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There's a clip on YouTube from a pilot of a 747-200 whose TCAS system issues a "confusing" RA:

What exactly is happening here?

For those who can't watch the video right now, here are some frames:

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The TCAS responded to the other aircraft not following the flight profile that was observed earlier. If you watch closely, you will see that the other aircraft started to descend as well after the first resolution advisory is given. The onboard TCAS responds by issuing the counter-movement to climb immediately.

The only thing that we do not know is what version of TCAS this is supposed to simulate, because newer TCAS systems can interrogate each other and issue resolution advisories not only based on own observed data, but also on data received from the opposite TCAS.

Related Question: How does TCAS work?

The entire situation could have been part of the training scenario, as this was done in a simulator, according to the YouTube description:

Practising TCAS RA manoeuvers during our MCC course in a Boeing 747-200 full-flight simulator. This one was quite confusing when the TCAS' voice advised to climb just after having asked to descend.

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    $\begingroup$ Even if it's modern TCAS, that could still happen - just because the other plane gets an RA, even if it's a climb RA, doesn't mean they necessarily won't descend. $\endgroup$ – cpast Jan 31 '15 at 3:56
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    $\begingroup$ In TCAS jargon this is called an RA reversal. (Ref p.33) $\endgroup$ – RedGrittyBrick Jan 31 '15 at 15:40
  • $\begingroup$ This appears to be a TCAS 7.0 based on the phrases used. "Descend, Descend" (two times) is only on v7.0 and 7.1 and "Adjust vertical speed" was only used on version 7.0 (older TCAS say "Reduce climb" and 7.1 says "Level off"). However even with a modern TCAS equipped, RA coordination only exists in certain circumstances. $\endgroup$ – Cody P May 27 '16 at 23:43
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See below for a description of how the TCAS (Traffic Collision Avoidance System) display is showing another aircraft approaching in each frame:

  • 900 feet below and climbing
  • 800 feet below and climbing
    • The traffic advisory (TA) gives the pilots the chance to avoid or at least anticipate conflict
  • 500 feet below and still climbing
    • The resolution advisory (RA) gives pilots instructions that should prevent a collision between the aircraft
    • The RA tells pilot to descend
      • They are level at FL270, so descending will allow the other aircraft to continue climbing above them
    • The TCAS in the other aircraft will tell them to keep climbing
  • 500 below and descending
    • The RA changes to "climb"
      • The other aircraft has started descending, contradicting the TCAS instructions
      • The pilot should climb now to avoid the other aircraft
  • 800 below and climbing
    • The RA changes to "adjust vertical speed"
      • There is now more vertical distance between the aircraft
      • The pilot should adjust their vertical speed to remain clear of the traffic
  • 700 below and climbing
    • Clear of conflict
      • The aircraft have passed each other by this point, without colliding, so TCAS and the pilots worked together to prevent (simulated) disaster

The TCAS display shows other aircraft as shapes depending on proximity and the risk of collision.

TCAS symbology
Source

The symbols in the simulator from the video are slightly different, but you can see the aircraft go from non-threat (first frame), to traffic advisory (second frame), to resolution advisory (third and fourth frames).

In addition to the symbol, basic information about the aircraft's altitude is displayed. The numbers represent the difference in altitude in hundreds of feet (negative means below your aircraft), and the arrow represents climb or descent over 500 feet per minute. See here for info about the symbols.

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