CONF iture asked :
Svarin, what’s your take on this ?
I will summarize very quickly here :
At the start of accident sequence, two apparently unrelated faults :
PROBE PITOT 1X2/2X3/1X3
WRG:ADIRU1 BUS ADR1-2 TO FCPC2
PROBE FLR is very likely due to icing of probes by high altitude ice crystals.
It triggers a specific 10 seconds ADR checking process by all three PRIMs.
Starts 02:10:05.
All three PRIMs work in Alternate 2 for 10 seconds.
WRG FLR affects PRIM2 by cutting it from ADR1 data. My take is this happens right at the start of above mentioned 10 seconds ADR checking process.
At the end of these 10 seconds,
each PRIM will either confirm and latch Alternate 2
or revert to Normal law, based on differences between ADR values. This is the purpose of this 10 seconds checking. This happens at 02:10:15.
My take is PRIM2 reverted
alone to Normal, while PRIM1 & PRIM3 latched Alternate 2. This is because PRIM2's set of ADR data, being different (only ADR2 & ADR3), would have allowed differences into the 50 kts range that triggers reversion to Normal law.
The sentence "the aircraft was in Alternate 2" makes no sense. The PRIMs compute flight laws, not "the aircraft". All three PRIMs are 99.9999% of the time in full agreement. Not this time.
NAV ADR DISAGREE is CMC stamped 02:12 and ACARS ground-received at 02:12:51, which means it happened on board between 02:11:58 and 02:12:43. Aircraft was downwards fast at that time. Therefore, the ADR DISAGREE condition
was not in effect to latch Alternate 2 for all three PRIMs at the start of pitch-up sequence at 02:10:16.
At 02:10:16, the pitch-up sequence and "zoom-climb" happened with (according to my research and "flimsy theory") PRIM1 & PRIM3 in Alternate 2 and PRIM2 in Normal.