Review of Policy: Resumption of Separation Following a TCAS RA

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Review of Policy: Resumption of Separation Following a TCAS RA

54TH ANNUAL CONFERENCE, Sofia, Bulgaria, 20-24 April 2015

WP No. 88

Review of Policy: Resumption of Separation Following a TCAS RA

Presented by TOC

Summary

This working paper is a review of a current provisional IFATCA policy with regards to the resumption of separation after a Traffic Alert and Collision Avoidance System (TCAS) Resolution Advisory (RA) event. A review of the TCAS policies of the Federation made at the 2014 IFATCA Annual Conference in Gran Canaria, Spain, did highlight several issues regarding the responsibility for providing separation after the completion of a TCAS RA manoeuvre. This working paper will address those issues and then propose a new policy recommendation.

Introduction

1.1  At the 2014 IFATCA Annual Conference, held in Gran Canaria, Spain, a working paper was presented where a systematic review of the TCAS-related policies of the Federation was made. Its main purpose was to update and/or cancel any outdated TCAS-policies.

1.2  One of the recommendations of that working paper, presented to Committee B/C combined, was to upgrade a current provisional policy regarding the responsibility for providing separation after a TCAS RA to full policy. However, after the presentation of the paper to the delegates, the ensuing discussion did highlight a number of outstanding issues which prevented the upgrade to full policy from happening. It was decided that a new review of the matter was required in order to propose a fully consolidated draft recommendation at the next IFATCA Annual Conference in Sofia, Bulgaria.

1.3  This paper discusses the issues raised at the 2014 IFATCA Annual Conference, paving the way for the draft policy recommendation at the end of the paper.

1.4  It should be noted that the term TCAS refers to a specific implementation of the Airborne Collision Avoidance System (ACAS) concept. TCAS II (software version 7.0 and 7.1) is currently the only available equipment that is fully compliant with ICAO ACAS II International Standards and Recommended Practices (SARPs).

1.5 The difference between the terms “Accountability” and “Responsibility”, albeit small, plays an important role in the discussion in Section 2.

They are described below for completeness:

a)  Accountability: a requirement to justify actions, decisions.

b)  Responsibility: an obligation to do something, as part of one’s job or role.

Discussion

2.1  The current provisional IFATCA policy on the resumption of separation following a TCAS RA reads:

After an aircraft has departed from its ATC clearance or instruction in compliance with an RA, or a pilot has reported an RA, the controller shall not resume responsibility for providing separation, until separation has been established for all affected aircraft.

 

2.2  That policy differs from the current ICAO procedures with regards to aircraft equipped with ACAS, which reads:

When a pilot reports an ACAS RA, the controller shall not attempt to modify the aircraft flight path until the pilot reports “Clear of Conflict”.

Once an aircraft departs from its ATC clearance or instruction in compliance with an RA, or a pilot reports an RA, the controller ceases to be responsible for providing separation between that aircraft and any other aircraft affected as a direct consequence of the manoeuvre induced by the RA. The controller shall resume responsibility for providing separation for all the affected aircraft when:

a)  the controller acknowledges a report from the flight crew that the aircraft has resumed the current clearance; or

b)  the controller acknowledges a report from the flight crew that the aircraft is resuming the current clearance and issues an alternative clearance which is acknowledged by the flight crew.

2.3  The difference revolves around the fact that the current IFATCA policy specifies that the controller shall not resume being responsible for separation until separation has been established following a TCAS RA. Whereas, on the other hand, the ICAO provisions determine that the controller becomes (again) responsible for providing separation immediately after the pilots report “Clear of Conflict”.

