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.
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;
RA Downlink in PASS
RA Downlink was part of the EUROCONTROL PASS project.
RA Downlink in FARADS
RA Downlink carried out the Feasibility of ACAS RA Downlink Study (FARADS) the objective of which was to assess the technical and operational feasibly, as well as safety aspects, of displaying ACAS RA information on Controller's Working Positions (CWP).
RA downlink research has recently been undertaken by the following SESAR (Single European Sky ATM Research) projects:
- Project 4.8.1 - Ground-Airborne Safety Net Compatibility (operational aspects)
- Project 15.4.3 - ACAS Monitoring
The RA downlink research undertaken by SESAR P04.08.01 has been inconclusive, as real-time simulation could have not been conducted.
How it helps
RA information is transmitted to the ground via different channels. This is referred to as RA downlink. RA downlink is used for offline monitoring of ACAS. More recently RA downlink is also used to display RA information on controller working positions. Typically, controllers only know about RAs when, and if, pilots report them. The controllers' situational awareness may diminish if an aircraft departs from its clearance as the result of an RA and the pilot doesn’t promptly report this.
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.