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Can a TCAS give a TAs (traffic advisories) if the intruder aircraft has a transponder in stand-by mode?

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No. If the intruder's transponder is in standby it will not respond to interrogations and will be invisible to the TCAS. TCAS can't act on targets it can't interrogate.

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    $\begingroup$ A TCAS in standby mode still replies to discrete interrogation. What would happen if TCAS1 had already registered the presence of TCAS2 before TCAS2 was switched to standby mode? $\endgroup$
    – mins
    Oct 22, 2017 at 11:39
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    $\begingroup$ @mins It might work if the intruder was close enough to be put on the track list before the transponder was set to standby. TCAS can only issue a TA or RA when the intruder is actively interrogated. But even then, if it slides off the track list and it's in standby, the TCAS won't re-acquire it as there won't be a short squitter to announce it. Going forward, if TCAS2 has ADS-B out and TCAS1 has hybrid surveillance/ADS-B IN, it would be possible to see the aircraft on the display. but without a TA or RA. $\endgroup$
    – Gerry
    Oct 23, 2017 at 23:42

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