2.4  TCAS is an airborne Safety Net (SNET) designed to work autonomously and independently of aircraft navigation equipment and any ground system used for the provision of Air Traffic Management (ATM) services. The system detects the presence of other aircraft, carrying a functioning transponder, which may present a threat of collision. If TCAS diagnoses an impending risk of collision, it issues vertical escape advisories (i.e. RA) to avoid such collision, thus functioning as a last resort airborne SNET. TCAS is not aware of the type of airspace it is operating in, or the legal separation requirements for that type of airspace. For example, an RA aims to achieve a vertical safe distance (ALIM) of between 300 and 700 feet at the closest point of approach (CPA), depending on the aircraft’s own altitude (at higher altitudes, a larger ALIM is used) but irrespective of the type of airspace.

2.5  Therefore, after a TCAS RA event, once the pilot reports “Clear of Conflict”, there might be a period of time where standard separation does not exist. IFATCA believes that the controller should not be responsible for that situation (irrespective of whether what caused the TCAS RA was attributable to the controller or not). In the view of the Federation, according to current provisional policy, only when separation has been established shall the controller resume responsibility for providing separation. This stems from the fact that TCAS is a “last-ditch” collision avoidance system, which aims to achieve an ALIM considerably smaller than the minimum vertical separation specified for a particular type of airspace.

2.6  However, there are three major issues arising from that view:

a) The current IFATCA policy states that “the controller shall not resume responsibility for providing separation” but it does not specify separation between which aircraft. According to the ICAO procedures, the controller ceases to be responsible for separation between the affected aircraft. However, the controller is still responsible for the separation of any other aircraft on frequency not affected by that TCAS RA event. The IFATCA policy does not reflect that point, which could potentially lead to misinterpretation.

b)  The clause “until separation has been established for all affected aircraft” does not specify how that (standard) separation is established (i.e. the minimum legal separation associated to a particular type of airspace). In a TCAS RA event, TCAS generates a “Clear of Conflict” message when it establishes that the collision has been avoided (according to TCAS II software parameters). At that point, the pilots shall report “Clear of Conflict” and return to the previous clearance [6], unless the controller issues a new clearance. This is of critical importance in the case where the previous clearance was the cause of the RA event. Therefore, in that particular situation, it can be inferred that separation would be established by the new clearance instructions issued by the controller. Pilots cannot provide their own separation, and TCAS is no longer involved at that point, therefore, controller intervention is required. This also matches IFALPA’s point of view, which believes that it makes sense for controllers to issue instructions without delay, even if the required separation standard is not yet re-established. Thus, if a controller intervenes, issuing instructions to establish standard separation, it should follow that he/she is the party responsible for providing separation.

c)  The final issue can be best explained through a practical example. Let us consider the case of controlled airspace, where the controller is responsible for providing standard separation between IFR aircraft. When a TCAS RA-event takes place, the controller ceases to be responsible for the separation of all affected aircraft and TCAS takes over, issuing collision avoidance advisories to such aircraft in order to achieve a safe vertical miss distance at CPA. When TCAS generates the “Clear of Conflict” message, it stops being involved and, for IFR flights in controlled airspace, another party has to be responsible for providing separation from that point onwards. Of the parties involved, the pilots do not have the required overall situational awareness to do so and, as mentioned in 2.6 b), they cannot provide their own separation in controlled airspace. The controller is in the best position to be responsible for separation once the pilots report “Clear of Conflict”. This again matches IFALPA’s point of view, which believes that, even when full ATC separation is not yet established, it must be acknowledged that the air traffic controllers are the ones with the best overview (and control) of the current traffic situation.

2.7  It is accepted that there are traffic situations immediately after a TCAS RA event (when the pilots report “Clear of Conflict”) where aircraft are not separated by standard separation. However, that should not prevent the controller from providing an ATM service from that point onwards which can include, according to the type of airspace and flight rules, the provision of standard separation. Even if a controller resumes responsibility for separation following a TCAS RA event, he/she cannot be held accountable for the traffic situation resulting from TCAS intervention. However, the controller might be held accountable for the traffic situation that caused the TCAS RA in the first place, but that would be determined by an independent investigation and is beyond the scope of this working paper.

