So it was all the German pilots fault.
A single cause airliner accident is rare. Most are the result of the multiple causes.
The following is mainly taken from the BEA report into the loss of D-AXLA.
The traces for the AoA sensors show them working untill approx FL320. The traces then show that they did not moved again. (Page 33).
The latest reports refers to an aircraft wash which suggests that they were contaminated with water and as a result were frozen into the position that they were at that time.
As seen with other accidents, ice evidence can melt after a crash making definitive statements about icing difficult.
The first request for the slow speed check (including an alfa prot check) was made by the Air NZ Captain (who was overseeing the test flight) at approx FL120 which accorded with the advisory paperwork. (page 19) shortly thereafter, the A/C begins a continuous decent taking it out of the advised altitude for the low speed test.
The handling Captain suggested that the low speed check be done later or skipped. This at 5000'. (page 20).
At approximately 4000' the handling pilot asked the Air NZ Captain if he should conduct the slow speed check and this was affirmed. The handling pilot (who was also following ATC instructions and discussing nav with his F/O) eventually initiated the low speed check at about 3000" (page 20).
Due to what? configuration? control law? inoperative sensors? Alfa protection did not initiate. There is no reference in the report as to whether or not the crew had any expectation of this outcome. The aircraft stalled, eventually diving into the sea, which would indicate that they did not.
x x x x x x
From what has now been published the post painting checks for clearance of external sensors were not carried out adequately, eventually leading one way or another to the failure of the AoA sensors to function correctly. This failure played a part in events that has yet to be explained.
Alfa protection did not occur.
Inadequate training? This crew should have been prepared for Alfa floor protection failure and the check captain appears to have been unaware of the altitude requirement.
Inadequate paperwork? The paperwork should clearly identify the risk of Alfa floor failure. The Customer Manual assumption is that the Alfa protection will work.
If the test had been carried out at say FL120 the question remains would the crew have been able to recover the aircraft. What is shown in the traces published on page 34 is the disconnect between the pitch inputs of the handling pilot and the position of the elevators which in the final instance were diametrically opposed to each other. The french have intimated that the programing of the flight control computers is not an issue. Was this the result of the Aoa failure?
A single cause airliner accident is rare. and this isn't one of them.