From the final report:
It would also seem unlikely that the PNF could have determined the PF’s flight path
stabilisation targets. It is worth noting that the inputs applied to a sidestick by one
pilot cannot be observed easily by the other one and that the conditions of a night
flight in IMC make it more difficult to monitor aeroplane attitudes (pitch attitude
in particular). In addition, a short time after the autopilot disconnection, the PF’s
statement that he had the controls and his reaction to the initial deviations observed
(in particular in roll) may have led the PNF to change his action priorities. Identification
of the failure appeared to become a priority over control and flight path monitoring.
Consequently, he was unaware of the climb.
Control of the flight path does not correspond to what is expected. The amplitude
of the actions may doubtless be explained by the highly charged emotional factors
generated by the unexpected autopilot disconnection in the context of the flight.
From the CVR:
2 h 10 min 30,0 Stabilise
2 h 10 min 30,7 yeah
2 h 10 min 31,2 Go back down
2 h 10 min 32,2
According to that
we’re going up
2 h 10 min 33,7
According to all
three you’re going
up so go back down
2 h 10 min 35,2 okay
2 h 10 min 35,8 You’re at…
2 h 10 min 36,4 Go back down
2 h 10 min 36,7
It’s going we’re
going (back) down
2 h 10 min 38,5 gently
2 h 10 min 39,3
I’ll put you in in A T
T (*)…
So .. the PNF made remarks (give orders) to the PF to go back down and the PF give a positive verbal answer
Anyways .. the PNF can't see if the PF make good actions on the stick ...
How he can be sure that his orders are really followed by PF actions ?
By checking the instrumentations ?
Maybe by looking at the instruments and seeing that these would not show what he expected of them (because his order to the PF" back down" and the positive answer by PF) he began to believe that the instruments were no longer reliable
That can be a additional trouble for the PNF ... and alter momentarily is judgement of the situation ..