Part of the 777 engine failure checklist is to set the transponder to TA only.

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What is the purpose of this step?

  • $\begingroup$ @mins, Ah, thanks. Makes sense. So is the design such that an aircraft receiving an RA from one only transmitting TA will take more "drastic" action? $\endgroup$ – Simon Sep 25 '16 at 9:59
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    $\begingroup$ The intruder will be resolving in "uncoordinated RA" mode, the vertical rate should be increased as soon as the separating distance crosses thresholds (reference is the paying RTCA DO-185 document). From this document: "Other TCAS II aircraft can generate (uncoordinated) RAs. A “TA-only” aircraft will be treated as unequipped by other TCAS II aircraft. This will allow them full freedom to choose the most effective RA." $\endgroup$ – mins Sep 25 '16 at 11:29

The TA mode is selected when there is an issue, basically telling the 'other' aircraft that you cannot maneuver if a RA situation occurs. The other aircraft has to do all the maneuvering (as they can still generate RAs).

From Eurocontrol ACAS II bulletin from Dec 2012:

In the TA-only mode,the equipment still performs the surveillance function (i.e. it scans for proximate traffic) but it will only generate TAs. Other TCAS II aircraft can still generate uncoordinated RAs against the aircraft which has its TCAS II in TA-only mode.

The TA-only mode may be explicitly specified in operating procedures as a response to in-flight malfunctions such as an engine failure with consequent aircraft performance implications or a pressurisation system failure followed by an emergency descent.

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    $\begingroup$ I would think the uncoordinated response will occur sooner to provide more separation, is that true? $\endgroup$ – wbeard52 Sep 25 '16 at 14:06
  • $\begingroup$ Where "you cannot manoeuvre" mainly means you can't achieve the rate of climb normally expected when "climb" instruction is given. $\endgroup$ – Jan Hudec Sep 28 '16 at 18:57

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