Review policy of ACAS / TCAS

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Review policy of ACAS / TCAS

42ND ANNUAL CONFERENCE, Buenos Aires, Argentina, 17-21 March 2003

WP No. 94

Review policy of ACAS / TCAS

Presented by SC1

Introduction

1.1  As part of the 2002/3 work programme, SC1 undertook to review existing policy on ACAS / TCAS.

1.2  IFATCA has already developed policy for ACAS / TCAS. However, in the light of the recent incidents, existing TCAS policy is being reviewed by a number of organisations. It is therefore necessary to ensure that existing IFATCA policy on TCAS is still applicable.

1.3  The purpose of this paper is to review and, if applicable, amend IFATCA policy on issues concerning the use of ACAS / TCAS.

Discussion

2.1  Existing IFATCA policy on ACAS states:

“IFATCA recognises that the development of airborne collision avoidance systems should be encouraged. However it must be accepted that the primary means of collision avoidance within a controlled airspace environment must continue to be the air traffic control system which should be totally independent of airborne emergency devices such as ACAS. TCAS devices should not be a consideration in the provision of adequate air traffic services.”

 

It is recommended that the above policy is retained. The provision of safety nets, including ACAS, should not be taken into account when determining the overall safety of an ATS system. This policy is in line with EUROCONTROL’s Safety Review Commission who have drafted the following policy statement:

“Any safety benefit which may be provided by a safety net shall be considered as an additional overlay to that provided by the ATM system. The ATM system must be able to demonstrate whatever ATM safety minima and aviation level of safety are considered to be necessary, without reliance upon the safety benefit expected to be provided by safety nets.”

2.2

“The use of automatic airborne collision avoidance systems should allow for safe operation within different types of airspace, with different ATC procedures and with different aircraft equipment capabilities – without detriment to the ATC service or to aircraft not fully equipped.”

 

IFATCA should recommend standardisation of ATC procedures to accommodate ACAS. A common ATC procedure to be applied in response to ACAS events should be established throughout the world to promote a universal understanding of how ACAS events should be handled. Some discussion has taken place regarding the wording of the existing policy above. This discussion has focused upon the interpretation of “…with different ATC procedures…”. The outcome of the discussion is that this phrase refers to the variety of ATC procedures in place at different ATS Units across the world and is not intended to refer specifically to ATC procedures to be employed in the event of an ACAS event. The existing policy therefore does not preclude the introduction of a common ATS response to ACAS events to be used on a global basis.

2.3

“The inevitable changes in ATC procedures, techniques and phraseologies resulting from the introduction of airborne collision avoidance and traffic alert systems should be compatible, not only with a controller’s responsibilities for providing positive separation, but also with a controller’s ability to discharge them.”

 

The above policy is still regarded as being applicable and should be retained.

2.4

“All MA’s should urge their National Administrations to assemble, disseminate administer and maintain a comprehensive ACAS training package for ab-initio and regular refresher training.”

 

It is considered that the availability of suitable training packages is essential to ensure that controllers throughout the world are presented with information on how to respond to ACAS events in a common and effective manner. The above policy should be retained.

2.5

“At airports where Simultaneous Intersecting Runway Operations (SIROs) are occurring, aircraft on departure and arrivals inside the final approach fix, TCAS should be operated in the TA mode only.”

 

During discussions regarding the above policy it has become clear that TCAS Version 7.0 software ensures that TCAS will operate only in Traffic Alert (TA) mode when below a height of 1000ft. Additionally, TAs are suspended when the aircraft is below a height of 300ft. The policy specified above is supplementary to the function of TCAS Version 7.0 software and can be considered to be superfluous as the software will ensure that TA Mode only is available when below a height of 1000ft.

It is proposed to delete the above policy.

2.6

“Collision avoidance transmissions should not exclude, by virtue of their priority, transmissions directly concerned with establishing and maintaining tactical separation.”

 

It has been determined that this policy was put forward as part of WP 78 – Estoril 1984. Whilst this Working Paper addressed some emerging issues on TCAS, it also contained information about Mode S developments. The policy statement above refers to Mode S transmissions and is not considered applicable to policy on ACAS / TCAS.

It is proposed to delete the above policy.

2.7

“Whenever a manoeuvre contrary to an ATC clearance is made as a result of a collision avoidance advisory, the pilot shall inform ATC as soon as possible.”

 

This statement is similar in wording and the same in intent as a statement within ICAO PANS-ATM (ref:para15.6.3). IFATCA only creates policy where there is no existing ICAO policy or where the IFATCA view differs from that of ICAO. In this case, the recent publication of ICAO policy, which it should be recognised was influenced by IFATCA representations, means that the existing IFATCA policy is now redundant.

It is proposed to delete the above policy.

2.8

“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.”

 

This policy item has been the subject of much debate since the last Conference. At present, the view of SC1 is that there is still not enough information available as to alter the current IFATCA position, namely the opposition to the downlinking of advisories generated by ACAS. SC1 is concerned that the information would not be presented to the controllers on a timely enough basis to be of operational benefit. The time lag between the ACAS advisory being generated by the airborne equipment and being displayed to the controller is dependant upon the type of datalink infrastructure that is in place, e.g. HF Datalink, VDL-Mode 2, VDL-Mode 4. There could be instances where the controller is not presented with the information about the ACAS advisory until several seconds after it has been generated by the airborne equipment. This outdated information could cause confusion to the controller as the advisory will not necessarily reflect the events either being displayed on the Situation Display or being relayed over the R/T. In some situations, it may be worse to have outdated information than any information at all. It is understood that there is research underway by organisations, including EUROCONTROL, to investigate the feasibility of downlinking ACAS advisories. Once the outcome of these studies is known, IFATCA should revisit this policy item.

