Lonewolf_50, TAT probe freezing.
This is (was) a ‘relatively’ common problem occurring on several aircraft types. It was most prevalent on the BAe146 which suffered engine roll back due to ice crystal icing in the compressor – all engines since modified.
The most usual erroneous TAT indications, as with the A330 incident, are where the temperature stabilises at zero (ice or water ice) due to the blockage. The temperature change is relatively slow in comparison to the assumed ‘near instantaneous’ pitot blocking.
“…seeing alt low a few hundred feet … prompts pilot to make what is initially a small correction to climb back to FL350?”
Yes, very much my point; this would provide a reason for the initial nose up input, but not answer the subsequent control actions.
Amongst this we should consider that the FDs had probably failed (cf VH-EBA), thus the crew might be looking for a cued parameter to follow. Also, consider what the VS might show after an ‘instantaneous’ 300 ft descent and attempted recovery (inertial / air data mix IIRC).
What happens to Mach No? cf VH-EBA.
What happens to MMO/VMO computation? It’s most likely to be independent of the airspeed display as the value is also used by the FWC for audio warning; would the warning still work?
As for IRU, I knew of one design which had an air data correction in the unit (I forget why), but during testing, a sharp pull up with a rapid loss of speed and quickly increasing altitude caused the ADC software to detect a ‘failed’ (out of tolerance) condition and shutdown both ADCs, which in turn shut down both IRUs. The system design was changed before certification!
The point is that ADC seems to get into many dark corners of modern systems, thus we might not be sure of what the effects of simultaneous ADC malfunction might be.