Interesting article which calls attention to the Trimmable Horizontal Stabiliser having remained virtually at full nose-up from very early on; calling attention to the fact that the same thing had happened before, to an A320 near Perpignan in November 2008:-
One question lies with the trimmable horizontal stabilizer (THS). The critical phase of the flight, from autopilot disengagement to the crash, lasted 4 minutes 23 seconds. During the last 3 minutes 30 seconds, the position of the THS went from 3 degrees to 13 degrees nose-up and then remained unchanged. Yet, from about 2 minutes before the crash, the pilot flying switched to pitch-down inputs.
So, as French website aerobuzz.fr pointed out, why did the THS stay in such a nose-up setting? This may hint at the flight control law being no longer “normal” but in a mode (“alternate” or “abnormal”) where the autotrim function is deactivated. In that instance, the crew has to trim the stabilizer manually. In an A320 accident that took place near Perpignan, France, in 2008, the crew’s failure to recognize this situation contributed to the catastrophic chain of events. In the case of AF447, the crew did mention “alternate law” in the recorded conversation.
History of Flight AF447 Puzzles Experts: AINonline
Also found an article on the Perpignan accident. This confirms that the investigators determined that "a progressive pitch-up deflection of the horizontal stabiliser as the A320 decelerated" had contributed to the accident.
The investigators concluded that water in the AoA sensors (which they thought had probably been introduced by sloppy maintenance on the ground) had frozen at altitude and caused the malfunction:-
"With the workload mounting in the cockpit, because the aircraft was on approach to Perpignan, the low-speed test commenced. But the blockage of the angle-of-attack sensors resulted in an underestimation of the limit speeds for the A320's angle-of-attack protection.
"The crew waited for the triggering of these protections while allowing the speed to fall to that of a stall," says the BEA. It says this "passive" wait for the protective systems, a lack of awareness of the risks, and confidence in the operation of the aircraft's systems "tend to show" that the captain and the ANZ pilot started the manoeuvre as a "demonstration of the functioning" of the angle-of-attack protection "rather than as a check"."
Although the stall warning sounded, and the captain increased thrust and pitched the aircraft nose-down, the configuration of the aircraft - notably a progressive pitch-up deflection of the horizontal stabiliser as the A320 decelerated - and the failure to understand the jet's behaviour, resulted in the situation deteriorating and the crew's losing control of the stall recovery. All seven occupants were killed when the A320 struck the water, just 62s after the stall alarm.
Sensor icing caught out A320 crew in Perpignan crash