Thanks for the replies, PJ2 and Takata.
First:
"Hm, it may be "unexplained" but I don't think it is inexplicable. I think the TCAS indicated "failed" because of the absence of good data from ADIRS 1. I don't think it's more complex than that."
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Each LRU, Line Replaceable Unit, as in the ADIRU, is a conglomeration of separate, often independent, functions, i.e.:
Altitude - independent computation, save for low speed trim.
Airspeed - depends on altitude for computation.
Vertical Speed - depends on altitude/rate
Pitch
Roll
Heading
Ground Speed
Lat/Long
etc.
Each of these functions outputs one or more words on a common Arinc 429 data bus. The receiving device recognizes and sorts the words by their three digit identification labels. Each word has its own SSM, Sign Status Matrix to tell the receiving device its health. The receiving devices may also receive the same words from the other sensors, to use in case of Fail Warn from the primary, or for comparison.
A blocked pitot, for example, will not disable altitude computation, nor IRU functions. This was seen in the Delta/NW incident.
TAM and others loss of airspeed AND altitude indication to the pilots indicates a different situation entirely. It provides evidence of a static source problem, such as frozen static lines, rather than iced pitot, or in addition to iced pitot.
GB