In normal law speed scale displays VLS, V Alpha prot and V Alpha max speed. In alternate and direct law speed scale is same. It displays VLS and Vsw only.
VLS Speed:
VLS is the lowest selectable speed for the autopilot and autothrust. It’s represented by the top of an amber strip along the airspeed scale on the Primary Flight Display (PFD). VLS for the selected landing configuration (CONF 3 or FULL) is also displayed on the Flight Management System (FMS) APPR page. VLS is computed by the Flight Augmentation Computer (FAC) based on aerodynamic data. VLS increases with extension of speed brakes.
V Alpha prot Speed:
It is the speed at which angle of attack protection becomes active. It is represented by the top of a black and amber strip along the PFD speed scale, in normal law.
Read more: https://aviationthrust.com/alpha-floor-protection-and-high-angle-of-attack-protection/
V Alpha max Speed:
It is the speed corresponding to maximum angle of attack in normal law. If you pull back control stick completely and hold, aircraft with reach and maintain V alpha max speed. Now if you release the sidestick, aircraft will pitch down to maintain V alpha prot speed.
Vsw Speed:
It is the stall warning speed and is displayed only in alternate and direct law. There is no automatic protection associated with this speed. It is represented by a red and black strip along the speed scale.
Does A320 aircraft stall immediately below Vsw (stall warning speed)?
In alternate and direct law no protection against stall is ensured. Here pilot has to ensure the protection based on stall warning or buffeting. In both the law stall warning is set at certain angle of attack with is lower than the angle of attack corresponding to stall. This gives pilot required margin to act and prevent aircraft stall.