2.8  There are other events in ATC where a controller might face a traffic situation where standard separation does not exist for reasons beyond his/her control. However, that does not mean that the controller ceases to be responsible for providing or establishing separation. One example would be the unauthorised penetration of controlled airspace by an aircraft not under the control of the controller, i.e. unknown traffic causing a loss of separation. The controller is given a situation where standard separation does not exist but he/she is responsible for establishing standard separation as soon as practically possible. It is clear, in that case, that the controller will not be held accountable for the loss of separation. The same logic should apply to the traffic situation of the aircraft affected by a TCAS RA once the RA has ceased and the “Clear of Conflict” has been reported, i.e., the controller shall be responsible for establishing standard separation if it does not exist at that point.

Conclusions

3.1.  A review of the provisional IFATCA policy on the resumption of separation following a TCAS RA was deemed necessary after the issues raised at the 2014 IFATCA Annual Conference. The main purpose of this working paper was to present the arguments behind the current provisional policy and to explain in detail the arguments in favour of proposing a modified policy, refining and improving the current one.

3.2.  In particular, after a TCAS RA event, the provision of standard separation, and consequently the responsibility for such task, must be undertaken by one of the parties involved in the operation. Although, given TCAS intervention, the controller has not created the traffic situation existing as soon as the pilots report “Clear of Conflict” (which could be a situation where a loss of separation exists), IFATCA and IFALPA both agree that, of all parties involved, the controller is best placed to establish standard separation as soon as possible. Standard separation shall be established, as stated in the current policy, and the only way of achieving it is by positive control on the part of the controller, issuing new clearances if required.

3.3.  Like in other ATC scenarios, a situation where standard separation does not exist due to actions by parties other that the controller does not result in the controller not being responsible for the separation of aircraft. This is consistent with the fact that in an ATC environment where separation has to be provided between all aircraft, there is always one party or operator responsible for that task.

3.4.  Therefore, the proposed modified policy must state clearly that the controller is responsible for separation after the pilots report “Clear of Conflict”. In addition, acknowledging the fact that standard separation might not exist when the “Clear of Conflict” is reported, the new policy must include the proviso that the controller shall establish standard separation between all affected aircraft.

3.5.  Finally, the Federation believes that the proviso in 3.4 should be included in the current ICAO procedures with regards to aircraft equipped with ACAS. In order to facilitate that possible inclusion, and given that IFATCA’s point of view fundamentally aligns with that of ICAO, the modified policy must match, insofar as possible, the aforementioned ICAO procedures.

Recommendations

4.1. It is recommended that the IFATCA provisional policy on page 3213 of the Technical and Professional Manual that reads:

After an aircraft has departed from its ATC clearance or instruction in compliance with an RA, or a pilot has reported an RA, the controller shall not resume responsibility for providing separation, until separation has been established for all affected aircraft.

is replaced by the following full policy:

Once an aircraft departs from its ATC clearance or instruction in compliance with an RA, or a pilot reports an RA, the controller ceases to be responsible for providing separation between that aircraft and any other aircraft affected as a direct consequence of the manoeuvre induced by the RA. The controller shall resume responsibility for separation and establish standard separation between all affected aircraft when:

a)  the controller acknowledges a report from the flight crew that the aircraft has resumed the current clearance; or

b)  the controller acknowledges a report from the flight crew that the aircraft is resuming the current clearance and issues an alternative clearance which is acknowledged by the flight crew.

References

Review of TCAS-Policy, 2014 IFATCA Annual Conference, WP 95, 2014.

IFATCA Technical and Professional Manual, AAS 1.1, 2014.

ICAO International Standards and Recommended Practices, Annex 10, Volume IV, 2007.

ICAO Procedures for Air Navigation Services – Air Traffic Management (PANS-ATM Doc 4444), 15.7.3, 2007.

Eurocontrol ACAS II Guide, 2012.

ICAO Procedures for Air Navigation Services – Aircraft Operations (PANS-OPS Doc 8168), 2007.

Last Update: May 14, 2020  

May 14, 2020   53   Jean-Francois Lepage    2015    

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