What is the correct procedure to follow if someone is intercepting you (or you are the F-16 driver intercepting someone else, for that matter) and you (or they, if you're the interceptor) get a TCAS RA in the middle of it all? I'm thinking to cases like this situation where an interceptor may interpret the interceptee's TCAS-triggered maneuvers as signs of noncompliance with the intercept...yet the interceptor may not know just what prompted TCAS to issue the interceptee the RA, and the interceptee must comply lest it is an interloping third party on a collision course!

  • $\begingroup$ I rather doubt that F-16s have TCAS. And, when he wants to avoid showing up on yours, he probably knows to turn off his Mode 3 (or perhaps just his Mode C, to avoid an RA) in order to accomplish that. $\endgroup$
    – Ralph J
    Commented Jul 5, 2015 at 3:58
  • $\begingroup$ @RalphJ -- I'd expect the interceptor to strangle their squawk, yes -- but that didn't happen in the Tabatinga case, and there's also the question of what happens if a third party flies into the middle of things and triggers a TCAS RA from the interceptee. $\endgroup$ Commented Jul 5, 2015 at 4:00
  • $\begingroup$ That case was about 4 years ago, and lessons are learned from such things. As to any 3rd-party-induced RA, that's got to be a pretty remote possibility, an RA and an intercept both happening at the same time??? Short answer, follow your procedures for the RA, and if you think you are being intercepted, get in touch with ATC ASAP. $\endgroup$
    – Ralph J
    Commented Jul 5, 2015 at 4:05
  • $\begingroup$ Remote -- but not impossible, especially if you're performing an intercept in a busy Bravo...and yes, you have most of the pieces of an answer here, although I wonder if our resident Rhino Driver has something to say about this topic from the interceptor's POV. $\endgroup$ Commented Jul 5, 2015 at 4:24
  • $\begingroup$ @RalphJ: Why wouldn't an F-16 have TCAS? I would imagine that the Air Force would rather not lose a multimillion-dollar fighter jet to a collision if it could avoid it... $\endgroup$
    – Vikki
    Commented May 5, 2018 at 3:23

1 Answer 1


The correct procedure for the interceptee would to follow the TCAS Resolution Advisory as they have generally no idea that they are being intercepted.

The interceptor must ensure that no TCAS RA is generated by switching his transponder to the appropriate mode. This has been standard procedure since the introduction of TCAS.

TCAS works by interrogating other aircraft's transponders. These interrogations are done in both Mode 3/A and Mode C as well as in Mode S (UF-0). To ensure that no TCAS RA is generated the interceptor's transponder must not reply to Mode C and Mode S interrogations from the interceptee. Since the source of Mode C interrogations is not encoded in the interrogation, the transponder must not reply to any of these interrogations including those from radar.

Technically a transponder could be designed to selectively not reply to Mode S interrogations from the interceptee only, however I think it is common to switch off TCAS Mode-S replies, or even switch off Mode S entirely.

  • 1
    $\begingroup$ Indeed, in any type of military formation only one aircraft has an active squawk/active TA/RA squawk for just this purpose. Since the interception is an intention formation for all practical purposes, the same thing would happen $\endgroup$
    – SSumner
    Commented Jul 6, 2015 at 2:01
  • $\begingroup$ This is inconsistent with the general requirement that all aircraft squawk in Mode 3/Mode C while in a Mode C veil and in certain other situations (DC SFRZ, R-zones etc). U.S. military aircraft performing an intercept within FAA airspace will squawk 0000; I don't know if TCAS systems treat that code any differently. $\endgroup$
    – KeithS
    Commented Jul 6, 2015 at 17:57
  • $\begingroup$ @KeithS TCAS doesn't treat A0000 differently as far as I know. But if the aircraft is only replying on Mode A and not on Mode C there will not be a TCAS RA since that requires altitude (and hence Mode C or Mode S) $\endgroup$
    – DeltaLima
    Commented Jul 6, 2015 at 22:13
  • $\begingroup$ But you still have the legal requirement to operate in Mode C while in a veil, which exist around any Class B airport which are in most major cities including DC. So as the interceptor vectored to investigate an SFRZ violation, do you squawk Mode C and risk a TCAS RA by the interceptee indicating noncompliance, or do you switch to Mode A and handle IFR separation by alternate means (VFR, reporting altitude to ATC vocally)? $\endgroup$
    – KeithS
    Commented Jul 7, 2015 at 2:00
  • $\begingroup$ @KeithS -- never mind the possibility of having to do the intercept in actual IMC! $\endgroup$ Commented Aug 5, 2015 at 22:30

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