Originally Posted by
Dalex64
My question is, why doesn't the software realize, especially when in cruise, that pitch and power haven't changed much, GPS altitude is relatively stable, that a big increase in airspeed is simply illogical?
It does. That is what happened as far as we know today (tomorrow maybe we'll know more) - from BEA original reports on the ACARS messages:
This message, transmitted by the FCDC2 (EFCS2), means that the FCPCs (or
PRIMs) triggered one of the speed monitoring processes: they have detected
a decrease of more than 30 kt in one second of the “polled” speed value
As I read it, the computers will tolerate a transient (<10 seconds) period of dodgy values and revert back to normal if everything comes back into line - but if the values stay out of expected range, you are in alternate law etc for rest of the flight. Note that the BEA use the phrase "
one of the speed monitoring processes" - which implies to me that there are other cross-checking processes as well as this one.
One thing we might find out tomorrow is whether the pitots failed in a way that slipped through this monitoring - maybe a symettrical and gradual speed increase or decrease - leading the plane to be already doing the wrong thing by the time the fault is detected.