We know that the failure was Pitot tubes
The AP was alerted of this failure and so he disconnected
The FBW system was alerted and so go in alternate law
Is the crew was alerted of this failure (for take appropriate actions .. so follow the procedure in force for IAS) and when in the time scale of the event ... ?
The AP in FMGEC was disconnected by 2 by 2 comparison of the speed data from ADIRUs.
The PCPCs reverted to ALTN LAW by comparison of the median/average value of the speed data from ADIRUs.
The crew is alerted by comparison of the speed displays (
NAV IAS DISCREPANCY) if the a/c was updated to the highest standard available (we know it is was not!).
The message NAV ADR DISAGREE didn't show up in phase 1 of the event because -unfortunately- it didn't exceed the delta/time limits.
This crew at that time needed to do the comparison themselves and take the appropriate action (pitch and power) like it was done for many years.
An exact failure description is simply not feasible, even the ADIRUs don't know the PITOT icing, the only know (by comparison) that Pt is not equal.
Above that all pitot failures will lead to the same appropriate action: pitch and power.