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Background |
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Avoiding Collisions |
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The Problem |
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How RA Downlink may help |
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Current Work |
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Early Adopters |
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The safe flow of air traffic requires close cooperation between flight crews and controllers. This cooperation becomes particularly crucial if, for whatever reason, the separation between two aircraft is lost and urgent steps are needed to restore it.
Controllers are normally provided with a Short Term Conflict Alert (STCA). STCA is a ground-based safety net that provides the controller with a warning of an imminent separation violation. An STCA warning is nominally generated 90 to 120 seconds in advance of the closest point of approach between the aircraft concerned. STCA does not provide any conflict resolution advice.
Flight crews have an Airborne Collision Avoidance System (ACAS, also commonly referred to as TCAS - Traffic Alert and Collision Avoidance System) to help them avoid collisions. ACAS produces vertical collision avoidance advice in Resolution Advisory (RA) messages and displays it to pilots 15 - 35 seconds in advance of potential collisions. RAs instruct the pilots to manoeuvre the aircraft vertically. ACAS RAs are automatically coordinated between the aircraft involved.
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When ACAS issues a Resolution Advisory (RA) to the pilot:
- The pilot shall immediately manoeuvre the aircraft in compliance with the RA.
- He must also report the RA to ATC as soon as workload permits.
- Departure from the current clearance in response to an RA ends the controller's responsibility for separation.
- After receiving a pilot's report about the RA, the controller is not allowed to issue any clearances to affected aircraft.
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During the RA pilots are busy dealing with the urgent issue at hand:
- 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.
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Whenever an RA is generated in the cockpit, the aircraft's transponder provides detailed information about the nature of the RA for possible downlink to the ground. Currently, air traffic controllers are only made aware of ACAS RAs if pilots give them radio notification. Therefore, the controllers' situational awareness may be diminished if the aircraft departs from its clearance as the result of an RA.
If proved feasible and safe:
- RA Downlink will provide an automatic notification to the controller about RAs generated in the cockpit.
- Whereas the controller might, for whatever reason, miss the verbal RA report from pilots, RA Downlink information will remain on the controller screen until Clear of Conflict.
- RA Downlink will be more dependable, more structured and often faster than current voice communications.
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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.
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RA downlink research is now progressed in the following SESAR (Single European Sky ATM Research) projects:
- Project 4.8.3 - Ground-Airborne Safety Net Compatibility (operational aspects)
- Project 15.4.3 - ACAS Monitoring
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Early adopters of RA Downlink shared their experience during the 2009 RA Downlink Workshop.
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Last validation: 11/05/2011
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