It disengaged, moved them to alternative law and one of them (the co-pilot, IIRC) didn't realize it and pilot and co-pilot gave different stick inputs. The purpose of a single button would be to put the plane in a known, easy to reason about state, even if it's not the most stable or the easier one to fly.
> The purpose of a single button would be to put the plane in a known, easy to reason about state, even if it's not the most stable or the easier one to fly.
The "known" is the problem there. The pilots there were continuously misinformed about the plane speed due to that iced measurement devices. That is what plane "knew" and what the pilots "knew" in the storm.
The autopilot handed over control to human, but then human drove it too high (the "law" here means "mode of operation"):
"The pilot continued making nose-up inputs. The trimmable horizontal stabilizer (THS) moved from three to 13 degrees nose-up in about one minute, and remained in that latter position until the end of the flight."
"A second consequence of the reconfiguration into alternate law was that stall protection no longer operated. Whereas in normal law, the aircraft's flight management computers would have acted to prevent such a high angle of attack, in alternate law this did not happen. (Indeed, the switch into alternate law occurred precisely because the computers, denied reliable speed data, were no longer able to provide such protection—nor many of the other functions expected of normal law).[55] The wings lost lift and the aircraft stalled"
> one of them (the co-pilot, IIRC) didn't realize it
But the co-pilot definitely knew that the autopilot disengaged:
"At 02:10:05 UTC the autopilot disengaged" ... "As pilot flying, Bonin took control of the aircraft via the side stick priority button and said, "I have the controls.""
The best was not even to enter that storm.