Graybeards said (at
post 1549 that:
"Ref: "There are five official, damning reports on A330 and A340 aircraft operated by
major airlines suffering complete failure of all flight instruments in flight. Pitot probes
iced up in meteorological conditions at night, by day, in cloud and in clear air."
The early failures reported by AF447 ACARS all seem to be explainable by iced pitot tubes, save
for the TCAS Fail. TCAS does not use airspeed, just altitude via the transponder.
Air data Altitude failure will cause the transponder to revert to Mode A. I'm not sure if it would report failure of its altitude input to the CMS/ACARS, but it didn't report a Fault.
Transponder reversion to Mode A will not cause a TCAS Fail condition, but a TCAS OFF condition.
Therefore, we have a TCAS Fault that occurred nearly simultaneously with the airspeed sourced faults, but unrelated."
It would appear that some are flummoxed by the TCAS failure msg on ACARS (TCAS needing and acquiring static pressure only).
Correct me if I'm wrong but, depending upon exactly
how the ADIRU derives and passes the static pressure, the pitot icing failure may be related.
i.e.
Pitot pressure = dynamic pressure (i.e. IAS) plus static pressure
The ASI derives a normally correct airspeed by balancing the pitot derived static against the static pressure sensed from the static ports (and fed into the ADIRU as a digital signal by a transducer).
IAS (or dynamic pressure) = pitot pressure minus static port sensed pressure
So if your static lines ice up due to trapped water freezing as you climb, then those
two differently derived static pressures will become increasingly unequal and the ASI will wind back to zero (in a climb/opposite in a descent). In the cruise, the IAS will simply "freeze".
So it depends which static pressure is being fed to the ADIRU and at what stage? Is it simplistically static port sensed pressure (with its built-in pressure error) or is it the resolved average of the two differently sourced static pressures? Once the pitot ices over, that average would be duff.