If the VOR works and the TACAN works, why doesn’t the VORTAC work?

Question

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.

IH870 receives bot the VOR and the TACAN, but it complains about the VOR functionality. What happened?
Is it possible that the VOR is malfunctioning, while the TACAN works like a charm?

stretch · Answer

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.

Answered by stretch on December 26, 2021

If the VOR works and the TACAN works, why doesn’t the VORTAC work?

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