Originally Posted by
DJ77
It is Dozy Wannabe’s opinion that the THS rightfully moved to 13 deg up in AF447 because it’s what the PF requested. It is apparently not Dozy’s opinion however that the THS should continue to trim up in normal law when high AoA protection is active and the pilot pulls on the stick, since the software prevents it. Where is the logic?
The main difference is that in Normal Law, all the inputs to the FCU and FMC can be trusted - therefore it is likely that the computers will know more about the plethora of aircraft parameters at any given time than the pilots, who are relying on an instrument scan and therefore only a subset of available data at any given time.
Alternate 2 is only triggered when one or more of the flight surface, sensory or computation systems have failed. At that point the system is designed to acknowledge that the computers' ability to assess the situation is impaired, and give complete control to the pilot. This presumes that the pilot knows his aircraft and how to handle it (which makes the assumption that Airbus designed their aircraft systems around the idea that pilots are largely numpties not only wrong, but pretty ironic in hindsight).
In essence, when you get ADR DISAGREE and Alt 2 clicks in, the computers are saying to the pilot "I can't trust these readings and I don't have your problem-solving skills. You can not only see the instruments but can also see outside. I'll help you keep the shiny side up based on what I know, but if I'm wrong you can override me". Unfortunately in this case the pilot made control inputs way in excess of what was appropriate, overrode the "soft protections" in doing so and stalled the aircraft, apparently because he was not appropriately trained.