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RA Downlink
From SKYbrary Wiki
| Article Information | ||
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| Category: | Safety Nets | 
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| Content source: | SKYbrary | 
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| Content control: | EUROCONTROL | 
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Contents |
Description
Resolution Advisories (RAs) are issued to the pilots by the Airborne Collision Avoidance System (ACAS) (also referred to as TCAS - Traffic alert and Collision Avoidance System) if a risk of collision is identified. The RA takes precedence over ATC instructions and the controller should not interfere with the resulting collision avoidance manoeuvre.
Until recently controllers have only known about RAs when, and if, the pilot reports them. Therefore, the controllers' situational awareness may be diminished if the aircraft departs from its clearance as the result of an RA.
RA Downlink
However, when an RA is generated the aircraft’s transponder can provide detailed information about the RA for downlink. If proved feasible and safe RA downlink will provide faster, more structured, automatic notification of RAs, which will remain on the controller screen until ACAS issues a “Clear of Conflict” message.
Pilots often fail to notify ATC or delay their reports. It is estimated that 25% of RAs are never reported to the controller and 25% of RAs are notified very late. Pilot reports (when they do happen) are often lengthy, unstructured, incorrect or incomplete, requiring repetition or clarification from the controller. If controllers do not know that the aircraft is responding to an RA, they might issue an instruction in an attempt to resolve the conflict. That instruction could contradict the RA.
Potential benefits of RA Downlink include:
- Reduced likelihood of contradictory clearances.
- Improved situational awareness.
- More up-to-date traffic information, especially to other aircraft in the vicinity.
- Better post conflict traffic planning
Safety Recommendations
Safety recommendations concerning RA downlink have been made by a number of organisations and advisory groups. Some of them are listed below:
- In 2001, following an incident in Copenhagen, the Danish Accident Investigation Board recommended that it should be considered whether controllers should have access to Mode S transponder RA information .
- In 2003, the High Level European Action Group for ATM Safety (AGAS) advised to investigate the feasibility of RA Downlink.
- In the Überlingen accident report, BFU (German Federal Bureau of Aircraft Accidents Investigation) recommended the initiation of RA Downlink development.
Previous Studies of RA downlink
EUROCONTROL in 2003 launched the Feasibility of ACAS RA Downlink Study (FARADS) project. The aim of this project is to investigate the technical feasibility and the operational benefits of providing the RA information to the controller in a comprehensive way.
The FARADS project is not the first initiative investigating the possibility of displaying RA information at the Controller Working Position (CWP). Studies on the feasibility and operational benefits of RA downlink have been conducted in the USA, in France, and in Japan.
In 2005 and 2006, a number of RA downlink real-time simulations have been conducted. The simulations provided evidence that RA downlink can increase controllers’ situation awareness and limit the likelihood of controller’s issuing a clearance to the aircraft responding to the RA. In contrast, no negative effects of RA downlink on the controller’s performance were found (e.g. over-focusing on the RA event).
The FARADS Study, completed in 2007, concluded that RA Downlink is technically feasible and may deliver safety benefits. However, these benefits could not be quantified due to insufficient data.
Since the completion of FARADS, EUROCONTROL and other organisations have undertaken monitoring work to provide that data and examine the interactions between ground based and airborne safety nets (STCA vs. ACAS). That is being done in the scope of EUROCONTROL PASS Project and now is being carried on by SESAR.
Developments as of July 2010
In July 2010, downlink was available in some commercial systems and a number of ANSPs (in Europe and elsewhere) had decided to implement it.
There were no ICAO provisions regarding RA downlink and concepts of operations varied between places where RA downlink had been implemented.
Related Articles
- STCA
- Loss of Separation
- Level Bust
- ACAS Regulation and Procedures
- ACAS Training
- ACAS Resolution and Traffic Advisories
- ACAS: Guidance for Controllers