We know, that the aircraft maintains stick free 1g, and that SS commands a change of g blended with pitch rate starting below 210 knots. In a newspaper article from the early A320 flights i remeber, that the ratio is 50/50 at 150 Knots. The speed responsible used for this changeover was faulty, down to 60 knots which would equal nearly a pure pitch rate change. But the aircraft was traveling still at over 230 knots in the regime where only g command should be present. I could not find any reference from where we could draw a conclusion, wether this different SS command would have influenced the outcome of any SS order. There is reason, that this changeover takes place in the low speed regime, what influence does it create wehen this changeover is taking place when the airframe is still above the change over regime in degraded mode?
As I understand the system it doesn't work quite like this. The BEA report says:
In alternate 2 law, the longitudinal control law remains a load factor law and the lateral control law is a direct law. In the specific case of alternate 2B law, some coefficients used in the longitudinal flight control law become speed-independent and are set for the maximum speed for the aeroplane configuration (330 kt in clean configuration). This hardly modifies the behaviour of the aeroplane in comparison to normal law, but can nevertheless induce an unusual response dynamic when the aeroplane has an abnormally low speed for the configuration.
From all the diagrams I have seen describing how C* works the pitch rate/g mix is a
feedback term. S/S movement
always commands a load factor as the
steady state output just as the BEA report says. What the pitch rate/g term does is change the shape of the
transient response between initial stick movement and achievement of the final desired 'g'. The origins lie way back in aircraft handling research when observers noticed that pilots tended to base their opinions on what constituted 'good' behaviour on how the pitch rate transient varied at low speeds and how the 'g' transient varied at high speed. Then somebody thought if that is what they rate as good why can't we give it to them?
So when in Alt 2B the system gains default to their 330 kt (Vmo) values the effect would be that the system would be trying to achieve the 'ideal' transient response (remembering that the Airbus intent was to make the pitch response the same at all speeds) using high speed 'g' base gains but at much lower speeds. This is what I think BEA are getting at when they say the aircraft has an unusual response dynamic in this situation.
Since the transient 'g' response to elevator movement would be much crisper at high speed I think this would lead the system to apply less elevator (to drive the pitch acceleration) than it would normally use at these low speeds, so the aircraft response would be more sluggish.
Anyway, that is how I see it working
