, process and display the downlink to the appropriate control positions”- and “Compatible with all ground based safety nets”

A TCAS RA downlink system could only be successfully implemented on an ATC system that was suitably advanced and automated to handle such inputs, and thus would likely to already have other safety nets such as a STCA (short term conflict alert) system. Obviously most scenarios that would generate a TCAS RA would also generate a STCA alert prior to the RA. The parameters of the STCA should be set such that the STCA normally activates prior to the TCAS RA to enable the ATCO to rectify the situation.

2.2.2.1. The architecture of the system must be such that the TCAS RA is displayed upon the individual HMI of the sector having “jurisdiction” or control authority for the affected aircraft as well as on all other sectors having their HMI selected within range. Similarly, the system must be able to handle the progression of alerts for individual aircraft from STCA to TCAS RA without any delay or over-writing or clutter of displayed alerts, and with sufficient differentiation between a STCA and a TCAS RA alert to enable the ATCO to immediately recognise their different responsibilities in relation to each type of alert.

2.2.2.2. The architecture of the system must be such that the TCAS RA would always take precedence over other alerts or alarms that may be displayed for that individual aircraft.

2.2.2.3. Where both visual and aural alerts are in use, the visual display of a TCAS RA alert should be accompanied by a simultaneous aural alert, sufficiently different from other aural alerts and alarms that may be activated at the ATCO’s HMI. Special care should be taken into consideration not to over-alarm the ATCO’s HMI as this can negatively influence the ATCO’s reaction upon the alarms.

2.2.3. “Nuisance and false alerts must be kept to an absolute minimum”

A vital aspect affecting the ATCO’s response to a TCAS RA is the validity and timeliness of the alert being made known to the ATCO. In the case of verbal reports received from aircrew, there is no doubt that a response to that RA is underway, however it has been well documented that there can be significant delays in this information being communicated to the ATCO.

2.2.3.1. In the case of TCAS RA downlinking, the ATCO should be aware of the situation earlier than through the verbal communication from the aircrew, but no longer has the certainty that the aircrew are responding to the alert. The integrity of any implemented TCAS RA downlinking system must be established to such a degree that the ATCO must be legally and professionally entitled to assume, during all displayed alerts, that the aircrew have received and are responding to the same alerts, and that the ATCO will at that point in time cease to pass instructions. It is crucial that this aspect be clearly understood by all, in that the display of false TCAS RA alert on an ATCO’s HMI would cause the ATCO to cease transmitting otherwise valid and required control instructions to an affected aircraft on the assumption that the displayed TCAS RA is genuine. This is one of the primary reasons why IFATCA is opposed to downlinking of any advisories generated by ACAS, and wishes to see important issues surrounding this procedure addressed in a satisfactory manner prior approving the procedure.

2.2.3.2. A possible solution to the nuisance and false alerts problem could be that before the TCAS RA is downlinked the aircrew has to interact. This interaction should as well, be the confirmation from the crew that they react according the RA.

2.2.4. “Clear and unambiguous controller legal responsibilities”

This is of vital importance to ATCO’s and the main driver for this professional working paper presented by PLC. Currently ICAO rules state the ATCO ceases to be responsible for separation once an aircrew has advised they are manoeuvring in response to a TCAS RA until such time as the air crew report clear of traffic and resuming cleared level. As the ATCO shall not transmit any flight profile changing instructions or clearances it is evident that ATCO’s responsibility for separation provision ceases until the air crew has reported clear of traffic and has resumed the cleared flight level.

Were TCAS RA downlinking to be implemented, this definition of ATCO responsibility during a TCAS RA response could be extended to encompass TCAS RA “events” whether communicated to the ATCO verbally by the pilot or by TCAS RA downlink.

2.2.4.1. The ATCO must not be placed in a position of issuing contradictory instructions to the TCAS RA. In automated ATC systems that are capable of displaying downlinked TCAS RA’s, it is likely that a Short Term Conflict Alert (STCA), when available, would have activated prior to the TCAS RA being received. Consequently it is likely that the ATCO would have initiated avoidance instructions that may subsequently be contradicted by the TCAS RA. In such cases the ATCO’s legal responsibilities must be clearly detailed. Control or avoidance instructions should cease immediately upon a TCAS RA being displayed on the ATCO’s HMI, and from that moment the ATCO ceases to have responsibility for separation for the affected aircraft, even should it subsequently transpire that the TCAS RA alert was false. The ATCO must be legally entitled to assume that the relevant aircrew of all aircraft that may be affected, have received the same TCAS RA and are responding to it, without any verbal RT exchange taking place and that at that point in time he/she will cease to pass instructions, and be protected by the full extent of the law in doing so.

2.2.4.2. In light of the above, 2.2.3.2 becomes even more meaningful. If a manual interaction of the crew is the trigger of the downlink and if the crew only does this interaction when following an RA then the legal implications are the same as if the crew had advised verbally that they are following an RA.

2.2.4.3. As a TCAS RA alert, communicated either verbally or non-verbally, requires the opposite of normal reactions expected of an ATCO to an alert, i.e. to cease action or take no action as opposed to the normally expected reaction of taking positive action in response to the alert, ATCO’s who may be affected by such a system should receive initial and on-going refresher simulator training in the use of the system. In particular such training should cover the scenario of an STCA alert, requiring immediate ATCO action, progressing to a TCAS RA alert, requiring the ATCO to cease attempts to resolve the situation.

