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Here https://www.tailstrike.com/270680.htm is the transcript of the CVR of the Itavia Flight 870 (Ustica Massacre), but some communications have been omitted; I’ll use this site: http://www.noidellitavia.it/176334379.

I read (I’m translating):
20.23.38 =ROMA= 870 what’s your distance from Firenze? [FRZ was a VORTAC, now is the LOMED point]
=IH870= Approaching Peretola, 15 miles south [from FRZ].

=IH870= 870 is exactly aligned with the Firenze radial, heading 153 [FRZ – Bolsena (Amber 14), BOL is now the GIKIN point].

It’s evident that both the VOR and the TACAN are working. But:

=IH870= We have to change our mind about the Firenze VOR functionality
=ROMA= Yes, it's actually not going very well.
  1. IH870 receives bot the VOR and the TACAN, but it complains about the VOR functionality. What happened?
  2. Is it possible that the VOR is malfunctioning, while the TACAN works like a charm?
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  • $\begingroup$ TACAN is a military technology not used by airliners, so the flight wasn't using TACAN to begin with. $\endgroup$
    – GdD
    Jul 24, 2020 at 9:48
  • $\begingroup$ @GdD How do he know "15 miles south"? The TACAN is used as DME. $\endgroup$
    – Cristiano
    Jul 24, 2020 at 10:37
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    $\begingroup$ @GdD It is normal for civilian aircraft to use the DME part of TACANs $\endgroup$ Jul 24, 2020 at 10:46
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    $\begingroup$ That wasn't clear win the question @Cristiano. DME is only one part of TACAN, you may want to be more specific. $\endgroup$
    – GdD
    Jul 24, 2020 at 11:34
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    $\begingroup$ VOR, TACAN, and DME are three entirely different pieces of equipment. The fact that they're all located at the same place and referred to by the same identifier doesn't change that. There's no such thing as "the DME part of TACANs", they're two entirely different things. So, yes, it's entirely possible for the VOR part to fail while the DME still works or vice versa. Is that what you were asking? $\endgroup$ Jul 24, 2020 at 21:33

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DME is a subset of TACAN. DME transponders and interrogators are abbreviated TACAN transponders and interrogators. Civilian aircraft only use DME but that just means they ignore the rest of the TACAN ground station broadcast when they interrogate a TACAN ground station. A ground station will never have both DME and TACAN transponders: if it has TACAN, DME is redundant. It's possible for VOR to malfunction while a co-located TACAN or DME still works. and vice-versa.

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  • $\begingroup$ +1 for the question #2. Any idea for the question #1? I mean: is it possible that the onboard instruments show a wrong radial? $\endgroup$
    – Cristiano
    Jul 25, 2020 at 22:06
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    $\begingroup$ I thought I answered question #1 with my last sentence. Wrong radial, as seen by the aircraft is possible. It would depend on what went wrong with the ground station. Unless it was way off I don't know how the aircraft would notice. I think in this specific case it stopped working altogether. $\endgroup$
    – stretch
    Jul 26, 2020 at 16:11
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    $\begingroup$ "Wrong radial, as seen by the aircraft is possible. It would depend on what went wrong with the ground station". The VOR integrity is monitored on the ground: "The monitor shall transmit a warning [...], and either remove the identification and navigation components from the carrier or cause radiation to cease if any one or a combination of the following deviations...", and for bearing the deviation is 1° (though some national AIP require less than 1°). (ICAO Annex 10.) Perhaps in 1980 things were different. $\endgroup$
    – mins
    Jul 27, 2020 at 16:08
  • $\begingroup$ Very interesting, thank you, but I'm not able to find your citation. Would you be so kind to tell me the exact volume and page? $\endgroup$
    – Cristiano
    Jul 27, 2020 at 20:04

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