Originally Posted by Machinbird
But lets think a bit about the characteristics of a pitot tube that has iced up and closed the drain hole
If you really want to consider this case, then, you'll need TWO pitot tubes iced up the same way and exactly at the same time for your system not rejecting the wrong data (but it would reject the third pitot, displaying the good airspeed, as wrong) -ie. something easy by pressing a button on the sim panel, but not so replayable in real life (possible though as Airbus worried).
Originally Posted by Machinbird
There the aircraft pitches up into a high speed stall, freezing the RTL unit at the pre-existing correct value, since the high AOA triggers ADR disagree results similar to the Perpignan A-320.
I'm really unable to follow you here: any "high speed stall" won't freeze the RTLU, only wrong air data will.
Also, if the aircraft would wrongly pitch up, the end risk would be a low speed stall if both wrong airspeed retained were in reality lower than the system would think... not a high speed one.
Next, if your pitots are really clogged for inducing the system in error, you'll need to clean them again before they would record some wrong values due to any "stalled/spin" attitude (and now trigger unreliable airspeed, freezing RTLU at Mach .80, as mm43 quoted above).
Moreover, Perpignan is a really bad case of reference as this whole sequence was way too short as to even close a correlation window and to valid or reject any airspeed discrepancies [meaning, you can have many intermittent UAS alerts popping up in the cockpit without having a real/solid UAS event being triggered at the maintenance computer level]
This whole explanation is way out of reality. This is mostly the only thing acertained during this flight: they had suffered some REAL airspeed issues and the aircraft systems seems to have responded correctly by switching to Alternate Law 2.
The question remaining is not how the system performed but how the crew reacted to it. By studying similar unreliable airspeed cases, you'll see that the system is working as expected (as it is predictable) but you can't say the same about the crew reaction.
S~
Olivier