The two fundamental questions posed by Shockwave appear to be:
a. What is spiral stability?
b. Why does the AB330 roll back to wings level in an over speed - over bank, but only back to 33 degrees when in high alpha-over bank.
Although I am neither an aircraft designer nor an AB330 expert, I think that both questions can probably be answered from first principles. (But I could of course be wrong)
FIRST QUESTION
In order to answer this question we need to examine lateral stability, directional stability and the interaction between the two.
The term lateral stability refers to the manner in which an aircraft responds to (rolling) disturbances about its longitudinal axis. When banked an aircraft will sideslip and it is the response to this sideslip which determines lateral stability. If it rolls away from the sideslip it is positively laterally stable and will tend to level itself. If it rolls into sideslip it is negatively laterally stable, and any bank will be self increasing. If it neither rolls into nor out of sideslip it is neutrally stable and will neither return to nor diverge from the wings level condition following a disturbance.
The term directional stability refers to the manner in which an aircraft responds to (yawing) disturbances about its normal axis. Following a disturbance in yaw an aircraft sideslips in the opposite direction. If it then yaws into the sideslip it is positively directionally stable and will tend to correct disturbances in yaw. If it yaws away from the sideslip it is negatively directionally stable and disturbances in yaw will be self increasing. If it neither yaws into nor out of the sideslip it is neutrally directionally stable and will neither return to nor diverge from its original heading following a disturbance.
The term spiral stability refers to the combined effects of lateral and directional stability. Following a disturbance in yaw or roll an aircraft will sideslip. If its directional stability is stronger than its lateral stability it will yaw into the sideslip more than it rolls away from it. This will decrease the airspeed over one wing and increases that over the other, producing a dissymmetry of lift, rolling the aircraft towards the sideslip. This will cause further yawing and rolling to that side, such that the aircraft enters a wide spiral dive. This condition is termed spiral instability.
If lateral stability is stronger than directional stability then the aircraft will roll away from the sideslip more than it yaws into it. If it then rolls beyond wings level it will sideslip in the opposite direction. The process will then be repeated, with the aircraft rapidly banking from side to side in the phenomenon of Dutch Roll.
Most commercial jet aircraft possess dihedral, sweepback and large fins. Sweepback and large fins both increase directional stability, whilst all three factors increase lateral stability. Such aircraft are generally prone to Dutch Roll which suggests that they possess a fair degree of positive spiral stability.
SECOND QUESTION
If an aircraft is spirally stable it will tend to roll away from a bank rather than yawing into it. This means that continuous aileron deflection is required to maintain any given bank angle. If at any given bank angle, the ailerons are returned to neutral, the aircraft will tend to roll wings level. It is probably this effect that is referred to in the AB330 manual. In effect the protection system is reducing aileron deflection in order to permit the inherent spiral stability of the aircraft to roll it away from the bank. It is of course possible that the AB330 employs artificially induced spiral stability, in which case the ailerons would need to be deflected to correct the roll.
The question of why the system takes the aircraft back to wings level in an over speed -over bank condition, but only back to within limits (33 degrees) in a low speed, high alpha-over bank situation is probably safety related. The high alpha-over bank is most likely to occur when manoeuvring at relatively low altitude. In this situation an automatic (slight) reduction of bank angle would increase turn radius only slightly, whereas a wings level correction would stop the turn completely. If the purpose of the turn was to avoid a collision, its automatic cancellation might have disastrous consequences.
In the over speed -over bank condition the aircraft is subject to overloading due to two factors. These are high dynamic pressure due to excessive airspeed and high load factor due to excessive bank angle. Although the load factor at the quoted 33 degrees would be less than half of the JAR 25 limiting value(1.2 compared with 2.5), the combined effects of dynamic pressure and load factor might approach critical values. If the situation was caused by an upset during high altitude cruise, the reduction in Mcrit caused by the high load factor would make the need for corrective action even more pressing. Under these circumstance rolling to wings level would immediately reduce load factor and increase Mcrit, enabling increased alpha and/or spoilers to be used to reduce speed.