Greenspinner,
Thanks for your input, being a Boeing man I could not see that Airbus would have any other power supply for the ISIS than from the hot battery bus, so of all the ACARS transmitted messages the one relating to the ISIS grabbed my attention.
It is very easy (Boeing and Airbus) to, during the conversion ground-school, simplify the various systems under the " need to know" banner.
With regard to standby horizons (as we all used to call them) it amazed me during my 777 conversion, that technology had coaxed a laser to spin and act as a gyro (the way the Boeing SAARU was described), or as avspook describes the ISIS has an internal Fibre Optic Gyro (FOG) with no moving parts.
Concentrating on standby attitude indication (putting aside the other data that the ISIS provides to the crew), as long as there is battery power these "gyro's" should continue to proved basic standby indications of attitude.
What the Boeing course didn't cover was did the SAARU (and ISIS in the case of an Airbus) being unlike the old horizon with gimbals have any "limits" in roll and pitch, beyond which the internal checking would have suggested a fault and generated a maintenance message? If not, in the case of the ISIS, could the message have been generated by "erroneous" airspeed data being circulated within the ISIS, with the crew still having a downgraded ISIS still providing basis attitude indication to the crew?
Only Airbus can answer these questions, rather than us, as professional pilots on this forum, who have only been subjected the the "need to know" brainwashing technical courses!
What the communications from Airbus , Air France and the investigators yesterday led us to believe is that they have (perhaps), even without the flight recorders found, a common thread that could have produced so many pieces of electronic equipment having been downgraded/failed... unreliable airspeed.
Lets hope they find the recorders...