Originally Posted by Dozy
The problem (as you state below regarding another point, and as I think I said in my original reply) is with false positives. If a sensor jam or genuine technical error causes the stall warning to sound when the aircraft is neither at or near the stall, that solution would prevent the pilot from trimming up when he wanted or needed to. You're then faced with the option of holding attitude with the primary controls and thrust (which would be fatiguing) - or if the limit applied to autotrim only, forcing pilots who are not used to trimming manually to do so at altitude with an abnormal situation on their hands.
So Dozy, how do you explain the AOA inhibit of nose up trim in Normal Law? Why did the engineers decide that was sensible? Wasn't it because you could actually trim an aft cg aircraft into a stall?
What happens when the trim stops is you have to hold a pressure. If you don't want to hold the pressure, you can still reach down with your free hand and turn the trim wheel. No big deal, but it will have to be a conscious decision.
The protections are a separate subsystem entirely from the annunciation/warning subsystem. There is no overarching logic connecting them, which makes implementing such a change considerably harder. I can see where you're coming from, I just think that a hard limit on autotrim under certain flight control circumstances would make more sense.
There are all manner of things that happen in a FBW aircraft that are not annunciated. If nose up trim stops because it senses a limiting AOA, that is a form of annunciation in itself, and from a piloting standpoint, that is very intuitive..