In the interim, it is recommended that the above policy be retained until further information is available for consideration.

2.9

“All Member Associations should ask their National Administrations to introduce, as a matter of urgency, procedures to absolve their controllers of responsibility for any losses of separation caused by an aircraft executing an RA (Resolution Advisory) from the point in time where the RA was initiated to that point in time where the aircraft concerned have been cleared to, and are maintaining new altitudes and/or level spacings. (These altitudes may well be the pre-RA altitudes of the aircraft concerned)”.

 

This policy statement is similar in wording and the same in intent as a statement within ICAO PANS-ATM (ref: para 15.6.3). IFATCA only creates policy where there is no existing ICAO policy or where the IFATCA view differs from that of ICAO. In this case, the recent publication of ICAO policy means that the existing IFATCA policy is now redundant.

It is proposed to delete the above policy.

2.10  In previous Working Papers on the subject of ACAS / TCAS there has been discussion that ACAS should only be considered as a Safety Net and not as a ‘separation tool’. It is generally recognised that ACAS is “a last minute device not to be used for ATM purposes”. ACAS certainly falls within the IFATCA definition of Safety Nets. New policy is required to unambiguously state that ACAS should only be considered as a Safety Net.

2.11

“IFATCA recommends that National Administrations ensure that pilots should receive compatible training on action to take upon receipt of a Resolution Advisory, regardless of the region that they are operating in or the country from which they originate”.

 

The recent tragic mid-air collision over the Swiss-German border has indicated that different ACAS techniques are being taught depending on the country / region in which the aircrew are based. Discussion on the above policy statement indicated that training alone might not resolve the problem of pilots reacting in differing way to Resolution Advisories. In addition to differences in training, variances in company operating practices and legal requirements of states have been identified as a potential source for inconsistent responses to Resolution Advisories.

In order for ACAS to be a successful safety net it is essential that all parties concerned react in a consistent manner to advisories provided. Discussions regarding how best to ensure that pilots react in a consistent manner to Resolution Advisories should be held with IFALPA as soon as practicable.

Conclusions

3.1  TCAS has been mandated within some states for many years. However, recent incidents have highlighted the fact that TCAS, despite undergoing progressive enhancements, is still not infallible as a safety net. The introduction of additional safety nets within the ATM system, such as the implementation of STCA, is to be encouraged as human error is unlikely to ever be eradicated despite advances in training techniques and practices.

3.2  The implementation of safety nets within the ATM system has tempted controllers, managers and regulators to use the application in the calculations to determine if the ATM system is ‘safe’. However, the stance that has historically been taken by IFATCA and ICAO alike, and that needs to be maintained, is that such systems are safety nets and as such are a “last ditch” defence against collision with other aircraft, obstacles, or terrain.

3.3  This review of IFATCA policy on ACAS needs to be repeated on a regular basis to ensure that policy reflects developments in the applications and the way in which they are used.

Recommendations

It is recommended that:

4.1  IFATCA policy, IFATCA Manual page 3 2 1 2, para 1.1., being:

“At airports where Simultaneous Intersecting Runway Operations (SIROs) are occurring, aircraft on departure and arrivals inside the final approach fix, TCAS should be operated in the TA mode only”.

Be deleted.

4.2  IFATCA policy, IFATCA Manual page 3 2 1 2, para 1.1., being:

“Collision avoidance transmissions should not exclude, by virtue of their priority, transmissions directly concerned with establishing and maintaining tactical separation”.

Be deleted.

4.3  IFATCA policy, IFATCA Manual page 3 2 1 2, para 1.1., being:

“Whenever a manoeuvre contrary to an ATC clearance is made as a result of a collision avoidance advisory, the pilot shall inform ATC as soon as possible”.

Be deleted.

4.4  IFATCA policy, IFATCA Manual page 3 2 1 2, para 1.1., being:

“All Member Associations should ask their National Administrations to introduce, as a matter of urgency, procedures to absolve their controllers of responsibility for any losses of separation caused by an aircraft executing an RA (Resolution Advisory) from the point in time where the RA was initiated to that point in time where the aircraft concerned have been cleared to, and are maintaining new altitudes and/or level spacings. (These altitudes may well be the pre-RA altitudes of the aircraft concerned)”.

Be deleted.

4.5 Following be accepted as IFATCA policy:

“ACAS should only be considered as a ‘safety net’.

References

5.1  WP 88 – Jerusalem 1995.

5.2  WP 78 – Estoril 1984.

5.3  WP 79 – Christchurch 1993.

5.4  ICAO. PANS-RAC 2.19.

5.5  ICAO. PANS-OPS (Doc 8168), V1, Part VIII, Chapter 3.

Last Update: September 29, 2020  

March 22, 2020   260   Jean-Francois Lepage    2003    

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