2.3. IFATCA has policy on “Automation Controller Training” on page 4 3 2 5 of the IFATCA Manual as well as on “Airborne Collision Avoidance Systems training” at the same page. The ACAS training policy states that:

“All MAs should urge their national administration to assemble, disseminate, administer and maintain a comprehensive ACAS training programme for ab-initio and regular refresher training. Such training should at least consist of:

a) Definition of ACAS (TCAS);

b) Technical description and cockpit displays;

c) Pilot reactions to Traffic Advisories and Resolution Advisories;

d) Controller reactions and legal responsibilities;

e) Phraseologies;

f) Experience of simulated ACAS (TCAS) events in an aircraft simulator or on video.”

 

2.4. IFATCA’s legal policy (paragraph 1.2.7, section 1.2 Legal Liability, page 4 4 1 3 of the IFATCA Manual) says:

“A controller shall not be held liable for incidents in which a loss of separation occurs due to a resolution advisory issued by an automated system.”

 

2.4.1. PLC believes that this policy adequately covers the legal liability aspects of the downlinking of TCAS RA’s.


2.5. Other Professional Issues

2.5.1. Until this point PLC mainly reviewed and endorsed IFATCA’s technical policy voted at last years’ Conference from a professional perspective and reviewed the existing professional policy. Exchanges of views between members of TOC, members of PLC and HFS raised additional ideas and concerns about downlinking TCAS RA’s.

2.5.2. Seen from a safety aspect, are there any benefits in downlinking TCAS RA’s and displaying that information to the controllers’ HMI? If so, can IFATCA be against this downlinking as a concept? Should all TCAS RA’s be displayed to the controller or only those upon which the crew is performing a manoeuvre away from the instruction or clearance issued? Should the downlinking be fully automatically or initiated by a pilot’s intervention?

2.5.3. Safety-wise speaking PLC can see two advantages in downlinking TCAS RA’s in addition to the verbal reports of the pilots involved. One advantage is the possible gain in time the information reaches the ATCO’s involved to react. Additional research is needed to see whether this gain in time is of that crucial importance to allow certain disadvantages to enter the safety net systems. Another advantage PLC discovered is where two aircraft are involved that are under the control of adjacent ATC units and as such on different ATC frequencies. Both units can receive the advisory simultaneously where there is a possibility that by RT only one unit is advised.

2.5.4. Not all TCAS RA’s should be downlinked and displayed to the controller. Most of the current RA’s, in Europe roughly 75% of them, are so called “preventive” RA’s. These occur when the aircraft is about to level of at the cleared level with traffic separated vertically just above or below. These RA’s do not require a pilot interaction contradictory to the clearance/instruction issued and can from a controller’s perspective be considered as nuisance. From the same perspective, these RA’s, reported by RT, are nuisance as well.

2.5.5. As ATC is mainly interested in a timely awareness of a pilot action in deviation with the clearances/instructions issued, triggered by the RA, the emphasis of the downlinking should be put to the pilot’s action and not to the RA as such. Therefore PLC is of the opinion that the downlink should be initiated by a pilot’s interaction and not by an automated system.

Conclusions

3.1. IFATCA is opposed to the downlinking of any advisories generated by ACAS, and wishes to see important issues surrounding this procedure addressed in a satisfactory manner prior approving the procedure.

3.2. Where any TCAS RA downlinking system is to be implemented the legal liability of an ATCO should be clearly defined such that the ATCO is no longer responsible for separation or the provision of avoidance advice or any instruction issued once a pilot has reported responding to TCAS RA or a TCAS RA downlink has displayed this information on the ATCO’s HMI, whether or not such HMI display of TCAS RA subsequently proves to genuine or false.

3.3. Where TCAS RA downlinking system is to be implemented other safety nets such as STCA should be in place and their system parameters should be set to alert the ATCO sufficiently in advance to enable the ATCO to rectify the situation.

3.4. Where TCAS RA downlinking is to be implemented, the ATCOs should receive initial and ongoing refresher simulator training in responding to STCA and subsequent TCAS RA alerts.

3.5. The system architecture should ensure that any TCAS RA alert displayed is done in a manner that is immediate and clear to the ATCO and does not display simultaneously with any other alert or display for that aircraft.

3.6. PLC believes that existing IFATCA policy adequately covers the legal liability aspects of the downlinking of TCAS RAs.

Recommendations

4.1. That TCAS RA downlinking remain the subject of regular review for the purposes of IFATCA awareness of developments and updating IFATCA policy as required.

4.2. That IFATCA continue to strengthen both it’s technical and professional/legal policy to stipulate ATC system architecture requirements and procedures, inclusive non-automated procedures (ie: voice communication), with regard to display of TCAS RA alerts on an ATCO’s display, and/or notification of aircraft manoeuvring as a result of TCAS RAs.

4.3. That IFATCA take appropriate steps to urge industry and national administrations to develop, maintain and administer a comprehensive ACAS training program for air traffic controllers. Such training should consist of at least:

a) Definition of ACAS (TCAS);

b) Technical description and cockpit displays;

c) Pilot reactions to Traffic Advisories and Resolution Advisories;

d) Controller reactions and legal responsibilities;

) Phraseologies;

f) Experience of simulated ACAS (TCAS) events in an aircraft simulator or on video.

4.4. This paper be accepted as Guidance Material.

Last Update: September 29, 2020  

March 26, 2020   228   Jean-Francois Lepage    2005    

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