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When the TCAS issues a Resolution Advisory (RA) which contradicts an ATC instruction, the RA takes priority.

  1. Are controllers aware (notified on their screen) that a RA has been issued between two converging traffic?
  2. If yes, are the controllers also able to see the RA instruction issued to each specific aircraft?
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  • $\begingroup$ Is there a difference between TCAS and any other traffic system including looking out the window? $\endgroup$ – user6035379 Jan 15 '17 at 17:42
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    $\begingroup$ Exactly because of this the Überlingen mid-air collision took place. The TCAS issued an RA to the crews, the ATC stepped in and gave exactly the opposite orders. One air crew followed TCAS, the other the ATC - both planes climbed and collided, killing 71 people. $\endgroup$ – jklingler Jan 17 '17 at 11:28
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In general ATC is not aware of any RA occurring in aircraft, and is neither aware of the RA content. The crew informs ATC using a phraseology described in ICAO Doc 4444 PANS-ATM, Chapter 12, Para 12.3.1.2:

  • When the RA occurs, and the crew must deviate from ATC instructions, they inform ATC with "TCAS RA" when possible.
  • If an ATC instruction is received, which contradicts a current RA, they inform ATC with "Unable, TCAS RA".
  • The crew then informs ATC when the RA has ended, with "Clear of conflict, returning to (the ATC clearance)", and "resumed" when done.

As @J.Hougaard mentioned, there are some experimentation going on to downlink RA to ATCO display, but nothing mandatory, the information displayed varies and somehow conflict, or introduces a confusion, with ICAO recommendations. So far it seems Czech Republic, Hungary and Japan have a downlink capability. More on Eurocontrol site.

When informed, ATC doesn't provide further instructions to this aircraft before receiving the "clear of conflict" confirmation.

RA always takes precedence on ATC and must be followed by the crew, even if the crew has the intruder in sight and another safe maneuver could be executed as well. The reason is because there is no certainty that the aircraft they have in sight is the conflicting one, or is the only conflicting one.

See instructions to ATC and RTF Phraseology associated with TCAS RAs.

(Indeed a pilot can deviate from anything, but must later demonstrate that they were justified in doing this).

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    $\begingroup$ Some ANSP's are experimenting with downlinking RA's to controller working positions via mode S. It will likely be implemented in some areas within a few years. $\endgroup$ – J. Hougaard Jan 15 '17 at 17:59
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    $\begingroup$ "RA takes precedence" only when the crew has been trained on the use of the system. I worked at a carrier that operated at least one airplane (mine) with RA capability but I was not legally able to take action on an RA that I received. The regs may have changed since but I recall this particular limitation becasuse it was just so stupid. $\endgroup$ – acpilot Jan 15 '17 at 18:11
  • $\begingroup$ @acpilot: interesting :-) I can't answer for all jurisdictions, but at least in EU, ACAS II v7.1 are the only ACAS allowed, are mandatory for MTOW 5.7 t or 19 pax. Then EU-OPS 1.398 mandates SOPs to contain a requirement to use RA in flight, and "the commander or the pilot to whom conduct of the flight has been delegated must ensure that any corrective action indicated by the RA is initiated immediately". $\endgroup$ – mins Jan 15 '17 at 18:40
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It is technically feasible to alert ATC of the TCAS advisory via an automatic TCAS downlink:

"An automatic downlink for the TCAS – which would have alerted the controller that a TCAS advisory had been issued to the aircraft under his control, and notified him of the nature of that advisory – had not been deployed worldwide at the time of the accident." (source)

The above implies this system is available in some places in the world, but was not deployed worldwide in 2002 at the time of the Überlingen mid-air collision. I don't have information about the current status of worldwide deployment.

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